Morning All, Ive just started a new job, and we have some prototype parts made by rapid prototyping, in (I think) polycarbonate. The process involves laying down 'slices' of the part in consecutive layers and building the part up. This leads to a pattern of the layers being visible when the part is finished, and the end appearance is not quite upto 'niceness' IYSWIM. sanding with wetndry paper is ok but takes a lot of time, so does anyone know of a better way to smooth out the surface and leave it nice?
Depends. If it is an SLA (stereolithography) it is made from a liquid epoxy cured by laser in build layers of around 0.1mm, and is normally very smooth - almost as good as a finished moulding in some cases. They are sometimes lightly blasted to remove the 'build ladders', then finished with wet & dry.
If it is an SLS (selective laser sintering) they are usually made from cryogenically ground nylon powder, again fused together by laser, but in build layers of around 0.3mm. Appearance is always a bit rough,like a casting or sandblasted part, as you get gaps in the particles, as opposed to the seamless liquid build of an SLA. An SLS is almost always blasted to smooth it down a bit.
SLAs are fantastic for a fit and finish check, as the tolerances are very good, but the parts can be quite fragile. An SLS is good for functional testing as they are tough as old boots, but tolerance is usually around +/- 0.3mm.
For appearance modlels, parts are lightly blasted then coated with a high build primer and painted and polished.
Ive been to the rapid prototyping dept now (part of Loughborough Uni as it happens) and its a process which extrudes a bead of PC/ABS (about 30%PC) around the part and then infills it. fascinating to watch. The main reason for asking, which I forgot this morning, is that some of the parts are white, which goes a nasty gray colour when you use ordinary W&D on it. I remember White ALox wheels, which of course would leave white grit if any. Can you get white W&D?
I'll stand corrected for being such a smartarse then. Sounds likes the RP process they're using is FDM (fused deposition modelling), in wich case the minimum build layer thickness is about the same as SLS at 0.25-0.3mm.
This is what an SLA comes out like, the SLA is on the left, and the moulded part on the right.
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I don't know if you can get 'white' wet & dry, but you could try using a medium Arkansas stone instead, as these are white/translucent, and use it with plenty of water. Might be a little bit of overkill though.
We've got a FDM and we've played with improving the surface finish by quickly dipping the part in solvents then allowing the excess to evaporate. We've had good results using IPA on ABS but there is a bit of a knack to it. It still needs a bit of sanding but this can take a lot of the high spots and petterning off.
Try experimenting a bit with whatever dissolves polycarbonate.
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