The plan is to rough mill oversized blanks of a motor end plate (roughly 125mm diam x total of 37mm deep) with deep lugs that I have to make, use it as a pattern for a casting (home foundry), and then finish (cnc) mill the casting.
I could tell you some answers to that question, for instance mylar or even polyethylene films, but as you'd have to kill me after I won't :(
It won't work, especially as a foundry blank, the very rough initial result will expand slowly for days, and then be dimensionally unstable for several months. After all that, it will still have some too-big holes in it too, and be very uneven.
Use polystyrene foam instead, it's cheaper, better, much more regular and much less toxic than PU foam (which gives off cyanides when heated), especially the home-made sort of polyurethane foam.
"Blue" polystyrene foam building foam is particularly good, fine grained, strong and rigid, but even ordinary expanded polystyrene foam insulation should do, and it will be much much much better than polyurethane from a can or two-pack. Promise.
Sheffield Insulations for example will usually supply blue foam, try
for a local branch and give them a ring.
"Sweet dreams are made of Anything that gets you in the seam And I feel like I'm Seventeen again." Eurythmics
BTW, blue foam mostly comes in 2 foot by 4 foot sheets (or larger), and should cost under £20 for a 50 mm thick sheet. A similar-sized sheet of ordinary expanded polystyrene foam should cost well less than a fiver :)
-- Peter Fairbrother
Always retaliate first.
Pre-emptory retalitation works! Iraq didn't attack the us with chemical weapons, because we got in there first and STOMPED them!
Kingspan are the manufacturers of a foam insulation product usually foil clad that comes in a range of thicknesses according to U value needed. It is similar to the generic 'blue foam' referred to by others apart from the foil and being a mid to dark biscuit colour.
A comment from someone who works in the polyurethane industry. PU foam either rigid or flexible does not give off cyanide when heated unless at very high temp in the absence of air (=pyrolysis). It is cured polystyrene which evolves cyanide on combustion. However the
2-pack urethane systems do give off some diisocyanate as the foam cures and inhalation of that must be avoided. The DIY rigid foam cans ("one component foam") can also release a small amount of diisocyanate so if used, make sure there's plenty of ventilation until the foam has fully cured. IMHO Bob Minchin's advice is the best - get some ready made rigid polyurethane foam such as Kingspan. It will cut with a fine saw and you can pin the panels together with wooden meat skewers.
Some years ago CDT teachers were encouraging kids to carve blocks of polystyrene and use them as a formers for simple foundry casting with aluminium - like lost wax casting. Sugggested items were things like lamp bases for subsequent finishing on the lathes. There were a few nasty cases of cyanide inhalation as the polystyrene flashed in contact with the molten metal.
Once upon a time I was a research chemist investigating UV degradation mechanisms in polyurethanes. I have also studied the condensation of substituted ethylenes, including styrene, and done work on many other polymers. That was all a lot of years ago now, but I haven't forgotten the basics:
The idea that polyurethane does not give off cyanide and polystyrene does is complete nonsense. The truth is the other way round.
It is said to be "spin" (actually lies) put about by the PU industry, but I don't know where the story actually came from.
Cyanide from burning polyurethane foam in furniture still regularly kills people, probably at least 50,000 per year worldwide. Yes, that many. There are zillions of studies, and you've probably heard that anyway. Polyurethane foam is used in furniture, not polystyrene.
Wrong. Pyrolysis is what happens when foam is used as a foundry blank, or at least in the lost-foam process ... but polyurethane foam _does_ give off cyanide both when it is heated, and when it burns.
A quick Google turns up:
" One of the proximal toxicants is HCN liberated from the nitrogen-containing polymers, like polyurethane,"
Or "Cyanide toxicity from the thermal degradation of rigid polyurethane foam"
has a nice list of the nasties involved.
That's not only incorrect, it's chemically impossible. Polystyrene does not contain the element nitrogen, cyanide does. As do polyurethanes.
Polystyrene can give off nasties like toluene and acrolein when heated or burnt - but not cyanide.
I've read the other posts [even answered one] but seriously if you want to use expanding polyurethane as a casting material then the mos readily avilable materials for the mould is polypropylene [PP], polyethylene [PE] or High Density polyethylene [HDPE]. These can be obtained in sheets by cutting up plastic bottles, buckets warming with a heat gun (on low setting) and flattening out.
How do you know you have PP, PE or HDPE? Simple - take a small piece, ideally about the size of a match, set fire to it [one of those mini torch type lighters is good for this] and blow out the flame.
TWO SAFETY POINTS-
Hold it in such a way to not burn yourself, molten plastic sticks like a bastard to the skin and burns until you get it off.
Do this somewhere well ventilated; if it is NOT one of these three, the fumes could be very toxic.
LOOK at the flame as it burns - If it is a bright sooty or smokey flame the plastic is usually polystyrene, PVC or polyurethane. When you blow it out the smoke will be black. This smoke will be toxic so stop the test here. PET also gives this result but is very hard to work and deforms badly at temperatures over 76 degrees Celsius so it's a dead loss anyway. If the flame is not sooty or too smokey [about the same as a candle] then CAUTIOUSLY smell the smoke. If it smells like candle wax then it's one of our three
A somewhat simplified explanation follows, the gory details involve a lot of math.
In a hydrocarbon flame - polystyrene, like petrol, is a hydrocarbon, made only from hydrogen and carbon atoms - the burning fuel can grab one atom from a molecule of oxygen, leaving behind a lone atom of oxygen (known as a radical). These radicals are highly reactive, they make oxygen molecules look inert by comparison, and they can react with an otherwise fairly inert nitrogen molecule from the air.
Cyanide is a compound of nitrogen and carbon, the cyanide ion has the formula CN-. There is no similar mechanism for carbon to react with nitrogen gas in a flame - there is no way for a carbon-nitrogen bond to form - which is why you don't get cyanide produced when petrol (or polystyrene, or candlewax, or diesel, or butane, or any other hydrocarbon) burns.
There have to be carbon-nitrogen bonds, as exist in polyurethanes or nitriles, already in the fuel before cyanide can be produced.
Note that smoke from burning polyurethane can contain concentrations of cyanide hundreds of times higher than the typical concentration of NOx in exhausts.