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Foam Casting?

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Foam Casting?

Unread postAuthor: Lockednloaded » Mon Aug 30, 2010 10:32 pm

I have recently made a 2" bore mortar that fires primarily NERF pocket vortex footballs. It fires great, but NERFs are expensive, and even when i attach them to a line I can still lose them.

So I want to replicate them. My Idea is to make some sort of mold around a NERF and fill it with expanding foam. This would both cut down the price and get rid of the problem of bulbs breaking away from fins.

I can't decide what to make the mold out of. Epoxy would be pretty pricey for a mold this large, and I'm not even sure I could separate the mold, so I'd probably just waste epoxy. I could try cast it in a 2" barrel with epoxy only surrounding the fins and tip.

What do y'all think? has it been tried? will it work?
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Unread postAuthor: Gaderelguitarist » Mon Aug 30, 2010 11:30 pm

Expanding foam like "Great Stuff" ?

I would go with your plan to cast in epoxy and pvc. The key is to make the mold smooth enough for easy release, obviously.
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Unread postAuthor: MrCrowley » Tue Aug 31, 2010 2:15 am

Could you cast from something like paper mache? I've heard of it being done before, not sure how they separate them though. Otherwise do you have a place you could go to get a plastic vacuum mold? My high school had a machine that could do it.

get rid of the problem of bulbs breaking away from fins.

I found that if you superglue them back in, even with 5min drying araldite, they will hold together. The fin hasn't come off since I glued mine.
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Unread postAuthor: chinnerz » Tue Aug 31, 2010 3:12 am

thats a good idea! what are the finished properties of this foam? is it rigid? is it sticky? is it durable?
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Unread postAuthor: kenbo0422 » Tue Aug 31, 2010 11:38 am

Diatomaceous earth, graphite, even an oily coating will help separate your parts from your molds.
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Re: Foam Casting?

Unread postAuthor: dewey-1 » Tue Aug 31, 2010 2:22 pm

Lockednloaded wrote:I have recently made a 2" bore mortar that fires primarily NERF pocket vortex footballs. It fires great, but NERFs are expensive, and even when i attach them to a line I can still lose them.


You are trying to replicate something like this.
http://www.bp-usa.com/FoamRocket.htm

If you do attempt a mold, you will have to get the CG (center of gravity) in the correct position of the rocket.
There are "mold release agents" available.

Several years back I researched doing this, but the conclusion was is;

It is easier and cheaper to buy the Mini-Vortex in the long run.
What I found to help protect the Vortex fins from the launch air pressure was to use 2-1/2 inch foam stress balls.
It also helps to act as a sabot to prevent air blowby. Use closed cell foam stress balls, not open cell.

You can usually find the Nerf Mini-Vortex for about $3 US at Dollar Stores.
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Unread postAuthor: clemsonguy1125 » Tue Aug 31, 2010 2:56 pm

I had thought about this and come up with the idea of using pipe insulation in different sizes and make the tail from foam backer rod with a wire in the middle.
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Unread postAuthor: Lockednloaded » Tue Aug 31, 2010 4:43 pm

Gaderelguitarist wrote:Expanding foam like "Great Stuff" ?

yes exactly.


MrCrowley wrote:Could you cast from something like paper mache? I've heard of it being done before, not sure how they separate them though.

Paper mach can be pretty rough, I don't think it is suitable for a mold.

chinnerz wrote:thats a good idea! what are the finished properties of this foam? is it rigid? is it sticky? is it durable?

You can buy different types, I'll have to experiment.

dewey-1 wrote:Several years back I researched doing this, but the conclusion was is;

It is easier and cheaper to buy the Mini-Vortex in the long run.
What I found to help protect the Vortex fins from the launch air pressure was to use 2-1/2 inch foam stress balls.
It also helps to act as a sabot to prevent air blowby. Use closed cell foam stress balls, not open cell.

You can usually find the Nerf Mini-Vortex for about $3 US at Dollar Stores.


The cans of expanding foam are ~5$, About what i pay for NERFs now, so I estimate I can get at least three rockets per can, and once i make a mold I can use it as many times as I want, so this should be alot cheaper than Mini-Vortexes once i get the mold working well.

sorry I didn't reply to all of your comments, but I think i got the bulk of them
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Unread postAuthor: clemsonguy1125 » Tue Aug 31, 2010 5:27 pm

But the problem with great stuff is it doe not cure inside enclosed molds, I had a topic on instructables about this same thing, Ill go see if I can dig it up.
Edit-here
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Unread postAuthor: jimmy101 » Tue Aug 31, 2010 6:20 pm

What about cutting the shells out of "poll noodles"? Not sure of their exact diameter but it should be pretty close to 2". A hot-wire cutter would make quick work of putting a decent nose on them.
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Kmart, Target etc usually have'm during the summer.

If you really want to make a mold then you might consider Plaster of Paris. It's dirt cheap. A good thick layer of oil might work as a mold release agent. Four pounds for $6 from Home Despot. (Be careful of the amount of heat released in a large mold, it may well melt a template NERF shell.)
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Unread postAuthor: clemsonguy1125 » Tue Aug 31, 2010 6:26 pm

If you go with plaster of paris, DO NOT use petroleum jelly as a release agent. It mixes with the substance and stops it from curing as well and even then it doesnt release the object well. It was a disaster when I tried to mold around a golfball. I could of made a mistake but its better or no to take that risk.
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Unread postAuthor: Technician1002 » Tue Aug 31, 2010 6:55 pm

Car wax works as a cheap mold release. The foam in a can does not cure in a closed container. 2 part foam works well in a container, but can cause high pressure opening or distorting the mold. I know as I use it in boats. 2 part foam is not soft like a pool noodle. It is hard like a Styrofoam ice chest, and will break like it. It is also lightweight like Styrofoam so it won"t fly very far.

Regular size pool noodles are just about the diameter of a tennis ball. They fit my tennis ball barrel perfectly. The closed cell foam makes lousy sabots as it shrinks under the pressure of the shot. (compressible) It will easily blow right past a golf ball in a tennis ball barrel at 100 PSI. Use half a pop or water bottle as a sabot instead of a pool noodle. It works much better.

Buy the Nerf. They fly further and are re-usable if you can find them. As a bonus, they sound great with the whistle.

Edit, If you do use 2 part foam, simply filling some pop bottles with foam and a little sand for balast should work great. many bottles fit the thinwall 2.5 inch pipe very well. I think I'm going to look into doing this myself.
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