Sample Chapter
from:
"COMPOSITE FACTS"
2.10.1
Simple Molds for Small Parts
Molds are needed to produce
most composite structures. Whether it is a foam core which gets wrapped with
reinforcing fabric of a sophisticated polished, durable mold, the composite
material needs something for support while it cures. Typically a production
mold is stiffer in structure than the part that is made in it to prevent the
mold from getting distorted. A good mold needs to have a hard, durable surface.
This will make demolding easier and reduces the chance for wear and damage.
For a homebuilder it is often useful to build one-time molds or plugs
for smaller parts, which are of complicated shape. If there are no special requirements
for good surface quality, it is often easier to save the work of making a
female tool. A male plug will do just as well where the cloth is laid over the
core. When making a male plug, keep in mind that the part is going to be larger
by the thickness of the plies that you apply. Plugs or one-time tools can be
made from a variety of materials which include foam, cardboard, plastic, wood,
wiremesh, plaster of paris or even sand. Wood and foam need to be covered
either with a smooth layer of gelcoat, or what is often as good, just packing
tape. Some tapes work excellently and do not even require release coating,
others might stick quite well to the cured resin, so try a sample before making
a real part. This applies of course to any type of mold you fabricate.
Make sure that the material
combination you use for core - mold - part is going to release from each other
by testing samples!
I speak from experience
that this is important if you don‘t want to lose a lot of work eventually. To
demold a part, loosen all the edges first by sliding something thin between
mold and part. You may need to stick small wooden wedges in where possible and
keep prying and pulling until it pops out. In some cases well aimed blows with
a hammer may be justified. If it should happen that you cannot get the part
removed from the mold, check the following. Make sure there are no undercuts
preventing you from pulling the part off the mold. If that is the case, you
have to cut the part in at least two pieces before it can be removed. A common
reason for parts to be reluctant to come off their molds is that the resin is
not cured well enough. In this case just wait. Storing the part in a warm, dry
place will help. Of course if you forgot the release coat, it will definitely get
tricky to separate part and mold, damage is likely to occur.