Mold making for vacuum forming

We just purchased a vacuum forming machine and we've tried a couple of different mold with it and still cant get a decent part that demolds easily. I machined a couple of molds from MDF but it clogs the vacuum when I place the mold over the vacuum hole. Is there a certain type of MDF, or another material, that is porous enough that the air leaks through it? So far we have only tried 1/8" ABS but we'd like to find some less expensive materials to use as well. I'm curious to know what other materials you are using and what temperatures you use for them. Thanks!

-David

David, It may be more an issue of your mold geometry rather than your mold material. We've made vacuum forming molds from wood, foam, tooling board, and aluminum. Sometimes we can get away with just raising the mold plug above the platen .04-.06. Other times we need to drill additional vacuum holes to feed down to the vacuum port.

Back when I worked in a vacuum mold shop we would cast the molds from something like this... - Alumilite Regular - Black # Then lots of little holes drilled as required.

Thanks for the replies. What size holes are you drilling in the mold? I've tried doing that but the plastic seems to find every little hole/crevice/irregularity in the mold. We've also tried raising the mold up some to let the air flow around it but the plastic sucks into that crevice too, making demolding very difficult. Are you demolding right away when the plastic is still warm? The most recent mold I made I put a 3 degree draft and still had trouble. What kind of foam have you used?

David, we usually drill vac holes at about .040" diameter and raise the plug off of the platen about that much if necessary. We actually just finished a production aluminum vac tool about 20" x 40" that had over 400 vac holes. They need to be strategically placed. If there are no undercuts on your mold then the part should come off fine. We do some lower volume vacuum forming as well and the parts are still a little warm when we pull them. You also dont want the part to shrink onto the mold.

Are you using anything as a mold release agent? The ABS seems to stick to the MDF.