Foam Cutting problems
 

Hi

I am having a problem with cutting pink foam. I have cut before white foam and blue HD foam. I assume pink and blue are same.

Conditions are as follows:

Hot wire ctter
50" bow
DC power supply
.020 SS wire
Automatic feed (gravity system)
1/16" aircraft ply templates

Foam:
3" High density pink (polystyrene)
47.5 long (span)
9.5" wide (chord)

Wing
47.5" span
1.035 thick

My problem is the core and blanks all take on a bannana sghape after removing weights, and there is a fair bit of stress when I straighten out.

Any thoughts, I have attached pictures[img][/img]


9.5" chord
0 degrees dihedral

RE: Foam Cutting problems
 

Hi,

I have not cut pink foam but have had a lot of experience w/white. If your shucks/cores are assuming a banana shape after cutting, I am assuming they were a banana shape prior to your putting on the weights. Is that correct? If so, it would seem that you have some stresses in the foam that simply go back to the way it was prior to the cut. You could recut your blanks to square them up and that way there would be no stresses and thus no banana shape. I have sheeted cores that did have a slight curvature prior to sheeting, but once sheeted, they cured perfectly straight with the weights applied during a cure. I would suspect your cores would skin straight too. If your cores only take on a banana shape AFTER you cut them, I would suspect that your heat setting is too high and you are melting the center section causing it to shrink and the thus getting a "Banana" shape. If this is the case, cut a wing and reduce your voltage. I don't how to suggest how much as I don't know your power source. You might want to try one wing half to find out. I think that is the way I would go first.

Deadstik....

RE: Foam Cutting problems
 

Hi

Don´t be afraid about this thing in pink foam it is quite normal just put the wing in a flat surface when you make your wing and put some weight over.

Marco Vergara

RE: Foam Cutting problems
 

This is what works for me. The way I check my voltage or "heat of the wire" is:

Assuming you have an adjustable power source, while holding the cutting wire horizontally, sight down the wire as you turn up the voltage. As soon as you see the wire start to sag, back off just slightly, until the wire just comes back straight. This always seems to be the perfect setting to me.

While cutting, I find it helps to have a partner. Also make sure you have graduated marks on your templates. As you cut, call out what mark you pass so your partner keeps up. He can either slow down or speed up slightly so you both are about at the same point. This even works on tapered wings - even though the tip template is smaller, the marks help so you finish the cut together.

I use a slight downward pressure on the bow as we're cutting. This tends to help keep the hot wire straight (between the templates) and keeps the wire from dragging in the middle (which can create a "banana" look to the core).

RE: Foam Cutting problems
 

I read somewhere that the bowing of the pink or blue foam ( one made by owen corning, the other made by dow) after a cut is because of the production process of the foam. I have cut a few cores and most of mine were bowed. After skinning the cores they stayed straight. I've only had one that was no good, had a very bad twist in it after skinning with 1/16 balsa. On a tech note, has any one used titanium wire. Bass Pro shop is carrying titanium leader wire and stainless in various thickness.(.012-.020)

RE: Foam Cutting problems
 

I have cut all three type of foam. Simple heat adjustment. Graduated templates makes it alot easier.

RE: Foam Cutting problems
 

The Pink foam is extruded, that is it is pushed through a die, so it could have some internal stress built into it at manufacturing time.

Bead foam sheets are cut from a large block and are less likely to have internal stresses in them.

Does the foam lay flat without the weights on it?

Try cutting a small strip off the top and bottom of the foam blank. Shave of a 1/4!QUOT! or less. If it curls after cutting, try to square up the now curled blank before you cut your wing.

Dave