14. rapid prototyping

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Short Term Course on ADVANCED MACHINING PROCESSES, July 01-05, 2013

Mechanical Engineering Department, MNNIT Allahabad, Uttar Pradesh (INDIA)

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RAPID PROTOTYPING - A MANUFACTURING CHALLENGE

Dr. Rajeev SrivastavaAssociate Professor, Mechanical Engineering Department, MNNIT Allahabad (U.P.)

Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a scale

model of a part or assembly using three-dimensional computer aided design (CAD) data. What is

commonly 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, a number ofdifferent RP techniques have become available.

Rapid Prototyping has also been referred to as solid free-form manufacturing, 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 an airfoil shape is put

into a wind tunnel. RP models can be used to create male models for tooling, such as siliconerubber molds and investment 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 of the nature of

RP.

There is a multitude of experimental RP methodologies either in development or used by smallgroups of individuals. This section will focus on RP techniques that are currently commercially

available, including Stereolithography (SLA), Selective Laser Sintering (SLS), Laminated Object

Manufacturing (LOM), Fused Deposition Modeling (FDM), Solid Ground Curing (SGC), and

Ink Jet printing techniques.

The reasons of Rapid Prototyping are

To increase effective communication. To decrease development time. To decrease costly mistakes. To minimize sustaining engineering changes. To extend product lifetime by adding necessary features and eliminating 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 giving engineering, manufacturing, marketing, and purchasing a look atthe product early in the design process, mistakes can be corrected and changes can be made

while they are still inexpensive. The trends in manufacturing industries continue to emphasize

the following:

1. Increasing number of variants of products

2. Increasing product complexity.3. Decreasing product lifetime before obsolescence.

4. Decreasing delivery time.
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Rapid Prototyping improves product development by enabling better communication in a

concurrent engineering environment.

Methodology of Rapid Prototyping

The basic methodology for all current rapid prototyping techniques can be summarized asfollows:

1. A CAD model is constructed, then converted to STL format. The resolution can be set tominimize 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 is then lowered by the

thickness of the next layer, and the process is repeated until completion of the model.

4. The model and any supports are removed. The surface of the model is then finished andcleaned.

STEREOLITHOGRAPHY

Highlights of Stereolithography

The first Rapid Prototyping technique and still the most widely used

Inexpensive compared to other techniques. Uses a light-sensitive liquid polymer. Requires post-curing since laser is not of high enough power to completely 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, was developed by 3D Systems of

Valencia, California, USA, founded in 1986. A vat of photosensitive resin contains a vertically-

moving platform. The part under construction is supported by the platform that moves downward

by a layer thickness (typically about 0.1 mm / 0.004 inches) for each layer. A laser beam traces

out the shape of each layer and hardens thephotosensitive resin.


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The Stereolithography (SLA) System overall arrangement:

Stereolithography Process

The sequence of steps for producing an Stereolithography (SLA) layer is shown in the following

figures:


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Uncured resin is removed and the model is post-cured to fully cure the resin. Because of thelayered process, the model has a surface composed of stair steps. Sanding can remove the stair

steps for a cosmetic finish. Model build orientation is important for stair stepping and build time.

In general, orienting the long axis of the model vertically takes longer but has minimal stair


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steps. Orienting the long axis horizontally shortens build time but magnifies the stair steps. For

aesthetic purposes, the model can be primed and painted.

During fabrication, if extremities of the part become too weak, it may 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 three figures show whysupports are necessary.

LAMINATED OBJECT MANUFACTURING

Highlights of 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

Laminated Object Manufacturing

The figure below shows the general arrangement of a Laminated Object Manufacturing (LOM,

registered trademark by Helisys of Torrance, California, USA) cell.


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Material is usually a paper sheet laminated with adhesive on one side, but plastic and metal

laminates are appearing.

1. Layer fabrication starts with sheet being adhered to substrate with the heated roller.2. The laser then traces out the outline of the layer.3. Non-part areas are cross-hatched to facilitate removal of waste material.4. Once the laser cutting is complete, the platform moves down and out of the way so that

fresh sheet material can be rolled into position.5. Once new material is in position, the platform moves back up to one layer below its

previous position.

6. The process can now be repeated.The excess material supports overhangs and other weak areas of the part during fabrication. Thecross-hatching facilitates removal of the excess material. Once completed, the part has a wood-like texture composed of the paper layers. Moisture can be absorbed by the paper, which tends to

expand and compromise the dimensional stability. Therefore, most models are sealed with a

paint or lacquer to block moisture ingress.

The LOM developer continues to improve the process with sheets of stronger materials such as

plastic and metal. Now available are sheets of powder metal (bound with adhesive) that canproduce a "green" part. The part is then heat treated to sinter the material to its final state.

FUSED DEPOSITION MODELING

Highlights of Fused Deposition Modeling

Standard engineering thermoplastics, such as ABS, can be used to produce structurallyfunctional models.

Two build materials can be used, and latticework interiors are an option. Parts up to 600 600 500 mm (24 24 20 inches) can be produced.


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Filament of heated thermoplastic polymer is squeezed out like toothpaste from a tube. Thermoplastic is cooled rapidly since the platform is maintained at a lower temperature. Milling step not included and layer deposition is sometimes non-uniform so "plane" can

become skewed.

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

Fused Deposition Modeling

Stratasys of Eden Prairie, MN makes Fused Deposition Modeling (FDM) machines. The FDM

process was developed by Scott Crump in 1988. The fundamental process involves heating a

filament of thermoplastic polymer and squeezing it out like toothpaste from a tube to form the

RP layers. The machines range from fast concept modelers to slower, high-precision machines.

The materials include polyester, ABS, elastomers, and investment casting wax. The overall

arrangement is illustrated below:

SOLID GROUND CURING

Highlights of Solid Ground Curing

Large parts, 500 500 350 mm (20 20 14 in), can be fabricated quickly. High speed allows production-like fabrication of many parts or large parts. Masks are created w/ laser printing-like process, then full layer exposed at once. No post-cure required. Milling step ensures flatness for subsequent layer Wax supports model: no extra supports needed. Creates a lot of waste.


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Not as prevalent as SLA and SLS, but gaining ground because of the high throughput andlarge parts.

Solid Ground Curing: An Introduction

Solid Ground Curing, also known as the Solider Process, is a process that was invented and

developed by Cubital Inc. of Israel. The overall process is illustrated in the figure above and thesteps are illustrated below. The SGC process uses photosensitive resin hardened in layers as with

the Stereolithography (SLA) process. However, in contrast to SLA, the SGC process is

considered a high-throughput production process. The high throughput is achieved by hardening

each layer of photosensitive resin at once. Many parts can be created at once because of the largework space and the fact that a milling step maintains vertical accuracy. The multi-part capability

also allows quite large single parts (e.g. 500 500 350 mm / 20 20 14 in) to be fabricated.

Wax replaces liquid resin in non-part areas with each layer so that model support is ensured.

Solid Ground Curing Process

The steps in the process are as follows.

First, a CAD model of the part is created and it is sliced into layers using Cubital's Data Front

End (DFE) software. At the beginning of a layer creation step, the flat work surface is sprayedwith photosensitive resin, as shown below:
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For each layer, a photomask is produced using Cubital's proprietary ionographic printingtechnique, as illustrated below:

Next, the photomask is positioned over the work surface and a powerful UV lamp hardens the

exposed photosensitive resin:


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After the layer is cured, all uncured resin is vacuumed for recycling, leaving the hardened areas

intact. The cured layer is passed beneath a strong linear UV lamp to fully cure it and to solidify

any remnant particles, as illustrated below:

In the fifth step, wax replaces the cavities left by vacuuming the liquid resin. The wax ishardened by cooling to provide continuous, solid support for the model as it is fabricated. Extra

supports are not needed.

In the final step before the next layer, the wax/resin surface is milled flat to an accurate, reliablefinish for the next layer:

Once all layers are completed, the wax is removed, and any finishing operations such as sanding,

etc. can be performed. No post-cure is 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 to produce graphic images. RP ink jet techniques utilize

ink jet technology to shoot droplets of liquid-to-solid compound and form a layer of an RP

model. Common ink jet printing techniques, such as Sanders ModelMaker, Multi-Jet Modeling,

Z402 Ink Jet System, and Three-Dimensional Printing, are presented in this section. Although

none of the these techniques have become as established as the Stereolithography (SLA) or

Selective Laser Sintering (SLS) systems, several show promise.

Sanders Model Maker

Exceptional accuracy allows use in the jewellry industry.

Accuracy is partly enabled by a milling step after each layer deposition.Plotting system is a liquid-to-solid inkjet which dispenses both thermoplastic and wax materials.

Compared to SLS and SLA, not as established.

The Sander ModelMaker

product is produced and distributed by Sanders Prototype, Inc. of

Wilton, NH, USA. Smooth cosmetic surface quality can be achieved by pre-tracing the perimeter

of a layer prior to filling in the interior. The supporting wax material is deposited at the sametime as the thermoplastic. A schematic is shown below
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:

Both the thermoplastic material (Protobuild) and the wax support material (Protosupport) areproprietary materials of Sanders.

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 SLA system, Multi-Jet Modeling uses a

96-element print head to deposit molten plastic for layering. The system is fast compared to mostother RP techniques, and produces good appearance models with minimal operator effort. The

main market that this system is targeted at is the engineering office where the system must be

non-toxic, quiet, small, and with minimal odor. The system is 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 Rapid Prototyping. The ability to producequick models means greater productivity for the lab and quick prototypes for customers. Sincemanufacturing parts is easy, almost anyone in the lab can produce a quality part without

extensive Rapid Prototyping experience.

THREE-DIMENSIONAL PRINTING

Binder is "printed" on unbound powder layerWithout 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 isanother technique based on the inkjet printing process. Binder is printed on a powder layer to

selectively bind powder together for each layer.


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Design and manufacturing biocompatible and bioactive implants and tissue Engineering

RP technologies gave significant contribution in the field of tissue engineering through the use of

biomaterials including the direct manufacture of bioactive implants. Tissue engineering is a

combination of living cells and a support structure called scaffolds. RP systems like fused

deposition modeling (FDM), 3D printing (3-DP) and selective laser sintering (SLS) have been

proved to be convenient for making porous structures for use in tissue engineering. In this field it

is essential to be able to fabricate three-dimensional scaffolds of various geometric shapes, in

order to repair defects caused by accidents, surgery, or birth. FDM, SLS and 3DP can be used to

fabricate a functional scaffold directly but RP systems can also be used for manufacturing asacrificial mould to fabricate tissue-engineering scaffolds.

RECENT AND FUTURE TRENDS

Recently this technique was used for the separation of Siamese twins who was borned by the

attaching of the skull. It is a very significant discovery in medicine and the first step on the way

to making other complex human organs. Further development in RP in tissue engineering

requires the design of new materials, optimal scaffold design and the input of such kind of

knowledge of cell physiology that would make it possible in the future to print whole

replacement organs or whole bodies by machines. There are also many new trends of applying

RP in orthopedics, oral and maxillofacial surgery and other fields of medicine.


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CONCLUSION

RP technology can make significant impact in the field manufacturing and recently in biomedical

engineering and surgery. Physical models enable correct identification of bone abnormality,

intuitive understanding of the anatomical issues for a surgeon, implant designers and patients aswell. A precise RP model facilitates the pre-operative planning of am optimal surgical approach

and enables selection of correct or appropriate implants. In the UK, RPT has been used to help

plan treatment in more than 20 patients; however, the cost of the modeling process is currently a

significant limitation to its use. Surgical procedures continue to be more effective day by day

with reduced risk and expense to both the patient and the hospital. This could help minimize the

problem of long waiting list and congestion in big hospitals by reducing referral cases.

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