Question: I have a royalite/plastic (not fiberglass) part(s) that I
want to make a reverse mold of so that I can lay up a new real
fiberglass/CF part from it, with a few mods to it along the way. What
is the best way/process to do this (ie easy, fast and cheap!)? Fill
the part with pour foam? Or some sort of fast rubber plug making
concoction-I have seen such, but don't know what it is (I want to
make more than one part from this 'plug')? Plaster of paris? (nah,
done that before). What considerations do I have here? Mold release?
Etc. Thanks for any advice! Marc
Sounds like a Cessna interior trim or exterior fairing :)
Fill the back side with concrete and make a fiberglass female mold of
the front side.
I made a mold one time of the front wheel faring.
Saran Wrap makes a good mold release. The surf board shops use liquid
PVC. You spray it on and it is water soluble when it comes time to
separate the part from the mold.
You can also get thin sheet rubber at McMaster Carr.
Paul Lamar
There are a number of basic considerations, but this is generally not a
complex process. I have done many of these with good results.
For a one-time mold, you will have the ability to machine both the mold
and the finished product, so neither the original nor the mold it self
have to be perfect.
1. The original piece (Plug) needs to be stable, holes filled, and
smooth. You can stabilize a flexible part with bondo, foam, clay, or
anything you have handy -- but you want the original to be straight and
flat -- but still flexible. This is the mold plug and you won't need it
after a successful mold is built. The rougher the plug, the more work
you will have to do on the mold and/or product.
- Be sure to think out how the edges of the mold, and product, will
need to be trimmed, which determines the overlap of glass beyond the
original dimensions. Sometimes you have to make some extensions of the
edges of the original piece with clay -- or use a piece of glass or
other smooth (waxed, Saran Wrap, etc) surface to set the product where a
flat surface can create the mold edge extensions.
2. The basic release agent is wax. Lots of it. There are special
mold-release waxes, but using 5+ applications of Johnson's will usually
be effective. As mentioned, a very light film of spray PVC (get it at
the boat shops) is water soluble and helps release complex parts. It
will not dissolve when the gel coat / glass is applied. Saran Wrap also
works, but will not have a smooth surface.
3. Complex parts with lots of curves and small areas are best molded by
vacuum bagging. This is a little more involved, but will result a in
far superior mold (and product) if this is needed. There are lots of
references on the web for the materials and technique.
4. Industrial molds use mold gel coat, but any marine store gel coat is
OK for a one-off. Epoxies are best, smell less, but are more
expensive. Polyesters are cheaper, but use plenty of ventilation and
skin protection. Same with the resin for the mold glass and product.
5. Just mix (catalyze) the gel coat and paint on a thick coat onto the
waxed/PVC plug, filling every nook and cranny. Use a light fiberglass
mat, or thin cloth (8-10 oz) as the first layer over sticky gel coat,
embedding it into all the nooks and crannies so there are no air
pockets. Layer 3-5 layers of glass, using heavier glass on the
outside. You can stuff small pieces of foam or glass into difficult
recesses. You can also embed pieces of wood or metal to support
certain areas or create a stable base for the finished mold. If the
piece is complex, don't make the mold too solid because flexing both
parts dramatically helps mold separation.
6. If not vacuum bagging, trim the excess glass with a razorknife as it
sets up: it will cut easily and not come off the mold if timed right.
7. Trim the final edges of the new mold, taking care to leave a little
ledge to allow the product to be of the correct size after trimming. If
PVC used, wet the mold/plug, or just find an edge to start separating,
and twist until they separate. Did you use LOTS of wax? :-)
8. Clean up the mold, repair defects, and repeat the process with the
WAXED mold, to produce a final product -- but the layers of glass are
now critical for both weight and strength. Using multiple layers of
lighter glass is usually best. 4-5 layers of 8-10 oz cloth will usually
be more than sufficient for a non-structural fairing or interior part.
Make some test pieces to try. You can add reinforcing strips to large
flat areas that need support, or foam+glass structural supports wherever
needed.
There are lots of references on the web, but this should get you going.
Scott Gettings
MElbourne, FL
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