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Anyone ever built a hot wire knife for cutting foam?

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  • Anyone ever built a hot wire knife for cutting foam?

    I need to cut a bunch of polystyrene foam into shapes for a body mold I am building for the tech club. I plan to build a table with a long arm to provide a throat for cutting 4'x8' sheets up to 2" thick.

    I plan to use this transformer from Menards, guessing 8 volts is plenty, I have seen people use battery chargers too. If 8 volts is not enough I can go up to 16 or 24. The dimmer switch will help to fine tune heat as well.

    If you have experience with this sort of thing I would appreciate some insight, tips, etc.
    If the women don't find ya handsome, they should at least find ya handy

  • #2
    I made one using an HO train transformer. It worked fine and was adjustable, similar to a rheostat switch.
    Kansas City area


    • #3
      yeah .. built one with stainless gardening wire and 12 volt supply. works ok, but your adjustable volt supply is better. if left on for more than some seconds without cutting something, the wire gets too hot and melts. also the part of the wire that goes beyond what ur cutting gets excessively hot, so size (length) matters. and u need to have it under some kind of tension or adjustment because as soon as it heats up it gets longer and "loose"


      • #4
        Yes, it worked quite well. You have to be able to adjust the voltage/current.

        You also will find that some foam curls/melts back from the wire, so it never re-joins. Other types (I forget which) like to re-weld.

        Ours was for just a couple uses, so it was made of 1 x 2 and c-clamps. Still worked fine. I think we found some nichrome wire in the lab and used that, but for short term use, even copper wire will work. You just don't want to have it too hot, and you need to be able to re-tension it or keep tension on it.

        Wire gets hottest in the middle of the span, so figure to cut with that part of the wire.

        EDIT: We used a transformer and variac to control current in the wire (wire temperature).
        Last edited by J Tiers; 03-12-2018, 11:45 PM.
        CNC machines only go through the motions


        • #5

          You don't say what wire you will use. Ni-chrome wire will hold up better and give you even heat over the length of wire and should not brake as easily vs another wire like say copper or steel.
          Just a FYI so that the project can be a success for the club.

          Mr fixit for the family


          • #6
            Stan at Bar Zee Industrial did one using a welder as a power source.



            • #7
              I made one to cut foam parts. It worked very well. The picture you gave above is basically what mine looked like. I just made it out of wood.

              For power, I just used one of my beater bench power supplies like this one (pic below) that allows you to control voltage and current. IIRC, ~5v with ~1-2amps would heat up a hot wire to instantly cut foam sheets. I just bought this wire on Amazon:



              • #8
                Done lots of hot wire foam cutting. You can use nichrome, kanthal, or stainless steel wire. If you can get kanthal, use that. I've used dryer element wire and been happy with that. What I've done is to make connections from the cutting wire directly to some copper sheet to help keep the temperature down at the ends, and to help keep the exposed portions of the wire from overheating too much. If you're working with known thicknesses of foam, you can double over the ends, leaving the single strand center portion not much longer than the thickness of the foam you're cutting. That helps to keep the temperature down at the ends as well. Make sure you have some way of automatically taking up the wire tension- spring load one of the ends. A good temperature to cut with is somewhat below the point where the wire would begin to glow. You need real amps, so a wall wart or anything like that isn't going to 'cut it'. Dryer element wire works fine, and will cut a gap wide enough that the foam doesn't usually weld back together, although sometimes you will have a problem with beads of plastic sticking to the wire and screwing up the cut. When you cut, try to keep a consistent speed and don't dwell anywhere.

                You'll need a power supply that can output 3 to 5 amps at anywhere from 2 volts to maybe 10 volts, depending on the length of the cutting wire. I've had good results using what they call a filament transformer, and running that from a variac. Gives a very positive result with fine control. What I use now is a modified transformer where I stripped the secondary and wound on my own, with a tap every 2 or 3 turns or so. Running all the taps out to a terminal strip gives an easy way to select the appropriate voltage for the particular gauge and length of cutting wire in use. To give fine control, I put a tap at every turn for about 5 taps, then a tap at every 3 turns after that for about another 5 taps, the at every 5 turns for another 5 taps or so. You can pick the voltage off from the beginning of the winding, then select a tap which gives just a bit too much heat, then move the first tap over to another terminal. The first few taps gives finer voltage control, while the last few taps give a wider voltage range. You can pretty much get within a half a volt or so using this method, and still have up to 20 volts or so for the longer cutting wires. This is just a plug it in and cut power supply, no electronics involved. I think for what you're doing a transformer with a 12v secondary and driving it with a dimmer capable of handling inductive loads would probably work ok. The transformer secondary needs to be rated for minimum 5 or 6 amps so it won't get too warm.

                Ventilation is important, just a reminder. I do my cutting outside when I can.

                As far as connecting wire, use at least a 14 ga, and heavier is better. Not only will a heavier gauge help keep the terminal ends cooler, it will give a better control over the temperature. I usually use monster wire or something like it, both for lower losses and better heat control, and for the flexibility if the cutting wire is mounted to a hand held frame.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                • #9
                  I have made several foam cutters for cutting foam wings for RC airplanes. I use SS fishing leader around .014 for cutting wire. I have used various power supplies to run them. The easiest to use is a car battery automatic charger. You can also use a car battery.
                  Sole proprietor of Acme Buggy Whips Ltd.
                  Specialty products for beating dead horses.


                  • #10
                    Here is my hot wire cutter that I use to make patterns for lost foam casting:


                    • #11
                      I like to use SS fishing leader wire


                      • #12
                        I got a piece of Rene 41 wire from ebay. Allows you to have the cutting wire real tight as its stronger at 1600F than stainless steel at room temperature or 10 times the Kanthal tensile strenght at high temp.
                        Location: Helsinki, Finland, Europe


                        • #13
                          If you're looking for small pieces of kanthal or nichrome, hit a ecig shop. It's used for rewinding exit coils.


                          • #14
                            Originally posted by achtanelion View Post
                            If you're looking for small pieces of kanthal or nichrome, hit a ecig shop. It's used for rewinding exit coils.
                            Are those exit coils the coils that keep exit doors closed with magnet locks? Sounds like a trap...


                            • #15
                              Ducking auto co wreck.

                              That should be ecig coils.