something is fishy

[jacquesm]

After some tests, I'm convinced now that the foam I'm using (regular pink styrofoam insulation) does not have a uniform density. The result of that is that as soon as the 'top' layer is cut off the rest of the foam will expand a little.

This may not seem like it's a big deal, but on a 3' blade the expansion causes a 1.5" drop at the tip of the blade (towards the tower). I'm not sure if there is an easy way to fix this, or if this is something that I can cure during the 'wrapping' with fibreglass. I'd rather have the shape stable rather than to have it under tension while it's being covered. That would make covering it a lot harder.

Also there may be suppliers of foam that has a uniform density. I think I have spotted some much larger billets in the local hardware store that they use for boat docks, I'll have a look at that on monday. They'll have the same basic problem but I can just shave the tops off both sides and get a reasonably uniform block like that.

Tomorrow and on Sunday I'll be off to Toronto.

[monte350c]

Hi Jacques,

I noticed that too, and found a web page a while back that described the manufacturers cutting larger blocks into the retail sizes using - you guessed it - an industrial size hot wire. I think the hot wire or whatever they use sort of seals the cut surface, and the 'natural' foam underneath is kind of held there under a bit of pressure after they cut it.

You hit on one of the problems I have not solved using my hot wire cutter - the wing pieces DO grow slightly after the cut. With your setup you may be able to solve this one by making more pink dust and cutting the outer inch or so off the block prior to starting a wing cut.

Ted.

[jacquesm]

thanks for the confirmation, I was feeling a little weird earlier today, it's like you're hallucinating. The effect was real enough, it took me hours to figure out what caused it and that it was reproducible. In the end the best tests are the simplest: fillet the top part of a piece of fresh foam: presto 1/2" bend on 2" of foam ! That clinched it.

Good to see I'm not the only one

I'll get back to the grindstone on Monday and fix it one way or the other.

My ears could do with a rest too.

[Dan M]

I don't think it's necessarily a density issue as much as a residual stress issue.

Since foam expands (even non-expanding foam expands a little) as it cures, and the outside hardens first, you end up with residual tension on the outer surfaces, and residual compression in the middle.

When you "skin off" the surface, you remove a tension field and the blade moves to find equilibrium again.

You get the same thing with steel, but since it contracts as it cools (both from delta-T and a nasty phase change) you get the opposite, tension in the middle and compression on the surface.

I don't know if there is an equivalent to annealing where foam is concerned.  You might try warming a piece a little, not enough to melt it, but enough to make the outside "relax a little" and be more accommodating to what the foam in the center is trying to do.

It might be a fun experiment to place a piece in a large pot of boiling water.  Once it is cooled, cut a little from on surface and see if the distortion is less pronounced.

I've also heard of using the exhaust from a car to gently heat up plastics.  It might work with foam.  Be careful if you try this.

Have Fun,

-Dan M

[jacquesm]

I don't know if this stuff is extruded or cut with a wire, but there sure seems to be a big difference between the center and the outside. What I can try to do is laminate a bunch of it together, then cut billets out of that. (in the 90 O rotated direction of course). That should even out all the stresses, and any expansion will be lengthwise and fairly uniform instead of in the plane in which the shapes are cut.

A rough estimate of the density change (by weight) is that the outside is almost twice as dense as the inside.

Warming it up a bit to relieve the tension is an interesting one, I'll try that with a little piece (kind of hard with an 8' by 2' slab to do it uniformly)

The exhaust trick has a high 'bad vibes' index associated with it, I don't know if I would want to try that one without thinking it through really thoroughly.

[nack]

I think it is breaking the surface tension that allows the distortion.  De-skinning the whole block before shaping might get the distortion out of the way before any critical cuts are made.  Or maybe the answer might be to carve the outline first, then jig it up and cut the airfoil?  It looked to me like that was what the model airplane guys were doing, but then again they are also working with tiny blades.  Maybe a combination of both?  Maybe a different foam - if you can find a release agent that spray foam won't stick to, you could try injection molding - maybe the first layer of fiberglass cloth would be the release agent?  Your table would come in very handy for cutting the molds.  Injection would also allow you to cast it around a metal spine.  Just a few random (semi)thoughts, I don't know that any of them would be useful, but you never know.  I have enjoyed following your saga this far, and really hope to see it to a happy ending.  

[jacquesm]

Injection moulding is an interesting one, I have briefly experimented with it (using poyurethane), and while it works in principle getting that to work reliably is going to be one painful delivery. I'll definitely get back to that once I have my machine up and running because it seems to be the way to go in the long run.