Stereolithography

FROM Mikell_P.Groover
fundamentals_of_modern_manufacturing

Stereolithography

This was the first material addition RP technology, dating from
about 1988 and introduced by 3D Systems, Inc. based on the work of inventor Charles Hull.
There are more installations of stereolithography than any other RP technology. Stereolithography (STL) is a process for fabricating a solid plastic part out of a photosensitive
liquid polymer using a directed laser beam to solidify the polymer. The general setup for the
process is illustrated in Figure 33.2. Part fabrication is accomplished as a series of layers, in
which one layer is added onto the previous layer to gradually build the desired threedimensional geometry. A part fabricated by STL is illustrated in Figure 33.3.
The stereolithography apparatus consists of (1) a platform that can be moved
vertically inside a vessel containing the photosensitive polymer, and (2) a laser whose
beam can be controlled in the x-y direction. At the start of the process, the platform is
positioned vertically near the surface of the liquid photopolymer, and a laser beam is
directed through a curing path that comprises an area corresponding to the base (bottom
layer) of the part. This and subsequent curing paths are defined by the STI file (step 3 in
preparing the control instructions described in the preceding). The action of the laser is to
harden (cure) the photosensitive polymer where the beam strikes the liquid, forming a solid
layerof plastic that adherestothe platform. Whenthe initial layeriscompleted, the platform
is lowered by a distance equal to the layer thickness, and a second layer is formed on top of
the first by the laser, and so on. Before each new layer is cured, a wiper blade is passed over
the viscous liquid resin to ensure that its level is the same throughout the surface. Each layer
consists of its own area shape, so that the succession of layers, one on top of the previous,
creates the solid part shape. Each layer is 0.076 to 0.50 mm (0.003 to 0.020 in) thick. Thinner
layers provide better resolution and allow more intricate part shapes; but processing time is
FIGURE 33.2
Stereolithography: (1) at
the start of the process, in
which the initial layer is
added to the platform;
and (2) after several layers
have been added so
that the part geometry
gradually takes form.
Elevator

greater. Photopolymers are typically acrylic [13], although use of epoxy for STL has also
been reported [10]. The starting materials are liquid monomers. Polymerization occurs upon
exposure to ultraviolet light produced by helium-cadmium or argon ion lasers. Scan speeds
of STL lasers typically range between 500 and 2500 mm/s.

The time required to build the part by this layering process ranges from 1 hour for small
parts of simple geometry up to several dozen hours for complex parts. Other factors that
affect cycle time are scan speed and layer thickness. The part build time in stereolithography
can be estimated by determining the time to complete each layer and then summing the times
for all layers.
>>>>>>>>>>>>>>>>>>
average scanning speed of the laser beam at the
surface, mm/s (in/sec); D ¼ diameter of the laser beam at the surface (called the ‘‘spot
size,’’ assumed circular), mm (in); and Tr ¼ repositioning time between layers, s.
In the case of stereolithography, the repositioning time involves lowering the
worktable in preparation for the next layer to be fabricated. Other RP techniques require
analogous repositioning steps between layers. The average scanning speed v must include
any effects of interruptions in the scanning path (e.g., because of gaps between areas of
the part in a given layer). Once the Ti values have been determined for all layers, then the
build cycle time can be determined:

3
Although these equations have been developed here for stereolithography, similar formulas can be
developed for the other RP material addition technologies discussed in this chapter, because they all use the same layer-by-layer fabrication method.

After all of the layers have been formed, the photopolymer is about 95% cured.
The piece is therefore ‘‘baked’’ in a fluorescent oven to completely solidify the polymer.
Excess polymer is removed with alcohol, and light sanding is sometimes used to improve
smoothness and appearance.

Depending on its design and orientation, a part may contain overhanging features
that have no means of support during the bottom-up approach used in stereolithography.
For example, in the part of Figure 33.1, if the lower half of the handle and the lower
handlebar were eliminated, the upper portion of the handle would be unsupported during
fabrication. In these cases, extra pillars or webs may need to be added to the part simply for
support purposes. Otherwise, the overhangs may float away or otherwise distort the desired
part geometry. These extra features must be trimmed away after the process is completed.