Making the Prototype, or "Plug."
The first thing you will need when making a new mold is a prototype, or
"plug." The material the prototype is made from is not critical,
but some properties are important. The prototype should be rigid and thermally
stable, with a hard and smooth surface. Foam can be used if it is covered
with fiberglass, but my experience has been limited to carving the prototype
from wood. I have used balsa and pine, although I have it on good authority
that Gelutong is the wood of choice. Balsa is too soft on its own, and pine
has a very horrendous grain structure. Whatever you use, you will need to
finish the surface appropriately to ensure a good mold surface and to make
sure you get your prototype out of the mold with minimal destructive force.
If you are carving from wood or foam, the easiest way to ensure a straight
centerline down the fuselage is to laminate blocks of wood (or foam) and
have a seam running down the middle. Use black paint to color the inside
faces before joining them to make the centerline more visible. Make top
and side view templates from heavy paper and trace the outlines on your
assembled blocks. Make your rough cuts to these profiles using a band or
scroll saw if possible. Make cross section templates from heavy paper or
cardstock. Knock down the corners with a drum sander, band saw, or by whatever
means necessary and convenient. Use the templates to guide the removal of
material. Once the rough shape is finished, block sand all the lumps away.
Dips can be filled with appropriate filler or putty. Don't use Bondo on
foam because the polyester resin base will dissolve the foam in no time.
Wing saddle fillets may be made from Bondo or thickened epoxy. If the wing
will mate with the top or bottom of the fuselage (as opposed to sliding
in from both sides like some sailplanes), then make hard points to set the
wing incidence and use the wing itself as a mold to make the wing saddle.
Cover the wing center section with Saran wrap or other plastic film and
wax it well. Apply a bead of Bondo or thickened epoxy around the wing seating
area. Press the wing into the Bondo, let it cure and presto! Pop the wing
away and carve the fillets to shape with your favorite tools. A mini drum
sander on a Dremel comes in very handy.
Once you're satisfied with the shape of the fuselage prototype, surface
finishing may begin. If you carved from foam, a fiberglass and epoxy skin
will be required. If you used wood, then a coat of laminating epoxy resin
should suffice. When the epoxy has cured in either case, knock down the
lumps and sand with progressively finer grades of paper. You may wish to
use a dark enamel or epoxy paint to help with the final finish. Wet sand
with 600 or finer grit, and then work with a polishing compound until you
have a mirror finish.
Apply a minimum of 6 coats of mold release wax in preparation for the mold
building process.
Click on the small picture to enlarge.
The parting board is 1/4" hardboard.
The hole in the parting board is cut slightly oversize, as usual. I then cut
some pieces of balsa which will act as cradles for the plug, and these are also
cut out (with a drum sander on a Dremel) such that the area where the plug will
rest is oversize. I then mark the centerline on the plug (if it's not already
visible) and I use stiff paper masking tape along the centerline, extending
past the front and rear of the part, and I make similar tabs in the middle.
I wish I had a photo of this! :) Anyway, the tape tabs should be such that the
plug will rest on these tabs, thus suspending the plug with its centerline aligned
with the surface of the parting board. Make sure the plug is well waxed and
PVA'd (depending on your release system).
Here you see the 1" square aluminum tube frame and 1/2" balsa formers.
Mix enough auto body filler (trade name Bondo... a 2 part polyester resin product)
to just coat the areas of the cradles where the plug will rest. The plug is
then pressed into the filler putty and held securely until the filler has cured.
I then flip the whole thing over so I can run pieces of masking tape along the
back side of the parting board, making a seal between the board and the plug.
Once this "dam" has been made, the filler can be applied to the gap.
It is fine to do a section at a time... the stuff sets fairly quickly, so it's
unlikely you'd be able to do the whole thing at once. I just use plastic spatulas,
credit cards, or whatever. You want the filler to come just above the level
of the board... you'll sand it flush later.
Here's how we get a nice seam around the part... automotive body filler.
Once you've gone all the way around and the putty has cured, carefully pop
the plug out. If you accidentally break a piece of putty out, repair that
now. Carefully sand the putty flush with the board, and then spray on some
sanding sealer or paint of some sort. Once this is sanded smooth, wax the
heck out of the parting board. Replace the plug, wax the whole deal, and then
you're ready to go.
Here's the surface coat, some chopped glass, and the first layer of fiberglass.
This is NOT one step... sorry. The surface coat is 908SC from Jeffco
Products. The laminating epoxy is resin 1310-L6 with 3102 hardener, also
from Jeffco.
This is the almost finished product, showing the flanged, threaded inserts
for clamping the mold halves together.
This is what it looks like when you pop it up from the parting board. Leave
the original piece in until you have the second side finished. Wax the plug
and first side of the mold well, then repeat the process above to finish the
mold.
Now we are ready to make a fuselage in
our new mold.
I will be using the "Paint-In-Mold" method, where paint is transfered
directly to the part in the mold. The paint is "Rust-Oleum" gloss
white spray paint. The Epoxy is MGS L285/H285, which is a relatively fast-cure
room temp laminating resin with excellent strength and thermal properties.
MGS is available in the USA from several distributors. I got mine from Composite
Structures Technology. The fiberglass I'll be using I also purchased from
CST. The wax is "Dolphin Wax," available from H&M
Racing in Canada.
After waxing several times, I went ahead and painted the mold using Rust-Oleum
gloss white. You can see that there are little recesses which will require
special attention.
I filled the recesses and sharp corners with a thick mixture of epoxy and
microballoons. It's hard to see in the photo because of the white paint.
Once the filler has had a chance to set up a bit (not too much... just so
it's not going to move on you), mix up fresh epoxy and smear (or brush) it
all over the inside of the mold.
Lay some 0.6oz glass cloth in the mold and stipple it into place with your
fingers or a brush. This glass will make for a smoother surface and reduce
the tendency for pinholes.
Once the light glass is in place, trim the excess away with some curved scissors.
The surface glass may be trimmed flush with the part line all around. These
scissors came from the R/C car department of the local hobby shop.
Repeat the process for the main structural glass. In this case, I am using
3.6oz glass, which is still rather light. When the glass is in place and has
been saturated with epoxy, blot up any excess epoxy using paper towels. This
will reduce the weight of the finished part without sacrificing strength.
The glass should look almost dry. I have reinforced high-stress areas with
carbon tow.
Leave the glass oversize, and spread epoxy well past the partline. We will
trim the glass later, and the epoxy keeps the glass from fraying when you
cut it.
Oops... we need a special tool to press down the seams behind the canopy area.
A piece of balsa glued to a dowel is all it takes.
After the epoxy has begun to gel, you can trim the glass for the seams. Remember
to alternate... one side is trimmed flush while the other has a flap standing
high. The flap-side is cut with regular scissors. Make the flap about 1/4"
to 3/8" for most applications. You may need to experiment to get best
results.
The tail gets special attention again, since there is no way to overlap a
seam here. Mix up more epoxy/microballoons and spread it carefully around
the perimeiter of the tail. Spread some new epoxy on the flaps also. The halves
are now ready to join.
Next we clamp the mold together, making sure not to catch the flaps in the
process. They should slide inside so that they overlap on the other side.
If the epoxy has gelled a bit, you can actually bend the flaps towards the
inside of the fuselage and they will stay there. My mold uses incorporated
bolts, but I did not use enough... so I add C-clamps also.
We look inside (not easy to get a picture) and press the seams down. I use
a wooden dowel with the end rounded so that it slides easily. Use a flashlight
to guide you. Then you wait. And wait. And wait.
After 24-48 hours, you can open the mold. It looks really bad at first, but
this is just flashing and paint which will be trimmed off. You can use a razor
knife, files, sandpaper, etc.
When you are finished cleaning the seams, the fuselage is ready... and it
looks as good as anything you can buy!
