mfreebrey_product design principles-imts

8/13/2019 MFreebrey_Product Design Principles-IMTS

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Plastic Product Design Principles

Marc FreebreyTuesday 11 th September


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1810

1855

The right tool ?


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Molded components have changed the shape of everyday items

LighterRecyclableAesthetic Design

Plastic Product Design Principles

3D CADCAM systems have facilitated the

evolution of product design, and as a result, everincreasing geometry complexity . . . . .


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CIMData commissioned an International Trade Report listing the major consuming industries

for plastic injection molds. One specific study was to determine the amount of CAD data clean-

up work was required by the toolmaker .

Consuming Industries

Extent of model modification / clean-up (%)

Substantial 21

Moderate 26

Slight 37

None 16

Total 100

Source : CIMData

Consuming Industry Market Share (%)

Motor Vehicles 41

Electronics 16

Appliances 14

Packaging 10

Medical 6

Toys 4

All Other 9

Total 100


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The objective is to look at some key product design principles that will eliminate uncertainty

and ensure manufacturability for your next project. While each of the points are generic and

cannot be applied to every scenario, they provide a solid base from where to start your next

design.

Complexity v's Manufacturability

Consistent Wall Thickness

Draft Angle

Boss Design

Rib Design

Part Radii

Part Text

UndercutsLiving Hinges

Ventilation Slots

Gate Position

Material Choice


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Consistent wall thickness produces even material flow during the injection phase of the

molding cycle, helping to ensure uniform cooling and minimize warpage. Any major change

can cause molding issues such internal voids, surface sink marks, unpredictable shrink rates

and ultimately, longer cycle times.



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4 Draft

3 Draft

2 Draft

1 Draft

Draft angle is an important feature that allows a molded part to be extracted from the mold

cavity without issue. The high pressures of injection moulding and material contraction means

that it is often difficult to remove the part.

Draft Angle

For smooth surfaces,generally a minimum of 1

degree per side provides easyejection.

Textured surfaces are slightlydifferent as the non uniformtexture will drag and scuff ifinsufficient draft is applied.

As a general guideline, aminimum of 1.5 degrees per0.025mm depth of textureneeds to be allowed for - inaddition to the normal draft

amount.


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Bosses are common features in plastic part design as they offer strengthening properties and

provide alignment during assembly.

Boss Design #1

The boss thickness should be 60% of the nominal wall thickness. The boss height should not exceed 2.5 x the diameter of the hole in the boss. Corner bosses integral to side walls will result in excess material accumulation Tall ribs on a boss help material flow and venting, reducing the chances of air traps Gussets distribute the load applied (during screw/insert insertion) to a wider area, reducing

failure at the boss/wall junction

The top of the boss cancause an air trap and createa short fill or burn mark as a

result

Tall ribs on a boss helpmaterial flow and venting,reducing the chances of air

traps


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Try to avoid bosses that merge into the side walls as this produces thick sections, material

accumulation and ultimately, sink marks.

If the boss wall thickness must exceed the recommended value, adding a recess (0.3 nominal

part thickness) at the base of the boss will reduce the chances of sinking.

Boss Design #2

0.3 T

30

Avoid bosses that mergeinto the side walls as thisproduces thick sectionsRecess to be 0.3 of part

thickness to reduce thepossibility of sinking


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When creating rib patterns, it is important to remember that ribs are there to increase part

rigidity and should not be compromised for aesthetical reasons.

Rib Design

Rib thickness should be 60% - 80% of the nominal wall thickness. Maximum rib height should not exceed 3X the nominal wall thickness. To increase product rigidity, it is

better to increase the number of ribs rather than the rib height. Minimum spacing between ribs should be 2X the nominal wall thickness. Fillets at the rib base reduce stress, fillets at the top can aid material flow and minimise sticking within the

mold (Fillet radii applied to ribs should be no greater than 50% of the rib thickness). Cross ribbed patterns are preferred (if the design allows) as they offer greater stability and ensure uniform

stress distribution. Extra thick ribs should be cored out.


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A significant number of plastic parts fail due to sharp corners or insufficient radii. Sharp

corners create localised stress concentrations which will crack and cause premature part

failure. The addition of fillet radii to all sharp corners will not only reduce stresses, but also

improve plastic flow. As a general rule, at corners, the inside radius is 0.5 x material thickness

and the outside radius should be 1 x material thickness plus the part thickness - a larger

radius should be used if the part design allows it.

Part Radii

Excess materialaccumulation may lead to

voids or sink marks


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Text is usually one of the smallest details on a part and this makes it difficult to manufacture

(typically electrodes are required). Text should be embossed (upwards on the model) as this

represents a cavity in the mold and therefore easier to machine and easier to polish.

Part Text Up or Down ?

Text should be raised on thepart with a minimum of 2

degrees of draft


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Most undercuts cannot strip from the mold and therefore require additional mechanisms in

the mold to move certain components prior to ejection. This is typically performed using

slides, cams, lifters or collapsible cores all adding costs to the mold design. Clever part design

or minor design concessions often can eliminate complex mechanisms for undercuts.

Undercuts

Slide required for the holeson the side of the part

Lifter required for internalfixing on the underneath of

the part


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Undercuts #1 Snap-fit

A core pin eliminates theneed for side action when

creating snap-fit clips

Considerate part design can add value to the component with minimal impact on tooling costs.

Draw direction

Side action required


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Lifters


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Considerate part design can add value to the component with minimal impact on tooling costs.

Undercuts #2 Side Holes

Draw direction

Extending the slots over the top of acorner edge enables straight draw and

eliminates the need for side actionShielded ventilation slotsprotect internal circuitry


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A living hinge is a thin flexible hinge made from the same material as the two rigid pieces it

connects. Polyethylene and Polypropylene are considered to be the best resins for living

hinges, due to their excellent fatigue resistance and can flex more than a million cycles without

failure.

The thickness of a living hinge should range from 0.25 to 0.5 mm

Living Hinges

1.5mmR 0.1mm

R 0.75mm

0.2mm

0.3mm

Section showing suggested living hingedesign for Polypropylene & Polyethylene

Source : Efunda.com


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The gate location ultimately determines the filling behaviour, weld lines, shrinkage, warpage

and surface quality of the molded part. It is often preferred to gate onto the thickest section of

the component to reduce the possibility of sinking due to insufficient material packing.

Gate Location / Plastic Flow Front

Features that are normal to thematerial flow direction will

cause weld lines, back fill andunpredictable shrinkage

Material flow following thefeature direction


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Gate Location / Plastic Flow Front

Flow Leader Thickened sections (no

more than 25% ofnominal partthickness) to help drivethe plastic flow front.Typically used forfilling or packing issuesin areas furthest awayfrom gate location or

used to balance thefilling of nonsymmetrical parts

Flow Restrictor Small areas of reducedthickness (no morethan 33% of nominalpart thickness) to helpcontrol the plastic flowfront. Typically used tocontrol the fillingpattern and manageweld lines and air traplocations.


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Accepted Technology


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Successful designs are built on the knowledge of how the chosen resin will perform during the

molding process

Material Choice

Material / Resin Strength ImpactResistance

High TempStrength

Warp anddimensional

accuracy

Fills smallfeatures

Voids in thickareas

Sinking inthick areas

Acetal Medium Medium Medium / Low Fair Fair Poor Good

Nylon 6/6 Medium High Low Fair Excellent Good Fair

Nylon 6/6, Glass Filled High Medium High Poor Good Excellent Good

Polypropylene Low High Low Fair Excellent Poor Poor

High Density Polyethylene(HDPE)

Low High Low Fair Excellent Unknown Poor

Polycarbonate Medium High Medium / High Good Fair Fair / Good Fair

Acrylonitrile ButadieneStyrene (ABS)

Medium / Low High Low Good Fair Good Fair

Polycarbonate / ABS Alloy Medium High Medium Good / Excellent Fair Good Fair

Plybutylene Terephthalate Medium High Low Fair Fair Unknown Fair

Polystyrene Medium / Low Low Low Good Good Unknown Fair

Thermoplastic Elastomer Low High Low Poor Excellent Excellent Good

Acrylic Medium Low Low Good Fair Excellent Good

Source : Protolabs


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Trouble Shooting

Problem Cause Remedy

Brittleness

Wet materialOverheatingMolded-in-stressesPoor part designWeld-lines

Review drying procedureReduce barrel/nozzle temperatureIncrease barrel/nozzle temperatureEliminate sharp cornersIncrease injection pressureIncrease melt temperature

Warped parts

Part temperature differentialExcessive shrinkageOrientation of materialPoor part designEjection problem

Check mold cooling systemIncrease part packingChange gate locationAdd ribs or part thickness to improve stiffnessCheck for uniform wall thicknessIncrease cooling timeReduce mold temperatureIncrease ejector pin area

Flashing

Inadequate clamp tonnageHigh Injection PressureMisaligned platesExcessive vent depth

Use a larger machineReduce injection pressureAlign platesReview mold venting

Burn marks

Air trapped in cavityBarrel or nozzle overheatingShear heatContaminationHang-up in molding machine

Improve mold ventingCheck heater controlsReduce injection speedPurge barrelClean hopper dryerRemove and clean screw

Problem Cause Remedy

Weak weld-lines

Insufficient ventingInjection speed or moldtemperature too lowIncorrect gate location

Improve cavity ventingIncrease injection rate and tool temperatureRelocate gate or add overflow tab

Sticking in mould

Over packingMold design

Reduce injection pressureReduce injection speedCheck for undercutsInspect ejector systemIncrease draft in tool

Sinks or voids

Holding pressure / time too lowInsufficient feedGate freezing off or locatedimproperly

Increase hold pressure or timeIncrease shot sizeCheck gate dimension and location

Dimensional inconsistency

Shot to Shot variationMelt temperature variationInadequate packing

Maintain adequate cushionCheck for worn check ringCheck heater bands/controllers

Increase hold timeEnlarge gate to prevent premature freeze-off


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A project


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Automotive Instrument Panel

Specifications: Part weight 2.9 Kg 10 Injectors Manifold Injection Time 60 s 2300 Tons Injection Machine 32 Tons mold

600 Parting Surfaces

54 Modified Areas


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Injection Side Mechanics

10 undercuts on the injection side


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Ejection Side Mechanics

21 undercuts on the ejection side


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Cooling

36 different cooling circuits


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Numbers: 5740 Solids 1348 Screws 685 Cooling Channels 6 Part Changes 486 Hours (Design time) 3 Concurrent Designers

Vital Statistics


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Understanding the mold design & manufacturingprocess will help you design better parts.

Conclusion

Check List : Uniform Wall Thickness Adequate Draft Fillet Wherever Possible Basic Rib Fundamentals Basic Boss Fundamentals Understand Material Characteristics Knowledge Sharing Relationship With Molder


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Thank you for your time & attention

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