need to make a shell-type body for a toy

[fanatic]

I need to make a shell-type body to go over a frame with detailed interior mechanicals. I made the mold shown, and cast the model in white, but I can't slush cast as thin as I need for a body [this is a 20 inch long, 1/8 scale piece.] I thought of using the sculpt as a buck, and making a brass body, but I have no experience with that, and it seems too detailed and sharply curved for that technique. Anyone have any ideas how to get from here to there?

[Roger Zimmermann]

You have a good idea, but why are you not doing something easier first? You may try to do a shell with polyester and fiberglass but again, try first on an easy shape. Good luck!

[fanatic]

I had thought Fiberglass would be difficult to use with the deep cuts and sharp turn. Wouldn't I need to make a negative mold to lay the fiberglass into, so the outer surface is smooth? [the current mold is a 'pour in' type cavity mold and wouldn't work.] It is possible to section the model and make lay-up molds of the body in pieces similar to the real thing, but that would require four molds. Neither easy nor cheap.

[MODEL A MODEL]

Hello, I’m on my phone and can’t see your attachment

If you have extra RTV could you mold release your existing mold

Pour against it a second RTV

Then between the two molds make a “gasket” of whatever thickness you desire and pour?

This assumes no undercuts

Again I cannot view your attachment

-Don

[MODEL A MODEL]

Another way is to lay-up into your mold

Surface the B side of your lay-up

Apply a very thin film of Vaseline to the mold. Literally just apply and wipe

Return the lay-up and pour a second mold?

[fanatic]

There are a couple of problems with using the existing mold. The biggest one is, I tried to double my money, and so I made it into a sculpture, so it has a driver, whose helmet is in the way of laying up the deep inset air intake. Not entirely sure what you are suggesting with the gasket, Model A. Can you explain it a bit more? I've added some pics of the mold when you can view them.

[Egon]

Make a mold of silicone, it's flexible and can get in all cracks and easy to remove.

[MODEL A MODEL]

Hello Bob,

Well I shouldn't have made any suggestions without having seen your mold, but I am home now, and can view your attachments.

So, first thing forget the gasket idea, wrong geometry.

It looks like your plaster jacket mold, breaks into threes? A left, a right and a bottom? But the actual RTV mold is simply a top and bottom?

I assume that you have been pouring through the bottom?

Well, there are several ways to get a hollow part from your existing mold, the best is probably to start with just the bottom. Make sure its supported by your jacket mold, And layup fiber-glass in it alone. Personally I prefer epoxy , I like West Systems, but you could certainly do it with polyester.

Lay-up just the bottom don't lay up just to your parts edge but go a little past it and concentrate on maintaining a uniform thickness. Also be careful not to disturb your early lay-ups with later applications. If you are working with a surface coat? -be generous! It is with that material, that most of your details will be captured. -if your not using a surface coat / gel-coat, well? -think about it. Check with your supplier, if you are using the West System, look into the different additives for bulking out the resin .

Fillers , can be had for improving the density, weight, and hardness of any resin . Mixing in cotton flocking can help you to make a lay-up with ease.

As I said lay up just the bottom. Concentrate on a smooth even layer, let it set, and then trim back to your part line.

Do the same for the top, if you can split it into two halves that will greatly facilitate laying up those components. Concentrate on an even laying up. Go over your part line and when everything is cured, trim them, and assemble.

If you want to have a pourable, cast, with a consistent wall thickness? You'll need to make an additional RTV mold. Or two. Right now your mold would lock an interior core with no way of releasing it.

If the "head" is in your way? -you can fill that area of the mold with "Klean-Clay" -its a brand of oil based clay used by many of us to fill in patterns before pouring an RTV .

There are many considerations, but if all you want is a one off , shell? Try my idea. -If you want to produce a pourable shell of uniform thickness, and do several? You'll have to pour more RTV . -Or- get really good at Roto-molding!

[MODEL A MODEL]

A little more?

Fiber glass cloth can be cut into many small pieces. You shouldn't try to force large swatches of cloth into your mold. A friend of mine first will apply a layer of gel coat to the mold. being certain to fill in every nook and cranny. Let that tack up and if needed add more, or if satisfied that all the "trouble areas"
are filled, he will then wet out small triangles of a light cloth, and do his lay-up in small bits and pieces. Continually working all areas of the mold will help assure an even shell thickness.

Your sculpture looks very promising!

I hope this helps!

Good luck with your project!

-Don

[fanatic]

You figured out my mold system correctly, three part jacket mold and two part silicone, filled through the hole in the bottom. I need to check with Smooth-On and see if the mold material will handle fiberglass. I'd rather cut up a casting and make new molds than ruin this $200.00 mold. I also need to do some research on West System products. Am I using Gellcoat? I don't know; I've never done this before. [Sounds like I would want to use it.] Many thanks for the ideas and advice!

[MODEL A MODEL]

Having slept on it, the thought of doing the body in Styrene just like Jo has done with his Birkin Single Seater. I suggest you look at his build. Gluing small strips of Styrene together, shaping and forming as you go, has worked for him, and many others on this forum.

Its technique-wise very simple and not expensive. (Compared to Brass) Use a solvent type "glue" and work in a warm room.(Helps to promote quick evaporation) Be careful to not over saturate the plastic, and you can sand as you progress. -it is actually a fun way to develop panels with contours.

Again, I hope this helps!

-Don