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Hi all.
I have a hot knife cutter from hotwirefoamfactory.com, the 6" model, I think. The other night I decided to use it for the first time to cut some pink insulaton foam for some Aztec-style ruins I was planning to make.
I eteched out the dimensions I wanted, plugge din the cutter, and had at it. I discovered that the 6" wire was far too bendy to be reliable. I was expecting it to cut through the foam quickly and without a major anout of pressure. I was wrong. I had to press very hard on the foam to get the wire to cut, and by doing so, caused the wire to bow and bend. The resulting cuts were unreliable, at best.
Another phenomenon I encountered was what I can only describe as "hairing." lol. At the end of each cut, the foam would start to stretch, and "chase" the wire. This resulted in long strings of partially-melted foam growing outward from the cut. It looked like pink wispy hair was growing from the foam to the wire. While it breaks off the edges of the blocks very easily, I'd like to avoid having strands of wispy insulation foam all over my house. Is there any way to avoid this effect?

Having to work that hard is a sign that your wire is underpowered. The batteries might be weak, or there might be some buildup on the electrical connections.

Or, the tungsten wire is crappy quality. You might want to try getting a new tungsten wire from another source. I happy, healthy hot wire cutter cuts through that pink insulation foam easily. You won't set any land speed records, but you shouldn't have to push very hard at all.

Hmmm, that's a bummer. This is a plug-in version, and it's brand new.

It may be that you are simply trying to go too fast - try cutting much more slowly but with less pressure.

Well I used the GW foam cutter. If the wire is spanned tight and the current on, it goes through foam like a hot knife through butter. It is sometime hard to control where exactly the cut goes (needs afterwork with knife and file) and sometimes the frame gets in the way for long cuts. Otherwise I had no problems. I was just eager to avoid the burned foam fumes as they might be toxic.

Hmmm. The cutter does speed upwhen I use a sawing motion to cut, instead of drawing the wire straight down.

Right, I do that too slowly, not only to keep heated foam from sticking to the wire.

Having to work that hard is a sign that your wire is underpowered.

I've got a home-made one, and had to replace my original copper wiring leading to the tungsten wire in order to get the power through the tungsten instead of the copper.


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Wait, hang on, you have a hot knife, rather than a wire cutter?

Still sounds underpowered...

The problems you're having is because your cutter is underpowered. The strings are occuring because its incompletely melting the foam so its pulling light hot taffy. You should barely have to push to run your foam through.

Battery power is a bad idea when it comes to foam cutters. They just can't keep up to the current demands. All hot wire cutters use nichrome wire. Which is a nickle chrome alloy that acts as a kind of resistor. The resistance turns the DC current into heat. (Remember kids AC kills)

You can try getting a thinner piece of nichrome off of ebay. The thinner wire will burn hotter under the current load put out by your cutter, but will be more likely to break. Thicker wires are more durable but you have to pump more current through to get them hot.

There are some great articles on Terragenesis about building your own foam cutter. They recommend using a 9V walwart power supply but this can only handle the current load for so long before it fries. So you can only cut for a little while then shut the cutter down to let the walwart cool down. I built my own foam cutter using nichrome wire and an atx power supply.
An ATX has better over current protection and can be had cheap. You probably have a junk computer laying around ready to be recycled into a hot wire cutter. Steal the on off switch as well as it makes it nicer then having to unplug your unit every time your done cutting. The nice atx manufacturers even tell you which color wire is which voltage.

As for steadying up your cuts I recommend building a table model where the blade is mounted to a table. If you have a hand held unit I'd recommend mounting it in a bench vise to keep both hands free to control your cuts.

The other drawback of wire cutters is that as the blade heats up it stretches making it loose in the bow. So you may need to stop periodically to let the blade cool. I built my bow out of some old chair legs (hollow metal tubing) they can put a decent a mount of tension on my wire without allowing it to twist the bow. Those little piddly battery jobs have a pretty weak bow that just let the wire stretch out too much.

1) make sure your wire is clean. Sand it with some super fine sand paper. my tungsten wires get buildup on them and cut slower.
2) shorten your wire. Less surface aresa to displace heat means more heat in a smaller surface area. I'm not sure about your model, but most wire cutters suspend their wire on an aluminum frame which you can bend easily.

Thanks for the additional tips, all.
This is the exact model I own:

http://hotwirefoamfactory.com/product.php?productid=16167&cat=108&page=1

I'll have to contact the manufacturer and see about the underpower issue. Very odd.

In all of their videos they are cutting low density foam, and even in that it seems to cut very slow. It's probably way under power for high density foams (blue and pink insulation)