Devil’s in the Details

How to get the most from a rapid prototyping service bureau

Comments Off on Devil’s in the Details September 16, 2008
Mike McLeod

Since the advent of rapid prototyping roughly 20 years ago, companies like ZCorp, Stratasys and 3D Systems have striven to bring the cost of their equipment down to a level affordable to a wide market. Today, machines once priced in the hundreds of thousands are approaching the sub-$10,000 range. Still, spending thousands on a machine that may only be used occasionally doesn’t make financial sense to most.

Luckily, rapid prototyping (RP) service bureaus provide many of the benefits of ownership at a fraction of the cost. More importantly, they can help customers weave through the complex process of prepping their designs for output, choosing the right RP technology and even save money by fixing faulty CAD files or recommending design changes that will shorten build time. On the premise that an educated customer is the best customer, the following are some general guidelines on how to get the most from a RP service bureau.

Determine the model’s end use
It might sound obvious, but the first thing potential customers need to consider before approaching a service bureau is to think through exactly what purpose their model will serve. Of the three general technologies that encompass most of rapid prototyping—Stereolithography (SLA), Fused Deposition Modeling (FDM) and Selective Laser Sintering (SLS)—each has its advantages and disadvantages and require trade-offs to be made to find the right solution for the task.

“Which technology you go with is generally a question of how functional the model needs to be,” says Garry Campbell, president of Toronto-based Nova Product Development, a 15-year-old service bureau that offers all three technologies. “Is the model something you want finished for a trade show or a mock up of to show a client? Is it strictly something to check a fit or does it need to be dropped on the floor for impact testing?”

A common exhibit at manufacturing trade shows, SLA is probably the most familiar. SLA prototyping machines lay down hair-thin layers of liquid resin, one after another, until the part is formed. For those areas of a prototype that need temporary support while the part builds, SLA machines use either the same material as the prototype or a soluble gel that can be washed away after the resin is set by UV light.

The primary upside of SLA is its accuracy and ability to resolve fine detail. Layers in a typical SLA process measure 0.006 inches thick, but newer machines can attain 0.004 or less. Given this, Campbell says SLA is most appropriate for projects that require a high level of accuracy or need to look close to a finished plastic. Master moulds, for instance, are particularly well suited to SLA, as are tradeshow display models. Since SLA resin is relatively soft, the “stair-step” look common to all RP models can be sanded away. In addition, SLA machines can create multiple parts in the same build without adding substantively to the time it takes to create one part.

SLA’s downside is that its parts are more fragile than those made with SLS or FDM. And since it’s UV cured, sunshine and other light will continue to cure the resin. As a consequence, parts can become brittle and may not retain their accuracy over time. However, he says SLA build material has improved considerably over the past five years and is now much less susceptible to breakage or deformation.

For those who need a prototype that can be drilled, tapped or drop tested, FDM models are made from extruded ABS plastic that can be filled and finished to simulate the look of the finished piece while remaining strong enough to hold up under real world use.

“The selling point of FDM is that it can be installed and used as if it were a final part,” Campbell says. “And with the introduction of the new ABS plus material, which appears to be an ABS/polycarbonate blend, it’s even stiffer and more durable than standard FDM.”

On the downside, FDM machines offer lower resolution and don’t typically build more than one or two parts at a time. Still, for projects that require only a few parts with near-manufactured functionality, FDM may provide the most cost-effective solution.

Offering the best of both worlds in some respects, SLS builds the strongest parts at resolutions comparable to SLA. In this process, fine layers of nylon powder are laid down before a laser melts only those areas that compose the final piece. Any unmelted powder remains as support material that’s simple to remove. SLS machines can build multiple objects nearly as quickly as SLA while providing parts that, in some cases, can be used as final products.

“SLS is as close as you can get to a nylon injected piece without the expense of making moulds,” says Annette Kalbhenn, sales manager of New Hamburg, Ont.-based 3D Prototype Design Inc.

In addition, SLS isn’t restricted to one type of build material, says Richard Smeenk, general manager of Uxbridge, Ont.-based Agile Manufacturing.

“SLS has the broadest spectrum of material properties or types,” says Smeenk, whose company offers many RP options including SLS. “Material types can range from the flex material in a running shoe sole to a glass filled nylon which is very stiff.”

Keep manufacturability in mind
Once the model’s intended use is determined, the next step is to analyze its manufacturability, says Joe Van Pelt, president and owner of Pontypool, Ont.-based JVP Fabrication & Design. CAD is a great tool, he says, but too often allows designers to create idealized parts that can only exist on screen. “Just because you can draw it, doesn’t mean it can be made,” he says. As a tool and die journeyman for the past 40 years, as well as a mechanical designer, he can quickly spot problem areas and help clients rework their files to ensure a successful build and reduce costs.

With that in mind, he says designers need to check to make sure that all walls are at least 0.02 inches thick; that there is enough material around holes to sustain fasteners; and that holes are drill size so they can be tapped. In addition, he recommends people look for areas that don’t need to be solid, as a hollow section can be made as sturdy but without the extra material and build time cost. Pockets and angled walls will also up the costs because they often require additional support material.

“If you have an overhang that needs support but you don’t need the overhang, then why include it?” VanPelt asks. “If you are just doing it to be funky, get rid of the funky because it costs money.”

Another thing to keep in mind, Agile’s Smeenk points out, is that although RP technologies are becoming increasingly capable of producing minute details, they have their limits. For instance, SLS and FDM can’t reproduce standing features well below 0.030-inch. However negative features like holes and slots can easily be made down to 0.010-inch. SLA does not resolve standing features below 0.020-inch with the exception of high-resolution SLA, which can resolve positive features down to 0.010-inch and negative features like holes and slots down to 0.003-inch.

Provide high-res CAD files
No matter which process is chosen, RP machines accept STL files exclusively, so creating a high-resolution file is critical to producing parts with smooth curves, round holes and fine details. As a first step, designers should specify whether the STL files are in inches or millimetres and output all files in the same units. Beyond that, each CAD package has its own export procedures and options. For detailed instructions, check out the online resources for tips on proper STL file creation.

Even with these guidelines, bureaus warn that quirky CAD packages, modeling shortcuts and general geometry mishaps can all contribute to a faulty STL file. Common problems include surface gaps, overlapping/missing surfaces, reversed normals and intersecting triangles. Given how tricky STL files can be, most bureaus appreciate receiving native CAD, IGES or STEP files, as well. That way, if there is a problem with the STL file, they don’t have to put a project on hold while they wait for more materials. Typically, says Agile’s Smeenk, RP bureaus have specialized software that highlights and repairs common problems automatically.

“If that doesn’t fix it, and it often doesn’t when there are major problems, a technician on staff has to bring the IGES or STEP file into a package like SolidWorks so he can use surface repair tools to close the gaps and re-output the STL file,” he says. “That’s why we always want good STLs from our customers; we don’t want to have manipulate their data.”

Confidentiality is a Given
Finally, with all that CAD data in a third party’s hands, some companies may be concerned their proprietary product data will be compromised. But according to 3D Prototype’s Kalbhenn, keeping customer data safe is central to a bureau’s business.

“We take confidentiality very seriously,” she says. “We will sign an non-disclosure agreement no problem, and I’d be leery of any company that wouldn’t. If you are in the RP business to stay, then every customer you have is doing something that confidential. After all, nobody makes a prototype of something that already exists.”