For those interested-
As an Applications Manager for the DTM Corporation, I am obviously
quite interested in any rp-ml discussion of the Selective Laser
Sintering process. Perhaps you will allow me to clarify the recent
discussion of sintering vs. melting.
First of all, let me point out that in the strictest technical sense
of the word, our process is not true sintering. As I believe the
gentleman from Japan was trying to point out, true sintering takes
place at some temperature below the melting point of the material in
question. Adjacent particles are actually joined when their surfaces
become soft due to the effects of heat and pressure. Since there is
no pressure involved in SLS, you can see how we have a slight
misnomer.
Now, as to that porosity business. Most SLS materials yield a part
that is somewhat porous, and not just on the surface. This occurs
because, while the material is melted during the process, it does not
necessarily flow with ease. This means that while adjacent particles
may soften, swell, and then touch, the resulting union may not be
without gaps. Just how large the gaps are depends upon how well the
material flows in the small amount of time between being struck by the
laser and cooling to hardness. How well the material flows is in turn
dependent upon its' molecular weight and structure. More
specifically, density correlates with material approximately as
follows:
Fine Nylon- 100%
Glass Filled Nylon- 90-100%
Polycarbonate- 70%
TrueForm- 70-90%
Wax- 100%
RapidSteel (after copper infiltration)- 100%
As you may have deduced, each particle in a part will retain something
of its' original shape. This is one reason that SLS parts have a
"grainy" appearance before finishing. The other reason is that a few
non-melted particles that are adjacent to particles which are actually
struck by the laser will be fused to the part surface due to latent
heat effects.
About metals. The metals process is slightly different than that for
the polymeric materials since the laser energy provided is
insufficient to actually melt steel. In this case, each metal
particle (a sphere) is coated with a polymer. When the laser strikes
the material, it is this coating that melts and fuses to the coating
on neighboring particles. The steel particles are not directly fused
to each other until the part is taken through the "brown" stage of the
furnace process. At that point it is essentially a metal sponge,
which is then fully soaked with molten copper, becoming 100% dense.
Finally, it's still an .stl file. The minimum and maximum layer
thickness are material dependent, not machine dependent, and are
therefore unchanged. The laser is still a 50 watt carbon dioxide
tube.
I apologize for the length of this reply, but I hope it helps to clear
up any confusion on this subject.
Sincerely,
Will Pattison
Applications Manager, DTM Corporation
This archive was generated by hypermail 2.1.2 : Tue Jun 05 2001 - 22:37:41 EEST