Rapid Prototyping: No Longer Just for Design Engineers
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Rapid Prototyping

The widening capabilities of rapid prototyping can bring design and manufacturing engineers together earlier, improving collaborative efforts to achieve best designs for manufacturing.

No longer just for design engineers, rapid prototyping (RP) is becoming an increasingly important tool for manufacturing engineers in producing concept models, functional prototypes, and master patterns for tooling, injection molding, and casting. As the technology continues to mature, it yields greater versatility and variety of equipment, resins, and materials, including thermoplastics and metals. Rapid prototyping methods currently in use include stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), and three-dimensional printing (3DP), all of which can help shorten the design cycle and bring products to market faster.

"Engineers should not underestimate how far the technology has come in a few short years," says Terry Wohlers, president of Wohlers Associates, Inc. (Fort Collins, CO), an independent consulting firm that works closely with manufacturing organizations to identify improved approaches to rapid product development. "An almost unbelievable number of new technologies, materials, and enhancements are under development. Machines are faster, produce thinner layers resulting in better surfaces, and are more accurate. They also process a wider range of materials, some of which rival the properties of popular molded thermoplastics. A number of machines that produce metal parts are also developing impressively."

Shorter time frames and the accuracy, strength, and functional ability of test models make rapid prototyping an important tool for engineers, says Walter W. Fedun, president, W.W.F. Technical Services, Monticello, New York. Referring to a physical prototype rather than drawings can also benefit marketing, which has come to play a huge role in product development.

"Rapid Prototyping gives engineering the ability to take your 3D CAD model, go directly to an SLS rapid prototyping machine, and have a functional part in your hands as soon as the next day, in most cases," says Fedun. "The engineer now has a physical, functional part or assembly that can be placed on the table and reviewed with engineering, management, or marketing. This is accomplished without going to an expensive machine shop, getting scheduled, and then cutting or injecting metal or plastic to look at what your designer developed for you."

However, the level of adoption of the technology in manufacturing is still less than on the design side, according to Todd Grimm, of T.A. Grimm & Associates, Inc. (Edgewood, KY). "Rapid prototyping is usually owned and controlled more by design engineering, so manufacturing often doesn't see a design until it's too late," he says. But sharing rapid prototypes with the manufacturing team early will "bring cheaper, simpler, faster tools for production, and reduce product costs," Grimm says. A manufacturing engineer can spot problems on a model that might arise when machining the part. "The prototype might reveal a nasty undercut that may cost time and money in production or tooling, presenting an opportunity for redesign," he says.

Multiple Iteration

Rapid prototyping allows multiple iterations of designsmore testing, more feedbackto reach a way to make a part at less cost and in less time than traditional model-making and machining methods. For instance, Marina Hatsopoulos, CEO of Z Corporation (Burlington, MA), says that Delphi Automotive increased the number of iterations from three to nine in designing and testing automotive HVAC ducting with Z Corporation's desktop 3D printer. Model-making costs were reduced by 90% compared to building models from cardboard, which, she says, was "time-consuming, labor intensive, and subject to the interpretation of the model maker."

Advances in rapid prototyping materials have made it possible to rigorously test parts for form, fit and function. Resins have become more useful for functional testing, producing near real parts to give engineers a better idea of how a part will behave in production. As a result, engineers can see a rapid-prototyped part that behaves more similarly to the actual production part.

Elizabeth Goode, director of marketing, 3D Systems (Valencia, CA), notes that rapid prototyping is used for pressure analysis in wind tunnel tests to find stress points on parts ranging from automotive to aerospace to architecture. "We've seen stereolithography mockups to test stresses on buildings," she says. Rapid prototyping can also be used for wind tunnel tests to determine noise levels inside an automobile. Toyota, for example, conducts wind-tunnel testing to detect noise that would occur with various shapes of side mirrors made from a Z Corporation desktop 3D printer.

New Materials Offer Higher Performance

Materials currently used in rapid prototyping are "closer to production materials than ever before," according to Mike Rufo, of Design Prototyping Technologies (DPT, now 3D Systems Corp.), a rapid prototyping service provider in East Syracuse, New York. DSM Somos WaterClear10120, for example, is a new general-purpose stereolithography resin that is said to combine the optical clarity of many polycarbonates with durability, toughness, and impact strength. Due to its high optical clarity, the material is suited for see-through models of engine components, power tools, lenses, or any applications where it is essential to visualize the flow of liquids or gases. WaterClear10120 reportedly mimics the flexural strength of polycarbonate, the notched Izod impact strength of Nylon 66, and the tensile strength of ABS. Also available, Rufo says, are materials that mimic the flexibility of polyethylene (Somos 8100 series) and polypropylene (Somos 9100 series).

Resins are also providing smoother surface finishes and finer feature detail. More than 15 varieties of resins include wax, photopolymers, ceramic, nylon, and stainless steel. 3D Systems is also researching aluminum.

3D Systems Accura SI 10 material, used in models and patterns for investment casting, provides high green strength and humidity resistance and creates a pattern with a glossy top finish. Accura SI 40 material combines high temperature resistance with toughness that mimics Nylon 66. 3D Systems claims these parts can be "drilled, tapped, and bolted on for true functional testing. They can be used in under-the-hood applications without brittleness and breakage."

For SLS, 3D Systems DuraForm PA (polyamide) nylon material is said to offer exceptional tensile, flexural, and impact strength. When used as a model for surgical devices, it can be sterilized in an autoclave. Another SLS material, 3D Systems LaserForm, is a binder-coated powdered stainless steel. During the SLS process, the green part is baked to burn off the binder and infiltrate with bronze. The resulting pattern has approximately 60% stainless steel and 40% bronze for directly building metal parts or patterns for injection molding or die casting.

For desktop 3D printers, Z Corporation offers an infiltration resin which hardens the desktop part. The strengthened part can be machined, drilled, tapped, sanded, and painted like a finished part.

Rapid Manufacturing

A particularly interesting area, Wohlers says, is the use of rapid prototyping machines to manufacture finished parts. Low in cost and quick compared to conventional machining methods, "rapid manufacturing" permits limited manufacturing of specialized parts in lots as small as one. The idea is currently being explored, he says, in situations where requirements include relatively low quantities, small parts, and less-than-critical surface finish. "Parts that are hidden from view are good candidates," he says. "In each automobile, aircraft or electronic device, one can find hundreds of components that do not demand a high surface quality." Boeing subsidiary On Demand Manufacturing (Camarillo, CA), for example, uses rapid manufacturing to build nylon air ducts for non-commercial aircraft with 3D Systems' Vanguard selective laser sintering (SLS) system.

Another company had a service bureau build, on a Stratasys FDM Titan machine, a polycarbonate replacement pulley for a belt sander on a production line. The quick replacement part was built in a few hours and kept production running; without it, production would have been down for a few days.

Late last year, Z Corporation introduced its ZCast process, said to produce prototype cast parts in aluminum and other metals in 24-48 hours. ZCast uses a powder system composed of plaster and ceramic composite specifically designed for casting aluminum and other non-ferrous metals. The ZCast material enables a desktop 3D printer to fabricate a ceramic mold into which metal is poured to produce metal parts for low-volume manufacturing.

"Rapid manufacturing, given enough time, will take shape," says Grimm. "3D Systems Corp. sees big potential for rapid manufacturing, putting a number of its technologies under the umbrella term of "advanced digital manufacturing."

Rapid Prototyping Techniques

"Rapid prototyping technologies and innovations have become another tool in the entire developmental process," says Alan Peterson, Vice President, 3-Dimensional Services, Rochester Hills, Michigan. "As the processes are fairly mature, it comes down to 'how can we adapt our process in order to best serve the customer?' It may not sound glamorous or highly technical, but there are truly two factorsspeed and cost. How can I get it quicker and cheaper?"

Stereolithography is the most widely used rapid prototyping technology. A software program translates a designer's 3D CAD digital data model into STL format, which composes the information into thin cross sections, or layers. A UV laser beam selectively cures a photopolymer resin, causing the material to solidify in successive layers until the desired object is formed.

Recently, 3D Systems came out with its latest stereolithography system, the Viper si2, which is reported to build parts with smooth surface finish, high optical clarity, and thin, straight vertical walls. It is also said to have increased capability for producing small parts with extremely fine details.

Selective laser sintering (SLS) was initially developed and patented by The University of Texas, developed by DTM Corporation, and is now owned by 3D Systems. A CO2 laser melts and fuses, or sinters, powdered material (metal or plastic) layer by layer to create a durable, rugged solid object. Recent developments in SLS materials include the introduction by 3D Systems of Accura LaserForm ST-200, a specialty stainless steel composite that is said to produce durable and fully dense metal parts and tooling inserts in as little as three days. The material is suited for prototype and production applications.

Fused deposition modeling (FDM), developed by Stratasys, Inc. (Eden Prairie, MN), extrudes a thin bead of semi-molten plastic filament and deposits it, layer by layer, to build a prototype. Stratasys recently introduced an advance in the building platform of its FDM Titan that allows users to build models from high-performance engineering plastics such as polycarbonate (PC), ABS plastic, and polyphenylsulfone (PPSF), a flame-retardant material with high impact strength. By building with the actual material, such as polycarbonate or ABS, FDM permits more meaningful form-fit-function testing. "It's almost like finished parts that you can mill, tap and turn," says Patrick Robb, Stratasys product manager. "You can run the part for hours or days in, say, an engine."

Another interesting development is the introduction of 3D printing machines that combine inkjet technology with photopolymer resins. Accelerated Technologies Inc. (Austin, TX, and Erlanger, KY), is reportedly the first rapid prototyping service bureau in the United States to offer Objet Quadra rapid prototypes, said to build prototypes in layers 0.0008-inch thick. As inkjet heads deposit fine droplets of resin, a UV lamp immediately cures the deposited material, and the build platform is lowered by an amount equal to one layer thickness (0.0008 inch). Completed prototypes are said to have very fine detail and excellent surface finish.

Desktop 3D printing is the least costly and fastest RP method, but less accurate than higher-end, more expensive methods. Until recently, 3D printing equipment allowed only the form of a model to be tested, rather than fit or function. But last year, Stratasys introduced its Dimension 3D printer, a networked, desktop modeling system that allows users to build and test durable, ABS plastic models for fit and functionality. The Dimension is powered by Catalyst software (running on workstations using Windows NT or 2000), which imports STL files and automatically slices and orients the parts, creates necessary support structures, and plots a precise deposition path for the printer to follow. ABS material is fed into an extrusion head, heated to a semi-liquid state, and deposited accurately in layers as fine as 0.010 inch thick, according to Stratasys.

Another model, the 3D Systems ThermoJet printer, is about the size of an office copier; it employs hot-melt inkjet printing technology that deposits tiny droplets of wax to build 3-dimensional models layer by layer. Desktop printers from Z Corporation also rely on inkjet technology; they employ either a plaster- or starch-based powder-binder system.

Each RP technology has its limitations and strengths. According to Grimm, SLA offers good detail, accuracy, and surface finish, but falls short in strength and toughness. Selective laser sintering, on the other hand, offers durability and toughness while losing some accuracy and detail. Stereolithography is capable of producing 0.010-inch-wide features while SLS can replicate features of 0.025 inch.

Fused deposition modeling and SLS surfaces are said to exhibit rougher finishes than those with SLA. These parts can be smoothed with post-processing, such as sanding or use of solvents or adhesives. Grimm says FDM offers strong, tough prototypes of ABS with good detail, accuracy, and machinability.

Stereolithography requires adding a support structurethin ribs placed at 0.25-inch intervalsfor mounting the part while building it. After the build, the operator removes the supports, then hand finishes. Fused deposition modeling, likewise, requires a support structure to mount the part.

Rapid Tooling

Rapid tooling applies rapid prototyping techniques in creating molds and dies to reduce tool delivery and molding cycle times. A new RP technique accomplishes both rapid tooling and direct fabrication of metal parts. Solidica, Inc. (Ann Arbor, MI) has developed a direct-to-metal ultrasonic consolidation (UC) technology aluminum tooling process, which combines features of machining and rapid prototyping for producing functional parts. Its Form-ation 2030 machine combines ultrasonic material deposition with high-speed milling. The ultrasound merges layers of metal and creates metallurgical bonds, building tooling with feature-to-feature accuracy as fine as +/-0.002-inch in a large, 24-in x 36-in work envelope. The process eliminates the need to create a pattern or mold and then cast a metal part. Solidica is working on extending the technology to a variety of metals and applications.

Cast Metal Parts, Fast

Many designers and purchasers are surprised to learn how rapidly they can get high-quality, cast metal parts, according to Kristopher Klain, sales engineer at Prototype Casting, Inc., Denver, Colorado. One of the firm's customers recently had a requirement for a complex, cast aluminum housing with multiple post-machining operations. The catch was that he needed it in one week in order to meet his project deadline. Prototype Casting, using a modified rubber-plaster mold (RPM) process, shipped the order seven days after receiving the customer's 3D file.

Prototype Casting's RPM process is said to enable to firm to deliver magnesium cast prototypes in two to three weeks. In one recent example, an electronics manufacturer that had been a long-time customer required a thin-walled, complex magnesium casting with intricate post-machined features. The casting was towards the edge of Prototype Casting's capabilities, according to Klain, and required the firm to do some internal research and development with its proprietary process. "We were able to cast the difficult geometry they required and still had time to provide them with a couple of design iterations," says Klain.

The days of "one size fits all" manufacturing are gone, giving way to a future that promises mass customization, Klain says. As technology gives consumers more opportunities to order customized products, the amount of design work required will increase. This should, in turn, increase the number of prototyping requirements to be fulfilled.

"The very nature of prototyping presents unique challenges with each and every project," Klain says. "Many of the parts we see are cutting-edge technologies in our customer's particular industry; hence the need for prototyping and testing."

RP Service Providers

Prototype Casting, Inc., utilizes stereolithography to produce patterns for rubber tooling. The company also uses 3D printing technology for wax patterns of very small, intricate, high-tolerance parts. According to Kris Klain, the technologies give Prototype Casting a starting point from which to apply its expertise in manufacturing metal parts.

"Our rapid prototype process produces fully functional metal castings," says Klain. "Our prototypes are as similar to die-cast parts as can be achieved without actually using a die-cast production method, and require only a fraction of the investment needed for a production method. We provide parts in production metals, which increase the relevance of test data. We also provide our customers with full CNC machining capabilities and heat-treatment of parts."

In addition to advancements in rapid prototyping materials, the process used for procuring prototypes has been significantly streamlined, according to Mike Rufo of Design Prototyping Technologies (DPT, now 3D Systems Corp.). Implemented recently by DPT, the latest generation of web-enabled, on-line quoting/ordering allows users to obtain quotes for SLA, SLS, and urethane castings in real-time without uploading the files to DPT. Besides being easier and faster for the user, the system reportedly eliminates confidentiality issues by not requiring the files to be sent to DPT. Through the same interface, the user can send files to DPT to get quotes for injection molded parts and rapid metal castings.

"This capability affords the user the ability to modify parameters such as quantity, prototyping process, material, and finish level for a part or group of parts, and the price is updated in real-time," he says. "A user of rapid prototyping no longer needs to e-mail files to a vendor and wait for a quote; then ask for a succession of modified quotes to get exactly what they want." By combining rapid delivery capabilities with on-line quoting, DPT can put parts in the hands of customers two days after the customer placed the order.

W.W.F. Technical Services provides rapid prototyping services to companies in a wide range of industries, including semiconductors, medical, and advertising. The company also services inventors, engineers, architects, and land developers. "We've created scale model homes, office buildings that snap together, and even a replacement sprocket for a chain saw that is lasting longer than the original piece," says Walter Fedun, president.

"We also have the ability to show finished products in a rendered state, and fully animated," says Fedun. "This service our clients like because they can see how their assembly works and functions in real time."

Fedun says that W.W.F. Technical Services was once contacted by a customer who needed to make a presentation to management and marketing personnel four days later, showing the results of a new product line that was being developed. If the customer didn't have assembled parts on his desk for the Monday morning meeting, his job would be on the line, according to Fedun. The customer e-mailed 3D computer models of the assembly to W.W.F. Technical Services, which was able to make the required STL files and give the customer a fixed price quote and delivery date.

"We were able to produce, assemble the parts, and hand-deliver everything to the engineer Monday morning, on time," says Fedun. "The engineer started to assemble all the components for the appliance and (when finished) proceeded to plug it init worked. He brought the working assembly into the meeting, and management and marketing were happy." After deciding to go into production with the parts, the customer sent the SLS parts to an injection molding company that began creating the molds required. "I was told that the company had cut 10 weeks off their schedule," says Fedun. "Everyone was pleased with the final product we gave them and, as a result, we are doing almost all of their rapid prototype work."

Rapid prototyping capabilities of 3-Dimensional Services include stereolithography, selective laser sintering, laminated object manufacturing, and metal lamination. Its versatility enables the company to change and modify engineered parts quickly, and with flexibility. "Our systems minimize direct labor involvement and cost because they're unmanned during operation," says Peterson. The ISO 9001- and QS-9000-certified company provides a variety of manufacturing services in addition to rapid prototyping, from stamping and forming to casting, injection molding, laser processing, and machining. "We also provide design and engineering services, as well as assembly," adds Peterson. "We take part production beyond rapid prototyping and can provide prototype parts with manufacturing intent in mind."

Accelerated Technologies Inc. (ATI) offers stereolithography and selective laser sintering services, in addition to Objet 3D printing. The company utilizes some 18 rapid prototyping systems with more than 10 available materials.

Stratasys, Inc., is a manufacturer of rapid prototyping systems that are used for aerospace, automotive, military, consumer, and medical applications. The company's patented fused deposition modeling (FDM) process creates solid models directly from 3D CAD files using polycarbonate, ABS plastic, wax, or other materials. Stratasys also provides rapid prototyping services, including concept and functional models, silicone molding, and part finishing, upon request.

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