In many cases sealing is improved by a thin layer of insulating material called “buffer”. This is attached to one or both dies to insulate the material to be sealed from the die. This does several things: It lowers the heat loss from the materials to the dies; it compensates for small irregularities in the die surface and may help to make a good seal even if the die is not perfectly flat and it decreases the tendency to arc when too much time or pressure is used. In general, it makes a better seal with less arcing. Buffer material should have good heat resistance and high voltage breakdown. Many materials are used including: bakelite, Kapton, paper, glassine, Teflon, glass, mylar, silicone fiberglass, etc. Bakelite grade XXX about .010 to .030 inches thick can be used successfully in most cases. A strip of “Scotch” cellulose or acetate tape adhered to the shaped die is sometimes used to this advantage.