Which plastic for sanding and filing?

Polyacetal (Delrin being a brand name) is a good material for that, yes, BUT if you ever catch even a small bit of it on fire, it makes chokingly toxic formaldehyde fumes. I imagine that is why you hardly ever see it in consumer items.

Because UHMW is quite soft and the molecular chains are 4 to 6 million molecules long, sanding produces a hairy surface. There are ways to get what you want from it but you'll never get a polished surface. Machining can give a fairly good surface that looks "shiny" but will have tool marks to some extent.

Delrin....well, sort of. If you are looking for polished surface, probably not. Delrin can be a little soft so polishing can go like a dream or go bad really quickly if you happen to overheat a section. However you can get a good surface with hand tools and sanding that might be acceptable.

Maybe if you elaborate on what you are trying to do? Your description sounds more like you want acrylic or similar.

Koz

Richard Fergus> I was answering a neighbor's question today, and went back and looked

Depending on the desired app., acrylic (PMMA) has the added advantage that it can be compression moulded into complex shapes rather than always machining a part from a block.

bob g.

Koz wrote:

I am not necessarily looking for a polished surface, just not a hairy or rough surface. In other words, smooth to the hand, not polished to the eye. I guess a hand can feel down to 0.002" of roughness or thereabouts.

I was hoping that I could rough out the shape with a belt sander, and then sand it smooth, going stepwise from 80 grit to perhaps 400 grit.

Unfortunately, for various reasons I cannot explain the application, sorry.

Richard

Koz wrote:

Delryn, acrylic, syrene, polystyrene, polypropylene, polycarbonate, PVC, and ABS can all be machined or sanded to a smooth surface. Ditto phenolic, polysulfone, Noryl and many others. Some or most of these will probably not be suitable for your application for other reasons. If you prefer not to say what the application is, then you must define all of the relevant material characteristics and specs that are important or desirable, and be able to prioritize them. Just the mention of dishwasher rules out some of those mentioned above for various reasons. These might include other matters like temperature range, strength expected, how manufactured or assembled (glue, weld, threaded fasteners, ???), appearance (color, etc), chemical resistance (e.g. dishwashing detergent), etc etc.

You should be working with a good plastics supplier. He'll have a lot of questions. Another approach might be to find a similar product and find out what it's made of.