Rapid prototyping: state-of-the-art manufacturing.

Rapid prototyping is the direct conversion of an electronic computer-aided design model into a solid physical model. The creation of prototype models for design refinement is central to the new product development process. According to Ashley, "The most time consuming and costly stage of new product development is the creation of a prototype." Thus, new technological processes that reduce the time and effort needed to create prototypes will improve the efficiency of the design process. The time and effort in redesign that can be saved by finding and eliminating flawed components before mass production begins is tremendous. One critical error that slips through the design process and makes its way into the final dies or molds can cost a company many thousands of dollars and months to correct.

General prototyping methodologies

Two general methods - subtractive or additive - are used to produce prototypes during the design process. Prior to rapid prototyping, subtractive methods were most often used to create prototypes. New designs were modeled by starting with a block of material and cutting away that which was not part of the intended design. Some of the techniques used in this process include milling, turning, drilling, and electrical discharge machining. The advantages of subtractive processes include:

* Accuracy;

* Finish;

* Mass production;

* Material composition; and

* Part size.

Subtractive techniques, however, have drawbacks that cannot be overlooked. Subtractive techniques require the removal of material from a solid object, which can be difficult or impossible to do if the model has intricate designs.

Most rapid prototyping techniques are additive techniques. The additive techniques utilized by rapid prototyping offer some special advantages over subtractive methods used in conventional prototyping. Some of the most important advantages of the additive processes are:

* The parts can have an almost arbitrary geometric complexity;

* Fabrication can be set up with little or no human intervention; and

* The fabrication process can proceed with little or no human intervention.

Rapid prototyping methodologies

There are two general classes of rapid prototyping methodologies - layering methods and drop deposition methods. Within each class of rapid prototyping methodologies are several competing technologies. Some of the competing technologies in each class are:

1) Layering Methods

a. Stereolithography b. Laminated Object Manufacturing (LOM) c. Selective Laser Sintering (SLS)

2) Drop Deposition Methods

a. Fused Deposition Modeling (FDM) b. Droplet Deposition

Stereolithography

Stereolithography is the oldest of the rapid prototyping technologies. The SLA converts three dimensional CAD data of physical objects into a vertical stack of slices. A low-powered ultraviolet laser beam (HeCd laser) is carefully traced across a vat of photocurable polymer liquid, producing a single layer of solidified resin. The platform is then lowered to the next increment and coated with liquid polymer and the next layer traced. This process is repeated until the part is finished. The early SLA machines, 1988 vintage, had epsilon values for dimensional accuracy of about 400 microns, adequate for concept visualization and uncovering only basic design errors. By 1990, the dimensional accuracy had improved to 300 microns, and further improvements had the accuracy down to 200 microns by 1991. When new epoxy resins were formulated for the process in 1993, the accuracy began to approach 100 microns. This accuracy allowed for the prototype to be used for a pattern to make an investment cast metal prototype, representing a huge leap forward in technology. Using prototypes for investment casting patterns began to move rapid prototyping from the concept design phase into the fabrication of functional end-use prototypes.

Laminated object manufacturing

Laminated object manufacturing (LOM) is another layering technique used for rapid prototyping. This process is considered an additive technique though some say it is subtractive because it cuts away sections from the rolls of layering material used in the process. LOM uses a special heat-activated adhesive coated paper.

A laser beam cuts the paper in the outline of the shape of the cross section of the particular slice of the object. The waste paper surrounding each cross section is then crosshatched for easy removal after the prototype is created. The next layer is then bonded to the first and the laser cuts it in a similar fashion. The process continues, building the model layer upon layer, until the prototype is complete.

The LOM process produces an aesthetically pleasing prototype in a material that resembles wood. The advantages of LOM include:

* LOM models have better strength than plastic models, and they can be drilled, sanded, milled, and painted just like wood models;

* LOM models require no post-curing; therefore, they do not warp or shrink;

* LOM models require no support structures while being created. The scrap material surrounding the part is all the support structure required;

* LOM models can be used for investment casting using the "lost wood" process;

* LOM is the rapid prototyping technology that requires the least amount of site preparation; and

* A source of electrical power and a vent to remove exhaust smoke are all that is needed.

Selective laser sintering

Selective laser sintering (SLS) is a process whereby heat from a laser beam is used to fuse powdered material into a solid object. In the process, a computer guided laser passes over a vat of powdered material in a crosshatch pattern representing a two dimensional cross section of a "slice" of the model. When that layer is fused, the platform lowers into the powder and the top of the model is covered with another layer of powder. Then the laser fuses the next slice to the previous one, and so on until the model is complete.

Fused deposition modeling

Fused deposition modeling (FDM) machines extrude heated thermoplastic from a nozzle located above a table. The plastic is applied in layers, each building upon the next until the model is complete. This process is also called robotically guided extrusion.

Droplet deposition

Droplet deposition is a variation of selective laser sintering. Instead of a laser beam being used to fuse a powdered material together, a liquid adhesive is applied. This adhesive penetrates slightly and forms a hardened compound.

There are many more rapid prototyping techniques in addition to those previously mentioned including:

* Multi-jet modeling;

* Solid ground curing;

* Direct shell production casting (DSPC);

* Three-dimensional printing and disposition milling; and

* Ballistic particle manufacturing.

These techniques are all similar to those previously mentioned in that they either build by laminating layers, or by depositing and fusing droplets or particles.

Rapid prototyping market growth

Rapid prototyping has experienced tremendous growth since its commercialization in 1987. In 1995, the market grew to $295.1 million, a growth of 49 percent from the previous year. When the secondary market for the tooling, castings, and duplicate parts produced from the tooling is considered, the total 1995 market for rapid prototyping technologies exceeded $471 million.

The new sales total for rapid prototyping systems in 1995 was 526 units. In 1994, more rapid prototyping systems were sold than 1993 and 1992 combined. The exponential growth and acceptance of this technology across a wide spectrum of manufacturing concerns gives credence to the value placed on it by its proponents. Perhaps more importantly, uses for rapid prototyping are growing as technology improves.

Diverging markets

Already the rapid prototyping market has begun to split into two separate markets. One market is for small systems that can produce quickly and easily small prototypes strictly for design evaluation. The other divergent part of the market is for larger machines capable of producing prototypes to be used in the creation of investment casting molds, sand molds, or soft production dies.

Office rapid prototyping

The idea for office rapid prototyping machines stems from the idea that a machine can be used to create models that will only be used to convey design ideas for the development process. These systems are designed to be easy to use in an environment where the need for quick physical output is more important than high model resolution. In 1995, at least three new machines, ranging in cost from about $35,000 to about $60,000, were created to serve this market. These machines can create small models in a workspace of up to 10 x 8 x 8 inches.

The technologies used in these systems are based on three dimensional printing using thermoplastic/wax and deposition milling, and ballistic particle manufacturing. The main difference is that three dimensional printing builds by spraying layer upon layer of material, whereas the ballistic particle manufacturing technology utilizes a five axis printhead and sprays perpendicular to the model's surface.

Rapid tool making

The second part of the diverging rapid prototyping market is centered on the creation of production molds using rapid prototyping technology. This part of the market is often referred to as rapid tool making (RTM).

Advancement of the materials used in rapid prototyping and the increased accuracy of the more advanced rapid prototyping methods are enabling the use of this technology for the direct production of investment castings or soft production molds, such as plastic injection molds. RTM is a major leap forward in technological innovation. It has the potential of reducing tooling costs and development times by 75 percent or more. RTM is particularly useful when the tool geometry is such that it would be difficult to create the tool with traditional CNC machining techniques. Possibly more important though, by reducing the cost of tooling, RTM enables high-volume processes, such as injection molding, to be competitive at lower production volumes.

The most common rapid prototyping methodologies used by RTM include selective laser sintering and infiltration, and direct shell production casting (DSPC). With the laser sintering and infiltration technique, a tool is directly created by using a laser to sinter a polymer coated metal powder into the desired shape. This porous shape is then infiltrated with another metal with a lower melting point, usually copper, to form the finished tool.

The DSPC method differs in that the finished tool is not created directly by the RTM machine. Instead, only a mold is created from which the tool is cast. The DSPC method can be used to cast high-volume tools or it can be used for low-volume production applications. The DSPC process is not intended for high-volume production investment casting because conventional investment casting methods may be more economical.

Problems with rapid prototyping

As useful and as powerful as rapid prototyping technology is, there are some problems with this technology that must be overcome. One of the main problems is that of dimensional accuracy.

The best rapid prototyping technology available now can only produce models with an epsilon accuracy of about 100 microns. In a large number of applications, this accuracy level is sufficient, but in many cases it is not. Traditional CNC machining, by comparison, can produce models with an epsilon accuracy of about 40 microns. Obviously the accuracy of rapid prototyping must be improved before the technology can gain wider acceptance.

Many accuracy problems stem from the layering methods that rapid prototyping employs. This building of material layer upon layer tends to create a staircase or terraced effect when viewed from the edge of the laminations. In some cases, it is possible to mitigate the effect of laminations by proper model orientation during construction, and also by reducing the thickness of the laminations.

Another major problem inhibiting the adoption of rapid prototyping is its difficulty of use. The ability to produce good models depends often on the skill and expertise of the modeler. One problem often experienced by novice modelers is insufficient triangulation. This problem occurs when the geometry of the model is described with triangles that are too large. When this happens, curved surfaces feel crude instead of smooth. Another problem is that of improper tessellation, which occurs when the model's geometry is described in such a way that it violates one or more of the rules of the file format of the system.

The current standard in the file format for rapid prototyping systems is the STL format. The STL file format describes the geometry of the model as a list of the coordinates of many individual triangles that compose the outer surface of the model. This format was developed by 3D Systems for use with their first stereolithography systems. There have been other attempts to develop better file formats for describing model geometry than the STL format, but there is no actual industry standard. The wireframe object often used by CAD systems may cause problems in the file conversion because the surface integrity must be intact in the STL file.

Conclusion

In the nine years that rapid prototyping technology has been commercially available, the technology has undergone great change. It has evolved from a technology suitable only for creating concept models, to a technology capable of creating sufficiently robust tooling for high-volume mass production. This technology holds great promise to change manufacturing as we know it. New processes are under development for manufacturing systems spawned from this technology that will allow for the direct manufacture of metal component parts.

Competition among manufacturing firms is at an all time high. More firms than ever are changing their traditional ways of doing business because of competitive pressure. This pressure will increase in intensity as rapid prototyping technology becomes more affordable and more capable, and as more companies adopt this technology. Rapid prototyping technology has the ability to cut the design-to-market time by 75 percent or more in some cases. The rapid production and marketing of new designs will no doubt be used as a competitive strategy by some firms to gain market share. Consumers should expect to see a proliferation of new product designs across the spectrum of manufactured products.

The concept of a manufacturing process that can produce truly customized products as rapidly and easily as mass produced products may soon become a reality. The idea of direct manufacturing will allow companies to produce low-volume manufactured parts as required. Each part produced may be different, unique to each customer's specifications. Direct manufacturing technology is already on the horizon, and early adopters of this technology, when it becomes commercially available, will gain a competitive edge.

Much has been written about the changes in the competitive nature of manufacturing concerns. Many talk about shortened product life cycles, increased customer focus, and time-based competition. These rapid prototyping technologies will only serve to accelerate the competitiveness of manufacturing as the technology begins to infiltrate more industries. The potential for application of these technologies is limited only by human imagination.

For further reading

Ashley, S., "Rapid prototyping is coming of age" 1995, http://cadserv.cadlab.vt.edu/bohn/rp/M E-7-95/art000/html.

Ashley, Steven, "Prototyping with advanced tools," Mechanical Engineering, June 1994.

Ashley, Steven, "Rapid concept modelers," Mechanical Engineering, Jan. 1996.

Baily, M. J., "Tele-Manufacturing: Rapid Prototyping on the Internet with Automatic Consistency Checking," http://www.sdsc.edu/tmf/Whitepaper/whitepaper.html.

Baily, M. J., "The Use of Solid Rapid Prototyping in Computer Graphics and Scientific Visualization," 1996, http://www.sdsc.edu/tmf/Sig96Notes/Rp ForSciVis.html.

Hilton, Peter, "Making the leap to rapid tool making," Mechanical Engineering, July 1995.

Hinzmann, B. "The Personal Factory," http://www.mcb.co.uk/services/confren/dec95/rapdipd/hinzmann/back -gnd7.html.

Japisky, David, and Frank Olsofka, "Agile engineering accelerates design," Mechanical Engineering, Nov. 1993.

Killander L. A., "Future direct manufacturing of metal parts, with Free Form Fabrication," http://www.cadcam.kth.se/public/computer/fff general_rp/lenaak/paper.html.

Machine Design, "Shell making for rapid prototypes," Apr. 4, 1996.

Schmitz, Barbara, "The value of virtual product development," Machine Design, July 5, 1996.

Wohlers, T., The Arrival of RP and its Value to US Manufacturing, http://lamar.colostate.edu/~wohlers/georgia.html.

Wohlers, T., New Industry Report Finds 49 Percent Growth in Rapid Prototyping Market, http://lamar.colostate.edu/~wohlers/96state.html.

Wohlers, T., Rapid Prototyping: State of the Industry, 1994-95 Worldwide Progress Report, 1995, http://lamar.colostate.edul~wohlers/95state.html.

Phillip W. Balsmeier, Ph.D., is a professor of management at Nicholls State University in Thibodaux, Louisiana. Wendell J. Voisin is a graduate assistant and MBA student at Nicholls State University.