RAPID PROTOTYPING

ABSTRACT

Rapid prototyping processes are a relatively recent development. The first machine was released onto the market in late 1987.Rapid prototyping more accurately be described as layer manufacturing processes. An alternative term is free-form fabrication processes. These processes work by building up a component layer by layer, with one thin layer of material bonded to the previous thin layer. This is in contrast to conventional manufacturing processes such as milling machines that start from a solid block of a substance and cut material away to form the finished part. Rapid prototyping processes are driven by instructions which are derived from three-dimensional computer-aided design (CAD) models. CAD technologies are therefore an essential enabling system for rapid prototyping. The processes use different physical principles, but essentially they work either by using lasers to cut, cure or sinter material into a layer, or involve ejecting material from a nozzle to create a layer. A physical model is prepared initially which is further used for various purposes.

INTRODUCTION

Rapid Prototyping is a form of 'Layered Manufacturing' or 'Solid Free-form Fabrication' (SFF) pioneered in the United States. Rapid prototyping was described in rapid prototyping report in October 1992 as, "The fabrication of a physical, three dimensional part of arbitrary shape directly from a numerical description - typically a Computer Aided Design model, by a quick, highly automated and totally flexible manufacturing process"

Rapid prototyping has the ability to convert a computer-generated model into a prototype model or final component more quickly and at a much lower cost than conventional production methods. 

The solid model created as a result of the Rapid Prototyping process can be constructed from a variety of materials; these include plastics, wax, metals, sand, ceramics and paper. 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. The first RP technique, Stereo lithography, was developed by 3D Systems of Valencia, CA, USA and since then, a number of different RP techniques have become available.

NEED OF RAPID PROTOTYPING

The reason to use rapid prototyping is:

1.   To increase the effective communication
2.  To decrease the development time
3.  To decrease the costly mistakes
4. To extend the product lifetime by adding the necessary features and eliminating the redundant features early in the design processes.

METHODOLOGY OF RAPID PROTOTYPING

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

1. A CAD model is constructed, and then converted to STL format. The resolution 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 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 and cleaned.

GRAPHIC REPRESENTATION OF MODEL:

             Note that in the model above, the construct prototype and utilize prototype form a loop in which       multiple utilizations of prototypes provide feedback for the construction of ensuing multiple                     prototypes.

CLASSIFICATION OF RAPID PROTOTYPING PROCESSES

Rapid-prototyping processes can be classified into three major groups:

1. Subtractive,
2. Additive, and
3. Virtual

SUBTRACTIVE PROCESSES

Making a prototype traditionally has involved a series of processes using a variety of tooling and machines, and it usually takes anywhere from weeks to months, depending on part complexity and size. This approach requires skilled operators using material removal by machining and Hnis/cling operations (as described in detail in Part IV)-one by one-until the prototype is completed. To speed the process,subtractive processes increasingly use computer-based technologies such as the following:

1. Computer-based drafting packages, which can produce three-dimensional representations of parts.
2.  Interpretation software, which can translate the CAD file into a format usable by manufacturing software.
3. Manufacturing software, which is capable of planning the operations required to produce the desired shape.
4. Computer-numerical-control (CNC) machinery with the capabilities necessary to produce the parts.

ADDITIVE PROCESSES

Additive rapid-prototyping operations all build parts in layers, and as summarized in Fig., they consist of stereolitliograpliy, Multiiet/polyiet modeling, fuseddeposition modeling, ballistic-particle manufacturing, three-dimensional printing, selective laser sintering, electron-beam and laminated-object manufacturing. In order to visualize the methodology used, it is beneficial to think of constructing a loaf of bread by stacking and bonding individual slices on top of each other. All of the processes described in this section build parts slice by slice. The main difference between the various additive processes lies in the method of producing the individual slices, which are typically 0.1 to 0.5 mm thick and can be thicker for some systems. All additive operations require elaborate software. As an example, note the solid part . The first step is to obtain a CAD file description of the part. The computer then constructs slices of the three-dimensional part . Each slice is analyzed separately, and a set of instructions is compiled in order to provide the rapid-prototyping machine with detailed information regarding the manufacture of the part.  shows the paths of the extruder in one slice,

using the fused-deposition-modeling operation . This approach requires operator input in the setup of the proper computer files and in the initiation of the production process. Following that stage, the machines generally operate unattended and provide a rough part after a few hours. The part is then subjected to a series of manual finishing operations (such as sanding and painting) in order to complete the rapid-prototyping process. It should be recognized that the setup and finishing operations are very labor intensive and that the production time is only a portion of the time required to obtain a prototype. In general, however, additive processes are much faster than subtractive processes, taking as little as a few minutes to a few hours to produce a part.

TYPES OF RAPID PROTOTYPING PROCESSES

1. Stereolithography
2. Selective Laser Sintering   
3. Laminated Object Modeling
4. Fused Deposition Modeling
5. Three-dimensional Printing

STEREO LITHOGRAPHY 

Patented in 1986, Stereolithography started the prototyping revolution. The technique builds three dimensional models from liquid photosensitive polymers that solidify when exposed to ultraviolet light. As shown in figure beside, the model is built upon a platform situated 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 models’ cross section while leaving excess areas liquid. An elevator incrementally lowers the platform into the liquid polymer.

            A sweeper re-coats the solidified layer with liquid, and the laser traces a top fist. This process is repeated until the prototype is complete. Afterwards, the solid part is removed from at & rinsed clean of excess liquid. Supports are broken off & the model is then placed in an ultraviolet oven for complete curing.

            Stereolithography apparatus (SLA) machines have been made since 1988 by 3d systems of Valencia, CA. To this day, 3d system is the industry leader, selling more RP machine than any company. Because it was the first technique, Stereolithography is regarded as a benchmark by which other technologies are judged. Early Stereolithography prototypes were fairly brittle and prone to curing-induced warpage and distortion, but recent modifications have largely corrected these problems

LAMINATED OBJECT  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 & rolled up on spools. As shown in fig. above, a feeder mechanism advances the sheet over the build platform, where a base has been constructed from paper & doubled-sided from tape. Next, a heated roller applies pressure to bond the paper & then cross-hatches the excess area.

Cross-hatching breaks up the extra material, making it easier to remove during post-processing. During the build, the excess material provides excellent support for overhangs & thin walled sections. After the first layer is cut, the platform rises too slightly below the previous height, the roller bonds the second layer to the first, & 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 of paper, they must seal & finished with paint or varnish prevents moisture damage.

 SELECTIVE LASER SINTERING

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.

FUSED DEPOSITION MODELING

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.

 SOLID GROUND CURING

Developed by Cubital, solid ground curing (SGC) is somewhat similar to Stereolithography (SLA) in that both use ultraviolet light to selectively harden photosensitive polymers. Unlike SLA, SGC cures an entire layer at a time. Figure 5 depicts solid ground curing, which is also known as the solider process. First, photosensitive resin is sprayed on the build platform. Next, the machine develops a photomask (like a stencil) of the layer to be built. This photomask is printed on a glass plate above the build platform using an electrostatic process similar to that found in photocopiers. The mask is then exposed to UV light, which only passes through the transparent portions of the mask to selectively harden the shape of the current layer.  

3-D INK-JET PRINTING

Ink-Jet Printing refers to an entire class of machines that employ ink-jet technology. The first was 3D Printing (3DP), developed at MIT and licensed to Soligen Corporation, Extrude Hone, and others. The ZCorp 3D printer, produced by Z Corporation of Burlington, MA, is an example of this technology. As shown in Figure 6a, parts are built upon a platform situated in a bin full of powder material. An ink-jet printing head selectively deposits or "prints" a binder fluid to fuse the powder together in the desired areas. Unbound powder remains to support the part. The platform is lowered, more powder added and leveled, and the process repeated. When finished, the green part is then removed from the unbound powder, and excess unbound powder is blown off. Finished parts can be infiltrated with wax, CA glue, or other sealants to improve durability and surface finish. Typical layer thicknesses are on the order of 0.1 mm. This process is very fast, and produces parts with a slightly grainy surface. ZCorp uses two different materials, a starch based powder (not as strong, but can be burned out, for investment casting applications) and a ceramic powder. Machines with 4 color printing capability are available.

3D Systems' version of the ink-jet based system is called the Thermo-Jet or Multi-Jet Printer. It uses a linear array of print heads to rapidly produce roduce ABS plastic models 2.5-5 times fthermoplastic models (Figure 6d). If the part is narrow enough, the print head can deposit an entire layer in one pass. Otherwise, the head makes several passes.

Sanders Prototype of Wilton, NH uses a different ink-jet technique in its Model Maker line of concept modelers. The machines use two ink-jets (see Figure 6c). One dispenses low-melt thermoplastic to make the model, while the other prints wax to form supports. After each layer, a cutting tool mills the top surface to uniform height. This yields extremely good accuracy, allowing the machines to be used in the jewelry industry.

 Ballistic particle manufacturing, depicted in Figure 6b, was developed by BPM Inc., which has since gone out of business

FUTURE DEVELOPMENTS

Rapid prototyping is starting to change the way companies design and build products. On the horizon, though, are several developments that will help to revolutionize manufacturing as we know it.

One such improvement is increased speed. "Rapid" prototyping machines are still slow by some standards. By using faster computers, more complex control systems, and improved materials, RP manufacturers are dramatically reducing build time. For example, Stratasys recently (January 1998) introduced its FDM Quantum machine, which can paster than previous FDM machines. ²⁷ Continued reductions in build time will make rapid manufacturing economical for a wider variety of products.

Another future development is improved accuracy and surface finish. Today’s commercially available machines are accurate to ~0.08 millimeters in the x-y plane, but less in the z (vertical) direction. Improvements in laser optics and motor control should increase accuracy in all three directions. In addition, RP companies are developing new polymers that will be less prone to curing and temperature-induced warpage.

The introduction of non-polymeric materials, including metals, ceramics, and composites, represents another much anticipated development. These materials would allow RP users to produce functional parts. Today’s plastic prototypes work well for visualization and fit tests, but they are often too weak for function testing. More rugged materials would yield prototypes that could be subjected to actual service conditions. In addition, metal and composite materials will greatly expand the range of products that can be made by rapid manufacturing.

Many RP companies and research labs are working to develop new materials. For example, the University of Dayton is working with Helisys to produce ceramic matrix composites by laminated object manufacturing. ²⁸ An Advanced Research Projects Agency / Office of Naval Research sponsored project is investigating ways to make ceramics using fused deposition modeling. ²⁹ As mentioned earlier, Sandia/Stanford’s LENS system can create solid metal parts. These three groups are just a few of the many working on new RP materials.

APPLICATIONS

The rapid prototyping has found its wide application for the following:

1. Visualization

The very first application of RP was to create real-life models of computer generated designs.

1. Patterns for Castings (SLS)

       Rapid prototyping methods are increasingly being used to produce castings for metals and             silicone rubber. Selective Laser Sintering has enabled these moulds to become far more complex         than was previously possible.

1. Moulds for Vacuum forming (SLS plastic)

        Plastic Selective Laser Sintering is particularly useful for the production of moulds for               vacuum forming plastic parts.

     Despite this, the similarity was recognized and RP has been applied to the production of               detailed models of bone structures. These can be used to aid medical practitioners in surgical               planning, as templates in surgery as well as developments for joint replacements and cranio-facial        reconstruction.

ADVANATAGES

 The main advantages of these new processes will be:

1. It saves time and money in production and development of new products.
2. Easy transfer from a virtual model to a real part
3. Very quick compared to traditional techniques for small volumes
4. No tooling investment
5. True Flexible Manufacturing
6. Complexity will be independent from cost

 DISADVANTAGES

1. Poor mechanical properties of the solid models

2. Cannot be used as a production technique with  the current technologies

CONCLUSION AND FUTURE

             Since the end of 1995 there has been a steady reduction in the size and cost of rapid prototyping machines. Small, inexpensive 'desktop' rapid prototyping machines are now entering the market. One of the latest machines to enter the marketplace is the Actua 2100 from 3D Systems. This is a small, lightweight Fused Deposition rapid prototyping machine costing around £30,000. This process lays down very thin layers of plastic, similarly to ink on paper from an ink-jet printer. 

              The future looks very promising for rapid prototyping. The benefits for most applications far outweigh the disadvantages especially when they are used in the correct situation. The price and size are rapidly falling to the point where they will soon be commonplace in any manufacturing company. 

 The rapid prototyping technology is therefore advancing beyond the production of 'prototype' models for which it was originally used.

REFERENCES:

    RAPID PROTOTYPING .....Principles and Applications (2nd Edition)

1. By C K Chua
2. K F Leong
3. C S Lim (Nan yang Technological University, Singapore)

Link for PPT Presentation :-

http://www.slideshare.net/konalartist1/rapid-prototyping-48612222?qid=949f58c3-bc84-4355-8ca4-32f883ca4d98&v=&b=&from_search=9

Mr. KONAL

Mechanical Engineer
Ph. 09423632068

Director & Founder

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