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Note:
Since this article was published, we purchased three FDM modelers and two PolyJet printers. 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,
we often 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.
Stereolithography advantages
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 OK for appearance models for silicone
molds, master-casting patterns, or for part fit verification.
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