Stereolithography is the most widely used form of rapid prototyping
Also known as “solid freeform fabrication,” rapid prototyping is a term used to describe a range of fabrication processes where three-dimensional CAD (> CAD/CAM/CIM/CNC) data is used directly in the construction of components or objects.
The data from the CAD model is broken down into a number of thin layers which are then reconstituted by the cutting, fusing, or depositing of physical material, layer upon layer until a physical representation of the data exists. The main advantage of rapid prototyping is that it allows for the construction of highly complex geometry without the need for costly and time-consuming tooling. For this reason, it is particularly important for designers.
There are several different methods of rapid prototyping. In stereolithography (SLA), the CAD data is sent to an ultraviolet laser which scans the surface of a tank containing a liquid photopolymer which is hardened where the two make contact.
After the scan is completed, a platform on which the model sits is lowered into the tank by approximately 0.1 mm and the laser repeats the process with the next layer until the model has been completed. The platform is then raised to reveal the finished object, which must then be cured in an oven before any final finishing can take place.
Stereolithography is the most widely used form of rapid prototyping. It is considered to have the best surface finish and there is a wide range of materials available that can mimic commercially used plastics such as ABS.
Ceramic materials are also currently in development for the process. Fused deposition modeling (FDM) is the second most popular method of rapid prototyping after stereolithography.
It extrudes a heated filament of thermoplastic or wax through a heated nozzle attached to a mechanism which can move both horizontally and vertically.
The material is layered in a similar fashion to cake icing onto a bed below and successive layers are bonded by thermal fusion. The machinery is capable of producing support structures for overhanging elements which can be removed at the end of fabrication. While the surface finish has improved greatly over recent years, it does not have the resolution of stereolithography.
Three-dimensional printing (3DP) works in a similar fashion to selective laser sintering, except where SLS uses a laser to fuse a thermoplastic or wax powder, 3DP uses a liquid adhesive to create and bond the layers of powder together. The objects must then be treated with a hardener before they can be handled. Materials that can be used in this process include powder metals and ceramics.
Although the resolution and surface finish of the objects created using 3DP are limited when compared to other technologies, it is the fastest and cheapest form of rapid prototyping currently available.
Photopolymer phase change inkjets (PPCI) works in a similar way to normal inkjet printers. A print head containing a photopolymer is used to lay down a single layer of the object which is then cured using a UV light before depositing the next layer.
A second print head is filled with a support material to deal with overhang and undercuts within the object. The support material can be washed away with pressurized water at the end of the build. PPCI technology is able to produce very high resolution models as layers can be no more than sixteen microns thick and the finished objects do not require any curing or cooling.
However, it cannot currently produce large-scale objects and the properties of materials used in the process are limited when compared to other rapid prototyping technologies.
The limited physical properties and surface finish of materials used in rapid prototyping, as well as the relative expense and slow speeds of the technologies involved, currently make these technologies unsuitable for the mass production of components or objects.
These drawbacks have limited its use to the fields of product design (for concept development and product testing) and engineering (for tool production). However, increased commercial availability of the machinery coupled with improvements in material properties and resolutions have seen a number of designers and artists experimenting with the processes as a means of creating objects in their own right.
There have also been developments in the field of medicine where they are being used to create bone replacements for reconstructive surgery.
The University of Manchester’s School of Materials in the United Kingdom has produced a three-dimensional printer that can produce layers of skin using cells taken directly from a patient which can then be directly applied to wounds.