I also print coffee mugs. I have used ABS for a long time, and, given the statements here and elsewhere about bacterial growth sites, have been attaching my life to a thread. That's fine for me, but I don't make mugs for anyone else.
There are three areas of concern I've found so far:
- The plastic may be toxic,
- The machine itself, especially the nozzle, my deposit bits of lead (Pb) into the object, and
- The small gaps may harbor pathogenic bacteria.
The toxicity of the plastic can be addressed by researching the FDA website. ABS is generally safe, although additives could be questionable. Also, for some reason the FDA lists ABS as safe, but not for use with alcohol -- so much for beer steins. Since the actual MSDS for hobbyist filaments can be hard to get, I'm considering using a less controversial plastic. PETG isn't good enough -- it softens at the temperature of boiling water (I know, I've tried). My next attempt at non-ABS will be Poly Carbonate. I have a spool queued up, but it is as yet untried.
The nozzle can be replaced with a stainless steel nozzle, which does not contain lead. Perhaps because they aren't heated, no one seems concerned about brass feed gears in the extruder.
The layer gaps might be addressable by acetone smoothing. I haven't had great results smoothing mugs. The first problem is that after smoothing a lot of acetone remains in the ABS. Even after a couple of weeks, when hot water is poured into the mug the acetone in the plastic vaporizes and creates bubbles in the surface, which completely defeats the purpose. I've also seen more long-term age-related cracking in vapor-treated ABS. This could also be due to gradually losing acetone and the surface shrinking. If one were to use acetone smoothing, I would suggest post-conditioning the mug in a heated, partial vacuum chamber to encourage the acetone to escape.
The method I intend to try to seal the inside of the cup is to use a food-grade two-part epoxy. I haven't done it yet, so I don't know how it will work.