Rapid Prototyping Leads To Rapid Products

Prior to the late 1980's, one component of product development was much more expensive than it is today--expensive time-wise as well as cost-wise. When a company needed to verify a prototype for form, fit, or function, it often had to go through the manufacturing process just as if the product were full-scale, production ready. Tools were made, liquid metal was poured, solid metal was cut.

Design errors were very costly. The cut metal became scrap, and the design was returned to engineering for time consuming modifications.

But in 1988, a new technology was introduced, and a new industry began to evolve. The new technology was stereolithography, and the new industry was rapid prototyping. There always are, and always will be, evolutions and advancements to existing technologies. But there have been very few new technologies that have had such a significant impact on how we produce goods as has rapid prototyping.

Stereolithography is not the only rapid prototyping technique, although it was the first and is still the most common. Stereolithography was developed by Mr. Charles Hull who founded 3D Systems in 1986. From the time 3D Systems shipped its first six stereolithography apparatuses (SLAs) in 1988, 3D Systems has been the leading producer of rapid prototyping equipment.

Since then, other makers of rapid prototyping equipment have appeared. Though their products are conceptually similar, there are enough differences among them that they are classified as different products.

In the last few years, rapid prototyping technology has improved in a couple of key areas to broaden its applicability. Advancements in the technology enable it to create larger parts than in the past--now about 12 inches. Also, rapid prototyping accuracy--especially in stereolithography--has improved significantly, with accuracy levels approaching those of machining.

The industry that began to evolve is not only the industry of manufacturing rapid prototyping equipment to assist product manufacturers with research and development, it is also an industry of rapid prototyping service providers that can often justify the rapid prototyping equipment cost more easily than the OEM. There are scores of service providers, and because they serve a broader base of applications than an in-house rapid prototyping department, they are in a position to develop greater expertise than the in-house rapid prototyper

Stereolithography at Work

Stereolithography begins with a computerized design from a CAD system. The design is transformed into a computer file known as an .STL file, the industry standard. Many CAD systems now come equipped with .STL file output capability. The original data file should be in the form of a three-dimensional solid model, or at least, a surface model. If the user's CAD system does not have an option for .STL output, there are utilities that will produce an .STL model from an IGES surface model.

Using the .STL file in the personal computer-like control system, the SLA builds physical models one layer at a time. After slicing the data into thin cross-sections, a UV laser traces each successive cross-section of the object onto the surface of a vat of photosensitive resin.

The liquid plastic hardens only where touched by the laser beam. As it does, the model is lowered in the vat of liquid so that a new liquid layer spreads over the solidified layer. Then, the next contour is drawn by the laser. The process repeats until the part is complete.

A post-curing heat process is often required to completely harden the model. Finishing is performed if needed to create smooth surfaces on the model.

Using Rapid Prototyping

There are already many applications for rapid prototyping technology. Others are still waiting to be imagined. Together, the applications work to lower development costs, reduce time to market, and improve overall productivity and competitiveness--not to mention product quality.

First, the technology provides a means to visually inspect the design for form, fit, and function. For example, it enables a designer to check the assembly fits and mechanisms of a multiple component product. If the rapid prototype model doesn't look like the designer thought it would, the designer can reiterate the modeling process until the design is perfected.

Reiteration is not done just for design imperfections. It can also be done for market research. Manufacturers can ask people what they think of a model'' functionality as a precursor to shaping the design to market preferences. Costs for reiterations and design changes such as these with traditional technologies would be too high to consider.

Other uses include examining several design variations to select the most suitable. Another is to include a rapid prototype model with bid packages to avoid costly mistakes by suppliers. The best quality systems, using the best quality materials can model parts that can be used as functional prototypes.

One of the most common uses of rapid prototyping technology is to reduce the cost or the time to make tooling for die cast parts. It is ideal to verify design adequacy--to identify design errors before production. The further into the development process that design errors are found, the more expensive it is to correct them. The high cost of tooling makes it extremely important to identify design problems before beginning tooling. Tooling that is reworked because of design errors may be weaker from the process with a reduced life expectancy.

Rapid prototyping models are used in several ways to create dies for making multiple prototypes, or even limited production runs. Parts can be cast in variety of urethanes and epoxies from a silicone rubber mold produced with the help of a rapid prototyped model. The model is suspended over a mold box that is filled with silicone rubber. After it cures, the silicone rubber mold is carefully removed from the part to create a two-part mold capable of reproducing very fine details in urethanes or epoxies.

Injection molds can be created with the use of spray metal tooling. In this process, a thin metal shell is sprayed over a rapid prototype model. The sprayed model is surrounded with epoxy and mounted in a frame for added strength.

Rapid prototyping technology has also been combined successfully with investment casting. A negative image of a production part can be produced as an .STL file. This image is built by rapid prototyping into the mold shape for the production part. The mold shape takes on the role of the wax model usually used in investment casting. It is coated with ceramic, and the prototyping material is removed from the ceramic mold as wax would be. The hollow ceramic mold can now accept molten metal from the casting furnace to create a high quality tool for part production.

Plaster casting and sand casting have also been used successfully with rapid prototyping models.

When rapid prototyping technology is combined with other technologies such as digital reverse engineering, or direct modem data transfer from one company to another, productivity extends even further.

Prototype Express

Smith Sport Optics, a leading manufacturer of sports eyewear, approached Prototype Express of Schaumburg, IL with a serious design dilemma. While developing its V3 ski goggles, Smith found that the critical area between the nose and cheek had a large gap that allowed air to pass. Smith needed to find a way to quickly refine the fit and close the gap, while retaining a shape that would be compatible with many different faces.

Prototype Express is one of the largest supplier's of stereolithography (SL) prototyping services. They responded to Smith's dilemma by building an SL model of the goggles from Smith's CAD design data. This was Smith's first attempt to use rapid prototyping as an aid for their design process. In the past, Smith had used body putty to create replicas of their designs. The dimensions weren't accurate, and the softness of the putty didn't permit functional testing.

Smith decided to try rapid prototyping for a couple of reasons. First, it enabled Smith to verify the accuracy of the design before spending money on tooling. The complex geometry of the goggles is very difficult to create. There are no existing standards because there is such a wide range of face shapes the goggles must accommodate. And minor changes in the shape of the goggles could alter the effectiveness of the ventilation system, causing the goggles to fog easily.

Second, Smith used rapid prototyping for the speed at which a completed model could be built. Due to the seasonal nature of ski products, the new goggles needed to be on the shelf by early October. That deadline would have been impossible to meet with traditional methods, especially if they encountered any significant design issues. Prototype Express was able to have a finished SL model of the goggles in Smith's hands within three days.

The first SL model allowed Smith to quickly determine that the design needed to be altered in the nose region to minimize the gap. They immediately modified the CAD model to narrow the nose opening and make it fit closer to the face.

A second SL model was built to check the fit of the modified design. It took a total of three design iterations and SL models for Smith to correct the problems of the original design. All design iterations were completed within a two week period.

After the design was finalized, Prototype Express created eight functional prototypes in urethane. They created an SL model and then sanded, buffed, and textured it to simulate the finish on the production parts. This finished master pattern was then used to create a silicone rubber mold into which the urethane was injected to produce castings.

For the Smith project, a soft urethane material was used to simulate the stiffness of the elastomeric material chosen for production.

The entire process, from the creation of the third and final SL model, to the eight urethane castings, took only eight working days. The castings were then built into functional goggles that Smith could take out on the ski slopes for performance testing.

The prototypes verified that the modified design met requirements. Had any problems been found, however, the speed of the rapid prototyping process would have allowed Smith to go through another round of design iterations and still meet their deadline.

When they were certain that the design was sound, an injection mold for the frames was ordered at a cost of $125,000. Given that the average cost of rework is 10 percent of the cost of the tool, the two design changes made in the project would have cost approximately $25,000 and required several weeks for completion.

Prototype Express' use of rapid prototyping and urethane casting before creating injection mold tooling saved Smith several thousand dollars in rework costs, while minimizing delays in getting the new product to market.

Smith's product development process now routinely includes rapid prototyping techniques.

Other Rapid Prototyping Technologies

As mentioned before, there are a variety of other rapid prototyping techniques available. The costs of the systems vary, as does maintenance and modeling materials. Some produce models with better surface finishes than others, more rigid than others, or faster than others. Each rapid prototyping system needs to be evaluated for its appropriateness for a specific application before use.

OEMs that do not have enough in-house demand to justify buying the equipment can usually find a service provider with the right rapid prototyping tool. Discussions with the service provider will enable the OEM to decide if a specific process is suitable for a desired application. Because the industry is relatively new, innovative ideas to do things not previously thought of are commonplace.

Besides stereolithography, other rapid prototyping systems include Fused Deposition Modeling (FDM). Developed by Stratasys, Inc. of Eden Prairie, Minnesota, FDM is a system in which a semi-liquid thermoplastic material is heated and deposited as ultra-thin layers. The model builds upward and there is no post curing, as with stereolithography. It is an economical process sometimes used in offices.

Selective Laser Sintering (SLS) from DTM Corporation of Austin, Texas, traces the shape of the part to be modeled in a thin layer of powder. The laser sinters the powder together. As with stereolithography, after the laser passes, the platform lowers, another layer of powder is deposited and the laser makes another pass. There is no post-curing required.

Solid Ground Curing (SGC) from Cubital America of Troy, Michigan, uses large machines known as Soliders to expose design layers to photopolymer. Resin is removed from unexposed regions and replaced by wax which supports the growing model. The wax is removed after all the layers have been made.

Laminated Object Manufacturing (LOM) from Helisys, Inc. of Torrance, California, cuts rolls of adhesive coated paper to build a model. The paper is rolled over the target area from a supply roll to a take-up roll. After a laser cuts the design layer into the paper, the paper is indexed, bonded to the previous layers, and cut. The automatic process continues until all layers are cut. The model is then removed from the surrounding material.

Direct Shell Production Casting (DSP) from Soligen, Inc. of Northridge, California, uses a negative of an .STL to produce molds with cores from a CAD design of a part. It sinters ceramic powder layers on a descending platform with a piezo-electric inkjet head.

Sanders Prototype of Wilton, New Hampshire, developed 3D Printing and Deposition Milling (3DP) which also uses piezo-electric inkjet heads to deposit thermoplastic model material and wax support material. Two jets are used: one for the thermoplastic and one for wax. Each layer is milled to a desired thickness. The process produces ultra-smooth surface finishes.

Ballistic Particle Manufacturing (BPM) from BPM, Inc. of Greenville, South Carolina, jets micro-particles of molten thermoplastic from a piezoelectric nozzle to a defined location. As the particles harden, they build the model upward, particle by particle. The desk-side unit uses a five-axis, robotic head to position the nozzle.

American Industrial Casting of Cranston, Rhode Island, manufactures ultra-thin to standard precision investment cast components in all alloys including beryllium copper for Aerospace, Defense, Communications, Electronics, Mechanical, Medical, and Subminiature applications.

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