Cartridge Heater

The fit in the bore is the whole story

A cartridge heater has no medium to dump its heat into. It has one path out: the metal touching it. If that contact is poor, the heat has nowhere to go, the sheath climbs and the wire inside fails. Nearly every cartridge heater that dies early died of a bad hole, not a bad element.

Aim for 0.05 to 0.10 mm of clearance on the diameter — enough to slide in, no more. A bore drilled and left rough, or drilled 0.5 mm oversize “to be safe”, is the classic mistake. It feels safer at fitting time and halves the life of the heater. Ream the hole, blow the swarf out, and keep it clean.

Watt density has to match the metal, not the wire

A 50 W/cm² heater is not better than a 20 W/cm² one; it is a different tool. High density only works where the surrounding metal pulls heat away quickly — a solid steel die, a large aluminium platen. In a small block, in a part with cooling channels close by, or in anything that spends time switched on with the machine idle, high density will cook itself.

If you are unsure, ask for the lower density and a longer heater. The part reaches the same temperature; it just takes a little longer to get there.

Getting a seized heater out

Cartridges seize. Oxide grows in the gap between sheath and bore, and after a year the heater is welded in place by rust. Two things prevent it: a thin coat of anti-seize compound on fitting, and pulling every heater out once during a planned service rather than waiting until one fails. A heater that will not move usually has to be drilled out, and the hole is then oversize for good.

Never pull a cartridge by its leads. There is a crimp inside the seal, and it will come apart in your hand.

Where the lead exits

The lead is the weakest part of the assembly. It leaves the heater at the cold end and immediately meets whatever is around the tool — a moving platen, a hot surface, a sharp edge of the mould. Say at the time of order whether the leads should come straight out or turn a right angle at the head, and whether they need a stainless braid over them. Retro-fitting a braid or bending a lead hard at the seal is how leads break.

Bake the moisture out before the first run

Magnesium oxide draws water out of the air. A heater that has sat in a store room through a humid summer can read a low insulation resistance and trip an RCD the moment it is switched on. This does not mean it is faulty. Run it at a low voltage, or in an oven at around 150 °C for an hour or two, then measure again. The reading usually comes back.

A thermocouple reads where it sits, not where it matters

A cartridge with a built-in thermocouple reports its own sheath temperature, which runs well above the temperature of the part. If what you need to control is the mould surface, the sensor belongs in the mould, in its own hole, not inside the heater. The built-in sensor is a safety limit, not a process sensor.