There are quite a few things to consider when deciding which rapid prototyping process to use and these are mainly to do with the way each different process works and the materials they use.
SLS machines are different from SLA machines in that they use a powerful laser to fuse together granules of material. The loose powder acts as a support for the parts, which means that no support structures have to be removed, and parts to be built can be stacked on top of each other filling the cube. The result is that SLS parts are generally cheaper than SLA parts due to the economies of scales from the build process. The materials SLS machines use can be either nylon or metal depending on the type of machine and there are different types of materials within those groups. Regarding nylon, you can have standard Duraform Polyamide material, which is tough with some flexibility, or if you need something more rigid there is 30% glass filled nylon. A new material on the market is Duraform EX plastic, which comes in black or natural (off white.) This material is more flexible than PA and is good for living hinges and snap fit designs. The self-coloured black version is also useful if you need coloured parts without the cost of painting and finishing. One of the drawbacks with SLS is the porosity of the parts due to the powder-based material. It gives the parts a rough, granular surface finish, which, granted can be rubbed down and painted but this takes hours and is expensive to do well. This is where SLA parts should be the choice if you need cosmetically superior finishes on your parts or if you need to go onto make vacuum castings. However, the choices don’t stop there! There are many different types of SLA material and also machines. Machines can come in 250mm x 250mm x 250mm sizes (as our machine at Martello) and also huge mammoth sizes (Materialise.) and in-between. Stereolithography as a process has proved itself to be the most effective in building accurate parts with good surface finish and durable enough to be used as masters for tooling. Masters need to be finished for vacuum casting and SLA’s are by far the best parts to work on. Other processes such as FDM parts are very difficult when it comes to finishing, as the parts tend to de-laminate. This adds huge amounts of extra time and cost onto the job, which is no good for us as a bureau building masters for tooling!
SLA materials come in quite a few varieties all with certain advantages. The watersheds are great for holding their shape, which was an issue with SLA’s originally and can be lacquered to produce clear parts. Si50 is a good resin, which comes in grey or blue and produces very plastic looking parts but is not any stronger than the watershed resins. More technical information is on our web site on the datasheets page.
The thing to remember is that there will be a process by which you can get your parts built cost effectively and accurately and the best thing to do is ask around for some advice if you’re not sure. We have referred many projects onto other companies because we recognised that our polyurethane resin vacuum casting process wasn’t the most suitable for certain parts. For instance, very thin parts without features such as ribs or bosses parts maybe cheaper using vacuum forming or thick walled large mouldings maybe be better using RIM (resin injection moulding.)