High Watt Density Cartridge Heater

A reamed H7 bore, clean and free of scale. The air gap in a loose bore is an insulator: it traps the heat the cartridge is producing and pushes sheath temperature up sharply, which is by far the most common cause of premature cartridge failure. If your bores are worn, oversized or oval, specify a lower watt density rather than trying to compensate with more power.

Almost always heat or contamination at the termination. The manufacturer's own guidance is explicit: the lead exit should sit outside the insulation layer, not buried inside it, and the magnesium-oxide at the terminal end must be kept clear of moisture and contaminants or it loses dielectric strength and a leakage fault develops. Check that the exit is not enclosed in a hot pocket and that the leads are not being flexed at the seal.

Yes, for anything running above about 20 W/cm² or where the block temperature must be held closely. A thermocouple in the block reads the metal with a lag; one built into the cartridge reads much closer to the sheath, which is what protects the heater. For hot-runner work an integral type J or K is effectively standard.

In a dry store. The manufacturer's instruction is to megger the element before use and, if insulation resistance has fallen below 1 MΩ at 500 V, to dry it in an oven at around 200 °C until it recovers. Magnesium oxide is hygroscopic, so a cartridge that has sat in a damp workshop can read low without being faulty — dry it before you condemn it.