WORKSHOP TECHNOLOGY(JJ 204)

RAPID PROTOTYPING

MUHAMMAD ASYRAF BIN RAZALI06DKM11F1084

MUHAMAD TAUFIQ BIN RESDI06DKM11F1083

AFIQ ASYRAF BIN MANSOR06DKM11F1072

Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabr icate a scale model of a par t or assembly using three-dimensional computer aided design (CAD) data. What is commonly considered to be the f i rs t RP technique, Stereol i thography, was developed by 3D Systems of Valencia, CA, USA. The company was founded in 1986, and s ince then, a number of different RP techniques have become avai lable.

Rapid Prototyping has also been referred to as sol id free-form manufactur ing, computer automated manufacturing, and layered manufactur ing. RP has obvious use as a vehicle for visual izat ion. In addi t ion, RP models can be used for tes t ing, such as when an ai rfoi l shape is put into a wind tunnel . RP models can be used to create male models for tool ing, such as s i l icone rubber molds and investment casts . In some cases, the RP par t can be the f inal par t , but typical ly the RP mater ial is not s t rong or accurate enough. When the RP mater ial is sui table, highly convoluted shapes ( including par ts nested within par ts) can be produced because of the nature of RP.

There is a mult i tude of experimental RP methodologies ei ther in development or used by smal l groups of individuals . This sect ion wil l focus on RP techniques that are current ly commercial ly avai lable, including Stereol i thography (SLA),  Select ive Laser Sinter ing (SLS ® ) ,  Laminated Object Manufacturing (LOM™), Fused Deposi t ion Model ing (FDM), Sol id Ground Curing (SGC), and Ink Jet pr int ing techniques

INTRODUCTION

Sixt ies: The f i rs t rapid prototyping techniques became accessible in the later eight ies and they were used for product ion of prototype and model parts . The history of rapid prototyping can be t raced to the late s ixt ies , when an engineering professor, Herbert Voelcker, quest ioned himself about the possibi l i t ies of doing interest ing things with the computer control led and automatic machine tools . These machine tools had just s tar ted to appear on the factory f loors then. Voelcker was t rying to f ind a way in which the automated machine tools could be programmed by using the output of a design program of a computer.  

Seventies: Voelcker developed the basic tools of mathematics that clearly describe the three dimensional aspects and resul ted in the earl iest theories of algori thmic and mathematical theories for sol id modeling. These theories form the basis of modern computer programs that are used for designing almost al l th ings mechanical , ranging from the smallest toy car to the tal lest skyscraper. Volecker ’s theories changed the designing methods in the sevent ies, but , the old methods for designing were st i l l very much in use. The old method involved ei ther a machinist or machine tool control led by a computer. The metal hunk was cut away and the needed part remained as per requirements.  

HISTORY OF RAPID PROTOTYPING

Eighties: However, in 1987, Carl Deckard, a researcher form the University of Texas, came up with a good revolutionary idea. He pioneered the layer based manufacturing, wherein he thought of building up the model layer by layer. He printed 3D models by util izing laser l ight for fusing metal powder in solid prototypes, single layer at a t ime. Deckard developed this idea into a technique called “Selective Laser Sintering”. The results of this technique were extremely promising. The history of rapid prototyping is quite new and recent. However, as this technique of rapid prototyping has such wide ranging scope and applications with amazing results, i t has grown by leaps and bounds. 

Voelcker ’s and Deckard’s stunning findings, innovations and researches have given extreme impetus to this significant new industry known as rapid prototyping or free form fabrication. It has revolutionized the designing and manufacturing processes. 

Though, there are many references of people pioneering the rapid prototyping technology, the industry gives recognition to Charles Hull for the patent of Apparatus for Production of 3D Objects by Stereolithography. Charles Hull is recognized by the industry as the father of rapid prototyping. 

Present-day Rapid Prototyping: Today, the computer engineer has to simply sketch the ideas on the computer screen with the help of a design program that is computer aided. Computer aided designing allows to make modification as required and you can create a physical prototype that is a precise and proper 3D object.

1) Reduction in project cost and risk. 2) Can be used in different industries. 3) Easily the errors in previous design can be detected and errors can be rectified. 4) Only upon the complete satisfaction the complete product is designed. Factors like manufacturability, robustness and functionality of design are checked before sending it for production. 5) Greater visualization capabilities are improved right from the first stage if designing. This helps the user in knowing how the final product will look like. 6) All the designing flaws can be detected easily before the manufacturing of the product starts. 7) Manufacturer, designer and user can discuss the product and work forward to get the best product. This helps to give the user higher output product.

ADVANTAGES OF RAPID- PROTOTYPING

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) powderbased.

THREE MAJOR GROUP PROCESS OF RAPID PROTOTYPING

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

state. Through a process commonly known as curing, the liquid isconverted into the solid state

1 .LIQUID-BASED

3D Systems’ Stereoli thography Apparatus (SLA) Cubital’s Solid Ground Curing (SGC) Sony’s Solid Creation System (SCS) CMET’s Solid Object Ultraviolet-Laser Printer (SOUP) Autostrade’s E-Darts Teijin Seiki’s Soliform System Meiko’s Rapid Prototyping System for the Jewelry Industry Denken’s SLP Mitsui’s COLAMM Fockele & Schwarze’s LMS Light Sculpting Aaroflex Rapid Freeze Two Laser Beams Microfabrication

THE FOLLOWING RP SYSTEMS FALL INTO THISCATEGORY:

Except for powder, solid-based RP systems are meant to encompass all

forms of material in the solid state. In this context, the solid form can

include the shape in the form of a wire, a roll, laminates and pellets.

2 .SOLID-BASED

Cubic Technologies’ Laminated Object Manufacturing (LOM)Stratasys’ Fused Deposition Modeling (FDM)Kira Corporation’s Paper Lamination Technology (PLT)3D Systems’ Multi-Jet Modeling System (MJM)Solidscape’s ModelMaker and PatternMasterBeijing Yinhua’s Slicing Solid Manufacturing (SSM), MeltedExtrusion Modeling (MEM) and Multi-Functional RPM

Systems(M-RPM)CAM-LEM’s CL 100Ennex Corporation’s Offset Fabbers

THE FOLLOWING RP SYSTEMS FALL INTO THIS DEFINITION:

In a strict sense, powder is by-and-large in the solid state. However, it

is intentionally created as a category outside the solid-based RP systems

to mean powder in grain-like form.

3. POWDER-BASED

3D Systems’s Selective Laser Sintering (SLS) EOS’s EOSINT Systems Z Corporation’s Three-Dimensional Printing (3DP) Optomec’s Laser Engineered Net Shaping (LENS) Soligen’s Direct Shell Production Casting (DSPC) Fraunhofer ’s Multiphase Jet Solidification (MJS) Acram’s Electron Beam Melting (EBM) Aeromet Corporation’s Lasform Technology Precision Optical Manufacturing’s Direct Metal

Deposition(DMDTM) Generis’ RP Systems (GS) Therics Inc.’s Theriform Technology Extrude Hone’s PrometalTM 3D Printing Process

THE FOLLOWING RP SYSTEMS FALL INTOTHIS DEFINITION:

SRP (Subtractive rapid prototyping) is another process of transforming 3D digital model content into physical objects. Subtractive rapid prototyping is a low cost prototyping and parts manufacturing process.

The digital model is recreated and transformed into real world physical world that can be held in the hand. Subtractive rapid prototyping is a removal process; the process CNC machining from a raw stock block of material.

SUBTRACTIVE PROCESS

Additive rapid prototyping operations all build parts in layer. All of the processes describe in this section build part layer by layer. The main difference between the various processes lies in the approach taken to produce the invidual slice.

Rapid prototyping, in this method, requires an input in the setup from the computer files and in the initiation of the production processes. Following this stage, the machines generally operates unattended and provides a rough part after a few hours. In order to complete the rapid prototyping process, the part is then put through a series of finishing manual operation(such as sanding and painting). It should be recognized that setup and finishing operations are very labor-intensive and that the production time is only a part of the time required to obtain a prototype. Generally additive process are much faster than subtractive processes; they can take as little as from a few to few hours to produce a part.

ADDITIVE PROCESSES

Virtual prototyping, a totally software form of prototyping, uses advanced graphics and virtual-reality environments to allow designers to examine a part. This technology is used by a CAD packages to render a part, so that the designer can observe and evaluate the part as it is drawn.

The simplest forms of such systems use complex software and three-dimensional graphics routines to allow viewers to change the view of parts on a computer screen. More complicated versions will use virtual-reality headgear and gloves with appropriate sensors, to let the user observe a computer-generated prototype of the desired part in a completely virtual enviroment.

VIRTUAL PROTOTYPING

In this technique, filaments of heated thermoplastic are extruded from a tip that moves in the x-y plane. Like a baker decorating a cake, the controlled extrusion head deposits very thin beads of material onto the build platform to form the first layer. The platform is maintained at a lower temperature, so that the thermoplastic quickly hardens. After the platform lowers, the extrusion head deposits a second layer upon the first. Supports are built along the way, fastened to the part either with a second, weaker material or with a perforated junction.

Stratasys, of Eden Prairie, MN makes a variety of FDM machines ranging from fast concept modelers to slower, high-precision machines. Materials include ABS (standard and medical grade), elastomer (96 durometer), polycarbonate, polyphenolsulfone, and investment casting wax.

FUSED DEPOSITION MODELIN

Patented in 1986, stereolithography started the rapid prototyping revolution. The technique builds three-dimensional models from liquid photosensitive polymers that solidify when exposed to ultraviolet light. As shown in the figure below, the model is built upon a platform situated just below the surface in a vat of liquid epoxy or acrylate resin. A low-power highly focused UV laser traces out the first layer, solidifying the model’s cross section while leaving excess areas liquid.

Stereolithography

Developed by Carl Deckard for his master ’s thesis at the University of Texas, selective laser sintering was patented in 1989. The technique, shown in Figure 3, uses a laser beam to selectively fuse powdered materials, such as nylon, elastomer, and metal, into a solid object. Parts are built upon a platform which sits just below the surface in a bin of the heat-fusable powder. A laser traces the pattern of the first layer, sintering it together. The platform is lowered by the height of the next layer and powder is reapplied. This process continues until the part is complete. Excess powder in each layer helps to support the part during the build. SLS machines are produced by DTM of Austin, TX.

Selective Laser Sintering

BPM employs a technology called digital Microsynthesis.

1. In the first step of the process, molten plastic is fed to a piezoelectic jetting mechanims, almost like those of inkjet printers,

2. Next a multi-axis controlled NC(numerical control) system shoots tiny dropets of material onto the target, using the jetting mechanism.

3. Last, small droplets freeze upon contact with the surface, forming the surface particle by particle.

BALLISTIC-PARTICLE MANUFACTURING

In this technique, developed by Helisys of Torrance, CA, layers of adhesive-coated sheet material are bonded together to form a prototype. The original material consists of paper laminated with heat-activated glue and rolled up on spools. As shown in the f igure below, a feeder/collector mechanism advances the sheet over the build platform, where a base has been constructed from paper and double-sided foam tape. Next, a heated roller applies pressure to bond the paper to the base. A focused laser cuts the outl ine of the f irst layer into the paper and then cross-hatches the excess area ( the negative space in the prototype). Cross-hatching breaks up the extra material , making i t easier to remove during post-processing. During the build, the excess material provides excellent support for overhangs and thin-walled sections. After the f irst layer is cut, the platform lowers out of the way and fresh material is advanced. The platform rises to sl ightly below the previous height, the roller bonds the second layer to the f irst , and the laser cuts the second layer. This process is repeated as needed to build the part , which will have a wood-like texture. Because the models are made of paper, they must be sealed and f inished with paint or varnish to prevent moisture damage.

LAMINATED OBJECT MANUFACTURING

A much-anticipated application of rapid prototyping is rapid tooling, the automatic fabrication of production quality machine tools. Tooling is one of the slowest and most expensive steps in the manufacturing process, because of the extremely high quality required. Tools often have complex geometries, yet must be dimensionally accurate to within a hundredth of a millimeter. In addition, tools must be hard, wear-resistant, and have very low surface roughness (about 0.5 micrometers root mean square). To meet these requirements, molds and dies are traditionally made by CNC-machining, electro-discharge machining, or by hand. All are expensive and time consuming, so manufacturers would like to incorporate rapid prototyping techniques to speed the process. Peter Hilton, president of Technology Strategy Consulting in Concord, MA, believes that "tooling costs and development times can be reduced by 75 percent or more" by using rapid tooling and related technologies.  1 6  Rapid tooling can be divided into two categories, indirect and direct.

RAPID TOOLING

M o s t r a p i d t o o l i n g t o d a y i s i n d i r e c t : R P p a r t s a r e u s e d a s p a t t e r n s f o r m a k i n g m o l d s a n d d i e s . R P m o d e l s c a n b e i n d i r e c t l y u s e d i n a n u m b e r o f m a n u f a c t u r i n g p r o c e s s e s :

Va c u u m C a s t i n g : I n t h e s i m p l e s t a n d o l d e s t r a p i d t o o l i n g t e c h n i q u e , a R P p o s i t i v e p a t t e r n i s s u s p e n d e d i n a v a t o f l i q u i d s i l i c o n e o r r o o m t e m p e r a t u r e v u l c a n i z i n g ( RT V ) r u b b e r. W h e n t h e r u b b e r h a r d e n s , i t i s c u t i n t o t w o h a l v e s a n d t h e R P p a t t e r n i s r e m o v e d . T h e r e s u l t i n g r u b b e r m o l d c a n b e u s e d t o c a s t u p t o 2 0 p o l y u r e t h a n e r e p l i c a s o f t h e o r i g i n a l R P p a t t e r n . A m o r e u s e f u l v a r i a n t , k n o w n a s t h e K e l t o o l p o w d e r m e t a l s i n t e r i n g p r o c e s s , u s e s t h e r u b b e r m o l d s t o p r o d u c e m e t a l t o o l s .   1 7   D e v e l o p e d b y 3 M a n d n o w o w n e d b y 3 D S y s t e m s , t h e K e l t o o l p r o c e s s i n v o l v e s f i l l i n g t h e r u b b e r m o l d s w i t h p o w d e r e d t o o l s t e e l a n d e p o x y b i n d e r . W h e n t h e b i n d e r c u r e s , t h e " g r e e n " m e t a l t o o l i s r e m o v e d f r o m t h e r u b b e r m o l d a n d t h e n s i n t e r e d . A t t h i s s t a g e t h e m e t a l i s o n l y 7 0 % d e n s e , s o i t i s i n f i l t r a t e d w i t h c o p p e r t o b r i n g i t c l o s e t o i t s t h e o r e t i c a l m a x i m u m d e n s i t y. T h e t o o l s h a v e f a i r l y g o o d a c c u r a c y, b u t t h e i r s i z e i s l i m i t e d t o u n d e r 2 5 c e n t i m e t e r s .

S a n d C a s t i n g : A R P m o d e l i s u s e d a s t h e p o s i t i v e p a t t e r n a r o u n d w h i c h t h e s a n d m o l d i s b u i l t . L O M m o d e l s , w h i c h r e s e m b l e t h e w o o d e n m o d e l s t r a d i t i o n a l l y u s e d f o r t h i s p u r p o s e , a r e o f t e n u s e d . I f s e a l e d a n d f i n i s h e d , a L O M p a t t e r n c a n p r o d u c e a b o u t 1 0 0 s a n d m o l d s .

I n v e s t m e n t C a s t i n g : S o m e R P p r o t o t y p e s c a n b e u s e d a s i n v e s t m e n t c a s t i n g p a t t e r n s . T h e p a t t e r n m u s t n o t e x p a n d w h e n h e a t e d , o r i t w i l l c r a c k t h e c e r a m i c s h e l l d u r i n g a u t o c l a v i n g . B o t h S t r a t a s y s a n d C u b i t a l m a k e i n v e s t m e n t c a s t i n g w a x f o r t h e i r m a c h i n e s . P a p e r L O M p r o t o t y p e s m a y a l s o b e u s e d , a s t h e y a r e d i m e n s i o n a l l y s t a b l e w i t h t e m p e r a t u r e . T h e p a p e r s h e l l s b u r n o u t , l e a v i n g s o m e a s h t o b e r e m o v e d .

To c o u n t e r t h e r m a l e x p a n s i o n i n s t e r e o l i t h o g r a p h y p a r t s , 3 D S y s t e m s i n t r o d u c e d Q u i c k C a s t , a b u i l d s t y l e f e a t u r i n g a s o l i d o u t e r s k i n a n d m o s t l y h o l l o w i n n e r s t r u c t u r e . T h e p a r t c o l l a p s e s i n w a r d w h e n h e a t e d . L i k e w i s e , D T M s e l l s Tr u e f o r m p o l y m e r , a p o r o u s s u b s t a n c e t h a t e x p a n d s l i t t l e w i t h t e m p e r a t u r e r i s e , f o r u s e i n i t s S L S m a c h i n e s .

I n j e c t i o n m o l d i n g : C E M C O M R e s e a r c h A s s o c i a t e s , I n c . h a s d e v e l o p e d t h e N C C To o l i n g S y s t e m t o m a k e m e t a l / c e r a m i c c o m p o s i t e m o l d s f o r t h e i n j e c t i o n m o l d i n g o f p l a s t i c s .   1 8   F i r s t , a s t e r e o l i t h o g r a p h y m a c h i n e i s u s e d t o m a k e a m a t c h - p l a t e p o s i t i v e p a t t e r n o f t h e d e s i r e d m o l d i n g . To f o r m t h e m o l d , t h e S L A p a t t e r n i s p l a t e d w i t h n i c k e l , w h i c h i s t h e n r e i n f o r c e d w i t h a s t i f f c e r a m i c m a t e r i a l . T h e t w o m o l d h a l v e s a r e s e p a r a t e d t o r e m o v e t h e p a t t e r n , l e a v i n g a m a t c h e d d i e s e t t h a t c a n p r o d u c e t e n s o f t h o u s a n d s o f i n j e c t i o n m o l d i n g s .

INDIRECT TOOLING

To d i r e c t l y m a k e h a r d t o o l i n g f r o m C A D d a t a i s t h e H o l y G r a i l o f r a p i d t o o l i n g . R e a l i z a t i o n o f t h i s o b j e c t i v e i s s t i l l s e v e r a l y e a r s a w a y, b u t s o m e s t r o n g s t r i d e s a r e b e i n g m a d e :

R a p i d To o l : A D T M p r o c e s s t h a t s e l e c t i v e l y s i n t e r s p o l y m e r - c o a t e d s t e e l p e l l e t s t o g e t h e r t o p r o d u c e a m e t a l m o l d . T h e m o l d i s t h e n p l a c e d i n a f u r n a c e w h e r e t h e p o l y m e r b i n d e r i s b u r n e d o f f a n d t h e p a r t i s i n f i l t r a t e d w i t h c o p p e r ( a s i n t h e K e l t o o l p r o c e s s ) . T h e r e s u l t i n g m o l d c a n p r o d u c e u p t o 5 0 , 0 0 0 i n j e c t i o n m o l d i n g s .

I n 1 9 9 6 R u b b e r m a i d p r o d u c e d 3 0 , 0 0 0 p l a s t i c d e s k o r g a n i z e r s f r o m a S L S - b u i l t m o l d . T h i s w a s t h e f i r s t w i d e l y s o l d c o n s u m e r p r o d u c t t o b e p r o d u c e d f r o m d i r e c t r a p i d t o o l i n g .   1 9   E x t r u d e H o n e , i n I r w i n PA , w i l l s o o n s e l l a m a c h i n e , b a s e d o n M I T ’s 3 D P r i n t i n g p r o c e s s , t h a t p r o d u c e s b r o n z e - i n f i l t r a t e d P M t o o l s a n d p r o d u c t s .   2 0

L a s e r - E n g i n e e r e d N e t S h a p i n g ( L E N S ) i s a p r o c e s s d e v e l o p e d a t S a n d i a N a t i o n a l L a b o r a t o r i e s a n d S t a n f o r d U n i v e r s i t y t h a t c a n c r e a t e m e t a l t o o l s f r o m C A D d a t a .   2 1   M a t e r i a l s i n c l u d e 3 1 6 s t a i n l e s s s t e e l , I n c o n e l 6 2 5 , H 1 3 t o o l s t e e l , t u n g s t e n , a n d t i t a n i u m c a r b i d e c e r m e t s . A l a s e r b e a m m e l t s t h e t o p l a y e r o f t h e p a r t i n a r e a s w h e r e m a t e r i a l i s t o b e a d d e d . P o w d e r m e t a l i s i n j e c t e d i n t o t h e m o l t e n p o o l , w h i c h t h e n s o l i d i f i e s . L a y e r a f t e r l a y e r i s a d d e d u n t i l t h e p a r t i s c o m p l e t e . U n l i k e t r a d i t i o n a l p o w d e r m e t a l p r o c e s s i n g , L E N S p r o d u c e s f u l l y d e n s e p a r t s , s i n c e t h e m e t a l i s m e l t e d , n o t m e r e l y s i n t e r e d . T h e r e s u l t i n g p a r t s h a v e e x c e p t i o n a l m e c h a n i c a l p r o p e r t i e s , b u t t h e p r o c e s s c u r r e n t l y w o r k s o n l y f o r p a r t s w i t h s i m p l e , u n i f o r m c r o s s s e c t i o n s . T h e s y s t e m h a s b e e n c o m m e r c i a l i z e d b y M T S c o r p o r a t i o n   D i r e c t A I M ( A C E S I n j e c t i o n M o l d i n g ) : A t e c h n i q u e f r o m 3 D S y s t e m s i n w h i c h s t e r e o l i t h o g r a p h y -p r o d u c e d c o r e s a r e u s e d w i t h t r a d i t i o n a l m e t a l m o l d s f o r i n j e c t i o n m o l d i n g o f h i g h a n d l o w d e n s i t y p o l y e t h y l e n e , p o l y s t y r e n e , p o l y p r o p y l e n e a n d A B S p l a s t i c .   2 2   Ve r y g o o d a c c u r a c y i s a c h i e v e d f o r f e w e r t h a n 2 0 0 m o l d i n g s . L o n g c y c l e t i m e s ( ~ f i v e m i n u t e s ) a r e r e q u i r e d t o a l l o w t h e m o l d i n g t o c o o l e n o u g h t h a t i t w i l l n o t s t i c k t o t h e S L A c o r e .

I n a n o t h e r v a r i a t i o n , c o r e s a r e m a d e f r o m t h i n S L A s h e l l s f i l l e d w i t h e p o x y a n d a l u m i n u m s h o t . A l u m i n u m ’s h i g h c o n d u c t i v i t y h e l p s t h e m o l d i n g c o o l f a s t e r , t h u s s h o r t e n i n g c y c l e t i m e . T h e o u t e r s u r f a c e c a n a l s o b e p l a t e d w i t h m e t a l t o i m p r o v e w e a r r e s i s t a n c e . P r o d u c t i o n r u n s o f 1 0 0 0 - 5 0 0 0 m o l d i n g s a r e e n v i s i o n e d t o m a k e t h e p r o c e s s e c o n o m i c a l l y v i a b l e .

L O M C o m p o s i t e : H e l y s i s a n d t h e U n i v e r s i t y o f D a y t o n a r e w o r k i n g t o d e v e l o p c e r a m i c c o m p o s i t e m a t e r i a l s f o r L a m i n a t e d O b j e c t M a n u f a c t u r i n g . L O M C o m p o s i t e p a r t s w o u l d b e v e r y s t r o n g a n d d u r a b l e , a n d c o u l d b e u s e d a s t o o l i n g i n a v a r i e t y o f m a n u f a c t u r i n g p r o c e s s e s .

S a n d M o l d i n g : A t l e a s t t w o R P t e c h n i q u e s c a n c o n s t r u c t s a n d m o l d s d i r e c t l y f r o m C A D d a t a . D T M s e l l s s a n d - l i k e m a t e r i a l t h a t c a n b e s i n t e r e d i n t o m o l d s . S o l i g e n   ( u s e s 3 D P t o p r o d u c e c e r a m i c m o l d s a n d c o r e s f o r i n v e s t m e n t c a s t i n g , ( D i r e c t S h e l l P r o d u c t i o n C a s t i n g ) .

DIRECT TOOLING

A natura l ex tens ion of RP i s rap id manufac tur ing (RM), the au tomated product ion of sa lab le p roducts d i rec t ly f rom CAD data . Curren t ly only a few f ina l p roducts a re p roduced by RP machines , bu t the number wi l l increase as meta l s and o ther mater ia l s become more wide ly ava i lab le . RM wi l l never comple te ly rep lace o ther manufac tur ing techniques , espec ia l ly in l a rge product ion runs where mass-product ion i s more economica l .

For shor t p roduct ion runs , however, RM is much cheaper, s ince i t does no t requi re too l ing . RM is a l so idea l for p roducing cus tom par t s t a i lo red to the user ’s exac t spec i f ica t ions . A Univers i ty o f Delaware research pro jec t uses a d ig i t i zed 3-D model o f a person’s head to cons t ruc t a cus tom-f i t t ed he lmet .   2 3  NASA i s exper iment ing wi th us ing RP machines to p roduce spacesui t g loves f i t t ed to each as t ronaut ’s hands .   2 4  From ta i lo red go l f c lub gr ips to cus tom d innerware , the poss ib i l i t i es a re endless .

The o ther major use of RM is fo r p roducts tha t s imply cannot be made by sub t rac t ive (machin ing , gr ind ing) o r compress ive ( forg ing , e tc . ) p rocesses . This inc ludes ob jec t s wi th complex fea tures , in te rna l vo ids , and layered s t ruc tures . Spec i f ic Surface of Frankl in , MA uses RP to manufac ture compl ica ted ceramic f i l t e rs tha t have e ight t imes the in te r io r sur face a rea of o lder types . The f i l t e rs remove par t i c les f rom the gas emiss ions of coa l - f i red power p lan t s .   2 5  Ther ics , Inc . o f NYC is us ing RP’s l ayered bu i ld s ty le to develop "p i l l s tha t re lease measured drug doses a t spec i f ied t imes dur ing the day" and o ther medica l p roducts .  

RAPID MANUFACTURING

Although several rapid prototyping techniques exist, all employ the same basic five-step process. The steps are:

Create a CAD model of the designConvert the CAD model to STL formatSlice the STL file into thin cross-sectional layersConstruct the model one layer atop anotherClean and finish the modelCAD Model Creation: First, the object to be built is modeled

using a Computer-Aided Design (CAD) software package. Solid modelers, such as Pro/ENGINEER, tend to represent 3-D objects more accurately than wire-frame modelers such as AutoCAD, and will therefore yield better results. The designer can use a pre-existing CAD file or may wish to create one expressly for prototyping purposes. This process is identical for all of the RP build techniques.

BASIC PROCESS

Convers ion to STL Format :   The var ious CAD packages use a number o f d i ff e ren t a lgo r i t hms to r ep resen t so l id ob jec t s . To es t ab l i sh cons i s t ency, t he STL ( s t e reo l i t hog raphy, t he f i r s t RP t echn ique) fo rmat has been adop ted as t he s t andard o f t he r ap id p ro to typ ing indus t ry. The second s t ep , t he re fo re , i s t o conver t t he CAD f i l e i n to STL fo rmat . Th i s fo rmat r ep resen t s a t h ree -d imens iona l su r f ace as an as semb ly o f p l anar t r i ang les , " l i ke t he f ace t s o f a cu t j ewel . "   6 The f i l e con ta ins t he coo rd ina t es o f t he ver t i ces and the d i r ec t ion o f t he ou tward no rmal o f each t r i ang le . Because STL f i l es u se p l anar e l emen t s , t hey canno t r ep resen t cu rved su r f aces exac t ly. Inc reas ing the number o f t r i ang les improves the app rox imat ion , bu t a t t he cos t o f b igger f i l e s i ze . Large , compl i ca t ed f i l e s r equ i r e more t ime to p re -p roces s and bu i ld , so t he des igner mus t ba l ance accu racy wi th manageab l i l i t y t o p roduce a u se fu l STL f i l e . S ince the . s t l fo rmat i s un iver sa l , t h i s p rocess i s i den t i ca l fo r a l l o f t he RP bu i ld t echn iques .

Sl i ce the STL Fi l e :   I n t he t h i rd s t ep , a p r e -p rocess ing p rog ram p repares t he STL f i l e t o be bu i l t . Severa l p rog rams a re ava i l ab l e , and mos t a l l ow the use r t o ad ju s t t he s i ze , l oca t ion and o r i en t a t ion o f t he mode l . Bu i ld o r i en t a t ion i s impor t an t fo r s evera l r easons . F i r s t , p roper t i es o f r ap id p ro to types vary f rom one coo rd ina t e d i r ec t ion to ano ther. Fo r example , p ro to types a r e u sua l ly weaker and l es s accu ra t e i n t he z (ver t i ca l ) d i r ec t ion than in t he x -y p l ane . In add i t i on , pa r t o r i en t a t ion par t i a l l y de t e rmines t he amoun t o f t ime r equ i r ed to bu i ld t he mode l . P l ac ing the sho r t es t d imens ion in t he z d i r ec t ion r educes t he number o f l ayer s , t he reby sho r t en ing bu i ld t ime . The p re -p roces s ing so f tware s l i ces t he STL model i n to a number o f l ayer s f rom 0 .01 mm to 0 .7 mm th i ck , depend ing on the bu i ld t echn ique . The p rog ram may a l so genera t e an aux i l i a ry s t ruc tu re t o suppor t t he mode l du r ing the bu i ld . Suppor t s a r e u se fu l fo r de l i ca t e f ea tu res such as overhangs , i n t e rna l cav i t i es , and th in -wal l ed s ec t ions . Each PR mach ine manufac tu re r supp l i es t he i r own p rop r i e t a ry p re -p rocess ing so f tware .

Layer by Layer Construction: The fourth step is the actual construction of the part. Using one of several techniques (described in the next section) RP machines build one layer at a time from polymers, paper, or powdered metal. Most machines are fairly autonomous, needing little human intervention.Clean and Finish: The final step is post-processing. This involves removing the prototype from the machine and detaching any supports. Some photosensitive materials need to be fully cured before use. Prototypes may also require minor cleaning and surface treatment. Sanding, sealing, and/or painting the model will improve its appearance and durability.