Creating
a good mold
is the key to any form of casting. In vacuum forming with
composites this is especially true. Traditionally molds are created in
wax, plastic, or plaster and later in metal if large production runs
are intended. Due to cost and its ability to capture fine detail,
plaster is one of the most common mold materials. For medium
sized low
detail parts, I prefer autobody filler instead. The filler
has a
faster curing time, and is more forgiving to rough handling.
In
the next steps I will outline the process of creating molds suitable
for everything from valve covers to dash trim.
For any
parts with
paint or a finish, start by applying mold release to the parts entire
surface. For non-finished parts such as airboxes, cam
covers, or other plastic parts, start by applying a liberal amount of
WD40. Let the WD40 sit for a few minutes then dab up any excess.
With your
mold release
applied you can now start mixing your filler. Simply apply
the
filler to the surface or item you want to replicate. Extend
the
filler out at least 1" past the edges. Using your fingers do
your
best to assure there are no airspaces. Now wait 10-30 minutes
depending how you potted your filler.
Once the
filler has dried, the newly created mold should lift off with almost no
effort. Inspect the mold for any flaws which should be
addressed. In the above picture you can see the "U" in TURBO
did not completely transfer. This is easy to fix by simply applying a
little filler to the area and re-pressing it to the original piece.
Once you have a clean negative mold, you need to mount it on a flat
piece of wood. This assures when you draw vacuum it does not
crush or distort. To mount the mold, lay down a thick bed of
filler on a sheet of plywood 1" larger than the mold itself.
Next, press the mold into the layer of filler with the face
up.
Once the
mold mounted and dry, sand down any high spots that may cause your
fabric to bridge in the vacuum bag. In the above mold you can see where
I sanded off the higher edges especially at the bottom. Now
is also the time to lightly sand any places you had to touch up, or any
other rough spots. 220 grit seems to be about the best grit
for this. In the next how to I will talk about how to build a
budget vacuum bag system, and tips on how to best use your molds.
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