Abstract: A low thermal inertia die (10) must be heated to a specific set temperature to cut/seal plastics material. As heat energy is dissipated during cutting/sealing it is replaced by an equivalent amount of thermal energy from an electrical heating element (26) embedded in the die, adjacent the profiled working face (22) which in use contacts plastics material. The element (26) is covered in electrically non-conductive tape (28) and encased in flattened steel tube (30). A thermocouple (32) is also embedded in the die (10), with its tip (34) located to sense the temperature of a central region of the working face (22). The electrical current supplied to the electrical resistance heating element (26) is controlled in dependence upon the temperature sensed.