Johnny Geling asked:
>Does anyone has experience in using a diode-pumped solid state to cure
Acrylate resins?
>
>In a system using diode-pumped laser the energy is not constant but
pulses of high energy reach de surface. The frequency of the puls >rate
is 20 kHz. In the past al acrylate resin were cured with laser with a
constant energy. So the figure and facts of building parts using a
>pumped diode laser could be different than using the clasical
configurations.
>
>I can imaging that a energy pulse is that high that curing is to deep
for a proper use of STAR-WEAVE and STAR-WEAVE-like building >styles. So
that free krimp not possible is.
The laser exposure process should be very similar to the cw laser if the
energy per unit volume is kept the same. The only difference is the
time over which the energy is deposited. So long as there is no
reciprocity effect, the photon efficiency should be the same. The
amount of polymerization that results depends upon the diffusion of the
monomer in the bath where the viscosity is increasing as polymerization
occurs.
There is one major benefit to the pulsed laser system. Research at the
University of Dayton has shown that the embedded stresses, and the
resulting curl/warpage, decrease as the length of the vector drawn is
decreased. As a result, if all others factors are equal, the best
dimensional accuracy and the flattest surfaces are obtained by exposing
the layer with a series of unconnected dots and then filling in with
exposure between the dots. When we proposed this exposure pattern,
circa 1990, we named it the "bed of nails." It is fundamentally
different than the WEAVE family. The bed of nails develops a pillar
that is connected to the layer below and the shrinkage is in the
vertical direction. WEAVE first exposes a floating layer which is not
connected, so it can shrink horizontally "without" generating stresses.
WEAVE's cross exposure then attaches the layer to the layer below.
Supposedly, there is so little liquid resin left the shrinkage is small
and the stresses are small. The bed of nails exposure was originally
generated using a cw laser and an acousto-optic modulator to turn the
laser ON/OFF in short bursts. The pulsed laser is better suited for
this type of exposure. The rate of travel has to be fast enough that
the laser moves more than one laser footprint (typically 250 microns)
between pulses (>5 m/s). this can be reduced by either focusing the
laser to a smaller footprint or reducing the pulse rate.
The pulsed laser system is available in two photopolymer based systems,
one from 3D Systems and the other from EOS. The exposure pattern they
use may either be a bed of nails or they may overlap the laser
footprints to more closely resemble the cw laser system. You will need
to talk with the companies. A summary of our studies of laser stresses
for various exposure patterns was published in 1994 -
J. Ullett, R. Chartoff, A. Lightman, J. Murphy and J. Li, " Reducing
Warpage in Stereolithography Through Novel Draw Styles," the Fifth
International Conference on Rapid Prototyping, Dayton, Ohio 1994.
Allan Lightman
University of Dayton Research Institute
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