I was using a Windlock 2QC hotknife with a standard blade using perhaps a total of 5" of blade material (just cutting groves 2.5" deep, 3/4" wide for romex.  I was busying grooving with it, and then it stopped working.  I noted that the blade was sort of gunked up at the connection to the knife, so i used some sandpaper to clean up the blade, reinstalled it, and was working well once again.  This happened two more times (which I later realized was being caused by me using the connectors of the blade as a depth gauge against the foam), and then cleaning didn't work.  The unit was just inoperable. 

I called Windlock, who promply informed me that the knife was under warranty.  They even sent me a replacement loaner knife while I waited for mine to be looked at.  When the loaner came, I immediately noted that it didn't seem to get nearly as hot as the previous one - the blade used to glow orange when the unit was on (thereby making the cutting of the grooves go quickly).  I also noted that it stopped working after maybe 3-4 minutes of use, but if you waited a few minutes (I guess for it to cool down), it worked once again.

Then I had a need a need for a much deeper grove (installing some IC can lights in the "ceiling" of my LiteDeck garage floor).  I determined that I needed a groove 13" wide by 9" deep or so.  I took some plumbers tape, bent it to the proper configuration, cut it down narrower so as to fit into the slots provided by the knife, and turned it on.  It hummed louder than usual when on, but the plumbers tape eventually heated up and I was slowly (like very slowly) cutting through the foam.  I stopped after a few seconds, cut a smaller version (perhaps 6" wide by 4" deep), and had much better success.  Thinking that the core issue with the slow going with the longer plumbers tape was simply too much resistance, I then had the idea to use 12AWG solid bare copper wire to make my configuration, thinking that Copper was a better conductor than the galvanized strapping.

I cut a smaller section yet, albeit only slightly smaller, connected it to the knife, and turned it on.  My GFCI outlet immediately tripped.  It, in fact, didn't just trip, it died - nothing I could do would make it work again.  Remembering that the GFCI outlet was in fact a Harbor Freight Tools special and possibly of lesser quality - this was my first use of a HF GFCI, but I haven't always seen the best quality from their tools), I replaced the GFCI with another duplex receptacle (this time, a non-GFCI commerical-grade 20A version since I didn't have a GFCI non-HF receptable available), and went to work.  What a difference!  The copper wire worked famously as a "knife".  I cut for perhaps a total of 10 seconds, placed the knife down for a few seconds, went to use it again, and found it to be inoperable.  Great...so now I broke a SECOND hotknife. 

Am I using this tool incorrectly somehow?  It seemed at the time that the GFCI shouldn't have been able to trip since it is, after all, a GROUND FAULT circuit interruptor and the Windlock knife is only fitted with a two prong plug.  Now I wonder if it "took the bullet" for the knife and me just changing out the receptacle forced the failure of the knife.  The knife strikes me as not much more than a glorified toaster, but perhaps there is something more going on here.  Any ideas?