how do you make a mold - RC Groups
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Feb 02, 2002, 11:27 PM
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buckeyes1997's Avatar

how do you make a mold

im working on making a hand launched glider from foam, a carbon arrow shaft and foam fuse covered with fiberglass.
how thick should the fiberglass for the fuse be? also i was going to use the "lost foam" method to melt the foam after the epoxy glass hardens. anyone else try this or know of a better way?

matt clement
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Feb 02, 2002, 11:54 PM
Almost a Pilot
Mauilvr's Avatar
Hi Matt:
Check this out for "lost foam" info.
Feb 03, 2002, 01:16 AM
Registered User
chlee's Avatar

Lost foam method

I just made a HLG fuse using the lost foam method much like that listed on favonius's site. Works well. I suggest that you use around 3 layers of 1.4 oz or one layer of 3 oz fiberglass. I used only two layers of 1.4 oz and it left the fuse a little too squishy, so I had to add another layer after it all cured.
Also, consider using carbon tow or tape to stiffen the structure, especially along the bottom and under the wing mounting area. I added some tow on a whim, and it now seems that the fuse derives most of its strength from this carbon. Aim for below 2 oz for your fuselage.
Consider buying carbon or glass braided sleeves (available at or as mentioned in the favonius article instead of laying strips of glass like I did. It'll probably make your life much easier.

I used the following method for making a foam plug for a 16" pod with a 36" carbon rod with 1/4" diameter (trimmed later):
1) Decide the max width and height of your fuse. I picked 1.5" x 1.5".
1) cut two 0.75" x 1.5" x 18" rectangular piece of foam with a hot wire or band saw.
2) Make a sanding block with 1/4" width (diameter of carbon rod) and 1/8" depth (half the diameter of rod). Glue a wider piece balsa on top of this to act as a guard. The cross section of this sanding block looks like a T, and the sandpaper goes on the bottom of the T. Use this sanding block to a 1/4" wide, 1/8" deep groove, lengthwise and centered on one face of each foam block.
3) Sandwich the carbon rod in the grooves between the two foam blocks. Glue the foam blocks together. Now you've got a single 1.5" x 1.5" x 18" block with a 1/4" x 1/4" hole going through the middle.
4) Slide the carbon rod out of the foam block. Slice two 1" lengths off the end of this block. You will use these square 'donuts' later to align your tail boom with the pod.
5) Glue a small piece of scrap foam to one end the foam block so you won't have a hole where you're trying to shape a nose. Slide the carbon rod back into the foam block to add stiffness while you shape the foam.
6) Use bandsaw or hotwire to cut out wing mounting area. Then use razor plane and coarse sandpaper to shape the fuselage pod.
Do not completely sand a layer off the bottom. At least in the thickest part of the fuse, you need to maintain the original thickness between the carbon rod and the foam bottom.
7) When you're done shaping, slide the carbon rod out until only 2 or 3" remain in the foam block. Slip the two square 'donuts' over the remaining part of carbon rod. These donuts act as jigs that hold up the rod at a constant height, and align the pod and boom vertically (that's why you didn't sand an entire layer off the bottom in the previous step). Draw a line on your table and use the glue seams in the pod and donuts to align the boom horizontally.
8) When you are sure of alignment, use epoxy mixed with microballoons, or, if possible, foaming polyurethane glues like Pro-Bond, to glue the boom in. Use the donuts to keep everything aligned as glue cures.
9) Cover the pod with packing tape.
10) Cover the pod with fiberglass/resin. Reinforce the tailboom area well. After it's all cured, melt out the foam with gasoline or acetone. You might consider leaving the foam around the tailboom. I accidentally melted part of it out, so I packed the area with microballoons/epoxy and chopped glass.

Good luck.
Feb 03, 2002, 02:37 PM
Registered User
buckeyes1997's Avatar

how are you applying...

okay i have the fiberglass all around the foam. it was really hard to do and still looks "NOT PRETTY". is there a better way. i read your part abotu the sleaves and that sounds good but how do you get the glass to lay down on the fuse curves?
do you use several small (say 1" strips) layed along the fuse or do you try to wrap a single piece around it?
thanks i think this is a great thread for new people to learn.

also i would still like info about making a plug type mold, but the lost foam method is a great non-production type method.
Feb 03, 2002, 04:19 PM
Registered User
chlee's Avatar
I used 1" strips, like you said.

The sleeves are like those "Chinese finger traps"; the fibers are oriented on a 45 degree bias, so the sleeve can stretch lengthwise and shrink in diameter. It'll therefore shrink right around your fuselage curves. has some information about using molds. Unfortunately, it doesn't say much about making them. Here's the method that I plan to use sometime in the future (a compilation of methods from various online sources):

1) Glue two blocks of wood together (each is half the thickness of fuselage) with something like Duco Cement or Ambroid. Do not use wood glue, CA, or epoxy.
2) Shape the wood block.
3) Put the wooden plug in a ziploc bag with some acetone. The acetone will dissolve the glue. After an hour or so, take the plug out and separate the two halves. Alternatively, you could use silicon caulk in step 1, and simply peel/rub it off after shaping the wood block.
4) Wax and polish the outer surfaces of the plugs several times. If possible, spray with a mold release agent like PVA.
5) Get a flexible sheet of plexiglass and place it on a flat surface. Use silicon caulk to glue the flat sides of each plug half onto the plexiglass. Be sure to smooth the edges around the plugs, where the silicon squeezes out.
6) Use modeling clay to create a 'fence' around the plugs on the plexiglass sheet, so that epoxy won't flow everywhere.
7) Apply several layers of fiberglass over the plugs. Use finer grade cloth for the first layer, and you can use cheaper automotive grade glass for the outer layers. Build this glass up at least 1/8" thick.
8) After the resin cures, pop the mold and plugs off by bending the plexiglass. Then pop the plug out of the mold.
9) Follow directions in the scrollsander article.

Also see moldmaking articles in .

Hope this helps.
Feb 03, 2002, 11:27 PM
Almost a Pilot
Mauilvr's Avatar
I've made several plugs for fuselages using pretty much the same technique described by chlee. My results were more than satisfactory.
The key things to insure are:
  • Original plug finish should be as smooth as possible.
  • Be certain you can separate the original plug into 2 halves.
  • Apply enough wax/PVA to seal each half of the plug
  • Use a fine cloth on the first several coats to accurately capture the curves of the plug
  • Be certain no air bubbles are trapped - pits will result.
  • Final mold must be strong enough so it doesn't twist or become deformed
If you are patient and meticulous about the finish of the plug, you can produce excellent results that will allow you to successfully produce great fuselages in a short period of time.
Last edited by Mauilvr; Feb 04, 2002 at 02:06 PM.
Feb 04, 2002, 01:59 PM
Registered User
buckeyes1997's Avatar

great info guys

hey thats a lot of great info that everybody should read.
i do have another question. i have been to dj aerotechs workshop and watched them make their extremely high quality HLG and i saw how they make their fuses from the molds and i dont know if mine would work that well.
you take the 2 mold halves and push fiberglass thats wet all around the inside of the mold right? how do you keep it from crushing? also i cant seem to get the glass to lay down without moving after it starts to dry or worse wrinkle because its bunched up in places? the tubing fiberglass sounds awesome since it will stretch. how do you handle the bunching up where the fuse comes to a thinner point?
Feb 04, 2002, 02:00 PM
Registered User
buckeyes1997's Avatar

just thought of something

with your wood plug, why couldnt you heat up the plexiglass and melt it around the plug? should be very smooth and i would bet would contour very well when heated right?? i may give this a whirl.

okay i have actually tried this. with 1.30 worth of 1/16" plexiglass and some sockets and odd shapes, i placed the plexi on the items and put it in the oven on a pan at 500 degrees. it took about 5 minutes to melt and conform to the shape of the items. it was allowed to cool and was plenty hard enough to apply fiberglass for a mold. the only problem as that it bubbled throughout the sheet so there are several small dimples all over the mold. i think with the right temperature those can be minimized or even eliminated with a heat gun!! i will try to get rid of the bubbles or maybe put a layer of epoxy all over the inside, thus erasing the dimples?? i think this might be a great way to make quick molds. any ideas on temp control to help with the bubbles?
Last edited by buckeyes1997; Feb 04, 2002 at 07:00 PM.
Feb 04, 2002, 08:33 PM
Registered User
chlee's Avatar
This is very interesting.

Feb 04, 2002, 08:53 PM
RCGroups Author
SoarNeck's Avatar
I used the method described on the CST website to produce a pod mold for my new hand launch design. I replicated the basic shape of my Encore pod in balsa, then sealed it and used the "splitter plate" method to make the pod. It takes about three days to make a good mold.
Feb 05, 2002, 05:47 PM
Registered User
chlee's Avatar

I like those nice fairings you have for the wing LE. You didn't just carve those out of balsa, did you?

Feb 05, 2002, 07:10 PM
RCGroups Author
SoarNeck's Avatar
Here's a couple of shots of the plug, though it's a little the worse for wear since it been banged around a little.
Feb 05, 2002, 07:15 PM
RCGroups Author
SoarNeck's Avatar
The plug itself is made of three pieces of balsa glued together and shaped, and the fairings are Canadian Tire brand autobody filler. It's sealed with about 10 coats of mold release wax, and had a couple of coats of PVA applied before use. See for a description of the method used. Using the mold is a little challenging, since it has so many inside curves. The pod you see there was the first one out of the mold, and only used scrap 1.7 oz Kevlar to get an appreciation of how I'd have to cut the cloth.

I also recently made a mold of the nose of a Nova fuselage, and used the mold to make a removable nosecone. Same method.
Feb 06, 2002, 01:24 PM
Registered User
Hi guys :-)...

I make molds using a dentist tipe product. Is a powder wich you mix with could water, and it cure in 2 minutes!, with hot water is super fats... Here in Mexico the name of the material is; Alginato, in english, maybe alginat, ???, i try the dictionary but no alginato in.

Is super easy to use, as i say. Mixing, puring, presto. A nice exact mold of everything you like :-). This material can handle like 80-90 centigrades of hot water(i don't know how much temperture can handle, but the hoter i use is 90) when is "hard". The consistency is like a hard jelly. And no problem with acentone.

Well. I hope this help a little :-)...

Feb 06, 2002, 11:53 PM
Registered User
buckeyes1997's Avatar


that sounds like interesting stuff. ill look around online and see what i can find about that.

im going to try this very very soon.
1. make the two halves for the mold from wood.
2. melt plexy around each half
3. fill each mold with a layer of epoxy which gets rid of

i also think i will also take the two wood halves and make a fiberglass mold directly from the smooth wood halves.

i have a digital camera so ill start posting pics of tests on my website soon.

by the way how many layers of which weights would you use for a sailplane fuse? i have .75oz, 1.4oz, 6oz available.

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