The next step in rapid prototyping

Aug. 21, 2003
Stronger materials and new build techniques are changing the way companies use rapid prototypes.

Jean Hoffman, Senior Editor

Engineers at Airbus quickly generate models for wind-tunnel tests. They use a System 7000 to produce accurate parts that assemble into nearly complete subsystems, such as the landing gear.


The front-end loader, made of about 60 ABS parts and off-the-shelf fasteners, was built by engineers at Stratasys Corp. The model has the same ranges of motion as a real machine.


The coffee-carafe prototype is made of PPSF (polyphenylsulfone), an RP material with a glass-transition temperature of 450°F. The warmer plate operates at 212°F. PPSF has the best heat, chemical resistance, and strength among RP materials. It is used by FDM Titan systems from Stratasys Inc.


The 10-part turbine and housing were built by a gas-handling company on a Z810 machine from Z Corp. The complexity shows one trend in the use of RP parts.


The electric motor windings, shaft, and fan provide an example of parts produced on machines from Objet Geometries.


The casting core for a hydraulic component was made of Z-Cast from Z-Corp. by technicians at Griffin Corp. Traditional core material would not have allowed the detail. The cutaway shows the passageways formed by the core.


Rapid prototypes are available in as little as 24 hr from E-mail a model and the online system provides a quote. Users can have parts built on SLA machines from 3D Systems, FDM equipment from Stratasys, and the Z-Corp. way.


The valve body was printed and assembled by Griffin Industries for spacing studies at an agricultural equipment manufacturer. It was printed on a Z Corp. 3D Printer and then infiltrated with Z-Max epoxy to strengthen the part for drilling and tapping. As a finishing touch, engineers treated the part with a powder-coat paint, a spray on dry-powder paint. Static charge makes the paint stick to the part. Finally, they bake the part to create a hard, shiny finish.


Equipment from Stratasys uses fused-deposition modeling (FDM) in which thin layers of hot material are fused together. The equipment builds parts of ABS, polycarbonate (PC), and polyphenylsulfone (PPSF). Multiply values in the first four rows by 0.75 to estimate FDM properties.

Builders of rapid-prototyping equipment have done a good job publicizing the value of parts made on their machines. After holding an RP part you've designed, it takes only seconds to tell whether or not it's big enough or shaped right. Handling the part tells right away what should be done next.

But RP parts can also be assembled into nearly complete products. To some extent, that unsung capability comes from RP equipment that builds several connected parts and materials that closely approximate the strength of production plastics. Stratasys Inc., Eden Prairie, Minn., for example, has been handing out adjustable wrenches made of three parts built simultaneously. And Z-Corp., Burlington, Mass., has passed out full-complement ball bearings made on their equipment. There is more to these simple assemblies than meets the eye.

Fast and cheap

Several recent build improvements deserve mention. “Revisions to the software that builds RP parts improve the surface resolution so finishes are more acceptable right off the machine,” says Jerry Plath, an engineer at Prolific Plastics, a tooling and service bureau in Opelika, Ala. “Light sanding improves it further so it doesn't look layered at all,” he adds.

In addition, the newest equipment uses two materials, one for parts and one to support overhangs and undercuts. “The problem used to be with supports breaking,” says Plath. “When working with small parts, I had to be careful removing support plastic so as not to break the parts. But the support material is now water based, so a simple wash dissolves it. That makes final assembly easier,” he says.

Build tolerances have also improved to as tight as 0.001 in. That's better than the tolerances on production parts which are often ±0.005 in. What's more, the machines can make snap fits to aid in assembly.

Along with equipment and material upgrades, savvy engineers like Plath understand and preach the value of seeing design shortcomings in solid plastic before committing scarce dollars to production tooling. “We've become the plastics department for these companies we service,” he says.

Plath uses RP equipment for more than quick parts. For example, he doesn't charge a company for a prototype unless it's quite complex. The reason? “An RP part is more than an initial shape. Parts made on a Stratasys Dimension machine, for instance, cost about $6 to $7 per cubic inch. But when I show a part to a toolmaker, we look it over and identify features that will be difficult to shape and those that will be easy, and plan accordingly. Furthermore, I get a part weight, which helps in quoting. And after talking to the toolmaker, I can go back to designers with suggestions, such as, If you move this hole, we can save so much money in tooling,'” he says.

A few other RP benefits include:

  • Trimming costs. “Companies tend to see their products as a series of parts. But any good molder knows that if you've got six parts, the question to ask is: How can this be done with three? Reducing part counts trims production costs,” says Plath.
  • Avoiding assembly steps. When building assemblies, stronger RP materials can be drilled and tapped. But if you do that, design threads into the part and the machine will make them. This avoids secondary operations.
  • Building scaled parts. “You don't always need full-sized parts,” adds Plath. “For example, a golf-cart maker wanted me to build a bumper. This is a 3-ft-long part. To get a feel for the proportions, I reduced the part to 25% of original and built that. So instead of building an assembly the size of your desk, scale the part to more manageable dimensions to see if it will work,” he says.

Rapid cores

Engineers at Griffin Industries, a pattern and casting company in Green Bay, Wis., are pushing the manufacturing envelope by printing ceramic molds instead of just patterns. The process, called Z-Cast, was a cooperative effort between Z-Corp. and Griffin engineers. In a traditional casting, an RP pattern made of wax would be dipped in ceramic slurry which would harden. Melting the wax creates a hollow casting mold. “The Z-Cast method eliminates wax patterns and builds the casting mold using one of two materials,” says Spike Chronoski, an engineer with Griffin. “One is for nonferrous materials and the other, still under development, will be for heavier and hotter ferrous metals.”

Chronoski says his company is dedicated to fast turnarounds using several newer manufacturing methods. For example, they build complex cores for hydraulic tool bodies. Traditionally, these have been made in a cast-iron core box. This method includes designing air-ejection paths, rigging, and lead time. “And costs are significant,” says Chronoski. “But when we take a day to RP a core instead of making it in a core box, the whole casting, inside and out, can be done in six weeks instead of 12. And when we push ourselves, the job's finished in just three weeks.”

Accurate RP parts tell more in wind tunnel tests

Airbus in the United Kingdom plans to use RP components for wind-tunnel testing at its operations in Filton, Bristol. They recently purchased an SLA 7000 system from 3D Systems, Valencia, Calif., to make larger models with tighter tolerances and higher quality surface finishes than previously possible.

“We were already using RP technology through a service bureau,” says Martin Aston, wind-tunnel manager for Airbus. “But if we can build models more quickly, we can enter design cycles sooner, which gives aerodynamicists more time before committing to a test. Rather than outsourcing prototyping and model production, we needed to invest in our own to improve cycle times.”

Having an SLA system on-site lets the company react more quickly and more cost effectively. “We now try designs we never would have done before because the technology is on-site. Design ideas can be built overnight or over a weekend and tested the following day. If they don't work we repeat the process. Typical turnaround from a service bureau is four days,” says Aston.

Producing plastic models lets the company explore different shapes and curves on aircraft components, such as leading and trailing-edge configurations, flap tracks, pylons and nacelles. Improved geometric forms and dimensional stability means test data is more accurate. And having technology on-site lets project coordinators define and control geometry.

Thin layers produce smooth surfaces

RP machines from Objet Geometries Inc., Mountainside, N.J., uses really thin layers - down to just 16 microns. Eight jetting heads slide back and forth on the X axis depositing layers of photopolymer onto the build tray. UV bulbs alongside the jetting bridge cure and harden each layer as it's placed, eliminating additional postbuild curing.

The system works with two different materials, one for parts and one for support. Geometry of the support structure is preprogrammed to cope with complicated shapes, such as cavities, overhangs, undercuts, and delicate features. And positioning the support material needs no special programming.

The developer says any CAD model can be converted to an STL file for its Eden system. A preprocessing program suggests a build orientation, but users have full control over the build process. The technology lets the machine build several models in the time it takes other technologies to produce one. To shorten build times, the machine places the shortest dimension in the Z direction to reduce the number of layers.

The systems are safe for offices because model and support materials (proprietary photopolymer resins) are environmentally stable. Materials come in 4.5-lb sealed cartridges and cure to transparent and gray. The developer says running the Objet Studio Software is simple and intuitive so training isn't needed. Models are fully cured and can be examined and handled immediately after completion.

The material used in machines from Objet Geometries has a 20% elongation at break that lets it form snap fits. Materials are fully cured and need no postprocessing. And model surfaces readily accept paint.

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