Rapid Prototype Manufacturing Review

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Materials & Resolution
3D CAD File Conversion

General

Rapid prototyping is the method for the creation of parts derived from 3D CAD data. The derived part is decomposed into slices (down to 0.003 inches thick), and SLA, STL builds the part a layer at a time. The contour and interior of a slice are created using a laser to solidify liquid polymer.

The first step is to translate a CAD file into surface model. The current industry standard format for this data is called STL. Most industry CAD packages have built in translation for this type of file. The file is then sliced into many layers like a stack of playing cards. This is known as an slice file or SLI. These SLI files are loaded digitally into the machine which drives the motions of a laser. Parts can be expensive, driven by the 4-24 hour build times and the expensive materials. One consideration is the brittle nature of the SLA model. Care should be taken in handling the produced parts. I have found that the rapid prototype process very helpful in concept, fit and assembly review during the design and development process.

Design

Due to the nature of the process, rapid prototyping is a very easy process to design for. The primary difficulty lies in avoiding large, thin sections which tend to warp while curing. Trapped volumes should also be avoided. To overcome a trapped volume, there are techniques available, but these can impact build times, quality and design intent.

• Fill trapped volumes with the CAD model before file conversion.

• Change the geometry. The part can be built in sections This procedure will eliminate the trap, but once again, it can affect quality due to the bonding of parts.

• The part can be built with an manufacturing hole to the trapped volume. This will eliminate the trapped volume but, add unintended geometry to the prototype.