Liquid Nitrogen... for freezing... anyone have any thoughts?

[IslandTractor]

I hate it when that happens...!

Old joke: Two doctors are walking behind a man who has a funny limp. First doctor says it's probably an orthopedic issue. Second doctor says it's more likely a neurological issue. So they stop and ask the guy which one of them is correct. He replies "we're all wrong, I thought it was a fart".

 

[dstig1]

Someone above noted the big complication: CTE or coefficient of thermal expansion (or contraction in this case). The foam is going to not only shrink, but probably distort quite severely from LN2 as it will likely not cool uniformly. What you cut will not be what you get when it warms back up. It may also become very brittle as others noted. I would need to know the foam to look up the Tg and other thermal characteristics, if that info is even available. The larger Dewars are the standard you are going to find from a gas supplier. If you can find someone currently using LN2 (say from the supplier) you might be able to negotiate with them to borrow a cup o' LN2 to try it out. Just tell them you will return it in the air in gas form The other big caveat of renting Dewars is that they constantly offgas N2 as the LN2 heats up in the tank. You leave it long enough (several days to a couple weeks) and it will be all gone. So you need to use it right pronto when you get it. We use it at work on occasions for cooling test samples or other things such as analytical work.

Very complex shapes are fabricated from foam all the time, and another poster above noted that they deform the foam and then cut it straight. That is probably the route you need to explore. If you really only want a partner to do the fab work for you, then look for foam fabricators on the innerwebs. I'd bet you could find someone who will take a CAD file, your spec for the foam, and deliver you a part without too much effort. That also minimizes risk on your part from buying expensive equipment if the venture does not take off. You can think about doing your own manufacturing later on.

My $0.02

 

[SpringHollow]

Realize he is not talking about precision machining where you have to worry about 0.001". He could be off by 1/4" without any real issues.

There is no reason why you could not hook up a looped thermal wire to a CNC and let it cut the foam. But I could see that getting messy - depends upon the foam. Some behave better with that process than others.

A laser might be another possibility if the foam does not catch on fire. That would allow you to do 3D since you can adjust the power or focus so that you only burn off a small amount at a time.

Ken

 

[Arc weld]

I think liquid nitrogen would turn the foam to glass and it would shatter.

 

[GLyford]

at a certain point I think that making a mold is will be quicker. If you have to do a couple of processes and have time limitations for maching the foam then where is the advantage over the mold. I can't imagine injecting the foam into the mold is a time consuming or difficult process.

Oh, that's an idea!

Maybe machine a reverse shape in styrofoam or builder's foam. Either direct profile, or as a series of 2D "slices" which are laminated and then filled or finish sanded to remove the steps. Support the outside rectangular shape in a stiff wooden? plywood? box, then use the foam mold to cast two part expanding foam for the final product? I don't know what sort of support, surface, or mold release you will need for the two part foam, or if the right resistance weight of foam is available in a two part version, but it is one way to back into the project rather than tackle it straight on...

 

[SpringHollow]

Making a mold is not very realistic if each pillow is custom.

 

[GLyford]

That's why I was thinking builders foam or regular styrofoam. You'd reuse the casting box.

If the tradeoff is between doing it with a little more work/materials/expense, and not doing it at all...?

 

[Dadnatron]

The foam requires heat during curing process. Wax won't work.

My initial thought was to do what you have offered, however upon contacting a company in Canada, he said I should be able to mill the foam. I flew to Calgary and saw his system, and it worked, however, the foam he had available was closed cell and much more dense. (Both of which make milling easier) But I was intrigued enough to work on a solution and I haven't hit a solid wall yet. I'm still not convinced I'll need the freezing, but I know that it would help me at this stage.

Making a mold for each one would require either a different foam process or be so costly that it wouldn't be a feasible proposition for patients. I could never ask a patient to pay $6,000 for a pillow, given that each mold would cost $5K at least. Then the cost of production afterwards, then demolition of the mold. Not only that, but I want to be able to insure that I guarantee the pillow's efficacy. They won't be cheap, and I want to be able to 'fix' the ones which don't work for patients.

 

[Dadnatron]

Wire won't work for these shapes.

Laser and water jet won't work. Nothing that cuts in a straight line will work.

I've looked into compressed deformation cutting, and that was the second direction I was thinking about. However, these pillows are 3D both top and bottom, and no fabricator that I've found has a system to make them. I'd do it in a heartbeat if I could. Then I'd hire the guy and buy the machines.

I think I will be able to do this... with a 4D flat bed machine. Flipping the pillow for the back side contouring. Freezing might not be the direction I go.

I really think I need time on a flat bed CNC table. A bunch of foam, all my bits, and some very simple programs to just cut a few curves. Hopefully I'll be able to find someone with a machine and I'll be able to narrow a few directions.

 

[SpringHollow]

You can do 3D laser cutting. The power level, duration, travel speed, and focus all change the burning characteristics/depth of burn. The real question is how the foam is after being burned away.

You do not think submersing in water and freezing to a block of ice would work if you had troubles machining just the foam?

I assume a very fast spindle speed and fine flutes or grit to minimize grab probably cuts the foam best?

Ken