Solvent for expanding foam

The polyurethane foams will dissolve with acetone before they've cured. But once cured, mechanical removal is best. Burning will work, but the fumes aren't good for you.

Mark Rand RTFM

Screwfix do a cleaner for cured foam, not tried it myself and the "brush & scraper" included suggests it won't just wipe off!!

?_dyncharset=UTF-8&fh_search=32482 Jason

stuff:

You could try paint stripper Peter

I don't know how much rigidity some foam might add - it's gotta be something very thin!

This is the sort

Foams vary, but most of them are polyurethane-based. That doesn't necessarily mean that they are mostly polyurethane, it means the molecules stick together on order using polyurethane linkages, while the backbone of the molecules can be almost anything.

A good pure short-straight-chain polyurethane cannot be removed by any chemical means within the amateur's cookbook (and is almost completely UV resistant, about the only plastic which is) - but fortunately these aren't found in commercial foams of this type, too expensive.

However it's hard to say exactly what _is_ used in any specific case, so your results will vary according to the product used, the formula, the batch, it's age, the time of the month, the positions of the planets etc.

If your workpiece will stand it, a soak in almost-boiling caustic soda solution will weaken most foams to the point where they give up easily. After that it goes: methylene chloride with some caustic added, methylene chloride, you can try thinners and see.

For all these you will need a minimum of plenty of ventilation, eye protection, and gloves. A complete suit of tyvek or similar if done in quantity. Also, be very careful if you add water to hot caustic solution

- the solution can be at a temperature above the boiling point of water, and the added water can flash to steam causing you to be sprayed with boiling caustic soda ...

Have you thought about wax, or polycaprolactone (aka polymorph, the plastic which melts in boiling water)? Wax is easier to use, and cheaper, though both are reuseable.

-- Peter Fairbrother

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I don't know how bit the workpiece is but could you melt plumbers solder and pour in and melt out again for re-use?

Bob

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That should have read 'how Big' !!

stuff:

Perhaps another way would be to cast in low melting point alloy, I have use this in the past for bending concentric tube assemblies,(2 tubes side by side inside a third and managed to bend the whole lot than gently warmed it up to pout the metal out) The stuff is expensive, melts at about 80 C and can be reused and meled out in boiling water. Peter

A lot of the expanding foams are only removable by mechanical means once cured, they are very resistant against chemicals.

Can you not turn up a mandrel for the sleeve to fit onto?

Google for CERROBEND instead of foam

It might be what you are looking for.

One-part foam needs moisture from the atmosphere to cure so it'll only fil relatively small gaps ( afterwards. Is there a solvent that will disolve it or should I just

How about meltable mouldmaking compound such as

You should just be able to melt it, pour it in, do your machining, and melt it out again for later re-use.

Chris

Thanks for the suggestions.

The cylinder is 4" diameter and 5" long so expensive low melting point alloys are not an option. Nor is a mandrel really since the inside is stepped.

One option that I might look at is a wax/foam composite (wax as a base layer to even out the internal irregulaties with a foam inner.

And whilst this is a "home" job I can probably get access to a broader range of chemicals than most since SWMBO is a science teacher.

Charles

Doesn't wax shrink considerably when it solidifies, or am I thinking of something else?

Cliff Coggin.

Wax has been mentioned but has the problem of shrinkage, maybe you can use a mix of an inert filler and wax to counter this. Maybe something like cement, plaster, or talc and just enough wax to bind it mixed in. I was reminded of this by the repousse base recipe I used which was largely pine pitch with added tallow and talc IIRC.

Another thought, something I saw years ago around 1979 or so in a model shop in LA was IIRC some sort of fine glass bead or hollow sphere which was placed and then a superglue type adhesive used to bind them, the result was apparently a rapidly applied lightweight rigid but easily sanded filler. The model shop was making flying models as props for movies. I've not seen it since but then I don't think I've seen any professional plane model builders doing their work since. IIRC these days superglue adhesives have to be water soluble to some degree to aid removal of unwanted bonds.

Microballoons :

e.g.

Perhaps turn a wood mandrel that fits the smallest inside diameter and then pour wax in around it, while heating the cylinder with a hot air gun. Then after it has cooled, heat the cylinder again to melt only a thin layer and relieve any shrinkage stresses. You would have to take light cuts or cool the cylinder.

Hot melt glue is another option.

jsw

In article , Charles writes

Charles,

Found this discussion fascinating. Agree polyurethane foams are not the thing, the fumes given off on thermal decomposition are pretty toxic.

Have you considered using ice? You'd have to be pretty quick, but it's cheap enough to try; after all, you did say it was only a finishing cut, so there wouldn't be too much heat from the cutting. If you could find something to mix with the water it might stay solid for a lot longer - a bit like that Geoffrey Pyke chap in WW2 with his pykrete aircraft carriers; see

Perhaps a job best done in the depth of Winter?

Are you sure it is rigidity you want, not just damping? If the latter, stuff it full of plasticene or blu-tak.

David

Wax does shrink considerably - but having tried making candles it might not be a problem. In my experience the wax solidifies where it's in contact with the mould first so the liquid then flows to fill the shrinkage and leaves a big dimple in the open end - which can be filled with more hot wax - or avoided by extending the mould so that the dimpled portion can be cut off.

Russell

Charles, I doubt the foam would be very successful, even if you were to try it and in addition it is truely vile stuff to use. The 're-sealable can' is marketing twaddle in my experience. It's a one shot mess, it sticks like the proverbial to a blanket to everything except what you want and it goes everywhere and a few more places as well.

How about making a rough plug to go up inside the job from a Blue Peter toilet roll or a block of wood or whatever and then just filling the narrow gap between the core and the bore of the job with something a little more convenient such as the various suggestions to date.

The ice does sound a fascinatng proposition, if you have it in the deep freeze down to say -6 or so it would probably give you half an hour before warming up too much, I guess it depends on how agressive you machine requirements are. An additional lump of cardice inside once you take it out of teh freezer would give you quite a bit longer.

Richard