Selective Laser Sintering, Stereolithography, CNC Milling - How do they work?
All of these techniques are layer manufacturing techniques, i.e. the model is created layer by layer.
A laser sintering system produces a model from plastic powder. The process starts with a thin, evenly distributed layer of powder which is heated close to the melting point. Then a carbon dioxide laser scans the surface, heating all the grains comprising the model in that particular layer. Then the base plate moves down the thickness of one layer, the next layer of powder is spread out, and the process continues in that fashion. Finally, the cake of powder is cooled down, taken out of the apparatus, and loose powder is shaken off.
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Laser sintering model |
We have made use of this technology for evaluation purposes only; it will not be discussed any further.
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Stereolithography model production in progress.
The laser beam (blue) hardens the surface according to the layer. |
In stereolithography, one starts with a bath of a photopolymer, such as epoxy resin. Just the thickness of one layer, a baseplate resides below the surface. Then a laser scans the surface, hardening the resin in the coordinates included in the model for that particular layer. Then the base plate moves down for the next layer, and the process continues.
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Just finished model, still with supporting structure. |
Model production includes the additional hardening of a supporting structure since the hardened resin is still fairly flexible. When all the layers are completed, the base plate moves up, the model is taken out, rinsed, and placed under UV light for final hardening.
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Stereolithography model, ready for use. |
Finally, the supporting structures are removed manually.
Stereolithography is mainly used in craniofacial cases. When needed, we have a small company produce models of this kind.
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CNC milling system at Prüner Gang.
A model has just been finished. |
A CNC milling system consists of a computer for control and the milling system itself which is powered by compressed air. For model production, a block of rigid styrofoam (polyurethan) is cut to the approximate size of the model, including tolerance for fixation and milling. Then the block is rigged with mounts at the base and the top and inserted into the system. Milling itself consists of two runs: Premilling and final milling.
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Final milling in progress (Model of a child's spine). |
In premilling, the block is milled to the approximate shape of the model, using a normal drill. In addition to quick removal of the excess material, this step ensures that the fine drill will not break. In final milling (shown), the model is shaped layer by layer, from bottom to top.
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Milling models: skull consisting of two half shells and separate mandibula, spine; rigid foam block.
The spine model is the same as pictured above during milling in progress. |
Our clinic owns a CNC milling system, so this method is always available.
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