Nozzle heater of an injection molding machine

The nozzle heater of an injection molding machine is a small heater band, coil heater or cast heater fitted around the nozzle body. It keeps the melt in the nozzle at processing temperature between shots, because the nozzle tip touches the cooled sprue bushing of the mold on every cycle and would otherwise freeze. The nozzle is a heating zone of its own, with its own thermocouple and controller output.

Where the nozzle heater sits

The nozzle screws into the barrel front flange and runs forward to the mold. Its heater is clamped on the nozzle body between the flange and the tip, and its thermocouple sits in a hole in the nozzle body under or beside the heater. The drawing highlights the nozzle heater at the front of the barrel. The nozzle itself, its tip radius and orifice, is covered in the nozzle guide.

How the nozzle zone works

The nozzle zone behaves differently from the barrel zones:

• It has little steel and loses heat fast. The nozzle is small, sits in the open, and touches the mold, which is usually cooled. When the nozzle stays against the mold through the cycle, heat flows into the sprue bushing all the time.
• It gets no shear heat from the screw. The barrel's front zones get heat from the turning screw; the nozzle depends on its own heater and on the hot melt passing through it.
• It is usually set close to the front barrel zone. Too cold, and plastic freezes in the tip, giving cold slugs or a blocked nozzle. Too hot, and the melt drools or strings from the tip and can degrade.

On long or extended nozzles, one heater may not be enough to keep the whole length even, and two heaters or two zones are used.

Types of nozzle heater

Nozzle heater bands

Small ceramic or mica bands, the same construction as barrel bands in a smaller size, clamped with a bolt. They are a common type on general-purpose machines. The ceramic heater bands we supply include small sizes for nozzles. See heater band types for how ceramic and mica bands differ.

Coil heaters

A tubular heating element wound into a tight spiral that slides over the nozzle. Some have a thermocouple built in. They fit where a band would be too wide or the nozzle has an awkward shape.

Cast brass or aluminum heaters

The element is cast into a metal sleeve shaped to the nozzle. They spread heat evenly and are used on some nozzles where an even temperature along the length matters.

Signs of a nozzle heater problem

• The nozzle freezes: cold slugs in the part, a nozzle that has to be cleared at start-up, short shots after a pause.
• The nozzle drools or strings at the set temperature, or the reading does not respond when the set point is changed.
• The zone does not heat, or heats slowly while the output stays high.
• The temperature swings with each shot more than the other zones.
• The heater is coated in plastic or its terminals are burnt.

Our article on nozzle drooling, stringing and blockages covers the process side of these problems.

Why nozzle heaters fail

• Plastic leaks. Melt from a leaking nozzle seat or a drooling tip runs onto the heater, gets inside it or under it, and burns. This is the most common cause.
• Poor fit. A band that is not tight on the nozzle body runs hot and fails early.
• Mechanical damage from cleaning the nozzle with tools, or from the mold or purge guard.
• Wrong thermocouple position. A thermocouple on the outside of the band makes the controller hold the band, not the nozzle, at the set temperature.

Checking and replacing the nozzle heater

1. Retract the injection unit, isolate and lock off the heater power, and let the nozzle cool to a safe handling temperature, or follow your site procedure for working on a warm nozzle.
2. Measure the heater's resistance and compare it with its rating (R = V² / P), and test the insulation between the terminals and the shell.
3. Remove the thermocouple, loosen the clamp and slide the heater off. Clean burnt plastic off the nozzle body.
4. Fit a heater of the same diameter, width, power and voltage. Tighten it evenly on the nozzle body, and route the cable away from the nozzle tip and the mold.
5. Refit the thermocouple fully into its seat in the nozzle body.
6. Heat up, check the reading against a handheld probe, and re-tighten the clamp after the first heat-up.

The general procedure for bands, including stuck ones, is in replacing heater bands.

What to send for a replacement quote

1. The machine brand and model, and the nozzle type or length.
2. The outside diameter of the nozzle where the heater sits, and the length available.
3. The heater type (band, coil or cast), and its power and voltage from the label.
4. The connection type and lead length, and whether the thermocouple passes through the heater.
5. A photo of the old heater on the nozzle.

Send them with a quote request. For the rest of the heating system, see the heaters guide.