Hot Wire Foam Cutting

February, 2017

I’m working on my next robot and need a sturdy, but very lightweight material for the body.  I came across Smooth-On’s FOAM-iT Series of castable, rigid urethane foam and felt it would be a good fit for my needs.  I chose FOAM-iT 4 to get the balance of rigidity and weight I was seeking.

One my casting attempts resulted in issues when I did not properly estimate the amount of liquid to use to make the foam.  To save the mold, I pulled the top off the form and poured in some additional liquid and let it expand to fill the gaps.  Although certainly not ideal, it was an effective approach and I was able to save the mold (whew!).  The nice part about FOAM-IT is that you can add more and it usually fills nicely.  Unfortunately, the result was a bit of extra foam that needed to be cut off along the top.



There are many great videos online (like this and this) that discuss the use of hot wire to cleanly cut through foam.  They coach a gentle touch and let the hot wire do the work for best results.  So, I thought I’d build my own hot wire cutter to fix the problem with my foam mold since I had some extra Nichrome 60 wire left over from a previous project.


Small Hot Wire cutter

To fix the problem with the mold, I only needed the wire to be about 4 inches long.  I used some scrap 1/2″ plywood and quickly fashioned a Y-shaped handle.  I strung a small length of Nichrome 60 wire and used a spring to take up the slack since the wire expands when hot – I want the wire to be taught while I’m cutting and not bow.

I found an old PC wall plug and cut off one end to expose the wires.  I connected the black and white wires to each end of the Nichrome wire using some screws.  For some reason, I was feeling a bit fancy, and I decided to wire in an on/off switch (red latching button). I used zip ties to hold the wire in place.  Now I’m ready to start cutting.


I plugged the cable into my 500VA Variac transformer and flipped on the power.  POP!  Big mistake.  I forgot to check the voltage setting.  It was set to about 25 VAC which was a good setting for my previous project using a much longer strand of Nichrome wire.  The 6″ strand of Nichrome got red hot and snapped in less than a second – it sounded like a gunshot and freaked me out.  At this moment I realized that I wasn’t wearing any gloves or eye protection – stupid!  With an over-abundance of caution, I re-strung the Nichrome wire (good thing I had a lot), and slowly turned up the voltage starting at zero.  At about 11 VAC it was starting to glow.  Ok, now it’s finally time to start cutting.

My initial attempts to test on a small piece were met with a lot of resistance.  Unlike the videos where the hot wire glides through the foam, my experience was one where I had to use some force to move the wire through the foam which resulted in a very bowed wire.  The burning foam created a LOT of thick white smoke and resulted in a black charred finish on the foam.  At this point, I put on a mask to protect myself since I thought the smoke might not be the best thing for me to breath in.  A bit of sanding and the black charred foam was gone.  After the test cut, there was some thick black residue left on the Nichrome wire – I was able to get it off easily by scraping it on some cardboard while still hot.

I attempted to use the same hot wire on some Styrofoam.  In this case, it cut very easily with no effort and resulted in a perfect finish.  Ok, so not all foams are the same.  In my case, it was going to take some muscle.  After letting the wire cool, I remove the spring which was only making the wire bow worse.  I then went to work on my first real cut.


After about 20 minutes of slow cutting and LOTs of white smoke, I had finished half of the top.  Again, a bit of sanding was required to remove the charred finish.  It was looking pretty good!  The wooden form was helping to guide the hot wire across the top so I was getting a nice straight cut.  After another 30 minutes, I had finished all the cutting.  It was not an ideal way to cut the foam, but it was good enough for my needs.


Large Hot Wire Cutter

Next, I moved to the build of a larger hot wire cutter.  I used a different approach for this one that let me adjust the size of the wire (credit to Christopher Moore for this idea).  I skipped the fancy on/off button since I really didn’t need it – I could just use the on/off of the Variac instead.  I also skipped the spring and used an elastic band at the bottom to keep constant pressure on the wire to keep it tight.


My goal was to cut a 1/2″ square notch into the top of a 15″ wide foam piece.  I created some plywood guides to help me get a clean cut and clamped them onto the side of the piece.  I had to use a bit more voltage to get the longer Nichrome wire hot, but was able to dial that in quickly with the Variac transformer.

Unfortunately, this design did not work well at all.  Despite the elastic band keeping tension, there was a significant amount of wire bowing because of the pressure required to get a large piece to cut all at once.  The resulted was an uneven cut with a LOT of scorch marks left on the foam because I had to move much more slowly to cut such a long piece all at once.  After a lot of sanding I was able to get rid of some of the scorch marks, but the result was very poor and the 1/2″ square was more like a 1″ square.



Lessons Learned

  • Not all foam cuts the same.  Styrofoam and other “soft” foams may cut easily with a hot wire, but FOAM-iT 4 does not.
  • For longer, shallow cuts a long-bladed utility knife would probably result in a much cleaner and consistent cut for the large surface area I needed to cut.
  • Safety first – I got lucky I was not injured.  Wires break and electricity is dangerous if you don’t treat it with due respect.  Wear protection at all times!


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