thin wall technology

GE Plastics

Advanced Technology ForTHINWALL DESIGN & PROCESSING

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Contents

ii • Thinwall Technology Guide

IntroductionAbout GE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iiiAbout GE Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iiiAbout engineering Thermoplastics . . . . . . . . . . . . . . . .iv

Thinwall TechnologyWhat is Thinwall Technology . . . . . . . . . . . . . . . . . . . .1-2Thinwall Technology Benefits . . . . . . . . . . . . . . . . . . .1-2Thinwall Technology Market Opportunities . . . . . . . .1-3Standard Molding vs. Thinwall Molding . . . . . . . . . . .1-3Thinwall Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . .1-4Total Systems Approach . . . . . . . . . . . . . . . . . . . . . . . .1-5

Design ConsiderationsDesign Considerations . . . . . . . . . . . . . . . . . . . . . . . . . .2-2Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-3Mode of Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-4Design Strategies for Load Absorption . . . . . . . . . .2-5Design Strategies for Load Transfer . . . . . . . . . . . . .2-5Basic Impact Analysis . . . . . . . . . . . . . . . . . . . . . . . .2-6Advanced Impact Analysis . . . . . . . . . . . . . . . . . . . .2-6Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-6Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-7Part Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-9Ribs, Bosses and Gussets . . . . . . . . . . . . . . . . . . . . .2-9Sink Marks and Voids . . . . . . . . . . . . . . . . . . . . . . . .2-10Component Assembly . . . . . . . . . . . . . . . . . . . . . . . .2-11

Snap Fits/Press Fits . . . . . . . . . . . . . . . . . . . . . . .2-11Manufacturability . . . . . . . . . . . . . . . . . . . . . . . . . . .2-13Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-14

Warpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-17Mold Filling and Gating . . . . . . . . . . . . . . . . . . . . . . .2-18Flow Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-19

Melt Flow Index . . . . . . . . . . . . . . . . . . . . . . . . . .2-20Fill Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-20

Mold-Filling Analysis . . . . . . . . . . . . . . . . . . . . . . . . .2-20

Material SelectionIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-2Materials Requirements . . . . . . . . . . . . . . . . . . . . . . . .3-3Resin Flow Length . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-3Spiral Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-3Melt Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-4Impact Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-4Low Temperature Impact . . . . . . . . . . . . . . . . . . . . . .3-4Aesthetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5Heat Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5Flame Retardance . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-6Mechanical Integrity . . . . . . . . . . . . . . . . . . . . . . . . . .3-6LEXAN® PC Resins . . . . . . . . . . . . . . . . . . . . . . . . . . .3-7CYCOLAC® ABS Resins . . . . . . . . . . . . . . . . . . . . . . . .3-7CYCOLOY® PC/ABS Resins . . . . . . . . . . . . . . . . . . . . .3-7Property Considerations . . . . . . . . . . . . . . . . . . . . . . . .3-8

Flow vs. Impact . . . . . . . . . . . . . . . . . . . . . . . . . . .3-8Aesthetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-9Stiffness vs. Impact . . . . . . . . . . . . . . . . . . . . . . . .3-9Material Evaluation . . . . . . . . . . . . . . . . . . . . . . .3-10

Materials Portfolio . . . . . . . . . . . . . . . . . . . . . . . . . .3-10

ProcessingMolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-2Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-6Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-10

Finishing OperationsScrews and Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-2Ultrasonic Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-3Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-5

AppendixUL 1950 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-2Key Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-3

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2

Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-8

Sales Office . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-9

COVER: Thinwall SM technology is broad reaching andapplies to a variety of markets including telecommunica-tions, computers, business equipment, appliances andautomotive.

© Copyright 1998 General Electric Company

All statements in this guide are subject to the disclaimercontained on page 7-7.

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Introduction

Thinwall Technology Guide • iii

About GE The General Electric Company has its roots in the age of inventionwhen, more than 120 years ago, it was founded by pioneering inventorThomas Edison.

Closely following its founder’s philosophy of innovation and the creativeapplication of technology, GE has grown to become one of the largestand most diversified companies in the world.

Today, GE products and services make a positive contribution to virtuallyevery sector of commerce and industry. From jet engines to financialservices, from lighting and medical systems to factory automation,power generation, transportation and construction.

About GE PlasticsOf all GE businesses, one of the fastest growing is GE Plastics. Today,GE Plastics has emerged as the leading producer of engineering thermo-plastics. Through application development centers around the world,customers can access data from GE designers, engineers, and tooling,processing and finishing experts, utilizing the most sophisticatedequipment and systems available.

Working closely with customers is at the core of the GE Plastics’ businessculture. Today’s customers need to get the job done better, more cost-effectively and within tighter schedules. Having a concentration ofmolding equipment, testing laboratories and product specialists closeto the action permits a cross flow of information that can lead toimportant breakthroughs and exciting new product developments.

At the nucleus of this unmatched global technical network are the world-class facilities at GE Plastics’ headquarters in Pittsfield Massachusetts.Realizing that speed is the key to profitability today, these support services are backed up by production plants in several locations in the U.S., Europe, Australia, Japan and Mexico.


Introduction

iv • Thinwall Technology Guide

About Engineering ThermoplasticsThe advantages of high performance engineering thermoplastics have grown dramatically both from new material developments andthrough a new generation of design engineers.

Today, designers who have learned to “think” in plastics can take fulladvantage of their inherent benefits, rather than just simply translatemetal components into plastic. Some of the many potential benefitsoffered by plastics include:

• Consolidation of parts• Integrated system assembly• Molded-in assembly features• Unprecedented strength to weight ratios• Thinwall technology• Elimination of painting and other operations• Outstanding impact resistance• Excellent chemical resistance

Through re-thinking and re-design, many traditional metal assembliescan be produced in dimensionally stable plastics: with 50% fewer parts,engineered for automated assembly and offering a full range of impact,heat, electrical and chemical properties. This Thinwall Guide is oneexample of how designers can access and utilize the knowledge andexperience available from GE Plastics.


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Thinwall Technology

1-2 • Thinwall Technology Guide

This guide has been created to help address problems commonly associated with design, tooling, and molding of plastic parts in wallsections thinner than today’s nominal wall sections. Basic informationabout products and fundamentals of designing with engineering thermo-plastics utilizing “ThinwallSM” technology have been provided. With thisbasic knowledge, proper resin selection, coupled with good designpractice, should result in the development of a successful part.

What is Thinwall Technology?Thinwall Technology, or Thinwall, gets its name from one of the endresults that it provides, a thin wall section. The difficulty is deciding a wallsection at which a part goes from being “standard” or “conventional”wall thickness to “Thinwall.”

The portable electronics and notebook computer industries have estab-lished themselves as being rich in Thinwall applications. With wallthicknesses often less than one half of a millimeter, there is no questionthat these applications qualify as Thinwall. The smaller the part, typically,the easier it is to fill parts with these small wall thicknesses. Parts thathave different geometries, materials, and longer flow lengths, may notbe able to be manufactured at these low wall thicknesses, even withcurrent Thinwall technology.

The benefits associated with decreasing wall thicknesses below theircurrent values are still measurable and desired even if the final wallthickness is nowhere near those of the aggressive portable electronicsindustry. The techniques suggested within this guide can be applied to a wide range of markets and injection molded applications. Rather thansetting a cut-off value between “standard” and “Thinwall” thicknesses,use of this guide can help attain thinner wall sections.

Thinwall Technology BenefitsReduction of wall thickness has always been important and is an enablingtechnology in a variety of markets. For small hand held parts, thinnerwall sections help reduce weight and overall part size where these traitsare critical. For all size parts, Thinwall technology can also help enhanceproductivity by providing opportunities to reduce costs and cycle times.

The rapid growth in demand in the consumer electronics market forsmaller, lighter, faster cycling parts created a need for advancements in thinner wall-section applications and technology. Requirements continue to increase for lighter, more compact products, necessitatinghigh-performance housing designs with thinner wall sections – a criticalrequirement that is challenging engineers, resin suppliers, toolmakers,processors, and original equipment manufacturers (OEMs) alike.


Thinwall Technology

Thinwall Technology Guide • 1-3

Thinwall Technology Market OpportunitiesA popular market for Thinwall applications and engineering thermo-plastics has been portable electronics where demand for new productshas increased an average of 30% each year since the 1980s. Technologyadvances in the industry are frequent. New technologies and productstend to make current products obsolete quickly. The fast pace hasnecessitated short product design and life cycles. Thinwall technologyhas been advancing at an equal pace. With virtually each new productoffering has come a decrease in wall thickness. This has brought thewall thicknesses of many injection molded portable electronic applica-tions down to levels previously thought to be impossible.

An equally fast paced market with similar goals of light weight andsmallest possible package has been the computer notebook industry.It too has relied on Thinwall technology to not only meet these goals,but also to provide lower cost parts. Here, the parts have been largerand the machines to produce them have been more traditional thanthose used in the portable electronics market.

The technology developed within both of these markets is directly trans-latable to many markets. The trend has been to not only continue tolook for ways to drive wall thicknesses down in established markets,but also towards allowing thinner walls on larger and different parts.

Standard Molding vs. Thinwall MoldingAs an example, today’s injection molding market innovators are typicallymolding hand held applications at wall sections between 0.030 and0.060 in (1.0-1.5 mm), with the rest of the market molding at 0.065-0.100 in (1.7-2.5 mm). However, the gap between these two groupspossibly will increase in the near future as Thinwall technology becomesbetter understood and standardized. At that time, the market will likelybe molding between 0.060-0.080 in (1.5-2.0 mm), while the marketinnovators will be molding between 0.020 and 0.040 in (0.5-1.0 mm).

It is important to note that gains in wall-section reduction don’t alwaysoccur without investment – in this case, in tooling and machinery up-grades. Equally important is the fact that productivity and performancebenefits of reduced material usage, faster cycle times, and lighter weightcan often outweigh most added costs.


Thinwall Technology


Thinwall NomenclatureWhile it may be difficult to apply a thickness at which an applicationgoes from “standard or conventional” wall thickness to “Thinwall”, for thepurposes of discussion three sets of terms with dimensional referenceswill be used:

Standard Wall Thickness Technology [>0.080 in (2.0 mm)]Standard or conventional technology is represented by those applica-tions with wall thickness between 0.125 in (3.2 mm) and 0.080 in (2.0 mm), where conventional design rules apply. Standard engineeringthermoplastics are usually sufficient for these applications. In addition,processing and tooling are well understood and standardized. Muchinformation related to wall thicknesses in this range is contained inGE Plastics Injection Molding Processing Guide, PBG-135 and relatedproduct line publications. As parts become larger (longer flow lengths),Thinwall processing methodology applies, but if the standard wallthicknesses are used, conventional design rules should be applied.

One Step Thinner Wall Thickness Technology [0.080 and 0.050 in (2.0 and 1.2 mm)]Current Thinwall technology is heavily represented by applicationswith wall thicknesses between 0.080 and 0.050 in (2.0 and 1.2 mm);where design rules are transitional, where higher flow resins are required,and where processing favors high pressures and fill speeds that neces-sitate tooling changes. A majority of today’s Thinwall applications fallinto this category.

Dedicated Thin Wall Thickness Technology [<0.050 in (1.2 mm)]The most technically challenging thinner wall thickness technologyapplications are those with wall thicknesses below 0.050 in (1.2 mm).Such dedicated applications require new and unique design consider-ations. In many cases, applications with wall thicknesses <0.050 in (1.2 mm) involve the use of high pressure, high velocity processingtechniques that in turn often require changes not only in tooling, butin the molding machine itself. To address the requirements of Thinwallapplications in this wall thickness range, GE Plastics offers engineeringthermoplastic resin grades with very high flow and outstanding physicaland mechanical performance.


Thinwall Technology


Total Systems ApproachExperience has proven that risk increases when thinner wall applica-tions tare approached independently from a design, processing, ortooling point of view. Hence, it is important to treat all aspects ofThinwall technology in an integrated systems approach that does notseparate design, tooling, or manufacturing. When the inter-relationshipbetween these functions is clearly understood, the chances for successare the greatest.

GE Plastics welcomes the opportunity to work with industry players –OEMs, designers, engineers, tool builders, and processors – on expandingThinwall technology from an art into a science. When key players areinvolved at the outset of the product development cycle, both the teamand the application benefit from a wide range of real-world experience.Understanding current market challenges and anticipating futurechanges within Thinwall technology, GE Plastics has formulated specialThinwall resins and worked on new design and processing techniquesto minimize the risks of Thinwall applications.


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Design Considerations


Design ConsiderationsDesign is a key component of successful product development. Whenworking on applications with thinner walls, depending on the marketand the application, three main issues or goals will typically be of con-cern to the engineer:

• Impact • Stiffness

• Manufacturability,

As engineers move to increasingly challenging designs with wall thick-nesses of less than 0.050 in (1.2 mm), many “normal” design guidelinesand approaches change. Achieving required design features may bemore challenging as wall thickness decreases. Figure 1 reviews some of the design issues and options that are considered.

Figure 1. Thinwall Design Issuesand Options. Design Issues Design Options

High ImpactEnergy

Appearance Concerns

Flow LengthLimitations

Large Flat Surfaces

TightTolerances

Large StaticLoads

Extremely Tough

MinimumWeight

MinimumSize

ExtremelyRigid

Geometry Effects

ImpactModifiedMaterials

AssemblyTechniques

MultipleGates

ReinforcedMaterials

ProcessingTechniques

The following information generally applies to all applications wherewall thicknesses are typically between 0.080-0.050 in (2.0-1.2 mm). In many instances, information provided for these wall thicknesses can be translated from applications with wall thicknesses from 0.080-0.125 in (2.0-3.2 mm). Special notations for applications with wallthicknesses less than 0.050 in (1.2 mm) will be noted specifically.

} both of which relate to the mechanical performance of a component in its end-useenvironment; and,

which relates to mold filling and cooling performance and the part’s dimensional stability.


80706050403020100

0 0.02 0.04 0.06 0.08

Strain

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Impact Performance vs. Loading Rate

ε = 5 × 10-2 1/S ε = 5 × 10-1 1/S

ε = 10-4 1/S

ε = 10-2 1/S

1.61.41.21.00.80.60.40.2

0˚ 50˚ 100˚ 150˚ 200˚ 250˚

Impact Performance vs. Temperature

Temperature (F˚)

Tens

ile S

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tive

toRo

om T

empe

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Impact Performance vs.Part Complexity and Stress Rate

Uniaxial

Biaxial

Triaxial

Impact Performance – Brittle vs.Ductile Failure Mode

Impa

ct P

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Loading Rate: Low High

Brittle FailureDuctile Failure



ImpactImpact events take place when two or more bodies strike each other at speed (See Figure 2). Drops or falls where a component strikes theground are the most common impact requirement for plastic parts orassemblies. The response of an engineering thermoplastic part to suchan impact is affected by three main factors:

• Loading Conditions (Impact Velocity)• Temperature• Geometric Complexity of Part (Stress State)

Figure 2. Impact Performance.




All three of these factors are interrelated such that a complex partwith sharp notches may fail in a brittle mode at a high impact velocityand low temperature, but may fail in a ductile mode if temperature is increased or the velocity is decreased. Accurate design for impactperformance requires the use of appropriate analysis techniques and a good understanding of material properties.

Mode of FailureWhen a part is loaded to failure, it will fail either in a brittle or ductilemanner. The only other option is a combination of the two modes.Figure 3 illustrates the ductile and brittle behavior in biaxial instru-mented impact tests.

Brittle failure occurs when there is a rapid drop in load carrying capability after crack initiation, and little or no additional energy isneeded to propagate the crack. Brittle failure occurs at a stress level that is lower than the yield strength of the material and with minimalplastic deformation (yielding).

Ductile failure occurs when there is a relatively slow drop in load carrying capability as the material tears, and additional energy isrequired to continue to propagate the crack. Plastic deformation isalways present with ductile failures, which occurs beyond the yieldstrain of the material.

Figure 3. Ductile and BrittleBehavior. Displacement (in.)

Displacement (mm)

Load

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BrittleFailure Ductile Failure

0

500

1000

1500

2000

2500

3000

3500

4000

0

100

200

300

400

500

600

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0 5 10 15 20 25

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.800.70 0.90




Design Strategies for Load AbsorptionApplications utilizing unreinforced thermoplastics typically use a strategyof load absorption during an impact event. In the case of a plastic housingit tends to absorb most of the energy of the impact through deformation.This design strategy requires more package space, since the housingneeds to deflect without contacting the internal components. A part isgenerally considered to have failed if significant unrecoverable defor-mation is present after the impact event – even if internal componentsare still functioning.

When opting for this design strategy, it is important that internal components are secured tightly and do not move during the initialimpact event or this could result in a second impact event of these components into the housing. Snug fits and effective fastening canoften be used to attach the internal components to the housing and stillprovide sufficient room for the housing to deflect during impact. Allsharp corners and stress concentrators should be avoided in this typeof design to increase the impact performance of the unit and toreduce the chance of brittle failure.

Design Strategies for Load TransferIn other applications, reinforced thermoplastics will typically use a strategyof load transfer during the impact event. In this case, the plastic housingdeflects very little, transferring the impact energy directly into theinternal structures and components. Such a design requires virtuallyno additional package space, because the housing should not deflectsignificantly during the impact. In fact, it is important to minimizedeflection of the housing to assure that strains remain low and reducethe chance of failure of the housing.

When using this strategy, it is important to remember that the housingmerely has to be stronger than the weakest internal component. It isoften employed with notebook computers where some components,such as hard drives or LCDs, are not exceptionally robust.

Internal components should be fastened snugly so that secondary impactsbetween internals and the housing are minimized. Stress concentratorsand sharp notches should be avoided to increase performance. Generally,in load transfer designs, internal components fail before the housing.




Basic Impact Analysis As a quick estimate of a design’s impact performance, simple handcalculations can be used to determine how much energy must beabsorbed during the impact event. A common and useful assumptionis that all kinetic energy in the system prior to impact is converted intostrain energy during impact. This assumption allows a designers to calculate deflections and stresses for simple parts rapidly and is oftenuseful in preliminary design evaluation.

Figure 4. Advanced ImpactSimulation.

Incr

easi

ng T

ime

Brittle Ductile

Advanced Impact AnalysisNon-linear finite element analysis (FEA) techniques can be used tocharacterize the impact performance of a thin wall part. When material behavior is properly modeled, these techniques can providecorrelation with both simple test parts (like an impact tup) and complexreal-world parts (like telecom enclosures and laptops). The load/deflection response, including ductile and/or brittle failure, can bepredicted, helping designers assess design feasibility before tooling is cut and actual parts are tested. (See Figure 4.)

StiffnessThe need to achieve a desired deflection under a known load is acommon requirement for thermoplastic components. To determine a suitable approach for a given application, consideration must be givento the overall goals and limitations of the design, such as minimizingweight or package space. For example, in an application where partvolume or package space is limited, an engineer may not be able to




achieve higher stiffness by manipulating part geometry alone. A stiffermaterial or tighter assembly techniques may also be necessary to achievethe design goals within its limitations.

As wall thicknesses decrease the need to design in stiffness againincreases. For many applications significant use of ribbing is not feasible.Part stiffness must therefore be achieved in other ways.

One generally effective method to increase stiffness is to modify the part geometry. For example, using curved surfaces instead of flatplates will help limit deflection. Part stiffness can also be enhancedthrough careful design of assembly features in each piece of a largercomponent.

As wall sections become even thinner, in the range of less than 0.050 in(1.2 mm), the need to design-in stiffness increases even more. Currently,many of these applications are very compact hand held products and donot feature designs with large unsupported spans, therefore, reachingthe target stiffness value is achievable. By combining geometric featuresand assembly techniques, required stiffness can usually be attained with-out using reinforced materials. For larger parts, stiffness can be improvedthrough proper material selection, design, and assembly.

Material SelectionOne way to increase part stiffness with standard injection moldingapplications is to select a resin with a higher elastic (tensile) modulus.Glass or carbon-fiber reinforced thermoplastics provide 2 to 6 timesthe modulus of unreinforced resins. (See Figure 5.) Typically, thematerial selection decision must be made early in the design phasebecause it will influence not only what stiffness strategy is employed,but also which strategy is used to achieve the necessary impact per-formance. There is a direct relationship between an increase in amaterial’s elastic modulus and an increase in part stiffness.

It is important to remember that fiber-reinforced resins may haveanisotropic (directionally dependent) mechanical properties, behavein a more brittle fashion, increase tool wear, and have lower aestheticsthan unreinforced plastics. Use of these materials may require morecare during component design to ensure maximum performance.

Applications with thinner wall sections in which reinforced resins are often chosen tend to feature large unsupported spans and mustmeet stringent compactness requirements. A good example of such an application is found on the back of LCD panels for a notebookcomputer. In this case, common geometric stiffening features such as ribs and gussets cannot easily be incorporated due to packagingrequirements. So, stiffness is generally sought through the use of areinforced resin.




Unreinforced materials have been used successfully in many applicationswith wall thicknesses ranging from 0.080-0.050 in (2.0-1.2 mm). If thereare no large, unsupported spans, or if part geometry can be used toimpart stiffness (e.g., clothes iron), stiffness targets may be easily achieved.In parts with large spans, required levels of part stiffness are typicallyobtained through the general geometry of the part (e.g., using a curvedpart as the back panel on the LCD display), and by effectively attachingthis part to the rest of the components in the assembly.

With smaller hand held applications, where wall thicknesses are less than0.050 in (1.2 mm), unreinforced thermoplastics are usually selected,because stiffness is attained through size, geometry, and assemblyrather than material reinforcement. For larger applications, reinforcedmaterials can often be used to quickly increase the part stiffness. Theengineer must be cautioned once again that reinforced materials mayhave anisotropic (directionally dependent) mechanical properties. Timespent in designing in stiffness is well worth the effort if an unreinforcedproduct can be used.

Figure 5. Stress/Strain Curves forReinforced and Unreinforced

LEXAN® Resins.

Reinforced LEXAN® SP Resin

Unreinforced LEXAN® SP Resin

12000

10000

8000

6000

4000

2000

0 1 2 3 4 5 6

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10

20

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60

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Material Selection (Continued)




Part GeometryOne of the most effective ways of improving part stiffness is throughpart geometry. By using curved surfaces or including such reinforcingfeatures as ribs, bosses, and gussets, substantial increases in stiffnessare possible. Since flexural stiffness is proportional to the cube of theheight of a rib or of wall thickness, even the addition of small ribs orof small increments in wall thickness can provide a significant increasein overall part stiffness.

Ribs, Bosses and GussetsRibs, bosses, and gussets are common design features in applicationswith wall thicknesses ranging from 0.125-0.080 in (3.2-2.0 mm) as shownin Figure 6. Ribs can be used to greatly increase part stiffness. Bosseshave many uses, including facilitating assembly, accepting inserts, andhelping to position a part relative to a larger component. Gussets canbe used to increase the overall part stiffness, or to increase the stiffnessof particular features such as bosses or ribs. Gussets can also be usedto help local adjacent parts in a larger component. The design guide-lines for standard applications are well known and provide options fordesigning with these features. Thinwall applications bring their ownset of guidelines

Figure 6. Features DesignGuidelines for ThinwallApplications.

Conventional:Wall Thickness Thinwall:

trib ≤ 0.6 • twall

h ≤ 4 • twall

r ≥ 0.375 mmθ ≥ 1/2˚

trib ≤ 0.6 • twall ➝ twall

h ≤ 4 • twall

r ≥ 0.375 mm ➝ 0.6 • twall

θ ≥ 1/2˚ ➝ 1˚

trib ≤ twall

h ≤ 4 • twall

r ≥ 0.6 • twall

θ ≥ 1˚

tboss ≤ 0.6 • twall

h ≤ 4 • twall

r ≥ 0.375 mm

tboss ≤ 0.6 • twall ➝ twall

h ≤ 4 • twall

r ≥ 0.375 mm ➝ 0.6 • twall

θ ≥ 1/2˚ ➝ 1˚

tboss ≤ twall

h ≤ 4 • twall

r ≥ 0.6 • twall

θ ≥ 1˚θ ≥ 1/2˚OD ≈ 2 • ID OD ≈ 2 • ID OD ≈ 2 • ID

tgusset ≤ 0.6 • twall

h ≤ 4 • twall

r ≥ 0.375 mm

tgusset ≤ 0.6 • twall ➝ twall

h ≤ 4 • twall

r ≥ 0.375 mm ➝ 0.6 • twall

θ ≥ 1/2˚ ➝ 1˚

tgusset ≤ twall

h ≤ 4 • twall

r ≥ 0.6 • twall

θ ≥ 1˚θ ≥ 1/2˚

Ribs

Bosses

Gussets

0.080 to 0.125 in(2.0 to 3.2 mm)

0.050 to 0.080 in(1.2 to 2.0 mm)

< 0.050 in(1.2 mm)

+




Sink Marks and VoidsTo reduce sink marks and voids, ribs, bosses and gussets generallyneed to be thinner than the nominal wall-section in conventional appli-cations. In addition, sides of these features should have draft to facilitateremoval from the mold. To promote complete filling of the draftedfeatures, the maximum height of the feature should be kept below four-times the nominal wall thickness. Sharp notches should be avoided toreduce stress concentrations at the base of the feature. See a standardGE Plastics design guide for additional information on designing with these features.

Increasing a part’s moment of inertia by adding features like ribs, bosses,and gussets is a traditional means of adding stiffness. However, sincemaximum compactness of the final product is often a design limitation,and the addition of ribs and gussets may not be possible, stiffness maybe increased by using the overall part shape. For instance, with Thinwallparts, stiffness can be increased by selecting a curved surface instead of a flat one or by incorporating small ribs to accentuate styling orfunctional elements (raised or recessed features), such as the edge of a product logo or the border of a system component.

Design guidelines, for Thinwall applications where thickness is less than0.050 in (1.2 mm), are different than those for standard wall thicknessapplications above 0.080 in (2.0 mm). Refer to Figure 6 for Thinwalldesign guidelines.

For Thinwall applications between 0.050-0.080 in (1.2 mm-2.0 mm) theseparts represent a transition in feature design between standard designguidelines and Thinwall guidelines. At the upper limits of wall thickness,in general, design guidelines for wall thicknesses ranging from 0.080-0.120 in (2.0-1.3 mm) generally apply when designing features such asribs, bosses, or gussets. As wall thicknesses decrease to 0.05 in (1.27 mm),the greater design freedom characteristics indicated for dedicatedThinwall applications can typically be used.

Due to weight and ergonomic issues for small hand held Thinwallapplications, compactness is of great concern. With these Thinwallparts, ribbing strategies may take up too much space to be effective for increasing stiffness. Fortunately, hand held parts are small, so require-ments for part stiffness can be usually met using key stiffening features.For instance, by adding ribs to component features such as buttons,screens and logos, stiffness can be substantially improved. Stiffness isalso improved by selecting curved rather than planer surfaces.




For larger Thinwall applications, stiffness should be improved whereverpossible. Styling lines and curved surfaces can be used to improve theaesthetics as well as the stiffness of the part. Internal components canalso be used to further enhance the rigidity of the housing by carefullytying parts together. While achieving the desired stiffness with largeparts can be challenging, it is, for most applications quite possible.

For parts where wall thicknesses are less than 0.050 in (1.2 mm), thedesign suggestions for ribs, bosses, and gussets change since high injec-tion and packing pressures are used during molding. Part shrinkage,especially near the gate, is reduced, helping to limit sink marks andvoids. This, in turn, can allow design features to be made as thick as thewall-section. When these features are located far from the gate, however,traditional design guidelines relating to the thickness of design featuresto walls should usually be applied, because the pressure at that locationwill not generally be as high as at the gate. Also, to facilitate ejection ofthe part, draft angles on ribs, bosses, and gussets should be increased,because they will generally shrink less than they will at traditional wallthicknesses. To help minimize stress concentrators, radii at the base of the features can be increased.

Component AssemblyAnother way the stiffness of a part can be improved is through assemblyinto a larger component. By fixing two parts of a component together,such as a shoe box and its lid, the stiffness of the overall component isincreased versus the stiffness of the individual parts. In the case of theshoe box, if the component did not include the lid, or the lid merelyrested on the box without overlapping its sides, it would be significantlymore flexible. This same concept can be translated into many thermo-plastic parts, and especially Thinwall parts, to greatly enhance the stiffness of the overall component.

Snap FitsWith traditional design, snap fits are a good example of assembly features that improve overall component stiffness by tightly linkingtwo parts together. Snap fits also simplify assembly by reducing oreliminating screws or bonding operations and facilitating subsequentdisassembly and recycling. The same considerations as ribs and gussetswith respect to thickness, height, width and draft apply to snap fits.

With conventional and Thinwall applications alike, properly designedsnap fits are engineered so no permanent plastic deformation of thesnap finger results from its use. Typically, maximum strain of the snapfinger should not exceed 6% for unfilled resins and 1.5% for filled resins,although this value will vary depending on the specific grade of resin




used. The maximum bending strain of a snap finger can be calculatedusing standard linear beam equations, as long as the secant modulus, notthe elastic modulus, is used (See Figure 7). See a standard GE PlasticsDesign Guide, PBG-130, for additional information on designing withthese features.

Assembly features of a Thinwall part can also be used to greatly enhancecomponent stiffness (See Figures 8 and 9). For example, a shoe box isalways more flexible than the same box with a lid. Stiffness of thatcomponent can be further increased by more rigidly attaching the lid.A lid that is snapped down on the box will be much stiffer than onethat is merely resting in place. And a lid that is screwed into the boxwill result in a component that is even more rigid.

Snap fits, screws, ultrasonic welding, bonding, and the use of molded-ininterlocks can also be used during assembly to effectively stiffen a Thin-wall component. The design goal here is to have two or more parts act as if they are one part – i.e., to constrain all degrees of freedom atthe interface between the parts. This design approach may require thepart to hold tighter dimensional tolerances, since mating parts mustfit tightly together. Also, snap fits in Thinwall applications will usuallyappear more flexible due to deflection of much thinner sidewalls. Inconventional applications these same sidewalls are assumed to be rigid.

Figure 7. Snap-Fit Design.

ε = Strainy = Beam Deflectiont = Beam Thicknessl = Beam LengthB = Beam WidthEs = Secant ModulusF1 = Insertion ForceF = Cantilever Forceµ = Coefficient of Frictionφ = Insertion Angle

ε =3yt2l 2

F =yBt 3

4l 3Es

y t

B

l

Friction Normal

NormalForce

Cantilever ForceF1

φθ

Use secant modulus to calculate beam stiffness and insertion and pullout forces.

F1 = Fµ + tan φ

1 – µ tan φ( )

Snap Fits (Continued)




By using molded-in features to interlock pieces, component stiffnesscan be increased significantly. Tying into the stiff, internal componentscan also enhance rigidity.

When moving to thinner wall sections engineers need to be aware thatdesign strategies change and become more complex, and are furtherinfluenced by processing limitations and materials selection. A criticalchallenge in Thinwall design is to compensate for thinner wall sectionsby increasing component stiffness. This is accomplished through acombination of materials selection, use of geometric features, and carefulselection of assembly features.

Figure 8. Press Fit and SpadeInterlocks for Thinwall Assemblies.Spade InterlockPress Fit Interlock

Figure 9. Boss and Snap-FitInterlocks for Thinwall Assemblies.

Boss Interlock Snap-Fit Interlock

Front

Back

ManufacturabilityEnsuring that a part can be manufactured economically is the designer’sresponsibility. With adequate forethought and early input from manu-facturing, tooling, and the materials supplier, design engineers can helpreduce potential manufacturing problems and develop componentdesigns to improve the results from the injection molding process. Thedesign engineer must consider how the part geometry will affect, andbe affected by mold shrinkage. Also, the designer must consider how




Manufacturability (Continued)the part will be filled, and where it is to be gated. Mold-filling analysis canbe used early in the design phase to help assess part geometry andprocessing conditions.

As wall thicknesses are reduced to the range of 0.080-0.050 in (2.0-1.2 mm), the injection pressures required to fill parts increases. And asinjection pressure increases, so does the need for careful part design.To help maintain good dimensional performance, guidelines for goodmolding practices should be followed and observed very stringently.In addition, the use of mold-filling analyses becomes even more import-ant in thinner wall-section applications to help reduce the risk of creatingan unmoldable design.

For applications where wall sections are less than 0.050 in (1.2 mm), process-ability becomes a greater concern. While there has been good successin some markets at driving the average wall thickness down to these levels, and while it is usually still possible to predict mechanical perfor-mance, it is strongly advisable to assemble all stakeholders in the designtogether early in the development process. By bringing the part designer,materials supplier, processor, and tool builder together, it is more likelyto achieve a better design, faster, and with much less trial and error. Thisconcurrent engineering approach can also reduce the risks of moldingat thinner walls, helping to provide a design that will be manufac-turable and a part that will perform.

ShrinkageWith all applications to promote proper function, quality and aesthetics,thermoplastic parts must achieve and hold well-defined dimensionaltolerances. The dimensional performance of the part is largely controlledby the mold shrinkage.

The mold shrinkage applied to both conventional and Thinwall appli-cations is commonly thought of as solely a material property and is actuallya system property driven by processing conditions, part geometry andmaterial properties. This can be demonstrated by considering the shrink-age of a part as the sum of local shrinkages from all over the part. Theselocal shrinkages are controlled by the response of the material to thetemperature and pressure history at that point.

This history, in turn, is controlled by the process conditions set on the machine and the geometry of the part. So, the part geometry andprocess conditions set the temperature and pressure history at everypoint in a part to which the material responds, causing local shrinkage.When these shrinkages are summed up over the entire part, an overallmold shrinkage can be determined.




One of the key process conditions that influences shrinkage is the packingpressure magnitude and the packing time as illustrated in Figure 10.As the material cools in the mold, it shrinks. Packing pressure is usedto add more material to the old cavity to help compensate for this shrink-age. If the packing pressure is higher or the packing time is longer,more material can be pumped into the cavity to help compensate forshrinkage, which decreases.

Figure 10. Effects of PackingPressure on Shrinkage.

Injection Molding Time

Filling Packing

Pres

sure

Length


Filling Packing

Pres

sure

Length


Filling Packing

Pres

sure

Length


Filling Packing

Pres

sure

Length

Part geometry can play a profound role in the dimensional performanceof a part. The geometry on the flow path between any local point inthe gate helps determine the pressure history at that point. For instance,if the gate is located in a thin section of the part that freezes off quickly,packing pressure will not be maintained in thicker sections of the partlocated further from the gate, causing increased shrinkage in thesethicker sections. Another example is molding a center-gated disk andachieving a flat part. The tendency in this application is to overpackthe center (near the gate) and underpack the periphery. In that case,the perimeter of the part can shrink more than the center, warpagecan occur, and the part can become bowl-shaped.




Shrinkage (Continued)The response of the material can be seen using PVT diagrams, whichplot changes in specific volume (1/density) with temperature for variouspressures. Both the compressibility and the thermal expansion can beseen on these diagrams. Since the process and geometry dictate thepressure and temperature history at every point in the mold, the materialresponse can be roughly estimated locally using a PVT diagram.

Engineering thermoplastic materials typically used for computer andtelecommunications devices (such as LEXAN® polycarbonate resin,CYCOLAC® ABS resin, or CYCOLOY ® polycarbonate/ ABS resin) areamorphous polymers that have relatively low and predictable shrinkagerates (0.005-0.007 in/in [5.0-7.0 mm/m]). Unfilled amorphous resins alsotend to shrink isotropically (independent of direction). Because thesematerials do not tend to shrink extensively when processed correctly,the large shrinkage differentials that can lead to warpage are moreeasily avoided.

Fiber-reinforced thermoplastics exhibit anisotropic (directionally depend-ent) shrinkage based on orientation of the filler, which tends to line upin the direction of flow. Therefore, gate placement, which defines flowand cross-flow directions in the part, can have a significant impact onshrinkage as well as part stiffness. Orientation of the reinforcing filler inthe flow direction will reduce shrinkage values in that direction versusthe cross-flow direction. Care should be taken to place gates properlyto balance not only structural but also dimensional considerations whenworking with fiber-reinforced thermoplastics. See standard GE Plasticsdata-sheets for shrinkage values on fiber-reinforced thermoplastics.

As wall sections decrease to where thicknesses are less than 0.050 in(1.2 mm), the injection velocities required to fill these parts increase.The higher velocity combined with the thinner wall sections increasethe shear rate seen by the material during filling. In turn, this highershear rate will tend to enhance molecular orientation with reinforced andunreinforced materials, and fiber orientation with reinforced materials,leading to anisotropic shrinkage, where the material will shrink morein the cross-flow direction than the flow direction (See Figure 11).




To help reduce anisotropic shrinkage, parts should be adequately packed-out. Since Thinwall parts freeze-off very quickly, it is important to haveshort injection times in order to allow for adequate packing while thecore is still molten. This is especially critical for applications where wallthickness is less than 0.050 in (1.2 mm). It may also be necessary to uselarge gates (where gate thickness is greater than wall thickness) to promote sufficient flow of plastic through the gate during packing. If traditional gates (sized to 80% of wall thickness) are used, the gatecan freeze-off very quickly and reduce packing of the part.

WarpageWarpage of parts is caused by differential shrinkage throughout the moldcavity. During the filling phase, molten plastic is injected into the moldunder high pressure at the gate and the melt front is at or near atmos-pheric pressure. As the part fills, this pressure gradient remains, althoughit lessens during the packing phase. Higher pressure sections of the partwill tend to shrink less than sections held under low pressure, whichcan lead to warpage. Use of a sufficient packing phase as well as properpart and gate geometries will help reduce this phenomenon.

Figure 11. Shrinkage in ThinwallGeometries.Mold Wall

Direction of Flow

Molten Core3 mm Wall

Flow Front

1 mm Wall

Mold Wall

Molten Core

High Shear LayersMold Wall

Mold Wall

Frozen Skins




Mold Filling and GatingMold and runners should be designed and sized such that parts fillfrom thicker to thinner sections to reduce large, uncontrolled shrinkagevariations, which can lead to voids, sink marks, and excessive warpage.A single constant nominal wall is generally suggested, but when that isnot possible, parts should be gated into thicker sections to avoid trap-ping molten material that, uncompensated by packing pressures, willshrink more than the material surrounding it once the gate has frozenoff. Gates should be large enough (greater than the nominal wall) thatthe gate does not freeze off prematurely, before proper packing canbe accomplished. Gates should be positioned to achieve balanced fillingof the mold, which helps to reduce shrinkage differentials and warpage,and reduce injection pressure requirements (Figure 12).

Figure 12. Gate Placement andFlow Balancing.

Time (sec.)

Inje

ctio

n Pr

essu

re (M

Pa)

Inje

ctio

n Pr

essu

re (p

si)Unbalanced

Balanced

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 01.0

500

1000

1500

2000

2500

0

2

4

6

8

10

12

14

16

18

20

Unbalanced Flow

Gate

Balanced Flow




Flow LengthThe maximum length a material will flow into a mold will help deter-mine both the number and location of gates in that mold. It can alsoset a lower bound on wall thickness if, for instance, a single gate isdesired and the material must reach the extremities of the mold. Themaximum flow length is driven primarily by part geometry, processconditions and material properties.

Typically, the flow properties of particular resins have been indicatedby spiral flow data found in product literature. While these values maybe somewhat useful for comparing different materials, they cannot beused directly for design purposes with either conventional or Thinwallapplications.

Actual part flow patterns are typically a combination of three configu-rations – channel flow, (e.g., spiral flow geometries); radial flow (e.g., a center-gated disc); or planer flow (e.g., an end-gated rectangularplate) – with most parts having predominantly planer and radial flow(Figure 13). Using radial flow length estimates to determine maximumflow length, therefore, can be quite useful for the design engineer.

Figure 13. Basic Flow Geometries.

Radial Flow Geometry Planer Flow Geometry Channel Flow Geometry

Radial flow lengths at suggested processing conditions and varying wallthicknesses can be obtained from a GE Plastics’ representative or byconsulting the Engineering Design Database. Typically, as a rule of thumbradial flow lengths are approximately 70% of maximum spiral flowlengths for the same melt temperature and wall thickness. However, thisvalue can vary with material, wall thickness and processing conditions.




Melt Flow Index Melt Flow Index (MFI (ASTM D1238)) is a form of low-shear quality-control testing that has also been used to indicate resin flowability.However, since this value represents only a small portion of the complexrheological behavior of engineering thermoplastics, it is not, standingalone, a good measure for use either in design or for material comparison– especially when comparing resins from different families (e.g., LEXANpolycarbonate and CYCOLAC ABS resins).

Fill TimesFor Thinwall applications, the length of flow of a material into a partcan be improved by injection time. For applications less than 0.050 in(1.2 mm), this injection time will typically be between 0.1 to 0.5 seconds.Such short injection times require the use of high pressure, high speedinjection molding equipment. If longer times are used because ofmachine limitations due to speed or pressure, expected flow lengthswill probably not be achievable and molded in stress will be extreme.This situation can render an otherwise good design useless.

Mold-Filling AnalysisSince maximum resin flow length and mold filling are dependent onmany factors – e.g., material, part design, and processing conditions –mold-filling analysis tools can be very useful for assessing/evaluatingpart design and processing conditions. The use of these tools can helpeliminate expensive trial and error tool design and improve part quali-ty. Multiple configurations (e.g., part geometry, gating configuration,process conditions, material, etc.) can be electronically prototyped early in the design phase while changes are easier and less costly to make.

Mold-filling analyses can be used to help assess the processability ofThinwall parts. While accurate thin wall filling simulation presents somechallenges due to the very rapid injection phase, good correlation canoften be achieved between simulations and molding trials. Some of thecommercially available software even have Thinwall specific materialmodels to more accurately predict filling pressure. However, these




simulations will not currently predict material degradation during processing. If the process contains excessive residence time, melt tem-peratures or shear – all of which can cause material degradation – thesimulation will probably not accurately represent the magnitude of theinjection pressure. This is because degradation causes a viscosity change,which in turn changes the pressure requirements. However, filling andcooling patterns and pressure trends will not be affected.

When moving to thinner wall sections engineers need to be aware thatdesign strategies change and become more complex, and are furtherinfluenced by processing limitations and materials selection. A criticalchallenge in Thinwall design is to compensate for thinner wall sec-tions by increasing component stiffness. This is accomplished througha combination of materials selection, use of geometric features, andcareful selection of assembly features.

In many cases, when designing at the upper limits of Thinwall wherewall thicknesses are greater than 0.070 in (1.7 mm), conventionaldesign guidelines can be applied to features such as ribs, bosses orgussets. As wall thickness decreases to 0.060 in (1.5 mm), the greaterdesign freedom characteristics of Thinwall design guidelines can oftenbe used.


Notes


GE Plastics

Mat

eria

lSe

lect

ion

Thi

nwal

lSM


Material Selection


IntroductionNew products are frequently vital parts of a business strategy. Bringingthe right product to the market at the right time, at the right price is anenviable goal. Knowing the process for new product development andexecuting the steps flawlessly can provide a substantial edge. Manufac-turing with engineering thermoplastics continues to enable many newproduct developers to propose innovative solutions.

Proper materials selection is a critical step in the product developmentprocess. A properly specified material can enhance a product’s designand complement manufacturing. An improperly specified materialcan hamper proper function of the design and can render the productdifficult to process. Therefore, proper understanding of the material’sbehavior, processing constraints, and the end-use requirements arenecessary for material selection.

In its purest sense, materials selection is a process of elimination. The easiest way to begin is to define and rank key application require-ments, e.g. it must be tough, rigid, and have good aesthetics.

Next, these requirements need to be translated into physical properties,e.g. it must have good impact resistance, relatively high tensile modulus,offer a wide processing range, and provide a low gloss appearance.

Once key physical properties have been identified, a broad materialselection can be performed based on general performance of variousresin families. Finally, a more detailed selection is performed based onthe specific requirements of the part design and end-use environment.

Engineering calculations will be useful to help define and quantify key material properties required and to help further refine materialsselection. Actual molding of parts is always required to confirm finalpart design and performance and finalize materials selection. Resinsshould not be selected based solely on datasheet values alone, althoughthis resource can be useful for comparing various materials.


Material Selection


Understanding how important proper material selection is to successfulThinwall design and processing, GE Plastics has developed a wide rangeof materials to specifically address the requirements of a variety ofapplications. GE Plastics’ engineering thermoplastic resins have allbeen used successfully in various standard applications. For Thinwallapplications with wall thicknesses below 0.080 in (2.0 mm), highermaterial performance is often required. For these applications, specificLEXAN polycarbonate resin grades offer property profiles that includehigh strength, durability, and good processing characteristics, whileselected CYCOLAC ABS or CYCOLOY PC/ABS resin grades offer abalance of cost-effectiveness and performance. Many other GE Plastics’products have been chosen for use in various Thinwall applications. Contact your local GE Plastics representative or call 1-800 845-0600 foradditional information.

Material RequirementsEngineering thermoplastic materials used for Thinwall applicationsmust provide both processing freedom and the performance to withstand abusive end-use environments. The following properties are critical:

Resin Flow Length Resin flow length, is the distance an injected resin will flow before themelt front is stopped by freeze-off. It is a critical property for Thinwallapplications. However, the need for increased flow length must be balanced against the need to maintain other mechanical properties,such as impact strength. (For more details on flow length, see Designsection on pages 2-2 to 2-21.)

Maximum flow-length requirements for typical Thinwall applicationsin the range of 0.080-0.050 in (2.0-1.2 mm), such as small applianceproducts, are about 6 in (15 cm). For notebook computers flow-lengthrequirements might be in the range of 10 in (25 cm). Flow requirementscan be reduced by adding more gates, but doing so can create cosmeticproblems with additional knitlines. Sequential valve gating can be usedto reduce these issues.

Spiral FlowSpiral flow however, is the most common method of comparing materialflow length. Because spiral flow does not accurately model flow in mostapplications, it should not be used to establish the minimal wall thick-ness of a part. (For more details, see Design section on pages 2-2 to 2-21.)


Material Selection


Melt Flow RateViscosity (melt flow rate) has also been used to compare materials, butbecause of the nonlinear behavior typical of the test, and plastic mate-rials in general, results can be very misleading. Generally, CYCOLACresins have the highest flow lengths, followed by CYCOLOY resins andthen LEXAN resins.

Figure 14. Relative Flow Length vs.Wall Thickness (Typical).

Wall Thickness (in.)

Rela

tive

Flow

Len

gth

0.04 0.06 0.08

CYCOLOY High Flow Resins

Standard CYCOLOY & LEXAN SP Resins

CYCOLAC Resins

Standard LEXAN Resins

Impact Strength Impact strength is very often a critical property in Thinwall applications.Notched Izod values (ASTM D256A) of 12 ft-lb/in (640 J/m), or more,are typically used to meet toughness requirements. In general, experiencehas shown that notched Izod values are not always sufficient to properlycharacterize a material’s impact performance in Thinwall-section appli-cations. Notched Izod and Instrumented Impact values for the samematerial can vary significantly due to sample geometry and test meth-ods. Performance is more accurately measured by load/displacementcurves (similar to stress/strain curves) generated from instrumentedimpact testing. These load/displacement data found in GE Plastics’Engineering Design Database can help product designers and engineersbetter understand and evaluate a material’s impact performance. Actualpart molding and testing is necessary to determine whether a particularmaterial satisfies the needs of a particular application.

Low Temperature ImpactLow temperature impact performance can also be a critical property.Otherwise ductile materials can transition into brittle behavior basedon temperature for a given geometry and impact speed. In most caseswith Thinwall popular LEXAN resin and CYCOLOY resin, an impactmodifier is used to achieve ductile behavior at temperatures below -20°F (-29°C).


Material Selection


Aesthetics Aesthetics of Thinwall applications are often important for consumerappeal. Blemishes of any kind (sink marks, splay, blush, knitlines, etc.)are typically not acceptable. Lower gloss levels are generally preferred forthese products, and acceptable color shifts due to ultraviolet exposuretypically range between 1 to 2 ∆e. Painting is an option, but it can beexpensive.

StiffnessStiffness of a molded plastic component is a function of both materialproperties and design geometry and plays an important role in providinga high-quality feel. Typically, the unreinforced engineering thermo-plastics used in Thinwall applications have a flexural modulus (ASTMD790) of roughly 300,000 psi (2,000 Mpa). For reinforced products,flex modulus can range from 500,000 to 2,000,000 psi (3,500 to 13,800Mpa). Typically, reinforced products have less impact strength, so atradeoff between stiffness and impact must be made.

Heat ResistanceHeat Resistance is another important property in many applications fortwo reasons. First, in the case of an electronic device, it can generate heatduring normal operation, the plastic selected must not distort, sag orsuffer thermal aging during service. Typically, a UL* relative thermalindex (RTI [UL746B]) of 167-194°F (70-95°C) is sufficient to provideprotection, depending on the heat generated by the internals. Second,because the device could also be exposed to external heat (such as a closed car on a hot day) the material might have to also withstandthermal excursions without damage. Typically, a deflection temperatureunder load (DTUL [ASTM D648]) of 180°F (82°C) is sufficient to resistsuch temperatures. Data on these properties are in the UL ComponentDirectory. Data on other properties are in UL files for GE Plastics.

Note that the DTUL test is geometry dependent and the value canchange with wall thickness. While DTUL may be a useful property forcomparing materials in the same resin family, it is not directly useful inany engineering equation. Therefore, engineers may wish to consultdynamic mechanical analysis (DMA) curves to determine a more accurate description of a material’s thermal performance. DMA curveson GE resins and use of DMA information can also be found in GE Plastics’ Engineering Design Database.

*UL is a Registered Trademark of Underwriters Laboratories, Inc.


Material Selection


Flame RetardanceFlame retardance is mandated for many electronic applications basedon their power, voltage or current levels (see UL 1950 information inTable 9 in Appendix A on page 6-2). Examples are cellular productswhich require materials with a UL HB listing, while notebook computersrequire materials with UL 94* V-0 or V-1 listings. The industry standardfor most stationary electronics is typically a material listing of UL V-0and 5V.

Mechanical IntegrityMechanical integrity of a part is related to the end-use requirements,material properties, the part design, assembly layout and manufactur-ing methods. An appropriate assembly method will often help reducethe total weight of a portable unit. Snap fits, small screws and ultrason-ic welding are the typical methods of joining Thinwall parts.

Material characteristics that can impact the assembly method usedinclude knitline strength, strain-to-failure, notch sensitivity, flexuralstrength, screw pull-out/strip-out strength and weldability. (For furtherinformation, see the Secondary Operations section of this guide onpages 5-2 to 5-5.)

*Underwriters Laboratories, Inc. Standard 94 describes a vertical burning test to be performedunder laboratory conditions. In this test, specimens are placed in the flame of a laboratory burner,and the ability of the substance to sustain a flame over a specified period of time, upon removal ofthe source of the flame, is determined. The claims, representations, and descriptions regarding theflammability of the products described in this brochure are based on a standard, small-scale labora-tory test and as such are not reliable for determining, evaluating, predicting, or describing theflammability or burning characteristics of the products under actual fire conditions, whether theproducts are used alone or in combination with other products. Each potential user should deter-mine for himself/herself whether a particular test procedure is meaningful for a particular applica-tion and should run independent tests to determine whether the products mentioned in this paperare suitable for a particular application.


Material Selection


Material SelectionGE Plastics offers several resins which are commonly used in Thinwall applications.(See Figure 15.)

LEXAN ®PC ResinsLEXAN SP7000 series resins are a family of glass- and carbon-fiber-reinforced materials combining high stiffness with the same easy-flow characteristics of unreinforcedLEXAN SP resin products. By compensating for the loss in part stiffness due toreduced wall thicknesses, these higher modulus materials are often good candidatesfor consideration in notebook computer applications.

LEXAN SP resins are polycarbonate (PC) based polymers that combine high resinflow with the high-impact performance and outstanding mechanical properties ofstandard LEXAN resins. LEXAN SP resins are also available with impact modifiers(LEXAN ML6339R and ML6018R resins) for improved low-temperature performance.These materials can offer new opportunities for intricate Thinwall part designs.

Standard grades of LEXAN resin include tough and very versatile engineeringthermoplastics. A combination of processing and performance properties can provide design engineers with exceptional freedom to create functional, attractive,and cost-effective products.

CYCOLAC ® ABS ResinsCYCOLAC ABS resins can provide engineers the precise properties needed forThinwall applications without over- or under-specifying. They offer physical andeconomic advantages compared to other conventional design materials. They areoften used by manufacturers where high impact, excellent flow characteristics and well as heat resistance and modulus are desired.

CYCOLOY ® PC/ABS ResinsCYCOLOY HF resins provide high-flow capabilities for enhanced Thinwall process-ing. Coupled with the good impact strength, heat resistance, UV stability and cost/ performance value of standard CYCOLOY PC/ABS resins, this combination ofproperties makes the materials good candidates for consideration in small handheld parts applications.

Standard Grade CYCOLOY PC/ABS resins are versatile thermoplastic materialsoffering low-temperature ductility, excellent impact resistance, practical thermalperformance, excellent aesthetics, and good flow characteristics. These materialscan offer improved productivity with higher yields and better cost efficiency.


Material Selection


Property ConsiderationsAll of the GE resin families can offer some advantages for Thinwallapplications, but no one group of resins can meet every requirement.In selecting materials for particular applications, it is necessary to weighadvantages and make the best overall choice in order to balance stiff-ness, impact, and flow properties. Here are some tradeoffs to consider.

Flow vs. ImpactFor each standard LEXAN resin grade, there is a LEXAN SP resingrade with higher flow and equivalent impact strength (Figure 16).CYCOLOY HF resins will provide even higher flow than LEXAN SPresin materials, but there can be a compromise in impact strength.

Figure 15. Thinwall MaterialsComparison.

Flow

Le

ngth

Impa

ctSt

reng

th

Aes

thet

ics

Stiff

ness

Hea

t Re

sist

ance

Low

-Tem

pD

uctil

ity

Ultr

ason

icW

eld

Stre

ngth

Excellent Good Poor

LEXAN SP7000 Resins(reinforced)

LEXAN SP Resins(impact modified)

LEXAN SP ResinsLEXAN SP Resins

LEXAN Resins

CYCOLAC Resins

CYCOLOY HF Resins

Relative Performance

CYCOLOY Resins

Figure 16. Impact Strength vs. Flow Length.

LEXAN101

Resin

LEXANML6339R

Resin

LEXANSP1210R

Resin

LEXANSP1010Resin

CYCOLOYC1200HF

Resin CYCOLOYC1000HF Resin

Flow Length

CYCOLOYC1200 Resin

CYCOLOYC1000 Resin

LEXAN121

Resin

LEXAN141

Resin

Impa

ct S

tren

gth

Improved flow same impact

Improvedimpactsimilar flow


Material Selection


AestheticsFlow-length requirements on a material can be reduced by adding moregates to the tool, but this will create additional knitlines that are typicallynot acceptable for cosmetic as well as strength issues. Sequential valvegating can be used to reduce knitlines. For better aesthetics, use ofhigher flow materials with a single gate is suggested.

Materials with high-flow capabilities, such as LEXAN SP resins, can oftenpick up the finer texture needed for lower gloss applications – a resultthat typically cannot be achieved solely through processing techniques.LEXAN SP resin grades also create a more resin-rich surface, which inmany cases allows for the successful use of the 10%-glass-reinforcedLEXAN SP7602 resin grade without painting.

This can be an important advantage, since most reinforced materialsrequire additional secondary operations than their unreinforcedcounterparts to achieve optimal surface appearance.

Stiffness vs. ImpactA decision should be made early in the concept phase of productdevelopment as to whether to use an unreinforced material (whichwill typically provide better impact resistance but greater deflection,necessitating a design approach for stiffness), or a reinforced material(which will typically provide better stiffness but require a designapproach for impact). The decision is important because of the distinctadvantages and consideration associated with each material type andtheir effect on design. Key points are highlighted in Table 1.

Table 1. Unreinforced vs.Reinforced Materials Comparison.

Advantages

Unreinforced Resins Reinforced Resins

• Higher Impact Strength• Cosmetics

• Greater Stiffness• Higher Static Strength

Considerations • Lower Stiffness/ Higher Deflection

• Brittle Impact Failures• Possible Aesthetic Issues (painting may be required)

Unreinforced plastics can usually absorb and dissipate a tremendousamount of energy as they deform during impacts. Therefore, this featurecan be exploited during the design process to help protect internalelectronics from damaging loads or shocks. Conversely, higher modulus,reinforced products typically provide greater stiffness and resist deflection– critical features at thinner wall sections. Components using reinforcedresins must therefore be designed to minimize localized deflectionsand quickly transfer impact load throughout the rest of the unit.


Material Selection


Table 2. LEXAN Standard Resin Grades –

Typical Property Values.

Table 3. LEXAN SP Resin Grades –

Typical Property Values. Tensile Strength, yield,Type 1, 0.125" (3.2 mm)

Flexural Modulus,0.125" (3.2 mm)

DTUL, 264 psi (1.82 MPa)0.125" (3.2 mm)

DTUL, 264 psi (1.82 MPa)0.250" (6.4 mm)

Izod Impact, notched, 73˚F (23˚C)

Specific Gravity

UL 94 HB Flame Rating*

Mold Shrinkage flow,0.125" (3.2 mm)

LEXAN LEXAN LEXAN LEXAN LEXANTEST UNIT SP1210R SP1010 SP7700 SP7602 SP7604

METHOD ENG (S1) resin resin resin resin resin

ASTM D 638 psi (MPa) 8,700 (60) 8,700 (60) 15,100 (104) 10,800 (74) 13,300 (92)

ASTM D 790 psi (MPa) 345K (2,375) 345K (2,375) 967K (6,660) 570K (3,925 883K (6,085)

ASTM D 256 ft-lb/in (J/m) 16.0 (854) 12.0 (641) 1.8 (96) 1.1 (59) 1.0 (53)

ASTM D 648 °F (°C) — — — 199 (92) 198 (92)

ASTM D 648 °F (°C) 230 (110) 225 (107) 212 (100) — —

ASTM D 792 — 1.18 1.18 1.25 — —

ASTM D 955 in/in E-3 5-7 5-7 1-3 2-4 1-2

UL 94 in (mm) 0.045 (1.14) 0.042 (1.07) — — —

UL 94 in (mm) — — 0.048 (1.22) 0.030 (0.76) 0.030 (0.76)

* These ratings are not intended to reflect hazards presented by any material under actual fire conditions.

UL 94 V-0 Flame Rating*

PROPERTY

Tensile Strength, yield, Type 1, 0.125" (3.2 mm)


DTUL, 264 psi, (1.82 MPa)0.250" (6.4 mm)

Izod Impact, notched, 73˚F (23˚C)

Specific Gravity



LEXAN LEXANTEST UNITS LEXAN 141 ML6339R ML6018R

METHOD ENG (S1) resin resin resinASTM D 638 psi (MPa) 9,000 (62) 8,800 (61) 8,300 (57)

ASTM D 790 psi (MPa) 340K (2,345) 310K (2,135) 266K (1,830)

ASTM D 256 ft-lb/in (J/m) 15.0 (801) 15.0 (801) 10.6 (566)

ASTM D 648 °F (°C) 270 (132) 231 (110) 240 (115)

ASTM D 792 — 1.20 1.18 1.18

ASTM D 955 in/in E-3 5-7 5-7 5-7

UL 94 in (mm) 0.045 (1.14) 0.043 (1.09) —

PROPERTY

Material EvaluationWhile proper materials selection is always a very important step inproduct development, it is especially critical in Thinwall applications.As wall sections become thinner and processing becomes more chal-lenging, it is vital to select resins that have been specially formulatedfor high flow impact strength, adequate heat resistance, requiredflame retardance, assembly integrity and aesthetics.

GE Plastics has developed several materials specifically for use in Thinwall applications, which can provide designers and processorsa balance of necessary properties at an economical price as shown inTables 2 through 5.

Materials Portfolio for Use in Thinwall applications


Material Selection


Table 4. CYCOLAC Resin Grades – Typical Property Values.

Table 5. CYCOLOY Resin Grades – Typical Property Values.

Tensile Strength, yield, Type 1, 0.125" (3.2 mm)


DTUL, 264 psi, (1.82 MPa)0.125" (3.2 mm)

Izod Impact, notched, 73˚F (23°C)

Specific Gravity



CYCOLAC CYCOLAC CYCOLACTEST UNITS GPM5600 GPM6300 DSK

METHOD ENG (S1) resin resin resinASTM D 638 psi (MPa) 5,500 (38) 6,400 (44) 6,100 (42)

ASTM D 790 psi (MPa) 310K (2,135) 360K (2,480) 370K (2,550)

ASTM D 256 ft-lb/in (J/m) 6.5 (347) 4.5 (240) 4.0 (214)

ASTM D 648 °F (°C) 176 (80) 178 (81) 174 (78)

ASTM D 792 — 1.03 1.04 1.05

ASTM D 955 in/in E-3 5-8 5-8 6-8

UL 94 in (mm) 0.062 (1.57) 0.062 (1.57) 0.059 (1.50)

PROPERTY

Tensile Strength, yield,Type 1, 0.125" (3.2 mm)



DTUL, 264 psi (1.82 MPa)0.125" (3.2 mm)

Izod Impact, notched, 73°F (23°C)

Specific Gravity



CYCOLOY CYCOLOY CYCOLOY CYCOLOY CYCOLOYTEST UNITS C2800 C6200 C1200HF C1000HF C1200

METHOD ENG (S1) resin resin resin resin resin

ASTM D 638 psi (MPa) 8,500 (59) 9,700 (67) 8,300 (57) 8,400 (58) 8,800 (61)

ASTM D 790 psi (MPa) — — 340K (2,345) 360K (2,480) 340K (2.345)

ASTM D 790 psi (MPa) 390K (2,685) 390 (2,685) — — —

ASTM D 256 ft-lb/in (J/m) 8.0 (427) 10.0 (53.4) 11.0 (587) 10.0 (53.4) 12.0 (641)

ASTM D 648 °F (°C) 165 (73) 190 (87) 235 (112) 210 (98) 235 (112)

ASTM D 792 — 1.17 1.18 1.15 1.12 1.15

ASTM D 955 in/in E-3 4-6 4-6 5-7 5-7 5-7

UL 94 in (mm) — — 0.063 (1.60) — 0.063 (1.60)

UL 94 in (mm) 0.060 (1.52) 0.060 (1.52) — — —

* These ratings are not intended to reflect hazards presented by any material under actual fire conditions.

UL 94 V-0 Flame Rating*

PROPERTY


Notes


GE Plastics

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Processing


With the move to thinner wall-section designs, three key aspects ofprocessing need to be considered: molding, tooling, and machinery.With thinner wall sections, processing requirements change with respectto the use of higher pressure and speeds, faster cooling times, partejection and gating scenarios. Typically, the acceptable range for eachof these parameters becomes narrower and more precise at thinner walls,providing processors with greater challenges but also greater opport-nities. In this section, specific processing techniques and equipment forprocessing Thinwall parts are contrasted with those of more conven-tional wall thicknesses. Some of the significant differences are noted(See Table 6).

Parts with nominal wall thicknesses between 0.080 and 0.125 in (2.0-3.2 mm), typical with stationary electronics housings and a variety ofother applications including automotive parts, represent “standard”applications that are generally well understood and easily handled byOEMs and experienced processors in the plastics injection moldingindustry. As the wall section of a part decreases (0.080-0.050 in [2.0-1.2 mm]), the ability of the injected material to easily fill the mold cavityis also reduced. Extra measures are therefore required to help thematerial reach the end of the cavity before freeze-off and, at the sametime, retain enough molten core to pack out sink marks. To accomplishthis, typically injection pressures are increased and time-to-fill is decreased.As an example, if a part with a 0.120 in (3.0 mm) wall section is filledin 4 sec, a 0.060 in (1.5 mm) part may need to be filled in less than 2 seconds to achieve the same degree of pack-out at the end of fill.

The trend in small hand held parts housings has been to much thinnerwalls than have traditionally been utilized in injection molding. Wall-section thicknesses from 0.030-0.050 in (0.75 to 1.2 mm) are becomingcommonplace, and wall sections below 0.020 in (0.5 mm) are evenbeing witnessed.

MoldingQuite often, higher hydraulic pressure and shorter fill times are requiredto drive the molten thermoplastic materials into thinner cavities at asufficient rate. Where fill pressures of 12,000-14,000 psi (83-97 MPa) are commonly sufficient for midsize parts with 0.12 in (3.0 mm) wallthicknesses, 0.06 in (1.5 mm) wall thickness parts of the same size mayrequire pressures between 16,000-20,000 psi (110-138 MPa) to achievenecessary injection rates. (See Figures 17 and 18 on page 4-4.)


Processing


Key Factors

Hydraulicsystem

Controlsystem

Screw design

Processing

Time to fill

Cycle time

Drying

Tooling

Conventional Applications Thinwall Applications

Typical Wall 0.080 - 0.120 in(2.0 - 3.0 mm)

0.050 to 0.080 in (1.2 - 2.0 mm)

< 0.050 in (< 1.2 mm)

Machinery Standard High-end Customized high-end equipment

Inj. Pressures 9,000 - 14,000 psi(62 - 97 MPa)

16,000 - 20,000 psi(110 - 138 MPa)

20,000 - 35,000 psi (138 - 241 MPa)

Standard Standard Accumulators on injection & clamp units, servo valves

Standard Closed loop, microprocessor control over: injection speed,

hold pressure, decompression speed, screw RPM, back

pressure, all temps, (including feed throat & oil temperature)

Microprocessor controlled with the following resolutions: speed-0.40 in./sec

(1 mm/sec), pressure-1 bar, position-0.004 (0.1 mm),

time-0.01 sec, rotation-1 RPM,clamp force-0.10T (0.10t),

temperature-2°F (1°C)

Compression ratio: 2.0:1 to2.5:1, L/D = 20:1 to 24:1flights 5/10/5: Nitriding

not suggested

Compression ratio: 2.0:1 to2.5:1, L/D = 20:1 to 24:1

flights 5/10/5: Nitriding nottypically used

Compression ratio: 2.0:1 to2.5:1, L/D = 20:1 to 24:1

flights 5/10/5: Nitriding nottypically used

> 2 sec 1 to 2 sec 0.1 to 1 sec

40–60 sec 20–40 sec 6–20 sec

Dew point of -20°F to -40°F(-29°C to -40°C);

hoppers sized for materialthroughput





Standard Better venting, heavier construction, more ejector

pins, better polish

Extreme venting, very-heavy molds, moldinterlocks, precise mold surface

preparation, extensive ejection features,mold costs 30–40%higher vs. standard

Table 6. Comparison of Standard vs. Thinwall Processing.


Processing


Theoretically, as wall-sections drop in size, cycle times also drop,because there is less material to cool. Careful management of runnerand sprue, through redesign or the use of hot drops, may permit areduction in total cycle time. Typical cycles can then be in the rangeof 20 to 40 seconds.

At these wall thicknesses, closed-loop control over injection speed,transfer pressure, and other process variables can help to control theprocess for the filling and packing required to produce consistentquality parts.

Figure 18. Typical Cycle Time Range.

Wall Thickness

0 1 mm 2 mm 3 mm

0

10

20

30

40

50

60

70

Seco

nds

Figure 17. Injection Pressure andFill Time vs. Wall Thickness.

Pressure

Time

Wall Thickness

Tim

e Re

quir

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Fill

(Sec

onds

)

Inje

ctio

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100

0 ps

i)

0 1mm 2mm 3mm

0

1

2

330

20

10

0


Processing


As with conventional molding, proper drying and consideration ofmaterial residence time and temperatures in the barrel are required.Because the shot size for these applications may be smaller than is typical with conventional molding, material property degradation maybe accentuated due to overly long residence times for the material in the barrel.

With wall-sections of less than 0.050 in (1.2 mm), during processing, thetotal cavity time-to-fill values may need to be reduced even further. For most applications, fill times between 0.1-0.5 seconds are suggested.If fill times are longer than this, the material may simply “freeze-off”before the cavity is filled and packed. To drive the material at sufficientspeeds, injection units may need to generate pressures of 20,000-35,000psi (140-241 Mpa).

With proper heat management and due consideration to the sprue andrunner, cycle times can often be greatly reduced with wall-sections ofless than 0.050 in (1.2 mm). Total cycle times of 10-20 seconds should beexpected; these values can be even lower with other specialized equip-ment. Use of very fast cycle times and premium tooling make it prudentfor molders to select mold safety equipment such as video systems toensure there is no part hang-up in the mold. The use of robots may alsobe appropriate, since they can remove parts faster than gravity willallow them to clear the mold’s surfaces.

As wall thicknesses continue to drop below 0.050 in (1.2 mm), additionalcare must be taken in processing. Melt temperatures for these applica-tions should not exceed GE Plastics’ suggested processing temperatures.It is often tempting to exceed these temperatures in order to fill thecavity, however, this can be counterproductive. Too high a melt tem-perature and/or too long a material residence time in the barrel cancause a significant loss of the material’s physical properties and/orcreate aesthetic issues in the part.

With even the thinnest of walls, proper drying of the material is nomore or less critical than it is with thicker walls.

CAUTION: With much smaller shot sizes, material through-put should be suchthat the resin does not sit in the drying hopper for extended periods of time.Therefore, hoppers of suitable size or leveling switches installed in the hoppersshould be used to match projected material through-put.


Processing


ToolingTooling requirements for conventional applications are well docu-mented and understood by molders, tool designers and tool builders.P-20 steel is used extensively in the industry, and usually works ade-quately. For more detailed information about tool steels, refer to theGE Plastics’ Injection Molding Guides.

Significant changes must occur in tooling when moving to moldingThinwall applications. Because of the higher pressures used to mold atthinner walls, the tooling for Thinwall applications is usually stiffer andmore solid than that used for standard molding. For instance, the use ofheavier mold plates and support pillars is common (See Figure 19).

Mold interlocks are sometimes incorporated into the tooling to helpprevent flexing, misalignment, or other mold movement. Use of coresthat telescope into the cavity helps to reduce core shifting and breakage.Materials injected at high speeds may cause additional mold wear, whichshould be planned for when selecting mold materials.

With wall-sections of less than 0.050 in (1.2 mm), accuracy in tool build isa critical factor. Due to the very high pressures generated during process-ing, there is a greater susceptibility of the mold to flash and a higherpossibility of mold plate flexing. Tools for Thinwall molding applicationsmust therefore be built heavier than those for standard molding. Moldsupport plates 2 to 3 in (5 to 8 cm) thick are often required, with supportpillars under the cavities and sprue. Other desirable modificationsmight include:

• Preloading of the support pillars (typically 0.005 in [0.127mm])

• Extensive use of mold interlocks for better alignment and lateral support

• Application of #2 diamond polish to cores and ribs to help eliminate the problem of part sticking

• Improvements in part release via mold surface treatments such as nickel-polytetrafluoroethylene (Ni-PTFE)

• Use of more and larger ejector pins than with conventionalmolds to reduce pin pushing

• Strategic placement of sleeve and blade knockouts

With the very short fill times, typical of applications with wall-sectionsof less than 0.050 in (1.2 mm), makes venting more critical as well.Evacuation of gases to reduce burning is usually facilitated by extensiveventing of core pins, ejector pins and along the parting line (up to 30%of the edge of the part). Some processors have successfully sealed theparting line with an “O” ring in order to pull a vacuum on the cavityand help with gas evacuation.


Processing


Because of the reduced cycle times typical of Thinwall molding, ade-quate cooling of mold cores and cavities becomes more critical andchallenging. Some general guidelines to keep in mind are:

• Size and placement of cooling lines should be considered fromthe beginning when the tool is designed, not as the last itemadded into the mold.

• Non-looping cooling lines should usually be located directly in the core and cavity blocks to help keep the mold surface temperatures as consistent as possible.

• Instead of decreasing coolant temperature to maintain the desiredsteel temperature, it is generally better to increase the amount ofcoolant flow through the tool. As a rule of thumb, the differencein temperature between the delivery coolant and the returncoolant should be no more than 5-10°F (3-5°C).

• For better heat management within the tool, the use of thermalpins, beryllium copper, and other specialty metals is also suggested.

Figure 19. Typical Thinwall Tooling.

128

7 13

16

3

44

55

32

11

10

7

1

Cavity Side, Notebook Application Core Side, Battery Cover Application

1. Mold Interlocks2. Extensive Venting3. Hardened Cavity Blocks4. Non-Looping Cooling Lines5. Extra-Thick Mold Plates

6. Hot Drop to Cold Runner 7. Release Coating 8. #2 Polish on Ribs and Cores 9. Extensive K.O. Features10. Valve Gate on Appearance

Surface (Opposite Gate Well)

11. Tapered Walls12. Telescoping Cores13. Hardened Gate Inserts


Processing


All types of conventional injection sprues, runners, and gates can beused with most Thinwall applications. Large sprues and runners canhelp minimize pressure drops. However, they may also contribute tolonger cooling and total cycle times. When sucker pins and sprue pillarpins are used, they should be made as non-restrictive as possible toreduce pressure loss and excessive material shearing. When gatingdirectly onto a thin wall with a sprue, pin points or hot drop, etc., gatewells should be used to help reduce stress at the gate, aid in filling, andalso help reduce part damage when degating. And because of the veryhigh injection speeds that are often seen in Thinwall molding, gateinserts with a Rockwell (Rc) hardness of greater than 55 are typicallyused. Larger gates are generally better than smaller gates for materialflow purposes and they also can help reduce gate wear. Use of gatesthicker than the nominal wall is also common to aid flow and helpprevent freeze-off before proper packing takes place.

Figure 20. Area of MaterialThrough-Put: Valve Gate Examples.

Total area for material to flow through:

19.8 mm2(0.0314"2) 4.7 mm2(0.007"2)

1.0 mm (0.040")Wall Section

6.3 mm (0.250")Dia. Gate

1.5 mm (0.060")Dia. Gate

Thinner wall-sections of less than 0.050 in (1.2 mm), in general, reducea material’s flow length and a single gate may not be sufficient to fillthe cavity (Figure 20). It may need to be relocated to a more centralregion on the part, or multiple gates may be necessary. Multiple gates,however, will give rise to knitlines in the part. Knitlines are not as strongas surrounding regions of polymer and they may be undesirable froman aesthetic standpoint. Sequential valve gating can often be used tominimize knitlines.The location and number of gates are critical factorswhen designing Thinwall tooling and must be considered carefullybefore part and tooling designs are finalized.


Processing


With thin wall sections of less than 0.050 in (1.2 mm), hot manifolds canhelp reduce pressure loss seen in cold runner systems. When hot runnersare used, cycle times are not controlled by the cooling rate of the sprueand runner. However, hot runners need careful consideration. Theadditional material that is held in the manifold and hot drops betweencycles adds to the total residence time for the molten material. In orderto minimize pressure drops hot runners often require a minimum of0.500 in (13 mm) diameter inner passages with no sharp corners or deadzones where material can get hung up. They also require the use ofexternal heaters. Internal heaters should not be considered as an option.Because of the high pressures found in Thinwall molding, careful con-sideration must be given to the design and construction of the manifold.Finally, valve gates must be constructed for high pressure capabilityand be non-restrictive.

These suggested features for Thinwall tooling are generally add-onsrather than standard issue with the mold. Therefore, costs for Thinwalltooling may in fact run 30-40% higher than for more traditional molddesigns. However, these costs are often offset by increased productivity.

Tool steels harder than P-20 should be used in most applications withthin wall sections, especially when high wear and erosion are expected.(H-13 and D-2 materials have been used successfully for gate inserts.)Faster injection rates will make gas venting even more critical to pre-vent gas-entrapment burning. Additional vents are generally required,especially in areas where flow fronts converge and trap gases. Ventdimensions are typically 0.0008-0.0012 in (0.02-0.03 mm) deep by0.200-0.400 in (5-10 mm) wide. It may also be useful to vent core pinsthat shut off, as well as ribs, bosses, and ejector pins. Larger gates areoften helpful at these higher injection speeds by reducing materialshear and allowing for better packing.

Figure 21. Typical Thinwall Part Ejectors.Thinwall Ejector

LayoutConventional Ejector

Layout

Thinwall Parts Typically Require More and Larger Ejector Pins and Careful Attention to Placement

Ejector Blades

Sleeve Ejector


Processing


In applications with thin wall-sections part ejection may also be morechallenging due to:

1. minimal shrinkage across the thickness which tends to make ribs and other features stick;

2. faster injection rates, which make controlling packing more challenging;

3. thinner walls and ribs, which are more easily damaged; and4. higher pressures, which cause higher packing and less shrinkage.

To reduce the likelihood of pin punching and sticking of ribs, it iscommon practice to use more and larger ejector pins (up to twice asmany pins that are twice as large) than with conventional molding (See Figure 21). Draw polishing of all internal features, such as ribs, bosses,etc., is strongly suggested. Minimum draft angles should generally be1°/side + 1°/ 0.001 in (0.025 mm) depth of texture.

MachineryMost molding machines found in the custom molding shops today are adequate to meet production requirements for most standard wall-section thicknesses applications in the range of 0.125-0.080 in (3.2-2.0 mm). Based on the total projected area of the part and runners,machine tonnage in the range of 2-5 T/in2 (28-70 MPa) will usuallyprovide sufficient clamp pressures. Injection units will typically be runusing 35-70% total shot capacity.

Proper drying of material is important in order to maintain publishedphysical properties and to promote uniform viscosity of the materialduring processing. Dryers should typically be sized for the materialthrough-put of the job being run and provide -20 to -40°F (-29 to -40°C)dew point air at suggested temperature.

Screw design is an important and often overlooked factor for mold-ing of engineering thermoplastic resins. For LEXAN PC resin andCYCOLOY PC/ABS resin properly designed screws generally shouldhave the following characteristics:

• Compression ratio of 2.5:1 or less (2.0:1 - 2.5:1 are suggested).

• Flight configuration of 5/10/5 (Feed/Transition/Metering).

• 20:1 minimum length/diameter (L/D) ratio (suggested between 20:1 and 24:1).


Processing


• Nitriding is not suggested for screw, barrel, screw tip and compo-nents. Some nitrided components have led to degradation, discoloration and black specks in molded parts.

• For the screw tip, use of full-flow sliding check rings is suggested.

Standard microprocessor-controlled machines with closed-loop functionsare often suitable for most Thinwall-sections applications ranging from0.080-0.050 in (2.0-1.2 mm). Because the high injection pressures used tomold these parts necessitate the use of clamp tonnages in the range of4-6 T/in2 (55-83 Mpa), larger presses are generally required than aretypical for conventional molding. But these larger machines are usuallyfitted with larger barrels which must be properly sized to the application.

The injection presses required to mold most Thinwall applications are more specialized than those typically found in custom-molding facil-ities. In these Thinwall applications, tight control over injection of thematerial becomes critical. Injection fill times below 0.5 seconds do notleave much time for injection speed profiling or pressure cutoff. Also, atthese speeds and pressures, overpacking of the cavity can easily occur.A microprocessor-controlled machine with tight resolution is thereforevirtually mandatory.

In addition, the very high injection pressure used in most Thinwallapplications require machines with accumulators on the injection unitand a typical line pressure of 3,000 psi (21 MPa), which can generatefill pressures in excess of 30,000 psi (200 MPa). Requirements for veryrapid clamp movement also makes accumulators helpful in the clampunit. Effective control of the clamp and injection functions at thesehigh rates can usually best be handled with servo valves and very goodmicroprocessor control units. Overall, tight resolution and control oftimes, speeds, temperatures, and pressures will typically be necessaryin these machines to achieve consistent Thinwall parts with desiredtolerances and appearance.

The machine clamp unit for most Thinwall parts will be required toprovide a minimum of 5-7 T/in2 (69-97 MPa) force per unit of the projected area of the part and runner. Extra-heavy platens are usuallyneeded to reduce flexure below 0.005 in (0.127 mm). Machines withtie-bar-distance to platen-thickness ratios of 2:1 or less have been usedsuccessfully.


Processing


Small shot sizes used to mold Thinwall parts can lead to materialdegradation due to excessive residence time in the barrel. The barrelsize of the machine should have a shot capacity that is not too largefor the Thinwall application being molded. There are 200 T (181 t)molding machines with 5 oz (142 g) barrels that are currently beingused to mold portable electronics Thinwall applications. A minimumshot-size to barrel capacity of 40% is suggested. Since total cycle timesare often greatly reduced in Thinwall molding, it may be possible toreduce the shot size minimum down to 20-30%. However, such anapproach should be taken with caution, and parts must be thoroughlytested for property loss due to possible material degradation. If thebarrel capacity falls below this, processors risk significant materialdegradation due to excessive residence times. (This 20-30% rangetakes into account that the residence time will be shortened by thereduced cycle time.)

Special screw tips and check rings are usually not required for Thinwallapplications. In non-worn barrels, standard free-flow check rings haveperformed quite well at elevated pressures.

For Thinwall applications, processors and designers need to be awarethat there are critical tooling and machine changes that are required.It is important to note that while these changes rarely occur withoutadditional investment, the productivity benefits of reduced materialusage, faster cycle times, and greater yield can often outweigh the addedcost. Under these circumstances, the added investment can be amor-tized quickly.


GE Plastics

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Finishing Operations


With the exception of assembly techniques, secondary operations for both conventional and Thinwall molded parts will be similar. Likeconventionally molded parts, typical Thinwall assembly techniquesinclude the use of welding, screws, inserts or adhesives.

As with conventionally molded plastic parts, typical assembly techniquesfor Thinwall components include the use of mechanical (welds, screws,inserts) or chemical (adhesives) fastening systems. When mechanicalassembly of Thinwall parts is selected, several special considerationsare required for screws or inserts, or of ultrasonic welding.

For more specific information on standard secondary finishing opera-tions, please consult GE Plastics Design Guide and Product Line Guidesfor individual resin being used in a particular application.

Screws and InsertsThe use of screws and inserts remains the primary assembly method formost Thinwall parts, as it does for conventional molded plastic parts.The main advantage to this assembly technique is secure and cost-effective fastening and easy disassembly. Even with the use of smallerscrew sizes in some Thinwall sections, pull-out strengths of 100 lb (46 kg)or more can often be achieved. With some Thinwall parts, however,there can be a issue with low strip torque of screws – especially in thecase of smaller screws.

Bosses for screws and inserts put in with heat or ultrasonic welding area viable assembly option for many Thinwall applications (See Figure 22).However, the wall thicknesses of those bosses should not be less thanthe outer wall thickness, and it is possible to use outer diameters thatare significantly less than twice the inner diameters. Strip torques as lowas 2 in-lb (0.24 N-m) will require the use of precision drivers that can beaccurately set to these values. Further, care should be taken in definingthe gate location to reduce knitline weakness in the boss walls.




Ultrasonic WeldingThe use of ultrasonic welding as an assembly technique offers theadvantages of greater strength and stiffness in the assembled part, aswell as reduced weight due to the elimination of threaded fasteners.In many Thinwall applications, ultrasonic welding is now increasinglyused in the assembly of products which do not require disassembly for maintenance.

Ultrasonic welding of many Thinwall parts is different from that ofthicker wall parts. Relative weld strengths for the same material in thickand thinner wall sections can be significantly dissimilar, with muchgreater weld energies/times being required to achieve a full weld inThinwall parts – especially when walls are somewhat flexible.

Like conventional applications, weld strength is highly design-dependent. Other important factors affecting performance include:stiffness geometry (both as-molded and when under pressure andvibration), usage of near-or far-field welding mode, horn design andamplitude, and joint design.

Figure 22. Typical Boss Design.

d = nominal diameter of the screw holediameter = .75d - .9d depending on the material

2 to 2.5d

dholedia..3 to .5d




In a laboratory welding comparison using 2.25 by 3.75 in (57 by 95 mm)boxes with a 0.040 in (1.0 mm) wall, the following results shown inTable 7 were obtained.

LEXAN ML6339R/ML6018R Resin 750-800 340-360LEXAN SP7602 Resin 500 225CYCOLOY C1000HF/C1200HF Resin 350-400 160-180LEXAN SP1210R Resin 450 200LEXAN 920 Resin 400 180CYCOLOY C2950HF Resin 250 110LEXAN 121 Resin 250 110LEXAN SP1010 Resin 200 90

Material Welding Strength[lbs]* [kg]*

*The quantified weld strengths represent a material comparison, and not an end-use performancestandard such as that determined by impact loading from drop tests. At the same time, a relativelylow weld strength does not necessarily mean that an ultrasonically welded material cannot functionin a real-world application. It does however, confirm the need for end-use testing.

Table 7. Welding StrengthComparison for Thinwall Materials.




Table 8. EMC Coating Technologies (Shielding Systems).

• FormMetal-filled Paint System(Silver, Copper, Nickel)

Applied by Pressurized SprayRobotic or Human Application

• PerformanceVariable Shielding EffectivenessDependent Upon Metal Formulation,Good-Excellent (Ni<Cu<Ag)

0.05-1.0 Ohms/Square SurfaceResistivity, 25-75 Microns Thick

Good Abrasion Resistance

• LimitationsHuman Application LimitsConsistency

Line-of-sight Coating ProcessLimits Part Complexity

• AestheticsInternal Coating Only, Molded-InColor/Texture

Copper/Nickel Alloy Multilayer CoatingAvailable in Single-Sided andDouble-Sided Systems

Immersion Coating (Double-Sided) orSpray/Immersion Coating (Single-Sided)

Excellent Shielding Characteristics

0.03-0.04 Ohms/Square SurfaceResistivity, 3-4 Microns Thick

Excellent Abrasion Resistance

Double-Sided Electroless Plating RequiresAesthetic Outer Finish but Can Coat PartsIndependent of Complexity

Single-Sided Electroless Plating is DependentUpon Line-of-Sight Application Limiting PartComplexity

Single-Sided Electroless Plating CoatsInternal Surfaces Only, Molded-InColor/Texture Possible

Double-Sided Electroless Plating RequiresDecorative Color Coat, Textured Paints Available

Pure Aluminum Coating

Radially Dispersed EvaporatedMetal in Vacuum Chamber

Very Good Shielding Effectiveness

0.3-0.4 Ohms/Square SurfaceResistivity, 5 Microns Thick

Very Good Abrasion Resistance

Line-of-Sight Coating Process LimitsPart Complexity

Internal Coating Only, Molded-InColor/Texture Possible

Conductive Paints Electroless Metallization Vacuum Metallization

ShieldingWhen properly molded, shielding techniques used for most Thinwallcomponents are much the same as those used for thicker wall parts(See Table 8). However, it is important to note that stress levels can besubstantially higher in Thinwall designs unless care is taken duringboth the design and processing stages. For more specific information,consult the EMI/RFI Shielding information available from GE Plastics.


Notes


GE Plastics

App

endi

xT

hinw

allSM

UL

1950

Age

ncy

Req

uire

men

ts


UL AppendixTa

ble

9. U

L195

0 Ag

ency

Req

uire

men

ts.

Inte

rnal

s(e

.g. E

lect

rical

In

sula

tion)

Supp

lied

by L

imite

dPo

wer

Sou

rce

Supp

lied

by N

on-L

imite

d Po

wer

Sou

rce

Ther

mal

RTI≥

Max

imum

Oper

atin

g Te

mpe

ratu

re

Ther

mal

1.RT

I ≥ M

axim

um O

pera

ting

Tem

p.2.

If ha

zard

ous

volta

ge c

ompo

nent

is

mou

nted

to p

last

ic m

ater

ial d

irect

ly

and

is s

ubje

ct to

:- P

rimar

y vo

ltage

; (Ba

ll Pr

essu

re

Te

st a

t 125

˚C fo

r 1 h

r.)- S

econ

dary

vol

tage

; (Ba

ll Pr

essu

re

Te

st a

t 40˚

C >

Max

imum

Ope

ratin

g

Tem

pera

ture

for 1

hr.)

Flam

mab

ility

**94

V-2

Min

imum

Flam

mab

ility

**V-

2 M

inim

um o

r dev

ice

test

or

need

le fl

ame

Elec

tric

alN

one

Elec

tric

alEl

ectri

c st

reng

th te

st 1

min

.at

oper

atio

nal v

olta

ge

Mec

hani

cal

Adeq

uate

Mec

hani

cal S

treng

th

Mec

hani

cal

Adeq

uate

mec

hani

cal s

treng

th

for a

pplic

atio

n

Exte

rnal

s(e

.g. E

nclo

sure

s)

Supp

lied

byLi

mite

dPo

wer

Sou

rce

Port

able

or

Stat

iona

ry/F

ixed

(key

boar

ds, c

alcu

lato

rs,

cellu

lar p

hone

s)

Ther

mal

RTI ≥

Max

imum

Oper

atin

g Te

mpe

ratu

re

Flam

mab

ility

**94

-HB

Min

imum

Elec

tric

alN

one

Mec

hani

cal

Adeq

uate

Mec

hani

cal

Stre

ngth

Non

Lim

ited

Pow

er S

ourc

e:Su

pplie

d by

Non

-Lim

ited

Pow

er S

ourc

e Ha

zard

ous

Volta

ge (V

ac 4

.24V

Pea

k; V

dc .6

0V);

Haza

rdou

s En

ergy

(VA

> 10

0)

see

para

2.1

1 an

d 4.

4.5.

2 fo

r det

ails

Port

able

Devi

ce w

eigh

s le

ss th

an 1

8 Kg

(40

lbs.

) and

has

cor

d se

t (L

apto

p, P

alm

top,

Not

eboo

k et

c.)

Ther

mal

1.RT

I ≥ M

axim

um O

pera

ting

Tem

pera

ture

(MOT

mus

t not

ex

ceed

95˚

C du

e to

ski

n bu

rn c

onsi

dera

tion)

2.M

old

stre

ss re

lief h

eat s

oak

- 7 h

rs. a

t 10˚

C >

MOT

(7

0˚C

Min

.)3.

If ha

zard

ous

volta

ge c

ompo

nent

is m

ount

ed to

pla

stic

m

ater

ial d

irect

ly a

nd is

sub

ject

to :

- Prim

ary

volta

ge; (

Ball

Pres

sure

Tes

t at 1

25˚C

for 1

hr.)

- Sec

onda

ry v

olta

ge; (

Ball

Pres

sure

Tes

t at 4

0˚C

> M

.O.T

. for

1 h

r.)

Flam

mab

ility

**V-

1 m

inim

um a

t min

imum

wal

l thi

ckne

ss o

r dev

ice

test

3/4

" ye

llow

flam

e or

nee

dle

flam

e (A

V-2

mat

eria

l may

pas

s th

e de

vice

test

)

Elec

tric

al1.

If m

etal

ized

coat

ing

used

for E

M/R

FI s

hiel

ding

,th

en U

L QM

RX2

requ

ired

2.Pl

astic

< 1

3mm

(1/2

") fro

m a

rc in

a d

evic

e, H

AI P

LC2

3.Pl

astic

< 1

3mm

(1/2

") fro

m s

ourc

e of

tem

pera

ture

ig

nitio

n th

en H

WI P

LC3

Mec

hani

cal

1. Im

pact

- 5

ft-lb

s ba

ll im

pact

- 3 d

rops

, 1 m

eter

2. S

tead

y Lo

ad- O

pera

tor a

cces

s ar

ea 3

0N (6

.6#)

for 5

sec

.- E

xter

nal e

nclo

sure

250

N (5

5.5#

) for

5 s

ec.

Mec

hani

cal

1. Im

pact

-5

ft-lb

s ba

ll im

pact

2. S

tead

y Lo

ad- O

pera

tor a

cces

s ar

ea 3

0N (6

.6#)

for 5

sec

.- E

xter

nal e

nclo

sure

250

N (5

5.5#

) for

5 s

ec.

Stat

iona

ry/F

ixed

Devi

ce w

eigh

s m

ore

than

18K

g (4

0 lb

s.)

- Sta

tiona

ry -

cord

set

, mov

able

- Fix

ed -

hard

wire

d, n

ot e

asily

mov

able

(Des

ktop

Com

pute

r,

Mon

itor,

Prin

ter,

etc.

)

Ther

mal

1.RT

I ≥ M

axim

um O

pera

ting

Tem

pera

ture

(95˚

C)2.

Mol

d st

ress

relie

f hea

t soa

k - 7

hr.

at 1

0˚C

> M

axim

um O

pera

ting

Tem

pera

ture

(70˚

C M

in.)

3.If

haza

rdou

s vo

ltage

com

pone

nt is

mou

nted

to p

last

icm

ater

ial d

irect

ly a

nd is

sub

ject

to:

- Prim

ary

volta

ge; (

Ball

Pres

sure

Tes

t at 1

25˚C

for 1

hr.)

- Sec

onda

ry v

olta

ge; (

Ball

Pres

sure

test

at 4

0˚C

>M

axim

um O

pera

ting

Tem

pera

ture

for 1

hr.)

Flam

mab

ility

**5V

B m

inim

um (h

oles

allo

wed

) or d

evic

e te

st 5

" fla

me

(A V

-0 m

ay p

ass

a 5V

dev

ice

test

)

Elec

tric

al1.

If m

etal

ized

coat

ing

used

for E

MI/R

FI s

hiel

ding

;UL

QM

RX2

requ

ired

2.Pl

astic

< 1

3mm

(1/2

") fro

m a

rc in

a d

evic

e; H

AI P

LC2

3.Pl

astic

< 1

3mm

(1/2

") fro

m s

ourc

e of

tem

pera

ture

ig

nitio

n; H

WI P

LC3

UL

1950

Sta

tiona

ry/F

ixed

Equ

ipm

ent*

for T

hinw

all P

orta

ble

Glo

bal I

nfor

mat

ion

Tech

nolo

gy E

quip

men

tA

genc

y re

quir

emen

ts U

L195

0 (IE

C950

) (CS

A 2

2.2-

950)

*Th

is fl

ow c

hart

is a

bro

ad in

terp

reta

tion

of th

e UL

195

0 Ag

ency

Req

uire

men

ts. T

here

are

man

y re

quire

men

ts th

at h

ave

not b

een

show

n on

this

cha

rt; s

peci

fic a

nd in

depe

nden

t rev

iew

of a

ll re

quire

men

ts is

man

dato

ry. T

his

char

t is

inte

nded

to

s

how

the

gene

ral f

low

of t

he U

L 19

50 p

roce

ss. T

his

char

t is

not r

ecog

nize

d or

end

orse

d by

UL.

Thi

s ch

art r

epre

sent

s so

me

of th

e m

inim

um U

L re

quire

men

ts, s

peci

fic c

usto

mer

requ

irem

ents

may

be

diffe

rent

.**

Thes

e ra

tings

are

not

inte

nded

to re

flect

haz

ards

pre

sent

ed b

y m

ater

ials

und

er a

ctua

l fire

con

ditio

ns.



ULAppendix


Overview of UL1950 Portable/Stationary/FixedEquipment Key DefinitionsHigh-current Arc Ignition – Number of arc ruptures applied to thesurface of a material at a specified rate and distance that causes thematerial to ignite.

Hot Wire Ignition – Number of seconds it takes for a material to ignitewhen wrapped with wire with a specified resistance subject to a highlevel current.

Limited Power Source – Non-Hazardous Voltage, Non-HazardousEnergy (Vac ≤ 42.4V Peak; Vdc ≤ 60V; VA ≤ 100; See para 2.11 and4.4.5.2 for details).

UL 94 5V* – A 5" flame is applied to a test sample (flame bars & plaquespecimens) for five seconds on, five seconds off, for a total of five appli-cations. In order to be classified 5V the sample must not burn with the flame or glowing combustion for more than 60 seconds, after lastapplication of flame. For plaque flame ratings: 5VA* no holes allowedin plaque, 5VB* hole allowed. UL1950 accepts a 5VB flame rating.

UL 94 HB* – A 1" flame is applied to the free end of a specimen for30 seconds and then removed. The travel time of the flame from thefront is measured and a burning rate calculated in inches per minute.For specimens 0.120" to 0.500 burn rate not to exceed 1.5" per minuteover 3.0”. For less than 0.120" not to exceed 3.0" per minute over 3.0”.

UL 94 V-0* – A 3/4" flame is applied to five specimens for two ten second applications each. No single specimen can burn for more than10 seconds or glow for more than 30 seconds and the total flamingcombustion time for the group must not exceed 50 seconds. The UL94 V1* and V2* tests are conducted in a similar manner with the majordifference being longer allowable burn times. In the case of V2, drip-ping particles that ignite cotton are allowed.

UL Device Flame Test For Stationary/Fixed Equipment – Similar test-ing protocol to UL 94 5V specimen testing described above exceptflame applied to actual enclosure (end-product testing).

*This rating is not intended to reflect hazards presented by this or any other material underactual fire conditions.


UL Appendix


UL Device Test For Portable Equipment - 3/4" Flame (yellow) Test –The enclosure shall not flame for more than one minute after two 30 second applications of test flame, with an interval of one minutebetween applications of the flame. The results are not acceptable ifthe sample is completely consumed. 12mm (needle) Flame Test – Similar testing protocol to the 3/4" Flame (yellow) Test describedabove except a 12mm (needle) flame is applied to the sample.

Note: A 5V rating does not guarantee a V-0 and below rating andvice versa. The 5V test is designed more for quick high heat and flameelectrical failures while the V-0 tests are designed for more of a smol-dering type failure. If a material has a V-0 rating it will pass the V-1and V-2 requirements.

The foregoing is a summary only and is not intended to be a substi-tute for review of applicable standards. For more information call: 1-800-845-0600 and request the following:

GE Plastics Reference Guides:

• EMI/RFI Shielding Guide (CCS-002)• UL Recognition (PBG-UL6)


GE Plastics

Glo

ssar

yT

hinw

allSM


Glossary


Amorphous resins – Thermoplastics in which the solidified molecularchains exist in a random configuration. The overall structure is characterized by a general absence of any regular three-dimensionalarrangement of molecules. Amorphous thermoplastics, in contrast tocrystalline resins, tend to offer higher impact strength, greater warpand creep resistance, and lower shrinkage. Amorphous resins do nothave a distinct melting point, but rather have a softening range thatrepresents the transition between solid and liquid.

Anisotropic material – A material having properties (i.e., shrinkage,stiffness, strength, etc.) that are dependent upon direction. Typically,injection molded fiber-reinforced thermoplastics have different prop-erties in the direction and perpendicular to the direction of flow(cross-flow).

Check rings – Sliding rings on reciprocating screw tips that preventinjected melt from flowing back into the screw flights. The rings function much like check valves in a hydraulic system, which allowfluid to pass in one direction only.

Cooling pattern – Temperature gradients in the part or mold as the molten melt solidifies and cures. This pattern affects the residualstresses in the finished plastic part, and therefore its resultant mechanical and physical properties.

Cross-flow direction – Direction perpendicular to the direction offlow. The cross-flow direction of a center-gated disk, for example, isalways aligned with the circumference, perpendicular to the radialflow direction. In anisotropic materials, stiffness and strength proper-ties are usually lower and shrinkage rates are usually higher in thecross-flow direction.

Crystalline resins – Thermoplastics with molecular structures arrangedin a very regular repeating lattice structure. In contrast to amorphousthermoplastics, crystalline thermoplastics tend to exhibit greaterchemical resistance, lubricity, greater flow length, and higher shrinkage.Crystalline resins have a distinct melting point that marks the transitionfrom solid to liquid.


Glossary


Cycle time – In plastics molding, the period of time required for thecomplete sequence of operations on a molding press to produce oneset of parts from the mold. For the injection molding process, thesequence includes:

• closing and clamping of the mold• injection of molten resin into the mold• holding mold closed under pressure while plastic

cools or cures• opening the mold and• ejection of the part(s).

Degrees of freedom – Direction of possible movement. There are 6 degrees of freedom (DOFs): three in translation (x, y, and z) andthree rotations (about x, y, and z). Various assembly methods are usedto constrain different DOFs between mating parts. Maximum stiffnessis attained by fully constraining all 6 DOFs between these mating parts.

Dew point – The level to which air temperature must be reduced tocause water condensation. In preparing thermoplastic resins for use in injection molding, dryer units should provide at least -20°F (-33°C)dew point air at recommended temperature.

Engineering Design Database (EDD) – An interactive, customer-accessible computerized database generated by GE Plastics that providesa matrix of dynamic (multipoint) property data with interpolation/extrapolation routines. Engineers can use these data directly in engi-neering equations to estimate material properties at actual end-useconditions. To access EDD within the U.S.A., call GE Plastics at (800) 845-0600.

Far-field welding – Ultrasonic welding together of plastic parts inassembly, in which the joint created is more than 0.250 in (6.4 mm)from the area of horn contact.

Filling pattern – The path of the thermoplastic melt as it fills the injection mold. Mold-filling analysis software can be used to predictthese filling patterns.

Gussets – A geometric feature used to selectively stiffen anotherfeature. Typically, these are rib-like structures that can be placedbetween features to help tie them together and stiffen the overallstructure.


Glossary


Hot drops – Melt delivery units that bring the plastic melt from hotmanifolds into gates. During the injection cycle, plastic in hot dropsand hot manifolds will stay molten and will not freeze-off.

Isotropic – A material having properties (e.g. shrinkage, stiffness,strength, etc.) that are not directionally dependent. Injection moldedunreinforced thermoplastics typically have similar properties in alldirections.

Knitline – A line created wherever two flow fronts meet and freeze inthe mold. Knitlines may not only be aesthetically undesirable in someparts; they also represent an area of lower strength and potential partfailure.

Linear beam equations – Simple, text-book equations that rotate thegeometry and material of a beam to its stress, strain, and deflection.These equations are usually accurate for deflection not greater thanthe beam thickness.

Load absorption design – A design strategy for electronic products inwhich the aim is to have the plastic housing absorb most of the energygenerated by accidental impact events through elastic (reversible)deformation. This strategy requires increased package space, since thehousing needs to deflect without contacting the internal electronics.

Load/displacement curve – Chart that can be used to display impactperformance. It is useful in determining the impact failure mode (e.g. ductile or brittle) and maximum load during impact. It can alsobe converted into stress/strain data.

Load transfer design – A design strategy for electronic products inwhich the aim is to minimize deflection and thus transfer loads gener-ated by accidental impact events from the plastic housing directly intointernal structures and components.

Mold-filling analysis – Computerized simulation of the injection mold-ing process that can be useful for evaluating the part and processdesign before the design is finalized through prototype manufactureand testing.

Mold shut-off – Areas within the mold where moving steel meets steeland does not allow molten plastic to flow past. Also referred to as “kiss off,” the parting line steel is an example where the mold halves“shut off” against one another tightly and do not permit material toflow between the halves.


Glossary


Notched Izod strength – An impact value and test method that showsthe material’s relative ability to resist triaxial stresses created by impactloads. Although the most widely used impact test for plastics, NotchedIzod values cannot be used directly in engineering equations to predictpart performance.

P-20 steel – A common tool-grade steel widely used to build toolingfor conventional plastic injection molding applications.

Packing Phase – The portion of the injection molding cycle, after themold cavity is filled, when additional material is added to compensatefor shrinkage during cooling and to help equilibrate cavity pressuresto promote consistent, high quality parts.

Pin punching – In plastics injection molding, cases in which pins usedto eject the finished plastic part from the mold penetrate the part anddamage it.

Plastic deformation – Permanent deformation where the part does notreturn to its original shape even after the loading has been removed.This is in contrast to elastic deformation, which will reverse uponremoval of the load.

Pressure cutoff – The transfer point when the first stage (fill) hydraulicinjection pressure is stopped and the second stage (pack/hold) pressureis initiated. This is usually the result of a signal from the mold ormachine to activate certain hydraulic valves. The signal can be from a timer, pressure transducer, or position sensor, or may be computergenerated.

Relative Thermal Index (RTI) – Underwriters Laboratories’ relativethermal index – which provides guidelines as to how a plastic can be expected to perform at temperature over time. In testing, a heatedproduct is allowed to cool. A rating is then assigned based on how well the cooled item retains its shape following subjection to a certainamount of stress and pressure.

Residence time – The length of time a given amount of material residesin the barrel (and sometimes the hot manifold) before it is injectedinto the mold. Lengthy residence time exposure should be avoided,since it can lead to material degradation in the form of surface defects,and a reduction in molecular weight and property performance.

Rheological behavior – The complex response of a material during flow.Engineering thermoplastics typically exhibit non-Newtonian rheologicalbehavior, meaning that their viscosity is not constant with shear rate.These materials are shear thinning in that viscosity drops as shear rateincreases. The viscosity is also strongly dependent on the temperatureof the material.


Glossary


Secant modulus – Idealized modulus derived from a secant drawnbetween the origin and any point on a nonlinear stress/strain curve.The secant modulus is the line drawn from zero stress point to thecurve for a specific designated strain. It represents the stiffness of thenon-linear thermoplastic at a given stress/strain condition and leadsto a more accurate force and deflection calculation.

Servo valves – Components of a motion control system that use a feed-back device to precisely control injection molding functions such as mold clamping and melt injection.

Skin – The outer surface of the wall section of a thermoplastic part,formed during the filling stage of the injection molding process. Asthe molten plastic is injected into the mold cavity, it is quenched andsolidifies first on contact with the cooler mold surface, forming theskin of the part.

Spiral flow length – A test used to predict the distance a thermoplasticmelt will flow under specified pressure and temperature along aspiral runner in a mold. The test can define the “flow length” of agiven material, but is not an accurate predictor of actual materialperformance in a specific injection molding application.

Stress state – The measure of the complexity of the stress in a partunder load. A uniaxial stress state, found in a tensile bar under load, is the most simple form. A biaxial stress state, found in a Dynatup-styleimpact test setup, is more severe. A triaxial stress state, found in aNotched Izod test setup, is the most severe state. Stress state can beused to determine the severity of a stress concentrator.

Stress/strain curve – A graphical representation of the relationshipbetween stress and strain, traditionally displayed using data from tensiletests. The elastic modulus can be seen as the slope of the linear portionof the curve at low strains. The “yield” stress can be seen as the maxi-mum value of the stress.


Glossary


Total shot capacity – For any given injection molding machine, themaximum volume of melt (in3 or cm3) that can be injected by a singlestroke of the injection screw. Also commonly measured in ounces ofpolystyrene in the U.S.A.

Ultrasonic welding – An assembly method used for mechanical fasten-ing of plastic parts. The welding is accomplished through the applicationof vibratory mechanical pressure at ultrasonic frequencies. This vibratorypressure in the sealing area generates frictional heat that melts thethermoplastics and allows them to bond.

Weld energies – In ultrasonic welding, microprocessors can control thetime of the weld by calculating the energy (in joules) that has beenprovided to the weld.

DISCLAIMER: THE MATERIALS AND PRODUCTS OF THE BUSINESSES MAKING UP THE GE PLASTICS UNIT OF GENERAL ELECTRIC COMPANY, ITSSUBSIDIARIES AND AFFILIATES (”GEP“), ARE SOLD SUBJECT TO GEP‘S STANDARD CONDITIONS OF SALE, WHICH ARE INCLUDED IN THEAPPLICABLE DISTRIBUTOR OR OTHER SALES AGREEMENT, PRINTED ON THE BACK OF ORDER ACKNOWLEDGMENTS AND INVOICES, ANDAVAILABLE UPON REQUEST. ALTHOUGH ANY INFORMATION, RECOMMENDATIONS, OR ADVICE CONTAINED HEREIN IS GIVEN IN GOOD FAITH,GEP MAKES NO WARRANTY OR GUARANTEE, EXPRESS OR IMPLIED, (I) THAT THE RESULTS DESCRIBED HEREIN WILL BE OBTAINED UNDER END-USECONDITIONS, OR (II) AS TO THE EFFECTIVENESS OR SAFETY OF ANY DESIGN INCORPORATING GEP MATERIALS, PRODUCTS, RECOMMENDATIONSOR ADVICE. EXCEPT AS PROVIDED IN GEP‘S STANDARD CONDITIONS OF SALE, GEP AND ITS REPRESENTATIVES SHALL IN NO EVENT BERESPONSIBLE FOR ANY LOSS RESULTING FROM ANY USE OF ITS MATERIALS OR PRODUCTS DESCRIBED HEREIN. Each user bears full responsibilityfor making its own determination as to the suitability of GEP‘s materials, products, recommendations, or advice for its own particular use. Each usermust identify and perform all tests and analyses necessary to assure that its finished parts incorporating GEP materials or products will be safe andsuitable for use under end-use conditions. Nothing in this or any other document, nor any oral recommendation or advice, shall be deemed to alter,vary, supersede, or waive any provision of GEP's Standard Conditions of Sale or this Disclaimer, unless any such modification is specifically agreedto in a writing signed by GEP. No statement contained herein concerning a possible or suggested use of any material, product or design is intended,or should be construed, to grant any license under any patent or other intellectual property right of General Electric Company or any of its subsidiaries or affiliates covering such use or design, or as a recommendation for the use of such material, product or design in the infringementof any patent or other intellectual property right.


Literature


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® CYCOLAC, CYCOLOY, LEXAN, NORYL, ULTEM and VALOX are Registered Trademarks of General Electric Company.

™ The Weatherables and GE Select are Trademarks of General Electric Company.

A comprehensive 380-page guidethat provides in-depth informationto assist engineers design state-of-the-art equipment and componentswith GE engineering thermoplastics.Includes explanations of productgroupings and grades, applicationdevelopment assistance, proceduresand data relating to material selec-tion, design assistance, prototypingand processing considerations,assembly and finishing details. Alsoincluded is a glossary of commonlyused engineering terms that relateto plastics.

Complete access to over 3,000 pages of technical information isavailable on-line on the Internet.Users can navigate within GE Plastics’ www.geplastics.comaddress to access the literaturedescribed above as well as GE Select™, an engineering data-base containing product datasheets for over 500 grades of GE engineering resins.

We’re pleased to offer our customersan extensive library of product,processing, technical and generalliterature. From our popular DesignGuide to our in-depth Product andProcessing Guides, our literature isdesigned to act as a critical referencetool for our full product portfolio.To order a particular guide or aseries of multiple guides, please call1-800-845-0600 or access our on-line order center at www.geplastics.

A broad range of valuable infor-mation can be found in a host ofgeneral literature from GE Plastics.From our Global Resources and Six Sigma Quality brochures, to The Weatherables™ and new PolymerProcessing Development Centerbrochures, we offer you in-depthcoverage of our key business andtechnological initiatives.

A series of brochures containinginformation to assist designers and processors understand thecharacteristics of various polymerchemistries within GE Plastics’resin families. Includes informationto help select engineering thermo-plastics for specific applications.

Over 200 pages of informationcontained in 17 brochures to assistprocessors fabricating componentsusing injection molding processes.Includes general information onconverting engineering thermo-plastics together with specific processing details and moldingparameters for each GE resin familyand grade. Additional processingpublications containing informationon Thinwall,SM Gas Assist, Thermo-forming and Engineering StructuralFoam processing techniques arealso available.

Key product family products, markets and services are featured in a variety ofnew product-line brochures. Includes property and application informationon our Crystalline, Engineered Styrenics Resins, LEXAN®, ULTEM® andNORYL® product lines.

Provides comprehensive propertyprofiles and descriptive informa-tion on GE Plastics’ entire productportfolio with data on testing methods utilized to evaluate engi-neering materials.

General Guides

Design Guide

Product Guides

Processing Guides

GE Product Families

The Internet


Sales Offices


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AMERICASUnited States

GE PlasticsOne Plastics AvenuePittsfield, MA 01201 USATelephone: (413) 448-7110Cable: GEPLASTICS

California★ Western Business Region

4160 Hacienda DrivePleasanton, CA 94588Telephone: (510) 734-0161

Georgia★ Commercial Development Center

205 Scientific DriveNorcross, GA 30092Telephone: (770) 662-1000

IllinoisSuite 100, One Corporate Lakes2525 Cabot Drive, Lisle, IL 60532Telephone: (630) 505-2500

Massachusetts★ One Plastics Avenue

Pittsfield, MA 01201Telephone: (413) 448-7110

MichiganP.O. Box 5011Southfield, MI 48086-5011

★ 25900 Telegraph RoadSouthfield, MI 48034Telephone: (248) 351-8000

OhioSuite 660, 6000 Lombardo CenterSeven Hills, OH 44131Telephone: (216) 524-2855

TexasSuite 930, 5430 LBJ FreewayDallas, TX 75240Telephone: (972) 458-0600

Puerto RicoGeneral Computer BuildingP.O. Box 2010 Bayamon Puerto Rico 00960Road 174, No. 101Minillas Industrial Park Bayamon Puerto Rico 00959Telephone: (787) 288-2340Telefax: (787) 288-2348

BrazilGE Plastics South America S/AAv. das Nacoes Unidas, 12995 -20 andarBrooklin Novo04578-000 São Paulo, SP BrazilTelephone: (55) 11-5508-0500Telefax: (55) 11-5505-1757

CanadaGE Plastics – CanadaGeneral Electric Canada Inc.2300 Meadowvale Blvd.Mississauga, OntarioL5N 5P9 CanadaTelephone: (905) 858-5700Telefax: (905) 858-5798

MexicoGE Plastics Mexico S.A. de C.V.Av. Prolongación Reforma #490

4o. PisoColonia Santa Fe01210 Mexico, D.F.Telephone: (525) 257-6060Telefax: (525) 257-6070

EUROPEEuropean Headquarters★ General Electric Plastics B.V.

Plasticslaan 1 4612 PX Bergen op ZoomThe NetherlandsTelephone: (31) 164-292911Telefax: (31) 164-292940General Electric Plastics B.V.P.O. Box 1174600 AC Bergen op ZoomThe NetherlandsTelephone: (31) 164-292911Telefax: (31) 164-291725

AustriaGE Plastics AustriaPottendorfarstrasse 47A-2700 Wiener Neustadt, AustriaTelephone: (43) 2622-39070Telefax: (43) 2622-39047

France★ General Electric Plastics France

S.a.r.L.Z. I. de St. GuénaultBoite Postale No. 67F-91002 Evry/Cedex, FranceTelephone: (33) 1-60796900Telefax: (33) 1-60796922

Germany★ General Electric Plastics GmbH

Eisenstraße 565428 RüsselsheimPostfach 1364-65402 Rüsselsheim, GermanyTelephone: (49) 6142-6010Telefax: (49) 6142-65746

India★ GE Plastics India Limited

405-B, Sector 20Udyog Vihar Phase IIIGurgaon 122016 (Haryana)Telephone: (91) 124-341-801Telefax: (91) 124-341-817

Italy★ GE Plastics Italia S.p.A.

Viale Brianza 18120092 Cinisello BalsamoMilano, ItalyTelephone: (39) 2-61834-301Telefax: (39) 2-61834-305

Spain★ GE Plastics Iberica, S.A.

Avinguda Diagonal 652-65608034 Barcelona, Spain Telephone: (34) 93-252-1606Telefax: (34) 93-280-2619

United Kingdom★ GE Plastics Ltd.

Old Hall RoadSale, Cheshire M33 2HGUnited KingdomTelephone: (44) 161-905-5000Telefax: (44) 161-905-5106

PACIFICPacific Headquarters

GE Plastics Pacific Pte. Ltd.#09-00 GE Tower240 Tanjong Pagar RoadSingapore 0208Telephone: (65) 220-7022Telefax: (65) 326-3290

Australia★ GE Plastics (Australia) Pty. Ltd.

175 Hammond RoadDandenong, Victoria 3175AustraliaTelephone: (61) 3-9703-7200Telefax: (61) 3-9794-8563GE Plastics (Australia) Pty. Ltd.57/2 O’Connell StreetParramatta, New South Wales 2150AustraliaTelephone: (61) 2-9689-3888Telefax: (61) 2-9689-3530GE Plastics (Australia) Pty. Ltd.Legal and General Building206 Greenhill RoadEastwood, South Australia 5063AustraliaTelephone: (61) 8-8272-5044Telefax: (61) 8-8272-2479

ChinaGE Plastics – BeijingGeneral Electric (USA) China

Company, Ltd.3rd Floor, CITIC Building No. 19 Jian Guo Men Wai Ave.Beijing 100004, P.R. ChinaTelephone: (86) 10-6500-6438Telefax: (86) 10-6500-7476

GE Plastics – ShanghaiGE China Company9th Floor, Shartex Center88 Zunyi Road (S)Shanghai 200335, P.R. ChinaTelephone: (86) 21-6270-9623Telefax: (86) 21-6270-9973

Hong KongGE Plastics Hong Kong LimitedRoom 1008, Tower I, The Gateway25 Canton Road, TsimshatsuiKowloon, Hong KongTelephone: (853) 2629-0853Telefax: (853) 2629-0804

IndonesiaGE Plastics – IndonesiaKH Mas Mansyur Kav. 126Jakarta 10220, IndonesiaTelephone: (62) 21-574-4980Telefax: (62) 21-574-7101

JapanGE Plastics Japan, Ltd.Tokyo OfficeNihombashi Hamacho Park Bldg.2-35-4, Nihombashi-HamachoChuo-ku, Tokyo 103, JapanTelephone: (81) 3-5695-4861Telefax: (81) 3-5695-4859

Korea★ GE (USA) Plastics Korea Co., Ltd.

231-8, Nonhyundong KangnamkuSeoul 135-010, KoreaTelephone: (82) 2-510-6250/6000Telefax: (82) 2-510-6666/6224

SingaporeGE Plastics – SingaporeSales & Marketing Office80 Anson Rd., #38-00 IBM Towers Singapore 079907Telephone: (65) 223 -7022Telefax: (65) 223 -7033

TaiwanGE Plastics, Taiwan8F -1, 35 Min Chuan E. Road Sec. 3Taipei, Taiwan, R.O.C.Telephone: (886) 2-509-2124Telefax: (886) 2-509-1625

ThailandGE Plastics – Thailand21st Floor Thaniya Plaza Bldg.52 Silom RoadBangkok 10500, ThailandTelephone: (66) 2-2312323Telefax: (66) 2-2312322

★ Application Development Center


GE PlasticsWe bring good things to life.

General Electric CompanyOne Plastics AvenuePittsfield, MA 01201800.845.0600http: / / www.geplastics.comNBC-575A (9/98) CA

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