& Blow Moulding
are closely related to Pressing in that they rely on pressure forces into
contact with a mould. In vacuum forming this is done by sucking the sheet
material onto the mould whilst in blow moulding this is done by raising
the pressure to force the material against the mould.
Tooling costs are much lower than for presswork but the forces involved
are very much lower which limits the materials that can be used and the
The diagram below shows the equipment used for vacuum forming. Note that
the corners of the mould are rounded, tapered and air vents are included
in positions where air may become trapped. Whilst thermoplastic sheet
is the most common material, alloys of aluminium, zinc and stainless steel
can be vacuum formed although this is expensive. The mould can be made
from any material which can withstand the relatively low temperatures
and stresses involved.