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178 MICROSTEREOLITHOGRAPHY FOR MEMS

7.1.2 Stereolithographic System

All SL systems share the same basic elements or subsystems: the CAD design, layer-
preparation functions, and a laser-scanning or imaging system. A typical SL system is
schematically shown in Figure 7.7.
The CAD model is a 3-D representation of the object and is exported, most commonly,
into a file with a format called STL. Slice cross sections are produced from this file and
converging parts placed into a 'build' file. Process control software is then used to operate
the SL writing process according to the build file. Detailed information on CAD design,
STL file formats, part preparation convergence, contour slice, and so on can be found in
the references (Jacobs 1992, 1996).
A uniform, flat resin layer of the desired thickness is prepared for curing through
standard layer preparation. The liquid resin surface will be the foundation of each layer
of the SL model; therefore, the quality of the resin surface has a direct impact on the
accuracy of the final structure. Recoating blades, fluidic pumps, surface-finding lasers, and
robotic systems have all been considered to achieve a highly uniform resin surface. The
general requirements of the SL process are simple in concept, but their implementation
is quite challenging. The SL system should satisfy all the following requirements (Jacobs
1996):

• The resin surface must be maintained precisely at the focal plane of the imaging system
• The resin surface must be uniformly flat, leveled, and free of extraneous features
• The resin surface must be a controlled distance above the previously built cross section
of the part
The recoating and leveling systems work together and aim to achieve the desired goal
of providing a flat layer of liquid resin of proper thickness. A leveling system is used to
sense the resin height and permit adjustments. A level-finding system can vary from a

x-v scanner

2*0*
Photopolymer resin

Figure 7.7 Block diagram showing the basic elements of an SL system. From Jacobs (1992)