Machines and Materials

Our company produces 100’s of polyurethane pieceparts per month using silicone tooling, which we create from stereolithography models or machined patterns. We utilise high-end rapid prototyping technology and CNC machining to produce accurate masters for tooling.

Our SLA (stereolithography) machine is a 250 which means it has a build cube (area) of 250mm x 250mm x 250mm. However, the Z height actually only builds to about 240mm. The Z height is the length of build vertically.

Here is a video of our SLA machine:



It is constraints like this that make each project we look at a different challenge as there are several routes that you can choose to go down when looking at building parts using rapid prototyping technology. For example, some parts are better built using selective laser sintering, (SLS) because this process is more economical for low volume parts if cosmetic finish is not important. SLS machines build parts from granules of nylon, which are sintered by a laser, and this is why they have a rough and somewhat porous surface finish. The loose powder acts as a support for the parts being sintered so parts can be stacked on top of each other, which means the build is more economical. By contrast SLA machines you have only the space on the horizontal platform to fit the parts because SLA machines build parts using a vat of liquid. A laser curing off the material layer by layer builds SLA’s. The laser is programmed to trace out the horizontal slice of the part so as to build the parts shape and features. We have diagrams of how the processes work on our web site at www.martello.co.uk/rapid_prototyping.htm
When building rapid prototypes we often find that there are issues over the size of features and wall thickness that can be produced using the current technology. This is due to limitations such as the layer thickness, which is normally 0.1mm and the beam width of the laser on the machine.
Our machine’s beam width is .22mm, which means features sub this measurement would not form in the build. The issue is a lot of parts which are rapid prototyped are destined for injection moulding which can form a lot thinner walls and features than traditional rapid prototyping. However, the latest RP machines take the resolution of builds into new dimensions. SLA technology can go to 0.05mm layering on Viper machines and there are also Envisiontec machines with fine resolution layering which have found a niche in the jewellery market. The Objet Eden machine is also good for fine features and their latest machine (Connex) can now build in 2 materials.

Materials are also very important in deciding which process to use. SLS machines offer a choice of nylons. There is the standard Duraform Polyamide material, which means is also the cheapest as it is the most commonly run on the machines and then there is 30% glass filled nylon which is more rigid. A more flexible material is the newly release Duraform EX plastic (released in 2007) which comes in self-coloured black or natural (off white.) EX can be used for living hinges and snap fit designs.

There are also many different SLA materials. There is only one material on the market at the moment, which can withstand high temperatures and that is Bluestone. Another good material is si50, which comes in either grey or blue and hence looks more like a true thermoplastic even though it is no stronger than most other SLA materials. Our in house machine runs DSM Somos watershed 11110 resin, which is transparent with a green tinge but when, lacquered becomes clear. A major development in the RP world recently is the launch of the Connex 500 by Objet, which can build using two different materials at the same time so you can build rubber features into a rigid part in one build.