Cold Pins and Transition Zones – The Most Misunderstood Parts of a Heater

Look at a cartridge heater. The heated section is obvious. But at the end where the lead wires exit, there is a section that does not get hot. That is the cold pin or transition zone. Understanding this zone explains why some heaters burn out right at the exit and why others have uneven temperature profiles.

What are cold pins?

Inside a cartridge heater, the resistance wire ends at a nickel or nickel‑plated steel pin called the cold pin. This pin has much lower electrical resistance than the heating wire. It produces negligible heat. The cold pin then connects to the flexible lead wires. The entire cold pin assembly sits inside the unheated length of the sheath, typically 10–25mm long.

Why cold pins are necessary.

Without cold pins, the resistance wire would continue all the way to the lead wire connection. That connection would run extremely hot, melting solder or crimps and burning insulation. Cold pins keep the connection region cool enough for the lead wire insulation to survive. They also provide a mechanical anchor inside the MgO.

The transition zone – where heat begins.

The transition from cold pin to resistance wire happens inside the MgO. Ideally, the change is abrupt. In practice, there is a short transition zone (2–5mm) where the temperature rises from near‑ambient to full operating temperature. This zone experiences high thermal stress because the metal expands rapidly over a short length. Repeated cycling can crack the sheath right at the transition point.

Common failure at the transition zone.

Field failure analysis shows that 20–30% of cartridge heater failures occur within the first 10mm of the heated length, near the transition zone. Signs include a dark ring around the sheath at a fixed distance from the cold end, sometimes with a small crack. The cause is usually too short a transition zone or poor welding between cold pin and resistance wire.

How to specify transition zones correctly.

Tell the supplier the desired unheated length. For most applications, 20mm is safe. For very short molds where every millimeter matters, 10mm is possible but reduces reliability. Never ask for less than 8mm unheated length unless the heater is under 5mm diameter and low watt density.

Cold pins in multi‑zone heaters.

Some cartridge heaters have two or three separate heating zones with cold pins between them. These are used for long molds requiring different temperatures in different segments. Each cold pin creates a dead zone with no heat. The customer must account for these dead zones when positioning the heater.

Practical check for transition quality.

After receiving a heater, measure the cold resistance. Then heat the heater in open air at reduced voltage (50% of rated) while moving a thermocouple along the sheath. The temperature should rise steeply over a short distance (ideally less than 8mm from the end of the unheated section). If the temperature rises gradually over 15mm or more, the transition is poorly defined and the heater will have a long warm‑up lag.

A note on sealing the cold end.

The exit point where leads emerge must be sealed against moisture ingress. Silicone seals work up to 200°C. Epoxy seals up to 250°C. For higher temperatures, a ceramic or glass seal is required. A failed seal allows moisture to reach the cold pins and corrode them, eventually causing open circuit.

Cold pins and transition zones are not defects. They are essential design features. Respecting their limits-minimum unheated length, gradual power application, and proper sealing-pays off in longer heater life.