ic workshop materials 09 - rapid prototyping & manufacturing technologies

8/12/2019 IC Workshop Materials 09 - Rapid Prototyping & Manufacturing Technologies

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Reading Materials for

IC Training Modules

Rapid

Prototyping

&

Manufacturing

Technologies

ICPROFESSIONALTRAININGSERIES

Last updated at AUGUST 2009

Copyright reserved by INDUSTRIAL CENTRE, THE HONG KONG POLYTECHNIC UNIVERSITY


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Rapid Prototyping & Manufacturing Technologies

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IC Professional Training

Objectives:

To understand the importance and applications of rapid prototyping and

manufacturing (RP&M) technologies in the product design anddevelopment processes.

To understand the general function of prototype in the product design

and development process.

To appreciate the common types of rapid prototyping systems.

To understand the basic steps in RP process.

To learn the basic of Rapid Tooling process.

1. Introduction

In recent years, opening up local markets for worldwide competition has led to afundamental change in new product development (NPD). In order to stay

competitive, manufacturers should be able to attain and sustain themselves as

"World Class Manufacturers". The manufacturers should be capable in delivering

products in fulfilling the total satisfaction of customers, products in higher quality,

short delivery time, at reasonable costs, environmental concern and fulfill all the

safety requirements.

In many fields, there is great uncertainty as to whether a new design will actually

do what is desired. New designs often have unexpected problems. A prototype is

often used as part of the product design process to allow engineers and designers

the ability to explore design alternatives, test theories and confirm performance

prior to starting production of a new product. Engineers use their experience to

tailor the prototype according to the specific unknowns still present in the

intended design.

Rapid Prototyping technology employ various engineering, computer control and

software techniques including laser, optical scanning, photosensitive polymers,

material extrusion and deposition, powder metallurgy, computer control, etc. to

directly produce a physical model layer by layer (Layer Manufacturing) in

accordance with the geometrical data derived from a 3D CAD model. RP can

deliver working prototype at the early design stage of the new product cycle. Somanufacturers can use the working prototypes in bridging a multi-disciplined

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1.1.3 Engineering / Functional prototypes

In engineering development, Engineers will evaluate the form, fit and function of

the parts. The prototype will use the same material of the final product if

technology permitted. If exact material is not available, the materials with similar

mechanical, thermal and electrical properties are used. The SLS and FDM allow theuse of production grade plastic material, although the mechanical properties are

not exactly the same.

1.1.4 Samples for safety approval

For those products that are regulated by safety standards of importing country, at

lease 5-10 product samples have to be submitted to the laboratory for each

application. If pilot run of the product is ready, the finish products will be

submitted. However, in most case the project is still in tooling stage, and samples

by PU duplication may be used if the laboratory accepts. 5 to 10 duplicates can beproduced in a day. PU duplicate cannot be used as substitution if the part under

consideration is subjected to high temperature, voltage and mechanical abuse. In

this Rapid Tooling techniques can be used. Manufacturers have a choice to quickly

build a mould and produce a small batch of sample parts in actual production

material.

1.1.5 Marketing Samples

At a later stage when all design details are finalized and the project is in tooling

stage, the marketing department will start the promotion campaign. A lot of

samples may be distributed to clients. In this case, up to a hundred products maybe needed. Normally the samples will be made by RTV mould and PU duplication

technique. In many cases, these models are also used to take the picture that

appears in the package box or user manual.

1.2 Differences between conventional machining and rapid prototyping

Conventional machining can produce prototypes using metal removal method

with almost any type of engineering materials however there are only a limited

type of materials for any particular type of RP&M systems. RP&M systems are

usually used for making one or a few sample of prototype, while the number of

prototype produced in conventional machining can be adjusted as required. The

basic working principle of RP technologies is totally different from conventional

machining processes which build up a solid by mean of addition approach like

construction of the building. Hence the RP also started from foundation then

gradually increase in height until it reached the top layer.


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Fig. 1.1 Differences between conventional machining and rapid prototyping

1.3 Why RP&M

The application of the RP&M in the NPD can resolves the Fuzzy Front End the

messy getting started period of NPD process - would be relatively costly and

timely. However using RP&M enable manufacturers to schedule the right product

being developed in a timely manner is the most important winning strategy. The

combination of the RP&M and CAD technologies in blending with traditional

technologies and gradually formation of cross-functional team are the strategies

for survival or strategies for empowering organization, people and facilities.

1.4 Limitations of RP&M

High precision RP machines are still expensive at around US$ 100 thousands to 1

million and not easy to justify economically and many service bureaus in providing

the physical prototypes output services. However, as the RP technologies are

getting more mature while RP manufacturers are facing more head-to-head

competition, the price will go down significantly in the near future such as the

launch of V Flash priced at US$ 10 thousands. Most of the budget RP systems are

difficult to build parts with accuracy under +/-0.02mm and wall thickness under0.5mm. This is sufficient for prototypes built for concept evaluation and functional

test.

However, prototypes that will be used as master pattern for downstream

processes always require a much higher and consistent accuracy. Although there

are quite a large variety of materials that can be used in most RP processes, the

physical properties of the RP parts are normally inferior to those samples that

made in proper materials and by the traditional tooling. The RP parts are not

comparable to traditional Computer Numerical Control machined (CNC) prototype

parts in the surface finishing, strength, elasticity, reflective index and other

material physical properties.


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1.5 The future market profile of RP&M systems

The future market profile of the rapid prototyping/manufacturing industry will

have two specific niches. The first one will be focused on Digital Direct

Manufacturing (DDM) of engineering parts for not only testing form fit, functional

evaluation, but also can be directly used in products. The second niche will belooking at the market requirement in another point of view. It will focus on

concept modeling or 3-Dimensional printing for design verification, preliminary

marketing tool and manufacturability studies. In this case, the most important

considerations are speed and low cost. The accuracy and resolution requirement is

minimal so far it can provide the designer a reasonable representation of the

design.

2. Common Types of Rapid Prototyping Technologies

While there are many ways in which one can classify the numerous RP systems in

the market, one of the better ways is to classify RP systems broadly by the initial

form of its material, i.e. the material that the prototype or part is built with. In this

manner, all RP systems can be easily categorized into (1) liquid-based (2) solid-

based and (3) powder-based.

Liquid-based RP systems have the initial form of its material in liquid state.

Through a process commonly known as curing, the liquid is converted into the

solid state. The Stereolithography Apparatus (SLA) falls into this category.

Solid-based RP systems encompass all forms of material in the solid state. In this

context, the solid form can include the shape in the form of wire, roll, laminates,

pellets and powders. The Selective Laser Sintering (SLS), Three-Dimensional

Printing (3DP) Fused Deposition modeling (FDM) fall into this definition.

2.1 Stereolithography Apparatus (SLA) Liquid based RP

Stereolithography Apparatus (SLA) technology fabricates three-dimensional, solid

objects resin using a computer-directed, ultraviolet (UV) laser beam to cure

successive layers of photo-sensitive polymer resin in a vat.

SLA part is fabricated by laser spot instead of continues scanning on the resin

which forms a thin solid layer on the surface of the liquid resin through the

photopolymerization. Figure 2.1 shows the schematic diagram of this spot

scanning mechanism.

Polymerization is the process of linking monomers and oligomers into larger

molecules (polymers). When photoinitiators in the SLA resin are energized by the

laser spot energy exposure (Ec value in mJ/cm2) will undergoes the

photopolymerization process. It creates a thin layer of solid resin with thicknessabout 2-3 times thicker than the layer thickness (overcure). The total thickness of


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the layer is depending on the scanning speed of the laser and the Depth

penetration factor of the resin (Dp value in mm).

Fig. 2.1 Schematic diagram of SLA laser processing mechanism

2.1.1 Processes Description

In SLA process, the software firstly interprets and pre-process the CAD data and

slices it into a series of thin horizontal layers and converted to machine specified

control data files based on the part, building and recoating parameters. The

machine control data is then downloaded into the equipment for part building. A

perforated stainless steel building platform attached to a vertical elevator is

moved to the start position which is just below the resin surface.

An X-Y electronic motor driver optical scanning mirrors directs the laser beam,which cures the borders and cross sections of the built parts one layer at a time on

the surface of the resin. Photopolymers are converted into solid state instantly

after irradiation of laser beam

The elevator then lowers the newly built layer by a distance of one layer thickness

after a short period of time to allow the newly formed layer to increase the green

strength (pre-dip delay), the vacuum activated re-coater blade which is separated

from the surface of the resin by a predetermined blade gap then coats a new layer

of resin with thickness equal to the layer thickness on the newly formed solid layer,

and the process is repeated until the object is completed.

The elevator then rises out of the resin surface and the object is removed from the

vat for the post processing. Figure 2.2 shows a schematic diagram of SLA system.

Resin

Surface


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Fig. 2.2 Schematic diagram of stereolithography process

Fig. 2.3 Basic components of SLA system

2.1.2 SLA Material Liquid Resin

A variety of resin is available for SLA, each with its own advantage and weakness.

Typical SLA resins only react to a narrow bandwidth of UV ray, both Viper Si2 and

SLA-3500 employs a Diode-pumped solid-state (DPSS) lasers Nd:YVO4

(Neodymium Yttrium Vanadate) as a energy source of polymerization. Generally,

the resins need to be kept in an environment with tight temperature and humility

Laser concentrative UV beam to transom liquid resin

into solid state.

Elevator control the movement of platform upward

and upward in order to

Platform a steel plate with plenty of holes as the

basement for part building

Resin vat contain raw material to form SLA model

Mirrors control the path of movement of the laser

beam at X and Y axis

Sensor locate the coordinate and instant power of

the laser beam and feedback to the control unit for

fine adjustment

Sensor

Mirrors


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control in ensuring the reaction conditions. For example, the chamber of SLA is

maintained at 28C 1C.

Resins used in SLA process are mixture of photo-initiator, linkers, oligomers and

monomer mixture in liquid state, photonic energy source will trigger the chain

reaction of polymerization. Two parameters penetration depth (Dp) and critical

exposure (Ec), which are vary from different resins, are as the primary input to the

SLA machine for controlling the power and time of laser emission. The following

illustrate the underlying principle of the two important parameters;

Penetration Depth (Dp) and Critical Exposure(Ec)

When a beam of light hit the resin surface, it will cure a region of resin in the

shape of a bullet. The intensity of the beam determined the extent of reaction and

the size of the bullet.

The threshold exposure, Ec, is the energy required for the photopolymer changes

from the liquid to the gel phase. In the process, Dpand Ecis primary characters of

the resin provided by manufacturer. The SLA machine determine PLand Vson the

fry by the relation that the draw speed of the laser Vsproportional to (PL/EC)exp(-

Cd/Dp).

Cp Critical depth, given by the function

Cd=Dpln(Emax/Ec)

Dp The depth of penetration which the

power of laser decay to approx. Emax/3

Emax The power of laser at resin surface

Ec Critical Exposure, a primary resin

character, unit in mJ/cm2


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Physical properties of SLA material available in IC

Accura 25 Accura 55 Accura 60 Accura

Bluestone

Appearance White White Clear Opaque Blue

PenetrationDepth(Dp)*

4.2mils 5.2mils 6.3mils 4.1mils

CriticalExposure(Ec)*

10.5mJ/cm2 7.4mJ/cm2 7.6mJ/cm2 6.9mJ/cm2

Tensile Strength 38MPa 63-68MPa 58-68MPa 66-68MPa

Elongation atBreak(%)

13-20% 5-8% 5-13% 1.4-2.4%

@66PSI 58-63oC 55-58oC 53-55oC 65-66oC

@264PSI 51-55oC 51-53oC 48-50oC 65oC

@66PSI with120oC Thermal

Postcure

267-284oC

Hardness, Shore D 80 85 86 92

For more information about SLA material, please refer to 3D systems web site

(http://www.3dsystems.com/)


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2.1.3 Support Structure

Support usually need for liquid-based and solid-based RP systems in order to anchor

the part to the platform hence the part can be separated from the platform thus

preventing floating layers and make removal of the part became simple.

Also support is needed for overhanging and tilted surfaces hence minimize part curl

and stabilize the part while being built. Self support angle is a predetermined angle

for the particular SLA RP system such that tilted surfaces with this angle from the

horizon needs no support.

The supported surfaces are the overhanging down face of the part. Since supports

need to anchor the surface at the contact point. Hence the surfaces in contact with

the support are rough and coated with resin gel after the part build completed. So do

not place supports on surfaces where finish is important.

Fig. 2.4 SLA support structure, Left: curtain support. Right: fine-point support

Support can be classified according to the support structure and the type of surfaces

being supported. Support types include solid, box, web and fine point supports.

Besides supports classified according to the geometry of the down face include point,

line and surface supports.

SLA uses Fine-point support style as the default support style, however when long

and slender support is need then Curtain support style can be used to ensure the

strength of the support while the prototypes are build in the vat. Figure 2.4 shows

both SLA support styles.

2.1.4 Building Styles

The rapid prototyping systems usually bundled with software which can control the build

styles, generate supports, slicing of the STL files and creating control file for building

the prototype in the RP machine. There are Part build style, Support build style, Part

Recoat style and the Support Recoat style in the SLA software called 3DLightyear.

Part build stylecontrols the laser when drawing the part. Different part build style

can produce part in different patterns which include the solid part the exact build

style, the hollow part for investment casting Quickcast build style. The fast buildstyle which shortens the build time of the part by filling the part in alternate layers.

PartPart

Projection

Stand


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Support build stylescontrol the laser when drawing the support. Curtain support orFine point support can be created by choosing different support build styles in SLA

building platform.

Part recoat styles controls the recoating process including the dipping depth and

speed of the build platform and the speed and the number of recoats of the recoatblade.

Support recoat stylescontrols the recoating process while building the support.

2.2 Fused Deposition Modeling (FDM)

2.2.1 Processes description

Fused Deposition Modeling (FDM) was developed by Scott Crump, the founder of

Stratasys. It was commercialized by Stratasys in 1991. FDM process create component

by extruding material (normally a thermoplastic material) through a nozzle that

traverses in X and Y to create each two-dimensional layer.

Heaters which surround two separated nozzles keep the plastic at a temperature just

above its melting point so that it flows through the nozzle and forms the layer

according to the tool path. In each layer separate nozzles extrude and deposit

material that forms the parts and the support structure. The plastic hardens

immediately after flowing from the nozzle and bonds to the layer below. Once a layer

is built, the platform lowers, and the extrusion nozzle deposits another layer and the

process is repeated until the object is completed. Figure 2.5 show a schematicdiagram of FDM process.

Fig. 2.5 Schematic diagram of FDM processes


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Support structure can be removed manually or, when water soluble supports are

employed, they can be simply dissolved by put into particular chemical solution.

However, soluble support is only available for building ABS model, other high melting

point plastic material such as polycarbonate (PC) and polysulphone (PPSF) are not

applicable.

2.2.2 FDM material

The build material is usually supplied in filament form with 1.8mm diameter, but the

layer thickness and vertical dimensional accuracy is determined by the extrusion

nozzle diameter and the rate of feeding of the building material, which ranges from

0.018 to 0.008 inches. In the X-Y plane, 0.001 inch resolution is achievable.

Thermoplastic material is most often used which including Acrylonitrate Butadiene

Styrene (ABS), Polycarbonate (PC), polysulphone (PPSF), and investment casting wax is

designed for investment cast.

ABSi ABS-M30* PC-ABS PC* ULTEM908

5

PPSF*

Appearance Ivory White Tan

Tensile Strength

(MPa)

38 36 34.8 52 72 55

Tensile Modulus

(MPa)

1993 2413 1827 2000 2220 2068

Flexural

Strength (MPa)

59 61 50 97 115 110

Flexural

Modulus (MPa)

1797 2317 1863 2137 2507 2206

Notched Izod

Impact (J/m)

139 123 53.39 106 59

Heat deflection(oC)

95 96 326 127 153 189

Density g/cm2) 1.09 1.04 110 1.2 1.34 1.28

Elongation at

Break (%)

>10 4 1.2 3 4 3

Support

Structure

Soluble Soluble Soluble Bass Bass Bass

*Material Available in IC


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2.2.3 Support Structure

Supports in the form of solid type or box type are usually required for the Solid-based

RP system. Box type support is used in FDM. The support material used for FDM

depending on the type of model mater used for making the prototype and can be

classified into two main type soluble support and break away support. The solublesupport uses a "water-soluble" material which turns into liquid in an ultrasonic

heating tank filled with hot (70 degree F) amino based water solution for 4 hours, the

support is then dissolved in the tank. However the soluble support is only suitable for

ABS grade model material as shown in the table above. Break away support is used

for model material which melts at a higher temperature than ABS. Break away support

need to be removed by hand tools upon the completion of the building of the

prototype.

The self support angle of FDM is 45 degree and 4 types of support styles are available

for the creation of support namely Basic support, Sparse support, Surround supportand Break away support. The support curves can be created after the appropriate

supports style is determined.

Basicsupports will create supports under all part features that are exposed to air onthe underside. The top layer of each support column will have a solid fill and the

layers underneath will have a small air gap between toolpath passes. These passes are

called the support raster curves. This type support is particular suitable for model

with fine details in the down face however the build time is long and the more

support material is used compared with the Sparse support.

Sparsesupports will create supports that use less material than basic supports. This isaccomplished by creating a new raster fill pattern that has a much larger air gap

between raster. Like basic supports, the top layer of a support column is built with a

solid pattern of raster. The next several layers have the basic raster pattern that has a

small air gap. Progressing downward, a new sparse support region is used below the

several layers of basic support raster fill. The sparse support regions use a much larger

air gap when building the tool path raster. In addition, the region is surrounded by a

single closed tool path-perimeter curve.

Surround supports are used to surround small features or parts that require few

actual supports, but cannot stand upright on the platform during the part build.Surround supports will create the support tops just like basic supports but will also

surround every layer of the model with a minimum thickness of supports.

Break-awaysupports are similar to sparse supports but consist of boxes instead of acontinuous raster. There is no closed toolpath-perimeter curve around the break-

away supports. They are easier to remove than other support styles for some

materials but build slower than sparse supports. Break-away supports are not

recommended for use with WaterWorks.


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2.2.4 Toolpath

The software which bundled with FDM is Insight which transforms the STL file into a

CMB file. The CMB file contains information about the hardware setup, the tooth path

(Road) for building the part and the details which control the movement of the

building platform and the FDM head. Curves in Insight are group into different setnamely part set and support set. The set contain information about the fill up pattern

of the empty space inside the slice curve.

Fig. 2.6 Example of FDM Toolpath

The fill up pattern usually contain a perimeter and a raster fill as shown in figure 2.7.

Different fill pattern for support and model can be achieved by configuring the

parameters of the fill so 4 types of support styles is available to suit different

conditions.

Fig 2.7 Illustrations of Raster and Perimeter


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The fill up pattern of the part need to be investigated layer by layer to ensure there is

no overfill. Overfill occurs when too much material is squeezed into a geometry of

less volume. Underfill occurs when the software doesn't detect enough room in which

to squeeze the raster road. Overfill will hinder the parts surface finish or even cause

part to topple over. Hence overfill usually need to be fixed before the CMB file is

loaded to the machine for building. Figure 2.8 shows the overfill and underfill of

deposition.

Fig. 2.8 Descriptions of overfill and underfill

2.3 Selective Laser Sintering (SLS)

Selective Laser Sintering (SLS) was developed and patented by Ross House-holder in

1979, but it was commercialized by Carl Deckard at the University of Texas in Austin in

mid-1980s. DTM Corporation was first the company taking this technology into

commercial market and it was acquired by 3D Systems in 2001. The major competitor

of 3D systems; EOS, a Germany based company existed in the market in 1994, whichstill enjoys a significant market share. The major difference of the machine among 3D

systems and EOS is that 3Ds SLS machine can be processed multi material including

plastic, metal and resin sand but EOSs machine is targeted for single application, i.e.

EOS-S machine is solely for sand material processing.

2.3.1 Processes Description

The basic concept of SLS is similar to sterolothography, but the powdered polymer

and/or metal composite material is sintered or melted by a laser that selectively scans

the surface of a powder bed to create a two-dimensional solid shape. Three

dimensional object is then created by attaching together of the two-dimensional

layers.

As in all rapid prototyping processes, the parts are built upon a platform that adjusts

in height equal to the thickness of the layer being built and the commonly used layer

thickness is 0.15mm. After every layer of scanning, building platform lower one layer

thickness distant and a roller convey a new layer of material on top of the part surface

for the next scanning and the process is repeated until the object is formed. Figure

2.9 show a schematic of the selective laser sintering process.

Underfill raster

Ovefill Raster

Perimeter


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Fig. 2.9 Schematic of selective laser sintering process

(Source: CustomPartNet)

Since the sintering operation is performed by high power laser, the building platform

and the powder bed need to be pre-heated by infrared heaters and kept in a certaintemperature during processing to avoid part deformation. Nitrogen gas, as protective

gas, is launched into the working chamber throughout the sintering process to

prevent oxidization reaction. Unlike SLA, special support structures are not required

because the excess powder in each layer acts as self-support function while the part

being built except processing of metal powder.

Direct Metal Laser Sintering (DMLS) is the parented term used to describe the process

of metal powder sintering developed by EOS Company. The major competitive

advantage among DMLS and SLS provided by 3D systems is no further infiltration

process required in which SLS metal green part is required to be placed into a

furnace, temperature in excess 900 C, for removal of polymer binder and infiltrating

with bronze.


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2.3.2 SLS Material (Polymer)

Physical Properties Units PA PA-GF Alumide

Tensile Modulus N/mm 1700+/- 150 3200 +/- 200 3800+/- 150

Tensile Strength N/mm 45 +/- 3 48 +/- 3 46 +/- 3

Elongation at Break % 20 +/- 5 6 +/- 3 3.5 +/- 1

Flexural Modulus N/mm 1240 +/- 130 2100 +/- 150 3000 +/- 150

Charpy Impact strength kJ/m 53 +/- 3.8 35 +/- 6 29 +/- 2

Charpy Notched Impact

Strength

kJ/m 4.8 +/- 0.3 5.4 +/- 0.6 4.6 +/- 0.3

Izod Impact Strength kJ/m 32.8 +/- 3.4 21.3 +/- 1.7 NA

Izod - Notched ImpactStrength

kJ/m 4.4 +/- 0.4 4.2 +/- 0.3 NA

Ball Identation Hardness kJ/m 77.6 +/- 2 98 NA

Shore D-hardness kJ/m 75 +/- 2 80 +/- 2 76 +/- 2

Heat Deflexion t C 86 110 130

Vicat SofteningTemperature B/50

C 163 163 169

Vicat SofteningTemperature A/50

C 181 179 NA

Remark:PA is Polyamine (Nylon), GF is glass fiber reinforcement, Alumide is PA powder mixwith aluminum powder; the materials are used in EOS INT P system.


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2.4 Three Dimensional Printing (3DP)

The three Dimensional Printing (3DP) technology was invented at the

Massachusetts Institute of Technology and licensed to several corporations. 3DP

indeed is the innovation and further evolution of two-dimensional printing

technology. The process is similar to the Selective Laser Sintering (SLS) process,but instead of using a laser to sinter the material, an ink-jet printing head

deposits a liquid adhesive that binds the material. The following mainly describes

3DP technology which is employed in Z-corp 3D printing.

2.4.1 Process description

The 3D printing process begins with the powder supply being raised by a piston

and a leveling roller distributing a thin layer of powder to the top of the build

chamber. A multi-channel ink-jet print head then deposits a liquid adhesive to

targeted regions of the powder bed. These regions of powder are bondedtogether by the adhesive and form one layer of the part. The remaining free

standing powder supports the part during the build. The processes repeated and

solid model is competed and underneath inside the powder bed. Figure 2.10

show a schematic illustration of the 3DP process

Fig. 2.10 Schematic of 3D printing technology

After the part is finished, the loose supporting powder can be brushed away and

the removed by vacuum cleaner. Since the green part is very fragile, infiltration

of glue/ epoxy is needed to be applied on the part surface to improve the

strength of part. It should be noticed that any loose supporting powder should

be removed completely before performing infiltration to avoid generation of

ugly surface.

Input Binder Material

Print Head

Feedingpiston


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2.4.2 3DP Material

Material used in Z-corp 3DP include plaster, starch and ceramic powders, plaster

powder is commonly used for making concept design model, it is most suitable

for concept proofing as the build speed is very fast, typically 2-4 layers per

minute. Starch powder is particular used to produce master patterns forinvestment cast since it can be vaporized in high temperature. Ceramic powder

coated with resin is specially designed for making sand casting mould, i.e. Z-cast

is a special mixture of sand particle and resin for casting purpose.

Although the build speed of 3DP is relatively fast compared with other RP

processes, post-process is time-consuming and laborious. Infiltration is usually

performed by manual dipping of glue or epoxy resin. Wax-dipping has being

developed and utilized to speed up the post-process but the part strength is

much weaker than glue infiltration. A new powder system zp140 has been just

developed in which the parts can be cured by dipping into water or spaying onthe part surface.

3. Workflow of RP processes

There are basically three stages of building physical RP models based on the

CAD data, namely the pre-processing, building and the post processing.

Whatever the CAD model is generated by solid modeling or surface modeling

approaches, most of RP machine accept only STL file as digital data input in

which most commercial Engineering CAD system is capable to convert 3D CADmodel into STL format. The STL model is then slice into layers data set and

transfer to machine for building model. Completed RP model is then performed

corresponding post-processed operations such as cleaning, post-curing and

infiltration. Figure 3.1 shows the typical workflows of RP processes

Fig. 3.1 RP processes workflow


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3.1 Pre-processing

The first step in the RP&M process is virtually identical for all of the various

systems, and involves the generation of a three-dimensional computer-aided

design model of the object. A good, preferably solid CAD modeling or a total

enclosed surface water tight CAD model is a key component of success RPprocessing. The CAD file is then translated into a triangulation tessellated STL

format, which is the standard of the RP&M field. Figure 3.2 shows a typical

example of STL model which is composed of triangles and each triangle is

described by a unit normal vector direction and three points representing the

vertices of the triangle.

Fig. 3.2 Example of STL model

3.1.1 Verification and fixing of STL file

When one create 3D surface model using common surface modeler, there is very

little concern on the orientation of the surface normal, as most of the tool path

generation algorithms can detect the material side correctly.

However, when one generate STL data using these surface models, many

converters just use the normal data straight from the NURBS surfaces free form

surfaces, thus the STL files generated are not useable without repair.

Triangles in an STL file must all mate with other triangle at the vertex and must

be properly oriented to indicate which side of the triangle contains mass. Many

STL viewers like Magics RP from Materialise read a STL file to analysis and to

correct, the connectivity and gap in the three-dimensional triangle matrix. This

process is totally automatic and simple to operate. Figure 3.3 shows a bad and

good STL file.


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Fig. 3.3 Bad STL file is fixed Good STL file

3.1.2 Part Orientation

Part orientation has a significant effect on the final part quality and prototyping

cost. The switching between individual layers takes a significant part of the

overall building time and hence must be properly optimized hence to reduce to

building time and the building cost. The part should be orientated with minimum

height in order to reduce the number of layers.

For processes that need supports structures, part orientation should also be

optimized such that it would require minimal support hence reduce to buildingtime and the building material. The ultimate strength of the part will be affected

by its orientation within the print box. The part will be strongest along the Y-Axis

and the X-Axis and less strong along the Z-Axis which is illustrated in figure 3.4

Fig. 3.4 Effect of orientation to part strength

Hole exists in STL file Hole is filled by

Automatic Fixing


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Furthermore, staircase effect will be appeared on the near flat curve surface.

Hence proper orientation can produce a smooth external curve surface of the

prototype. In addition the minimum wall thickness of the part can only be built in

some particular orientation.

3.1.3 Support Generation & Editing

The rapid prototyping systems usually bundled with software which allows the

automatic creation and editing of the supports. The software will initially

generate the supports for all overhang regions based on the default support

parameters; figure 3.5 shows the problem of missing support for overhang

geometry. After the creation, the support structure can be individually modify,

edit, delete or add based on the part geometry. The region by region editing or

customization for supports generation has strengthened the essential support

and also minimizing the building of unnecessary supports.

Fig. 3.5 Result of missing supports for overhanging areas

Fig. 3.6 SLA Fine-point support

Support can reinforce delicate part while building and in the post processing

stage. However over support for delicate part will increase the difficulties of

support removal and even destroy the fine details on the down face. Figure 3.7shows an example of over support.

Down-facing Region

Up-facing Region

Stand

Projection

Desired part or model

geometry

Without supports, overhanging areas of

part may peel away and damage thewhole model


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Fig. 3.7 Over support of SLA part

3.1.4 Slicing (Layer thickness)

A STL model used for RP contains a collection of planar triangular surfaces. These

faces define an approximate boundary for the object. Horizontal layers of equal

thickness are produced while slicing to produce the outer boundary of the part

slice curve. Then void and solid region of the slice curve can be identified and

proper fill pattern can be created for part filling. Typical layer thickness of

commonly RP system is ranged from 0.05mm to 0.15mm.

3.1.5 Part Building

The prototype can be built in the RP machine according to the toolpath and

control codes generated by the software of the RP system. By laser scanning,

disposition, sintering, etc and under limited working envelope with closed

control of the processing environment the machine will start the build at the

bottom layer. Subsequent layer is added after the completion of the previous

layer until the final layer was build. Hence, RP process also refers to layer

manufacturing.

3.1.6 Post processing

Once the last layer on the part has been built, the prototype need to have

undergo some post processing processes such as removal of support, cleaning,

depowder, drain excessive resin, post curing in SLA, infiltration of resin/wax, etc.

The post processing is aimed to clean and reinforce the green part.

Additional surface finish procedures such as sanding, sand blasting, painting or

even electroplating are normally employed for cosmetic prototypes.


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4. Rapid Tool Production

Only when the production quantity is massive that the expensive tooling cost can

be justified. As a result, the way of how to produce tooling quicker and more

economically especially for small batch manufacturing becomes a big concern.

Furthermore, in the product design and development process, there is always inneed of some intermediate tooling to produce samples for marketing, functional

test, or production process planning and evaluation purposes. In this respect, RT

is the ideal mean to fit the needs.

Rapid Tooling (RT) is the result of combining Rapid Prototyping techniques with

conventional tooling practices to produce small quantity of plastic or metal

components from electronic CAD data directly or indirectly. Direct RT technology

such as Direct Metal Laser Sintering (DMLS) which fabricate production tooling

from CAD data whereas Room Temperature Vulcanization (RTV) silicone rubber

mould is the most commonly used indirect approach for plastic componentsduplication.

4.1 Type of RT Processes

Low volume (from 10 to 100)

RTV Soft Tooling

Intermediate volume (from 100 to 1000)

Metal filled Epoxy Tooling Direct Metal Laser Sintering

4.2 RTV Soft Tooling

RTV silicone rubber mould is one of the important kinds of soft tooling which is

an effective, high fidelity and inexpensive way to create multiple copies of a

master prototype part. Indeed, this technology has been used by the industry for

many years. The only thing new is that the master pattern is produced by the RP

technology. The other common kind of soft tooling material is tooling grade

Polyurethane.

This technology is not specially designed for a particular RP process. In fact,

master patterns produced from most RP processes are suitable to apply for this

technology.

RTV molds can faithfully duplicate details and textures present on the master

pattern. Apart from detail, geometry can also be fully duplicated from the master

part when prototypes are removed from the mould. Great care should be taken

to ensure that the pattern is in perfect condition. After RTV mould is completed,

it can be used to further produce limited quantity of prototypes with a wide

variety of material properties.


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Its application is mainly to produce plastics or metal prototypes in small batch by

vacuum assisted casting or gravity casting method. The casting materials

normally used are PU, polyester, epoxy, tin-lead alloy (200 C), pewter (230 C)

and zinc alloy (400 C).

The batch size is from several pieces to hundreds. Multiple moulds, sometimes,are required depend on the complexity of the parts. In fact, the ease of

producing multiple moulds is one of the advantages of this technology.

However, the process is tedious and required high skill workers attendance.

4.2.1 Process Description

One-piece mould approach

This process begins with a master pattern which normally output from RP system

directly. It requires to sand and polish the part surface well since the RTV mould

will reproduce any and all surface defects on the master pattern, and in turn will

transfer them onto the final model. Then, the pattern attached with gating

system is mounted in a mould box. Silicone rubber is mixed with specific amount

of hardener and poured into the mould box. Degassing in vacuum chamber is

preferred to avoid trapped air caused by mixing and pouring. After solidification,

the mould is then spitted into mould halves according to the predetermined

parting lines. Figure 4.1 shows the RTV mould making process of one piece

mould approach.

Fig. 4.1 Processes of making RTV mould (one-piece mould approach)


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Two-pieces mould approach

Firstly, master pattern is prepared and laid horizontally inside a mould box with

the parting line built up by hand with model clay. Prepared silicone rubber is

poured inside the mould box to form one half of the mould. After solidification,

clay is removed, the other half of the mould is produced by repeating the abovesteps with the master pattern turned upside down. Figure 4.2 shows the different

parting line surface of one-piece mould and two-pieces mould.

Fig. 4.2 Left; one-piece mould Right; two-pieces mould

Two-pieces mould is typically required when the parting line is difficult to be

determined. Two-pieces mould approach requires less skill compared with one-

piece mould. However, it requires double time to make the mould and the

dimension accuracy of part is poor than one piece mould.


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4.2.2 Type of RTV Silicone Rubber

RTV silicone rubber can be divided into two types; condensation and addition

types, the following table shows the differences among them.

Condensation Type Addition Type

Features Lower costs

Broader product range

Less sensitive to exact mix

ratio

Accelerator catalysts

available to speed up cure

Mixing of grades possible to

achieve desired hardness

Low shrinkage, below 0.1%

Marginally higher tensile

strength

Slightly tougher rubber

Need for careful and accurate

mixing

Good abrasion resistance

Can be accelerated using heat

Tolerant to the addition of

silicone fluid as a softener

Shrinkage Slightly higher than the addition

type

Low

When

TemperatureIncrease

Curing Time decreases

(5oC----7hrs;

25oC-----5hrs;

50oC-----4hrs(max temp))

Curing Time decreases

(50oC-----2hrs;

100oC-----30min;

150oC-----10min;)

When

Humidity

Increase

Curing Time decreases Curing Time decreases

When

Quantity of

Curing Agent

Increase

Curing Time decreases but there

are limits

Curing speed will not be altered

but will adversely affect the

physical properties.

*Quantity of curing agent must be

carried out as accurately as

possible.

Curing

Impediments

No Yes

Remarks By product will come out.

Dimensions of mould will be

slightly affected. (about 0.5%)

Warm the rubber mold to the

curing temperature before pouring

resin into a thermally cured the

rubber mold to raise dimensionalaccuracy.


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Mixing Ratiois a term used to state the amount of each material to be mixed ina multi-component material. The mixing ratios for two-part RTV products are

described on the individual product data sheets and are given as a ratio by

weight of each material.

Pot lifeis the length of time that a catalyzed resin system retains a viscosity lowenough to be used in processing. This is also known as WORKING LIFE or

USABLE LIFE.

Curing conditionis the condition which can provide the optimum properties fora silicone rubber. This typically depends on time and temperature.

4.2.3 Considerations of making RTV mould

To duplicate a good product by soft tooling, planning is very important as the

part may be blocked inside the mould if the parting surface is not well defined.

All holes should be filled by tapes to separate individual mould half before

pouring of silicone rubber.

For master pattern with flat surface, the parting surface should be located on the

flat surface and use plastic type as a marking. For master pattern with free-form

geometry, parting line should be located on the position which gives lowest

effect to the appearance.

It is important to evaluate the pattern if there are any undercuts that would lock

the casting in the mold. Small undercuts can be ignored since the silicone rubber

is flexible material. To deal with some deep holes feature such as boss, metalinserts can be used to replace the silicon material by attaching the inserts to the

part before pouring of moulding material.

4.3 PU Casting

Even RTV mould can be used for different material, PU is the most popular

material accompany with RTV mould for producing plastic components. There

are two different approaches for PU casting, Top-down approach (direct gate)

and Bottom-up approach (by gravity).

For Top-down approach, PU is poured into the mould directly through a gate

connected on top of the part. Firstly, weighted material and RTV mould are put

into a vacuum chamber for degassing. PU resins are mixed together inside a

vacuum chamber, it is then poured into the mould via a funnel and air is

reintroduced simultaneously to force the PU material into the mould. This

method is suitable for casting small components such as jewelry that has fine

detail. Figure 4.3 shows the processes of PU casing by this top-down approach.


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Fig. 4.3 Steps of PU casting

For Bottom-up approach, PU material is poured into the RTV mould and filled

into the moulding cavity by gravity. Vent channels are need to be added at the

top areas of part to allow air out. This method is most suitable for large casting

part in which the mould is unable to put inside a vacuum chamber. Figure 4.4

shows the different set up of top-down and bottom-up approaches

Fig. 4.4 Left; top-down approach Right; bottom-up approach

RTV Mould

Cavity

Vent channels

Material input Material input


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4.4 Epoxy Tooling

Epoxy tools are used to manufacture parts or limited runs of production parts.

Expoxy tools are used for:

Plastics injection prototype Mould

Mould patterns for casting

Vacuum forming moulds

Sheet metal forming moulds

Reaction injection moulds

Mould that is made of plastics is built from casting some special grade epoxy

resins directly onto the RP master model. This mould making method does not

require high precision machine tools as with conventional metal mould

production. This technology of direct transversal from the master model allows

large reduction in mould production costs and time.

In the past, plastics materials are not suitable for injection mould due to the lack

of strength and the high shrinkage during curing. Many problems arise such as

damage during mould making and moulding process. However, a special grade

epoxy resin is developed for better strength and stiffness. Epoxy resin is a

thermoset plastic reinforced with composite materials that can be cast to shape

before cured. This special grade epoxy resin is aluminum powder filled for

strength, stiffness and thermal conductivity improvement.

The mould made by this process is only suitable for injection moulding of

plastics parts. Common plastics materials like ABS, POM, etc. can be producedfrom this mould in small batch size up to 3,000 pieces.

4.4.1 Process Description

The process of producing epoxy tooling is somehow similar as making RTV two-

piece mould but usually double duplication techniques is used. A RP master

pattern is sanded and polished and the parting line is formulated by clay. A thin

layer of mould release agent is applied on the surface of the master pattern, RTV

is then poured into the mould box. The RTV is used as the negative mould

master patterns for casting of metal epoxy in forming the mould cavity andmould core. As the density and viscosity of epoxy resin is relativity high,

degassing is carried out by several times in order to extract out all of the trapped

gases. After pre-curing, the master pattern is removed and the mould halves are

put into oven step by step increasing the temperature to 280for post thermal

curing.

The mould halves are then turned to CNC machine for producing the sprue,

runner, gate and ejecting system. The mould is completed and ready for plastic

injection.


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4.5 Direct Metal Laser Sintering (DMLS)

Direct Metal laser Sintering (DMLS) builds solid metal parts directly from

powdered metals. It always used to build simple rapid tooling because of short

lead times, eliminate the cavity machining required. For advanced, cooling

channels and inserts can also be built in the rapid tools. Rapid tools using harder,tougher materials can be used to inject hundreds to thousands of plastic parts.

Fig. 4.5 Example of DMLS insert for injection moulding


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ic workshop materials 09 - rapid prototyping & manufacturing technologies

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