Thursday, September 4, 2014

Power Supply Convo...

I guess I'm not sure if I should keep posting these emails or not.. The questions seem to be common and I get them regularly. And I suppose not everyone reads the blog or news section, lol... BUT maybe it will help someone searching the web if the happen upon it. SO, here's another PS question.

We need to cut PU Foam having a density of 6 to 7 kg/ by using a straight
hot wire cutter.
As per suggestions, we have worked out to use  a  26 gauge NiCr wire of 2 feet length for a straight wire cutter using a power supply of 12 volt 2 amp.

I think you should anticipate at least 4 amps.. The 2 amp mark is really a minimum when it comes to cutting foam. Also, try to find a power supply that allows you to adjust the voltage.

When you start to cut foam, heat exchanges from the wire to your substrate (PUFoam). As the wire cools, it takes time to heat back up to a cutting temperature. In most cases what happens is your cutting speed slows dramatically, and most often will lead to wire breakage as people push harder to cut faster. 

With 3-4 amps you have a hotter wire, that can replenish heat faster, and cut faster without compromising your wire. Ideally the substrate (PUFoam) never actually touches the wire, it is the heat radiating around the wire (the kerf) that actually cuts the foam. 

Please let us know if we need output voltage as 12v DC or 12v AC.

AC versus DC, doesn’t matter. It is just important that you have enough current. Try to find a 24v4a variable power supply, and you should be all set!

Depending on where you’re at in the world, this power supply should suit all your needs and is rated for international use. 

Thanks for your guidance and are sure that will positively be of immense use to me.

I would like to tell you the exact use of hot wire foam cutter which I want to use for commercial
purposes to cut low density (ranging  from 6 to 7 kg/ PU Foam sheets of thickness varying from 2.5 mm to 6 mm stacked to a height of about 18 inches (i.e. say 100 sheets of 4.50 mm piled over one another) and cutting to smaller size sheets of size 8 X 18 inches from bigger sheets of size 32 x 72 inches.

So please suggest what type of nichrome wire and power supply would be required for a straight hot wire foam cutter so that large volumes of the above sheets can be cut.

It looks like you’re cutting wire will need to be 36” wide to make a comfortable cut. You always want to give a little extra cutting line to make it easier on yourself, but should be able to get everything to line up if you have a standard cutting jig or a stop-board. But I’m sure you can figure that out. 

Polyurethane foam is a little funny when it comes to cutting. Because it is an open cell foam, the little pockets of air between the foam matrix will actually suck up a little heat during your cut. Interestingly enough, the air acts like a little insulator during the cutting process and actually slows things down further.

At cutting lengths over 24” I tell people it is a good idea to go with 24 gauge nichrome wire. It is a little more stiff and is a little stronger than 26 gauge wire. Plus, the voltage/amperage requirements of 24 gauge is a little easier to deal with than 26 gauge nichrome wire. 

One draw back with nichrome wire at longer distances is that it may be hard to control the wire expansion and line “sag.” I’ve had some wavy cuts with larger cutters, but you can help prevent this by making the wire quite tight with a spring and use a higher temperature. A higher temperature helps less PU foam touch the wire during cutting, and the tight spring helps prevent line sag.

One other thought would be for you to use/try Rene Wire (link). Rene is sort of a “top of the line” cutting wire. It is highly resistant to breaking, won’t have as much line sag, and cuts well for both hobby and industrial purposes. For power supply calculations, 27 gauge Rene wire is similar to 28 gauge Nichrome wire.

If you want to make your own power supply, I would recommend the XFR3018E transformer from Jacobs-online. You’ll have to have a bit of electrical ability, and wire in a dimmer switch. I have a walk-through tutorial where I use a different transformer, but the images are essentially the same. 

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