Direct digital manufacturing (DDM) is a hot topic lately and rightly so. The catchall phrase describes several different additive technologies that let users build almost any 3D object they can imagine. But what should you know in dealing with shops that provide these services? Of the many techniques involved, how do you know which is best, or even whether a firm is reputable? A few rapid-prototyping and manufacturing service bureaus provide helpful insights. Service bureau

Alpha Prototypes builds prototypes out of ABS plastic or a plasticlike resin. “We use fused-deposition modeling (FDM) and stereolithography (SLA), as well as a technology that makes powder-fused objects,” says Operations Manager Peter McVermott. “FDM uses ABS plastic, SLA an ABS-like resin. And Z Corp.’s powder-fused method produces hard but nonplasticlike objects that are most helpful for things like architectural models.

Basically, McVermott says customers should ensure designs are in a 3D format to easily get quotes. “In traditional manufacturing, the standard is IGES files,” he says. “Rapid prototyping (RP), on the other hand, uses STL. The difference between the two formats can be explained with the analogy of raster versus vector graphics. IGES files let users make parts bigger or smaller. All the details stay the same. STL files, in contrast, can vary in resolution because they are made up of triangular shapes that form part surfaces. In exporting to STL format, less-experienced users might generate parts with too many or too few polygons. Too few, and parts need rebuilding. Too many, and file sizes can get huge,” he says.

Therefore, users who provide files with the correct resolution save service bureaus a lot of effort and themselves some money, says McVermott. “For example, those using Solid- Works’ export feature can select an advanced option and fine-tune the STL file so its resolution lines up with the layer resolution needed to build the part,” he says. This layer resolution, in turn, depends on the kind of machine. “For example, our FDM machine uses a 0.01-in. build resolution,” says McVermott. “It would be pointless to produce STL files with any finer details because they wouldn’t show up in a build anyway. SLA and PolyJet parts have a standard resolution of about 0.006 in., but PolyJet can print features as small as 15 microns.”

FDM targets applications in which high resolution is unnecessary, so it doesn’t work well with, for example, parts having walls under about 0.04-in. thick. “The technique produces a fairly strong part with ABS,” says McVermott. “However, when the machine extrudes a layer, the plastic where the layers touch is at the melting point, so layers don’t really bond together solidly. Instead, what happens is almost a laminating process.”

Part walls can’t be too thin with FDM because of the way the process works, says McVermott. “Imagine building a tube vertically as compared to horizontally,” he says. “The horizontal tube has build layers that run through the full length of the tube. It would be a lot stronger than the vertical-built tube where build lines run through the cross section. FDM typically suits midsize parts anywhere from a couple of cubic in. to 8 cu in. Engineers can check FDM parts for form and fit and perhaps to do some functional testing. To make the parts suitable for trade shows would require a lot of sanding and painting. Also, parts that get bent will fracture rather than stretch, as would ABS,” he says.

Expect an education
One part of a service-bureau’s job is to identify its customer’s level of understanding, says McVermott. “Rapid prototyping is a fast-moving business and there are always new technologies coming out, so it’s difficult to keep customers informed,” he says. “We still get a lot of companies that have never used RP before. We have learned through experience to walk customers like these through every step so they get what they expect.”

There is a lot of variation among bureaus on costs, says McVermott. “For FDM, most shops charge per cubic in. A lot of companies making SLA parts charge about $30 to $40/cubic in. and that is about standard. Because the methods are additive, there are no molds involved, so there is not a huge cost savings to do volume runs. From FDM up to the most expensive parts, costs range from around $10 to $60/cubic in. The best thing to do is get competitive quotes. And be sure to insist on a nondisclosure agreement so uploaded CAD files stay confidential.”

Also note that RP can produce cast urethane parts, says McVermott. Alpha Prototype, for example, makes positive casts using SLA and then constructs a mold from the built part. Such molds are intended for about 12 to 24 parts, maximum. “It’s a cheap way to closely approximate injectionmolded ABS for functional parts,” he says. “For example, we recently cast urethane parts to which a medical-device manufacturer attached electronics and then demonstrated at a show as finished products. Cast-urethane projects usually cost around $3,000 to $6,000. Consider the method a step between tooling and injection molding, and the prototype. Usually, runs of about 100 parts and up warrants injection molding.”

Finally, it’s important to determine the turnaround time, says McVermott. Typically, a small RP part takes about 24 hr to build, so including cleanup and queue time, any RP part should ship within three days.

A few ways to get metal parts
Investment-casting bureau Conceptual Reality says it’s critical to know what you are trying to learn from your prototype. The firm focuses on RP for casting functional metal parts such as automotive interior components, door handles, rearview mirrors, consoles, and bumpers. “And it helps to understand material properties for layered manufacturing. For example, most SLA materials are made from epoxies, which can bend and then break,” says engineer Drew Brown. SLS uses mostly nylons and glass-filled nylons that can produce more-functional parts compared to epoxies because heatdeflection temperatures are higher,” he says.

With investment casting, the pattern is burnt-out and destroyed in the making of the part. The RP pattern gets sprayed or dipped in ceramic slurry. The object then goes into an industrial furnace and the part inside disintegrates. What remains is a ceramic shell. This is filled with metal to make the part. Breaking the ceramic mold frees the part.

Another method is to sand-cast metal parts. Here, the RP pattern is used to make what’s called rapid tooling for sand or plaster molds. “These molds are for casting aluminum or zinc. A set of rapid tools for sand casting provides 200 to 300 parts. Investment casting, on the other hand, would require making 200 to 300 patterns, which can get a little expensive,” Brown says.

Investment casting targets relatively complex parts such as turbine blades and manifolds. “In the past, the process required cutting a mold from aluminum, then injecting wax into the mold,” says Brown. “The wax pattern becomes what is buried in the ceramic. The more recent method eliminates the need for production tooling. However, at some point, it makes more sense to make a tool than it does to make a pattern for every part. If you need 10 pieces, it’s probably better to use rapid tools rather than spending money on a mold.”

Design considerations for casting include ensuring that features are thicker greater than 0.020 in. “Designers still need to know about traditional manufacturing methods,” says Brown. “It’s possible to rapid prototype anything, but that doesn’t mean it can be mass produced. Take injection molding, for example. RP could easily make a hollow ball, not something that could be injection molded. RP is typically used to verify that things fit together properly. The best way to proceed is to design the part for injection molding and then RP the design to test for fit,” he says.

A common misconception is that functional parts are those that can be directly used on an end product, says Brown. “In fact, functional parts are those that meet all the design criteria. A good example comes from a coffee cup that holds water. That is functional. But if the cup must withstand 1,000°F, it’s not really functional,” says Brown.

In some cases, RP can correlate or even replace FEA, says Brown. For example, when making, say, a new plastic wrench, a shop could make an RP part, turn a hard bolt, and see where the wrench breaks. The designer then knows where to put more material or ribs. In that case, the RP part can be considered functional because it provides a lot of design information.

“Last but not least, when it comes to selecting a quality service bureau, find out how long the place has been in business,” says Brown. “Our shop, for example, has been around for 12 years. Without quality service and quality parts, shops don’t survive long. Also, find out what the shop does in-house. We do all our own moldmaking and DDM, which lets us mix-and-match for the most effective solution.”

Choosing a broker or a bureau
Brokers outsource all their work and bureaus operate in-house machines, often along with some outsourcing,” says C.ideas president Mike Littrell. “Brokers and bureaus specialize in certain technologies, but bureaus rarely maintain every available technology in-house. For example, with over 15 FDM and two Objet Eden machines, we are the largest independently owned FDM and PolyJet bureau in North America. Because we don’t own every RP process, we often outsource projects that are best suited to another technology. A good RP firm chooses the right process in the customer’s best interest. OEMs such as GE and Motorola have used us for years because they have faith in our credibility and knowledge. When done correctly, outsourcing can be valuable because DDM requirements can require more than one technology. It can be tricky choosing a trustworthy bureau. At C.ideas we only work with sourcing partners that produce quality products and maintain good business ethics. We stick with these suppliers for consistency and quality.”

Litrell suggests asking a potential bureau how long it has been in business, what services it offers, and the age of its equipment. Also ask the RP firm to provide examples of successful projects it has worked on in the past. “Honest firms admit to outsourcing and also guarantee their work. Lead times can be critical, so if a current bureau consistently misses the mark, you should consider using a different source,” he says.

When querying bureaus and brokers, treat conversations like an interview, says Littrell.” “When talking with a local bureau, ask to see its facilities. Large companies and government agencies often take a trip to meet project managers and see the equipment. This weeds out dishonest companies that claim they own machinery that they in fact don’t.”

Beware of online quoting pitfalls, says Litrell. “Online quoting has become a widely used method of pricing prototypes, but there are a lot of downsides. When you take the human out of the equation, poor material choices, less than optimal orientation, and bad part quality (due to bad STL files not checked before running) can often result. Also, lead times are inaccurate and not tied to the company production queue. Our company manually quotes each individual project to determine the best process, part orientation, and realworld lead times. Price is important, but a cheap prototype from an online quote can cost more in time and aggravation than it’s worth. Talking to a real person allows discussing budgets, schedules, and even negotiating better pricing for long-term projects. These are valuable and important topics not available from an online program.”