Materials to use for vacuum forming?

What kind of materials do most people use as molds for making things out of vacuum formed abs ive heard of people using wood or clay but can anyone give me a run down on a list of materials that can used for this or a better explanation?

 

Are you asking about bucks? If so, anything that won't crush under the pressure is fine. This can be cement (not Plaster-Of-Paris), wood/MDF, plastic or a combination of all three.

Tips for making bucks (where possible) -

1. keep the part wider than it is high.
2. avoid vertical side walls and add a at least 3 degrees slope to any vertical parts.
3. AVOID undercuts!