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© Dr. Des. Volker Ruhl
of such paraboloids, the voxels do overlap in transversal direction (see figure). The height of an epoxy paraboloid depends on the energy of the laser beam and the step period. A step period is the length of time, in hundred thousands of a second, that the laser beam remains in one place plus the time required to move it from one place to another. The larger the SP, the slower the drawing speed and thus the greater the depth of cured plastic (usually between 0.1 mm to about 0.5 mm). Is the hardening or curing depth of the polymerized resin greater than the defined layer thickness a vertical (conversal) overlap between the layers is achieved thus creating a three dimensional structure of interconnected epoxy paraboloids (see figure).
The chosen build style15 defines stepping distance, stepping period and imaging patterns which are traced by the laser over the vat. not only defines the measure of transversal and conversal overlap but also defines the imaging patterns These patterns range from a complete raster scan of overlapping spots, to spaced hatch patterns with deliberately unexposed areas (see figure). The part’s borders are drawn first and then the interior is scanned with any of several hatching styles. Usual hatching distance is 1mm, which is related with good utilization of the laser energy. The hatches can be chosen in angle of 60 deg., 120 deg. x,y-direction or any designed combination. The kind of build style not only largely determines the necessary imaging time, but is also largely responsible for the part’s mechanical properties as well as are an essential influence of the accuracy, especially in connection with shrinkage problems.
With this method it is possible to fabricate completely solid parts as well as honeycomb like, quasi hollow structures with an open hatch interior, as realized in 3D Systems QuickCast™ (see figure).16 This technique is only applicable to the laser-curing of liquid resin, because trapped resin material in the voids of the honeycomb pattern can drain out or be extracted after part building. Remaining captured resin between the hatched interior or not fully polymerized resin is then cured during postprocessing in which the so-called „green part” is exposed to UV light in a post-curing oven (see figure).
Laserlight merely cures thermosetting or thermoplastic photosensitive resins such as epoxy or acrilate polymers. Due to the fact that each cured material particle has to be scanned by the 0,2mm diameter wide laser beam in a point-to-point mode, the laser is limited to parts with low volume to surface ratio, meaning a small average scanning area per layer.
Restrictions to the laser-curing system
- specific clothing should be worn and safety recommendations need to be considered, because of the Imaging patterns which are traced by the laser over the vat. The part’s borders are drawn first and then the interior is scanned with any of several hatching styles. Usual hatching distance is 1mm. The hatches can be chosen in angle of 60 deg., 120 deg. x,y-direction or any designed combination. The skinn fill is a complete raster scan of overlapping spots on the parts surfaces to enclose all voids.
Selective curing methods offer the possibility of parts with up to 80 % of internal void. The trapped resin can be drained out at strategically set openings. In turn high-strength polymers may be injected into the voids for functional parts
The microsturcture of a laser cured part is a 3 dimensional mesh of interconnected polymer paraboloids. Transversal overlap and vertical staggering largely detetmine the parts mechanical properties as well as its shrinking, curling and warping behaviour