Better Be Running! Tools to Drive Design Success (part 2)

This is the second in our series of excerpts from “Better Be Running! Tools to Drive Design Success” by Dr. Ronald Hollis, President, CEO, and Co-founder of Quickparts.com (Atlanta, GA). Written for business managers, the book focuses on manufacturing processes, tooling choices, and production strategies that can help companies bring products to market faster. To order the book, go to www.betterberunning.com.

Stereolithography: The Lion’s Share of Rapid Prototyping

Stereolithography (SL) means to print in three dimensions. SL is the first and most popular liquid-based additive fabrication (AF) system that produces plastic parts from cross-sectioned CAD data. Electronic CAD design data is converted to an STL (Standard Tessellation Language) file format. Special software slices the CAD model into thin layers and creates build instructions for the machine. Layer by layer, the Stereolithography Apparatus (SLA) machine replicates a plastic physical model out of photo-curable resin. The resin turns into hard plastic whenever touched by an ultraviolet laser.

Background

SL allows you to create a 3D plastic object from a CAD model in several hours. Prior to this technology conventional prototyping methods could take days or even weeks. Whether you are a design engineer wanting to verify your concept or a manufacturing engineer needing form, fit, and function feedback, SL gives your team a quick, accurate way to convert virtual data into real objects. It allows you to test designs in their physical environment before committing to expensive tooling.

Stereolithography was invented by Charles Hull and made commercially available by 3D Systems, Inc. in 1988. Because 3D Systems was the first to market the SLA machine, many folks frequently misuse the term SLA to generically describe all rapid prototyping techniques and any liquid-based, UV AF process. One example of a process that produces parts similar to SL is made by Objet Geometries, Ltd. Objet machines produce parts using a technique known as PolyJet. This technique jets or sprays a photopolymer resin instead of using a vat of the resin and solidifies this resin with a UV bulb instead of a laser beam. The parts are similar to those made by SL but have much smoother surfaces. Since 1988, over 40 AF systems have entered the worldwide market.

Step by Step with SL

First, an operator loads the STL file from your CAD data into proprietary software, which digitally slices the model into thin layers of approximately 0.005 inch (five thousandths) and produces a removable, stabilizing structure to support the part during the build. Next, the physical process begins with a vat of photo-curable liquid resin and an elevator table in the vat, set just below the surface of the resin.

A computer-controlled optical scanning system directs the focused laser beam so that it solidifies the 2D cross section corresponding to the slice on the surface of the photo-curable liquid resin. The laser’s depth of penetration is greater than the desired layer thickness, and is known as overcure. Overcure plays an important role in producing solid SL models and also affects the build time.