Being clear about clarity

The most common prototyping plastics used are acrylic (PMMA) or polycarbonate (PC), although in production we also see PET, PS and PVC. You obtain clarity on a clear material through a high gloss finish and there are two methods for achieving this. For smaller part numbers it can be achieved through polishing, but as quantities rise we tend to move from polishing a part to polishing a tool that can then impart the required surface finish to the part. Obviously the nearer the manufacturing process can get to a smooth finish, the less polishing is required. The less polishing that's required, the less labour intensive the process and the less cost is incurred. From this perspective machining has an advantage over additive techniques, where the layering will always need to be dressed. Layers will vary in thickness according to the RP techniques used. A high resolution (0.05mm layer) Stereolithography (SLA) part for example, will require less work than a part built at 0.1mm layers, to achieve the equivalent level of clarity. Here are the options for creating clear or translucent plastic parts: Machining in PMMA or PC and hand polishing PMMA or PC that is machined will be a translucent, not clear. The further one improves the surface quality, the clearer the parts will become. Once polished the parts do not need to be lacquered and will retain their clarity even if subjected to UV exposure. PMMA has the advantage that it can be flame or vapour polished, reducing the time required to achieve clarity. Levels of translucency can be obtained (for example if you are trying to let light through in one direction yet hide the light source from the viewer) by blasting the part with a variety of media after polishing. Stereolithography in Waterclear or a similar resin, then dress and lacquer There are a variety of resins available for SLA that will produce a translucent part. However, post production and once the support is removed, it is necessary to dress out the layering and apply a clear lacquer if good clarity is required. As noted previously, building with fine layers will minimise the manual polishing involved. It should be noted that if exposed to UV these resins will take on a yellowish tinge with time. 3D printing with Objet in VeroClear and polish Relatively recently Objet introduced VeroClear, a solution for the production of transparent parts. The material can be polished or buffed after the support material has been removed, but may retain a slight purpleish tint. This will fade in time, but the process can be accelerated if required. Vacuum casting in a clear resin such as Axson PX522 from a silicone tool A number of manufacturers offer resins for vacuum casting that will produce a clear part. The process involves polishing the master model to achieve a high gloss finish, prior to casting the tool. Depending on geometry 10 to 25 parts can be taken from the tool before the tool surface deteriorates. Injection moulding with both aluminium and steel tooling As with vacuum casting (above) clarity is a function of mould surface quality. Higher quality steels are frequently selected when tooling clear parts as they will take a better level of finish, especially if producing lenses for example. Sometimes injection moulding will be the only option, as the correct material is required to assess light transmission. An example would be a lightpipe or lens. There is also an option for simpler geometries: Fabrication/bending and vapour polishing This is an option for parts with simpler geometries and can be a quick and inexpensive route to clear parts. The process is frequently used in point of sale applications as a high quality result can be obtained. Clear adhesives are readily available permitting more complex parts to be fabricated. Here are five tips for creating clear plastic parts: 1. If clarity is only needed in a few limited areas, specify this before production as it will reduce the cost compared to polishing the whole item. 2. The ability to polish is a function of accessibility. Even if SLA can build the geometry, polishing requires physical access to the surfaces. (This may require fabrication). 3. Polishing is labour intensive so quantities will dictate if tooling will provide a more economic solution. 4. If tinted material is not available tints can be applied by a coloured lacquer or by pigmentation if vacuum casting or injection moulding is being used. 5. Translucency will change with the colour of the tint or pigment selected, so run a test first. Tim Plunkett is director of Plunkett Associates.