The distinction you need to be aware of is that HDPE is a thermoplastic material, and you are worried about thermosetting materials. In the first, there is a chemical change as the plastic sets, and these include Phenol-formaldehyde, Urea-formaldehyde, Aniline-formaldehyde, Glycerol-phthalic-anhydride, Epoxies, Silicones, and the bases of Bakelite, Tufnol, Melamine, etc. They can sometimes be moulded as pastes (not yet reacted) or as mixtures of powders, but one set, that is it.
The stuff you need includes a selection of thermoplastics – stand by for alphabet soup – CA, CAB, HDPE, PA (nylons), PBTB, PC, PE (polythene), PETP, PMMA (Perspex), POM (acetal), PPO, PP, PS, PVC, PUR (polyurethane), SAN, SB, etc – and some synthetic rubbers. With a few exceptions, the setting plastics are hard and opaque, the remeltable ones are bendy and translucent, or even clear. And most of them are hopeless above boiling point.
So now you know what to ask for – which of the above list is available to hobby users, or suitable for your application, is something I can't help with.
Regards, Tim