STEREOLITHOGRAPHY

Stereolithography is a common rapid manufacturing and rapid prototyping technology for producing parts with high accuracy and good surface finish. A device that performs stereolithography is called an SLA or Stereolithography Apparatus Stereolithography is an additive fabrication process utilizing a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser light cures, or, solidifies the pattern traced on the resin and adheres it to the layer below.

After a pattern has been traced, the SLA's elevator platform descends by a single layer thickness, typically 0.05 mm to 0.15 mm (0.002" to 0.006").

Then, a resin-filled blade sweeps across the part cross section, re-coating it with fresh material. On this new liquid surface the subsequent layer pattern is traced, adhering to the previous layer. A complete 3-D part is formed by this process. After building, parts are cleaned of excess resin by immersion in a chemical bath and then cured in a UV oven.

Stereolithography requires the use of support structures to attach the part to the elevator platform and to prevent certain geometry from not only deflecting due to gravity, but to also accurately hold the 2-D cross sections in place such that they resist lateral pressure from the re-coater blade. Supports are generated automatically during the preparation of 3-D CAD models for use on the stereolithography machine, although they may be manipulated manually. Supports must be removed from the finished product manually; this is not true for all rapid prototyping technologies.

ADVANTAGES

• Unattended building process

• The system isvery stable. Once started the process is fully automatic and can be unattended until the process is completed.

• Good dimensional accuracy - The process is able to maintain the dimensional accuracy of the built parts to within +/-0.1mm.

• Good surface finish - Glass-like finishing can be obtained on the top surfaces of the part although stairs can be found on the side walls and curve surfaces between build layers.

• The process is of high resolution and capable to build parts with rather complex details

• 3D Systems Inc. have developed a software called "Quickcast" for building parts with hollow interior which can be used directly as wax pattern for investment casting

• It is the most widely used process in the RP field.

DISADVANTAGES

• Curling and warping - The resin absorb water as time goes by resulting curling and warping especially in the relatively thin areas.

• Relatively high cost (US$200-500K) -However, it is anticipated that the cost will be coming down shortly.

• Narrow range of materials - The material available is only photo sensitive resin of which the physical property, in most of the cases, cannot be used for durability and thermal testing.

• Post curing - The parts in most cases have not been fully cured by the laser inside the vat. A post curing process is normally required.

• High running and maintenance cost - The cost of the resin and the laser gun are very expensive. Furthermore, the optical sensor requires periodical fine tuning in order to maintain its optimal operating condition which will be considerable expensive.

APPLICATION AREAS

• Prototypes for concept models

• Form-fit for assembly tests and process planning

• Models for investment casting, replacement of the wax pattern

• Patterns for metal spraying, epoxy moulding and other soft tooling.