Processes that transform complete 3D models directly from
their virtual description in a computer into physical parts have been available since the mid 1980s, especially in the area of rapid prototyping. In all these processes it is common practice for the 3D model first
to be divided into very thin layers. These can then be generated two-dimensionally and when stacked on top of each other they form the three-dimensional body. The part produced can be subject to undercuts and cavities.
- With Stereo Lithography (STL) UV (Ultra Violet) hardening polymers are hardened in layers by a laser where material is located in the appropriate layer of the 3D model.
- With Selective Laser Sintering (SLS) polymer or metal powder is used instead of the UV hardening polymer and the particles are melted together by a laser.
- With Fuse Deposition Modeling (FDM) a melted liquid thermoplastic is applied in layers by a nozzle.
- With Laminated Object Modeling (LOM) the individual layers are cut out of paper using a laser and then glued on top of each other.
Common disadvantages to these generative processes are high technical expenses, the lack of accuracy and a limited choice of materials. Usually with one
process only one type of material can be processed (STL, FDM, LOM). Materials with special characteristics like high strength, chemical durability or transparency can not be used. This is especially true for
metals that are often needed in prototype building, but only can be processed as a sintered metal powder at limited strength with the SLS process.
Another well-known process widely used in the engineering field
to manufacture parts directly from 3D computer data is NC (Numerical Control) milling. By milling, not only the widest variety of materials, including metals, can be processed, but also the production
accuracy is about 10 times better than with the generative processes.
A disadvantage to the familiar NC milling process is that parts subject to undercuts or cavities only can be milled up to a certain
complexity using 5-axis milling machines, and that with the well spread and less expensive 3-axis milling machines the correct milling of these parts is almost impossible.
Such complex parts, with undercuts and
cavities, nevertheless can be built using a milling process when the 3D model is broken up into components in such way that each component does not appear to have undercuts or cavities. After milling both sides of
these individual components they can be joined together to form the complete part by using material-specific joining processes, such as glueing or soldering.
Without special software support, however, this
method demands a great deal of manual effort and requires a lot of experience, as the following problems arise:
- The cutting levels for breaking up the part into undercut-free components have to be determined manually, also the dimensions of the available base material have to be taken into consideration.
- The individual components can have very complex shapes and can be quite small, so that it becomes difficult to clamp them on the milling machine.
- The precise positioning of the components for milling on both sides is almost impossible.
- The tiniest displacement during assembly can lead to gross inaccuracies.
Millit now allows direct manufacturing of parts from 3D computer data by milling sheets of base material without the disadvantages described above.
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