To do vacuum forming, you need to:
- stretch a piece of plastic sheet over the mould
- suck the air from the mould
- heat the plastic until it softens enough to bend into
the shape of the mould.
To achieve this, I needed to arrange a vacuum chamber with a reasonable seal between the plastic sheet and the mould.
I drilled a large number of small holes through the deepest part of the mould, then flipped it over and
built up a thin frame
of stand-offs to make a "plenum"
behind the mould.
I then glued a sheet of
1/2" plywood across the back to finish the plenum chamber. A
hole in the back of this later will allow a vacuum pump to evacuate
The plastic sheet needs to
be held in place and a reasonable seal needs to be made between it
and the mould. So I put caulking around the rim and smeared a layer
around the edge of the mould.
The hinged frame closes with catches and holds the plastic in place.
I built a stand for the
mould, with a dowel pivot on the mould so it can be easily rotated.
The mould is quite heavy and I didn't want to be heaving it up and
down countless times.
Here it is with one of my many early failures
(this time due to not having an adequate heat source).
I later added a couple of handles to the sides of
the stand to make it easier to carry around.
I drilled a 2 1/4"
hole in a piece of 2x4 to take the hose of the shop vacuum. It's not a
very tight fit, but the suction holds it in place well enough.
I drilled a smaller hold in the back of the plenum
so the vacuum hose doesn't get pulled in and block the plenum.
Here's the shop vacuum hose in place.
At this point it's ready to go.
You need a powerful heat source that can be applied quickly and then removed just as quickly,
as the degree of softening is quite critical: too little and it won't get into the corners, too much and the plastic will
melt completely: a hole forms and you lose suction.