[julianh]

Has anyone thought about making E/G molds out of HDPE or UHMW PE plate? I have access to a 24x48 CNC router and I'm thinking I could achieve around 0.005" positional tolerance with decent flatness on all surfaces, maybe better on reference surfaces. One or two 4'x8' sheets of 0.50" HDPE at $100 a pop should be enough to make a decent mold and it would be reusable. Obviously wide spans would need to reinforcement to avoid sagging under hydrostatic pressure, but other than that the only major issue I can see would be the heat generated by the epoxy curing possible causing the HDPE to soften. The liquidus of HDPE is listed at ~260 F but the glass transition is something like -100 F so I'm not sure how it would act under the temperatures generated by the epoxy. I haven't read the entire E/G thread but I haven't seen any discussion of HDPE molds in what I head read.

[charlieslasher]

Any kind of plastic would be better than sealing wood moulds. I used glass and this gave superb finish and reasonable flatness. Exotherm isn't a problem once you have large mass and only 12% epoxy it wont heat up. Heating the mould post cure is good for extra strength and accelerate curing however.

[julianh]

Any kind of plastic would be better than sealing wood moulds. I used glass and this gave superb finish and reasonable flatness. Exotherm isn't a problem once you have large mass and only 12% epoxy it wont heat up. Heating the mould post cure is good for extra strength and accelerate curing however.

Did you have any trouble demolding? Glass is easier to cut and join than HDPE, and more dimensional stable. Also I think that mirror glass is flatter than regular float glass if you desilver it. However, the main reason that I am interested in UHMW is that epoxy does not stick too it at all, so mold release should not be an issue.