Your wire is way too hot. At the correct temperature, the wire should move through the foam at about 1/8" per second and there should be some very fine "whiskers" left attached to the foam. A variable amperage power source is most desirable. Get a 5 amp 120 volt to 24 volt-AC transformer to isolate you from touching a live wire and ground at the same time and accidently "offing" yourself. Then connect a common 120 volt light dimmer switch in series with the 24 volt circuit to adjust the heat. Then gradually turn up the heat until the wire just starts to cut the foam. Assuming you will be hand cutting the foam cores, you will need two people to cut the cores (unless you build a fancy geometric jig to exactly position the wire). Mark the cord line on each template. You will need to do this to accurately align the templates to avoid (or add) wash-out or wash-in. Both templates should be divided into the same number of increments and each increment numbered. For a constant cord wing where the two templates are identical, just mark 1/4" increments on both. If the wing cord tapers, then measure the cord and divide it by 2, then divide those sections again by 2 and again and again until the longer template has about 1/4" spacings. Then do the same for the shorter cord template so as to have the same number of increments. You run the wire for your end of the wing with the longer cord template and call out the numbers of the increments as the hot wire passes them. The second person guides the foam block along their template, feeding it at a rate to match wire location to the increment numbers you call out. This alignment/indexing process assures the wire cuts at a constant speed which is essential to getting a usable core. Also, make sure the templates have absolutely smooth edges, If the hot wire snags at any point. it will melt out more foam. Know that the wire needs to be tight to minimize the middle of the wire lagging behind during the cur. Start the cut at the leading edge so the wire has not had a chance to lag behind here, otherwise you will get a dished out leading edge. Also know that it is very difficult to cut a trailing edge to a feather thickness because the lag of the wire will end up cutting a concave scallop at the trailing edge. Cut the trailing edge of the core to match the thickness of the balsa piece to be attached to it. Good luck!