By Shea Gibbs, Managing Editor

Rapid prototyping (RP) options change rapidly. New technologies become available as quickly as old ones become obsolete. But there are a few processes that never seem to go away. So, depending on the casting characteristics you desire, it’s a good idea to have a handle on the primary ways to develop a prototype when you need to try out a cast component in your design yesterday.

Pick a Process

Several methods exist to create a workable prototype, but not all of them produce an actual casting. Two industries are at work in the process—rapid prototyping and rapid manufacturing—but not all RP results in the creation of a manufactured part, and not all rapid manufacturing starts with the development of a prototype. The two fields do cross paths, though, and the results can be a quickly produced, cost-efficient casting—if you are knowledgeable about the casting capabilities of the various prototypes.

One of the most common ways to receive a casting quickly is to start with a plastic pattern, which is printed using a computer data file. Three processes remain the standard in the creation of a plastic pattern—stereolithography (SLA), selective laser sintering (SLS) and fused deposition modeling (FDM). All boast advantages and have drawbacks owing to the different materials they use and ways in which they build parts. However, all three of the processes work on the same premise—building prototypes layer by layer, caking plastic on top of plastic until the piece is complete.

Built in Stereo

SLA allows one to create solid, plastic, 3-D objects from computer-aided design (CAD) drawings in a matter of hours. The process uses an ultraviolet (UV) laser to create successive cross-sections of a 3-D object within a vat of liquid photopolymer. First, a metal platform is placed on top of a vat filled with polymer (an epoxy resin). Before the build begins, the platform is moved to a point just below the surface of the resin. As the UV laser traces the layer in the polymer, the resin begins to cure on top of the platform, solidifying the part to be manufactured. When angled parts are constructed in this way, SLA builds rely on computer-designed support structures to maintain their structure.

SLA generally is considered the RP technology that provides the greatest accuracy and best surface finish. The CAD model can be divided into cross sections between 0.002 and 0.006 in. (about 0.005 and 0.015 cm), which reduces “stepping,” the appearance of stair-like transitions between layers.

The material used also can be the least expensive. Because the structure of an SLA part is 85% hollow (owing to its honeycomb-like makeup), the machines are the most efficient when it comes to material use.

"I have seen people quote [SLA machines] as low as $0.39 per cu. in. for pure material,” said Paul Miller of 3D Systems Corp.

Stereolithography most commonly is used to make parts through investment casting, as the prototype (top) can be attached directly to an investment tree (bottom).

However, finished SLA parts do have their downsides. The parts can be brittle (though they have made considerable advancements over the past few years), and they can warp over time. Their surface finish, while smooth, is also somewhat tacky, as some of the material does not cure completely. Uncured SLA material can be toxic if inhaled, so ventilation is necessary when working with the process.