The vacuum one is the most used. The maximum forming measure
that can be produced is 1500x2500 mm.
A negative or positive stamp is closed into an hot slab.
In between vacuum is made by taking away the air inside.Atmospheric
pressure (14.7 psi) is used to carry the hot slab (now malleable)
on the stamp, keeping it in position until it gets cold, lower
than the distorsion temperature degree.
The material distorsion dipends a lot on many things, but most
of all on:
1 - Uniform warming of the slab.
2 - Temperature of the stamp and kind of material used.
3 - Air scavenging velocity (vacuum and/or pressed air).
The more the stamp is hot the fastest is the evacuation or pression
of the air, and the better is the material distribution.
The working temperature overcoming must be avoided.
