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Foam Rubber 2

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CC9

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May 11, 2005
14
I would like to make some large model aircraft tyres - 7 3/4" in dia - (I cant buy them off the shelf). I can make a mold for the tyre, but was wondering if anyone out there knew what the best injectable foam/rubber would be. I need it to be extremely light, but firm. The smaller wheels I already have seem to be made from a firm aerated foam, black in colour with a smooth surface (not unlike rubber) but when cut it, I can see internally that the material is grainy/aerated. Anyone got any suggestions as to what it might be? Is there a material I could mix up and inject into my mold? It needs to be flexible, light but firm...the model will weigh about 40 lbs. Thanks.
 
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You could cut them from polyethylene (PE) foam, or cast them in polyurethane (PU) foam, or injection mould them in any number of materials, including PE and PU.

If injection moulded, they could even be gas injected to make them hollow.

Which way you go, really depends on tooling costs and production numbers, with cutting from sheet being cheapest initial cost but highest piece cost and injection moulding being the highest initial cost and lowest piece cost

It is a similar story with design freedom, with cutting from sheet giving the least freedom and injection moulding giving the most.

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thanks patprimmer.
I would like to try injection moulding. What is the material I need to purchase? I was thinking I could just fill some big syringes with some liquid form of this PE or PU and squirt it into the mould I have, at various points around the circumference....cross my fingers and hope for the best. These will be one offs...would the PE or PU set me back more than $150 ? I would really like to give this a go, but just need to source the correct foam. DOes the mould have to be made out of anything in particular ?...at the moment I am having one turned from very thick MDF(fibre board)which will be epoxy resined inside for a smooth finish...it will be in 2 halves, bolted together. Any advice always appreciated. Cheers.
 
CC9.

What you describe is not what is typically known as injection moulding, although I guess technically you are injecting material into a mould.

In injection moulding, you normally take a thermoplastic material, melt it in the barrel of a machine, then inject it into a mould at something like 10,000 psi of pressure. This requires pres ion fit and very strong moulds. This means they are also expensive.

The process you describe is casting, which was my second option in the previous post.

In casting, you take a liquid resin, pour or transfer it to a mould with minimal pressure, then cure the resin with heat or a catalyst.

Your easiest way would be to use polyurethane casting resins then inject them into a mould at low pressure with a syringe.

You will need to find a supplier of casting resins in your area. Suppliers to the fibreglass industry might be a good place to start your yellow pages search, or a google search for PU manufacturers. Bayer and BASF are the only ones I know from memory. Maybe Upjohns?

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
patprimmer
Many thanks for your advice. Will check the fibreglass shop for casting PU. Maybe there is an "expansion" type PU available whcich would give a smooth finish when it is forced against the wall of the mould?
thanks again.
 
They do have self skinning PU foams.

I forgot to mention that it should be a foaming resin.



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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
Thanks, I hope my local fg shop stocks this stuff. Just one question, which would be lighter, PE or PU ? or am I stuck with the PU because PE is not for casting? As you can tell, I have a mould and heaps of enthusiasm, but zero materials knowledge!
 
PE is not available in liquid form, so it must be melted and processed in the molten state. The molten state is still extremely viscous and it would take days for it to pour by force of gravity.

You are stuck with PU.

Huntsman and Dow are also suppliers of PU

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
Thanks for all your advice. Appreciated.
 
thanks btrueblood. awesome website. will have a good look around. doesn't seem that any of the tubes/discs of pre-cast PE are big enough, I require approx 8" dia x 3" width...do you know if this stuff is easily shape-able/carveable/sandable? if I went for the pre-cast option?
 
You could also cut them from blocks of PP or polystyrene foam.
 
CC9, sorry so late in replying. Yes, urethane is relatively easy to work, using wordworking tools and sanding, and you can also machine it with metalworking tools. All of which have the following caveat: as you get to softer and softer durometers, the PU will tend to "chip", especially if the loading gets too high. Use sharp tools.
 
thanks demon3 - pp - is that another name for polystyrene or a different type altogether?

btrueblood - thanks again, sounds good, maybe easier than casting. My local fibreglass guys have some pu liquid, so I'm tempted. Trying to find a local supplier for the blocks of PU
 
Hi CC9,

PP is short for polypropylene. You can get that as a foam too and it used as packaging material. It is more damage resistant that polystyrene foam. The polystyrene is stiff but once deformed it will not bounce back. The PP is less stiff but recovers its shape.
 
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