Quick Start for Rapid Prototyping

Nov. 15, 2002
The ability to construct accurate, easily modified models is helping make manufacturers more competitive.
The ability to construct accurate, easily modified models is helping make manufacturers more competitive.

Stereolithography, photochemical machining, laser sintering, and laminated-object manufacturing use 3D CAD data to produce models in hours. Most of these processes make parts from plastic. Models can be built from layers of liquid plastic, fused from plastic powders, or cut from partially cured polymer.

Though large manufacturers increasingly have rapid-prototyping capabilities in house, smaller firms generally work with service bureaus to obtain fast prototyping. Here are some things to keep in mind when working these outside vendors.

CAD drawings: What you send the service bureau ­ 2D drawings, CAD files, or STL files ­ determines the amount you pay in up-front processing costs. And these costs could vary dramatically depending on the CAD program the bureau uses to make your data machine ready.

For example, sending only 2D drawings of a part to be fabricated forces the service bureau to create the solid model from the prints. For relatively simple parts, it will probably take about as long to create a solid model with one CAD program as it would with another. But the situation changes dramatically as parts become more and more complex. Some CAD programs are just faster to work with than others. Feature-based or variational geometry modelers such as Pro/Engineer can usually generate models much more quickly than modelers based on Boolean operators. The difference in modeling time becomes more pronounced in complicated models that incorporate features such as sculpted surfaces, numerous bends and radii, and so forth.

Most service bureaus charge by the hour to create a solid model from prints. So the longer it takes them to create a model from drawings, the higher the cost. Medium-complexity parts might take eight to 12 hr of CAD time. Simple parts, from a half-hour to a few hours.

Surface models: Many surface modelers generate STL files. But for the few surface or wire-frame modelers that can't generate STL files, the service bureau often ends up creating a solid model from scratch, even when provided with a perfectly good surface model on disk. Starting from scratch is often easier than converting a surface model into a compatible format and making the necessary modifications.

Solid models: When a customer sends a solid model that has not been generated with the same brand of CAD program as used by the service bureau, there must be a conversion into a compatible format through an IGES transfer. The conversion process tends to be imperfect. The service bureau will still be forced to clean up the solid model before generating the STL file for fabrication instructions. This cleaning-up process typically involves adding features that sometimes get lost in the IGES translation, such as surface normals or information about certain kinds of radii.

Thus, it is good to ask the service bureau how cleanly it has been able to translate models generated by the CAD software used to generate the math models they will receive.

STL files: STL files created by most major CAD systems execute without any glitches. A few off-brand CAD programs do indeed create STL files that have problems, however. These problems typically consist of gaps on surfaces or areas where the fabrication software cannot identify the surface. When this happens, the service bureau typically goes back into the model and patches up these areas, then recreates the STL file.

Molds: Most RP parts made today are prototypes of molded components. Parts in this category are best fabricated by service bureaus that also have some experience in molding and, most do.

RP bureaus with molding experience can often provide advice about design factors such as adding drafting to the part. If the original model doesn't have draft, a sufficiently experienced bureau often can add this to the model. If one area of the part has particularly tight tolerances, they can also take this into consideration when they build the prototype by adjusting factors such as part orientation during the build or the shrink rate of the material.

Service bureaus generally shoot for a tolerance of ±5 mil/in. of part, but they sometimes can get this down to 1 to 3 mil/in. for special features.

Machining: A service bureau that has experience with machining will be able to give advice about the trade-offs for either machining prototype parts, or building them stereolithographically or with some other RP technology. Large part size and simplicity generally dictate a machined approach.

Pricing: Price-wise, parts small enough to fit in your hand should cost between $500 to $1,000, depending on the up-front processing needed. A part that fits in a 1-in. cube would normally be in the $200 to $300 range. For something inside a 6-in. cube, figure up to $1,500.

For RTV molds, a single-sided mold with a flat bottom might cost between $200 to $300. That figure would rise for medium complexity parts to $400 to $1,000.

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