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SLS© vs.
SLA Comparison
Material Properties:
The SLA (stereolithography)
process is limited to photosensitive resins which are typically
brittle. The SLS® process can utilize polymer powders that,
when sintered, approximate thermoplastics quite well.
Surface Finish:
The surface of an
SLS® part is powdery, like the base material whose particles
are fused together without complete melting. The smoother surface
of an SLA part typically wins over SLS® when an appearance
model is desired. In addition, if the temperature of uncured
SLS® powder gets too high, excess fused material can collect
on the part surface. This can be difficult to control since
there are so many variables in the SLS® process. In general,
SLA is a better process where fine, accurate detail is required.
However, a varnish-like coating can be applied to SLS® parts
to seal and strengthen them.
Dimensional Accuracy:
SLA is more accurate
immediately after completion of the model, but SLS® is less
prone to residual stresses that are caused by long-term curing
and environmental stresses. Both SLS® and SLA suffer from
inaccuracy in the z-direction (neither has a milling step),
but SLS® is less predictable because of the variety of materials
and process parameters. The temperature dependence of the SLS®
process can sometimes result in excess material fusing to the
surface of the model, and the thicker layers and variation of
the process can result in more z inaccuracy. SLA parts suffer
from the "trapped volume" problem in which cups in
the structure that hold fluid cause inaccuracies. SLS® parts
do not have this problem.
Support Structures:
SLA parts typically
need support structures during the build. SLS® parts, because
of the supporting powder, sometimes do not need any support,
but this depends upon part configuration. Marks left after removal
of support structures for parts cause dimensional inaccuracies
and cosmetic blemishes.
Machining Properties:
In general, SLA materials
are brittle and difficult to machine. SLS® thermoplastic-like
materials are easily machined.
Size:
SLS® and SLA parts
can be made the same size, but if sectioning of a part is required,
SLS® parts are easier to bond.
Investment Casting:
The investment
casting industry has been conservative about moving to RP male
models, so SLS® models made from traditional waxes, etc.
are preferred. Sterolithography.com, however, utilizes a special
process which allows SLA models to be more suitable for investment
casting. Since SLA resins do not melt but burn to form ash,
this special method modifies the build process so that the interior
of the model is hollow with a supporting latticework. When the
ceramic is fired, the modified SLA model collapses and any ash
is minimal because of the small total quantity of material.
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