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e.g., the result of a gap in triangle meshes

gap in mesh

The part produced has a problem with the hole, caused because the test

ray passed through a gap in the triangle mesh.

• Another problem can arise from mobius strip representations. Because the outside is defined by the order of node definitions, a mobius strip will lead to a back touching a front.

Picture of mobius strip

3D systems produces a software package to verify STL geometries

65.2 STEREOLITHOGRAPHY

•Invented by Charles Hull 1984

•Now developed by 3D systems Inc (90% of market in 1991)

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• The physics of the process is based on a photo-sensitive polymer that will harden when exposed to high intensity laser light

Focused laser light hardens polymer voxel

Polymer bath

• The process uses a vat of photopolymer with an elevator for the part to lower on. The elevator starts at the top of the bath, and drops down a layer at a time as the laser develops each layer.

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-to allow easy removal of the part from the platform

-unattached parts

•There are a few basic supports used,

Gussets - are used to support overhangs that would sag if not supported

projected edges - support overhangs that cannot be supported by a gusset

all are used to support large areas

65.2.2 Processing

•CAD files are converted to .STL files

•.STL files converted to slices using the parameters, such as,

-various parts on a platform (different .STL files can be mixed)

-blade sweeps per layer and sweep period, and “z-wait” to wait after recoating for mate-

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correct for this the tank has a fluid level detector that will control an adjustable reservoir that will add enough fluid to compensate for volume change.

•Layers vary in thickness from 0.002” to 0.020”. This is controlled by the amount the platform is lowered into the photopolymer. Thinner layers give smoother, higher tolerance parts (with polymer more cured) but these take longer.

•The laser is stationary, but optics and mirrors are used to guide the beam to x-y coordinates on the surface of the fluid.

optics

x-y scanning mirrors

laser

polymer layered model

elevated platform

vat of polymer

• The Slice Cycle

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laser

elevator platform

Process continues until done (elevator is lowered slightly to allow laser exposure of new layer.)

3. Wiper blade removes excess polymer and leaves consistent layers.

•After the part is done, the part raises above the fluid, and resin drains out. The elevator can be tipped to drain trapped volumes.

•After removal from the bath the part is cleaned off with towels and Q-tips, and hardening of the resin is completed in a curing oven.

•The laser - is often about 10-200 mW (more power is required for faster operation)

-often He-Cd or Argon-Ion to produce UV radiation about a 320-370 nm wavelength

•The optics - the user can set the focal distance of the laser to the range of the slice thickness.

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laser

mirrors direct beam to optics from laser

first lens expands beam

second lens focuses beam

x-y positioner directs beam across surface of polymer

focal spot is small, and high intensity,

developing polymer faster.

• x-y positioning - uses 2 computer controlled mirrors to reflect beam.

x positioning mirror

y positioning mirror

laser

x