Introduction to Rapid Pro to Typing

8/3/2019 Introduction to Rapid Pro to Typing

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Introduction to Rapid Prototyping

Rapid Prototyping (RP) can be defined as a group of techniquesused to quickly fabricate a scale model of a part or assembly usingthree-dimensional computer aided design (CAD) data. What iscommonly considered to be the first RP technique,Stereolithography, was developed by 3D Systems of Valencia, CA,USA. The company was founded in 1986, and since then, anumber of different RP techniques have become available.

Rapid Prototyping has also been referred to as solid free-formmanufacturing, computer automated manufacturing, and layered

manufacturing. RP has obvious use as a vehicle for visualization.In addition, RP models can be used for testing, such as when anairfoil shape is put into a wind tunnel. RP models can be used tocreate male models for tooling, such as silicone rubber molds andinvestment casts. In some cases, the RP part can be the final part,but typically the RP material is not strong or accurate enough.When the RP material is suitable, highly convoluted shapes(including parts nested within parts) can be produced because ofthe nature of RP.

There is a multitude of experimental RP methodologies either indevelopment or used by small groups of individuals. This sectionwill focus on RP techniques that are currently commerciallyavailable, including Stereolithography (SLA), Selective LaserSintering (SLS), Laminated Object Manufacturing (LOM), FusedDeposition Modeling (FDM), Solid Ground Curing (SGC), and InkJet printing techniques.

Why Rapid Prototyping?

The reasons of Rapid Prototyping are

To increase effective communication.

To decrease development time.

To decrease costly mistakes.

To minimize sustaining engineering changes.
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To extend product lifetime by adding necessary features andeliminating redundant features early in the design.

Rapid Prototyping decreases development time by allowing

corrections to a product to be made early in the process. By givingengineering, manufacturing, marketing, and purchasing a look atthe product early in the design process, mistakes can be correctedand changes can be made while they are still inexpensive. Thetrends in manufacturing industries continue to emphasize thefollowing:

Increasing number of variants of products.

Increasing product complexity.

Decreasing product lifetime before obsolescence.

Decreasing delivery time.

Rapid Prototyping improves product development by enablingbetter communication in a concurrent engineering environment.

Methodology of Rapid Prototyping

The basic methodology for all current rapid prototyping techniquescan be summarized as follows:

1. A CAD model is constructed, then converted to STL format. Theresolution can be set to minimize stair stepping.

2. The RP machine processes the .STL file by creating sliced

layers of the model.

3. The first layer of the physical model is created. The model isthen lowered by the thickness of the next layer, and the process isrepeated until completion of the model.

4. The model and any supports are removed. The surface of themodel is then finished and cleaned.


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Fused Deposition Modeling

Standard engineering thermoplastics, such as ABS, can be usedto produce structurally functional models.

Two build materials can be used, and latticework interiors are anoption.

Parts up to 600 600 500 mm (24 24 20 inches) can beproduced.

Filament of heated thermoplastic polymer is squeezed out like

toothpaste from a tube.

Thermoplastic is cooled rapidly since the platform is maintainedat a lower temperature.

Milling step not included and layer deposition is sometimes non-uniform so "plane" can become skewed.

Not as prevalent as SLA and SLS, but gaining ground becauseof the desirable material properties.


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Fused Deposition Modeling

Stratasys of Eden Prairie, MN makes Fused Deposition Modeling(FDM) machines. The FDM process was developed by ScottCrump in 1988. The fundamental process involves heating afilament of thermoplastic polymer and squeezing it out liketoothpaste from a tube to form the RP layers. The machines rangefrom fast concept modelers to slower, high-precision machines.The materials include polyester, ABS, elastomers, and investmentcasting wax. The overall arrangement is illustrated below


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Selective Laser Sintering

Patented in 1989.

Considerably stronger than SLA; sometimes structurally

functional parts are possible. Laser beam selectively fuses powder materials: nylon,elastomer, and soon metal;

Advantage over SLA: Variety of materials and ability to

approximate common engineering plastic materials.

No milling step so accuracy in z can suffer.

Process is simple: There are no milling or masking stepsrequired.

Living hinges are possible with the thermoplastic-like

materials.

Powdery, porous surface unless sealant is used. Sealant alsostrengthens part.

Uncured material is easily removed after a build by brushing

or blowing it off.

Selective Laser Sintering (SLS, registered trademark by DTMof Austin, Texas, USA) is a process that was patented in 1989 byCarl Deckard, a University of Texas graduate student. Its chiefadvantages over Stereolithography (SLA) revolve around material

properties. Many varying materials are possible and thesematerials can approximate the properties of thermoplastics suchas polycarbonate, nylon, or glass-filled nylon.As the figure below shows, an SLS machine consists of twopowder magazines on either side of the work area. The levelingroller moves powder over from one magazine, crossing over thework area to the other magazine. The laser then traces out thelayer. The work platform moves down by the thickness of one layerand the roller then moves in the opposite direction. The process

repeats until the part is complete.


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SLA vs. SLS: A Summarized Comparison

Material Properties: The SLA (stereolithography) process is

limited to photosensitive resins which are typically brittle. TheSLS process can utilize polymer powders that, when sintered,approximate thermoplastics quite well.

Surface Finish: The surface of an SLS part is powdery, like thebase material whose particles are fused together without completemelting. The smoother surface of an SLA part typically wins overSLS when an appearance model is desired. In addition, if thetemperature of uncured SLS powder gets too high, excess fused

material can collect on the part surface. This can be difficult tocontrol since there are so many variables in the SLS process. Ingeneral, SLA is a better process where fine, accurate detail isrequired. However, a varnish-like coating can be applied to SLSparts to seal and strengthen them.

Dimensional Accuracy: SLA is more accurate immediately aftercompletion of the model, but SLS is less prone to residualstresses that are caused by long-term curing and environmentalstresses. Both SLS and SLA suffer from inaccuracy in the z-direction (neither has a milling step), but SLS is less predictable


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because of the variety of materials and process parameters. Thetemperature dependence of the SLS process can sometimesresult in excess material fusing to the surface of the model, andthe thicker layers and variation of the process can result in more z

inaccuracy. SLA parts suffer from the "trapped volume" problem inwhich cups in the structure that hold fluid cause inaccuracies.SLS parts do not have this problem.

Support Structures: SLA parts typically need support structuresduring the build. SLS parts, because of the supporting powder,sometimes do not need any support, but this depends upon partconfiguration. Marks left after removal of support structures forparts cause dimensional inaccuracies and cosmetic blemishes.

Machining Properties: In general, SLA materials are brittle anddifficult to machine. SLS thermoplastic-like materials are easilymachined.

Size: SLS and SLA parts can be made the same size, but ifsectioning of a part is required, SLS parts are easier to bond.

Investment Casting: The investment casting industry has beenconservative about moving to RP male models, so SLS models

made from traditional waxes, etc. are preferred. 3D Systems has aprocess (dubbed "QuickCast") which allows SLA models to bemore suitable for investment casting. Since SLA resins do not meltbut burn to form ash, QuickCast modifies the build process sothat the interior of the model is hollow with a supportinglatticework. When the ceramic is fired, the QuickCast modelcollapses and any ash is minimal because of the small totalquantity of material.


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Stereolithography (SLA)

The first Rapid Prototyping technique and still the most widelyused.

Inexpensive compared to other techniques. Uses a light-sensitive liquid polymer. Requires post-curing since laser is not of high enough power tocompletely cure. Long-term curing can lead to warping. Parts are quite brittle and have a tacky surface. No milling step so accuracy in z can suffer. Support structures are typically required. Process is simple: There are no milling or masking steps

required. Uncured material can be toxic. Ventilation is a must.

Introduction to Stereolithography

Stereolithography (SLA), the first Rapid Prototyping process, wasdeveloped by 3D Systems of Valencia, California, USA, founded in1986. A vat of photosensitive resin contains a vertically-movingplatform. The part under construction is supported by the platform

that moves downward by a layer thickness (typically about 0.1mm / 0.004 inches) for each layer. A laser beam traces out theshape of each layer and hardens thephotosensitive resin.

The Stereolithography (SLA) System overall arrangement:


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Stereolithography Process


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Uncured resin is removed and the model is post-cured to fully curethe resin. Because of the layered process, the model has a surfacecomposed of stair steps. Sanding can remove the stair steps for acosmetic finish. Model build orientation is important for stair

stepping and build time. In general, orienting the long axis of themodel vertically takes longer but has minimal stair steps. Orientingthe long axis horizontally shortens build time but magnifies thestair steps. For aesthetic purposes, the model can be primed andpainted.

During fabrication, if extremities of the part become too weak, itmay be necessary to use supports to prop up the model. The

supports can be generated by the program that creates the slices,and the supports are only used for fabrication. The following threefigures show why supports are necessary:


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Ink Jet Printing Techniques

Ink jet printing comes from the printer and plotter industry where

the technique involves shooting tiny droplets of ink on paper toproduce graphic images. RP ink jet techniques utilize ink jettechnology to shoot droplets of liquid-to-solid compound and forma layer of an RP model. Common ink jet printing techniques, suchas Sanders ModelMaker, Multi-Jet Modeling, Z402 Ink JetSystem, and Three-Dimensional Printing, are presented in thissection. Although none of the these techniques have become asestablished as the Stereolithography (SLA) or Selective LaserSintering (SLS) systems, several show promise.

Exceptional accuracy allows use in the jewelry industry. Accuracy is partly enabled by a milling step after each layerdeposition. Plotting system is a liquid-to-solid inkjet which dispenses boththermoplastic and wax materials. Compared to SLS and SLA, not as established.

Sanders ModelMaker

The Sander ModelMaker product is produced and distributed bySanders Prototype, Inc. of Wilton, NH, USA. Smooth cosmeticsurface quality can be achieved by pre-tracing the perimeter of alayer prior to filling in the interior. The supporting wax material isdeposited at the same time as the thermoplastic. A schematic isshown below:


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Both the thermoplastic material (Protobuild) and the waxsupport material (Protosupport) are proprietary materials of

Sanders.


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Multi-Jet Modeling

Fast.

Office-friendly: non-toxic materials, small footprint, low odor. Simple operation: operates as a network printer in an office

environment. Models are primarily for appearance use. Compared to SLS and SLA, not as established.

Another product of 3D Systems from the makers of the SLAsystem, Multi-Jet Modeling uses a 96-element print head todeposit molten plastic for layering. The system is fast compared to

most other RP techniques, and produces good appearancemodels with minimal operator effort. The main market that thissystem is targeted at is the engineering office where the systemmust be non-toxic, quiet, small, and with minimal odor. The systemis illustrated below:


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Z402 Ink Jet System

Fast: one to two vertical inches per hour, depending on layer

density. Office-friendly: non-toxic materials, small footprint, low odor. Simple operation. Compared to SLA and SLS, not as established.

The Z402 is one of the fastest 3D printers known to RapidPrototyping. The ability to produce quick models means greaterproductivity for the lab and quick prototypes for customers. Since

manufacturing parts is easy, almost anyone in the lab can producea quality part without extensive Rapid Prototyping experience.

Three-Dimensional Printing

Binder is "printed" on unbound powder layer. Without milling step, work plane can become successively

skewed. Not as established as SLA and SLS.

Three-Dimensional Printing, developed by MIT and Soligen, Inc.,is illustrated below. It is another technique based on the inkjetprinting process. Binder is printed on a powder layer to selectivelybind powder together for each layer.


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Laminated Object Manufacturing

Layers of glue-backed paper form the model. Low cost: Raw material is readily available.

Large parts: Because there is no chemical reaction involved,parts can be made quite large. Accuracy in z is less than that for SLA and SLS. No milling step. Outside of model, cross-hatching removes material Models should be sealed in order to prohibit moisture. Before sealing, models have a wood-like texture. Not as prevalent as SLA and SLS.

The figure below shows the general arrangement of a LaminatedObject Manufacturing (LOM, registered trademark by Helisys ofTorrance, California, USA) cell:


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Material is usually a paper sheet laminated with adhesive on oneside, but plastic and metal laminates are appearing.

Layer fabrication starts with sheet being adhered to substrate with

the heated roller.The laser then traces out the outline of the layer.Non-part areas are cross-hatched to facilitate removal of wastematerial.Once the laser cutting is complete, the platform moves down andout of the way so that fresh sheet material can be rolled intoposition.Once new material is in position, the platform moves back up toone layer below its previous position.

The process can now be repeated.

The excess material supports overhangs and other weak areas ofthe part during fabrication. The cross-hatching facilitates removalof the excess material. Once completed, the part has a wood-liketexture composed of the paper layers. Moisture can be absorbedby the paper, which tends to expand and compromise thedimensional stability. Therefore, most models are sealed with apaint or lacquer to block moisture ingress.

The LOM developer continues to improve the process withsheets of stronger materials such as plastic and metal. Nowavailable are sheets of powder metal (bound with adhesive) thatcan produce a "green" part. The part is then heat treated to sinterthe material to its final state.

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Introduction to Rapid Pro to Typing