The process that builds parts with a laser beam.
We now have two FDM modelers (Dimension) and three PolyJet printers (Objet). Read about the rapid prototyping machines here.
There are many types of rapid prototyping machines and methods. At Omnica we use the CNC (computer numerically controlled) machinery method, which can produce “real” parts made from the actual engineering materials. However, when we need fast turn-around on complex shapes or models to be used strictly for appearance purposes, and we can't print them with one of our five RP machines, we sometimes contract with a service bureau for stereolithography (SLA) parts. There are at least ten different types of proprietary rapid-prototyping machines costing between $30,000 to $500,000 each. We choose an SLA process based on most the appropriate “build” material, and our design requirements. Generally, the machines built by 3-D Systems, first introduced in 1987, are considered to offer the best all-around combination of materials and part-building method.
Concepting with CAD
When we concept designs, first we create a CAD (computer-aided drawing) solid-model, which is a computerized 3-dimensional virtual object. When the concept is finalized, we save it as an STL file. That file is then electronically sent to an SLA service. Their proprietary software virtually “slices” the three-dimensional STL file, so the solid model is converted into thin (about the thickness of a human hair) horizontal sections stacked on top of each other. (Imagine a solid block of cheese that has been sliced into thin sections.) The stereolithography machine is now ready to read the modified file and physically build an actual three-dimensional model.
Building the stereolithography part.
The STL file, interpreted by the closet-sized, computer-controlled machine, guides the motion of a motorized ultraviolet laser, which is suspended above a pool of photosensitive liquid. The laser beam rapidly scans back and forth, “drawing” the shape of the first thin section on the fluid. It hardens to a depth of about 0.003” only where the beam strikes. Underneath, that section is supported by a metal platform. The platform and the cured section are then lowered below the surface of the pool until it is covered with fresh liquid, and the laser process is repeated. The laser continues to draw and build the layers. The platform and the hardened model move down, deeper into the pool of uncured fluid. Eventually, the submerged, stacked sections resemble the original solid CAD model. If, for example, the laser traced a series of smaller and smaller squares on top of each other, the built-up solid-model would look like a pyramid.
When the laser-writing process is over, the platform and the newly created object are raised from the depths of the pool. The solid object, now referred to as an SLA model, is cleaned, and finally cured in a UV chamber. The service bureau delivers it to us, and we inspect, and hand-detail the model. Next we give it to Andy March (now fifteen years with Omnica) who gives it a nice paint job. When it is completed, the SLA model looks just like the original computerized concept. Start to finish, from concept to handheld model, the process can take as little as three days.
A real advantage of stereolithography is that design complexity is not an issue. The only limitation is the resolution of the laser beam, and the thickness of the stacked layers. Build time, cost of material, and ever-increasing competition determine part costs. We can sometimes save money if we specify thicker (therefore fewer) sections. Part prices range from a couple hundred dollars on up. Over the years, competition between rapid-prototyping methods and SLA service bureaus have caused prices to drop dramatically. Part sizes can range from pea-sized to as large as a basketball.
Rapid prototyping with stereolithography is fast and relatively accurate, but the models generated by this method are usually not very robust (but much better than they used to be). They are excellent for appearance models for silicone molds, master-casting patterns, or for part fit verification.