Heater bands and thermocouples of an injection molding machine

The heaters of an injection molding machine are the electric heater bands clamped around the barrel and the nozzle, the thermocouples that measure the temperature of each zone, and the temperature controller that switches the bands on and off. Together they bring the barrel up to processing temperature before production starts and hold every zone at its set point during the cycle. The numbered drawing shows a barrel with its nozzle heater, ceramic heater bands, terminal boxes, clamp bolts and thermocouples; this guide explains what each part does, how the system fails, and what to send when you need a replacement.

How barrel heating works in the molding cycle

The heaters do two different jobs:

1. Heating up. From a cold start, the bands supply all the heat. The controller runs each zone up to its set point, and the barrel then needs time to soak so the heat reaches the bore and the plastic left inside. Many controllers block screw rotation and injection until every zone is within its tolerance band, because turning the screw on cold plastic can break the screw tip or damage the screw.
2. Production. Once the machine is cycling, the plastic gets its heat from two sources: the bands, through the barrel wall, and shear heat from the screw turning and from back pressure. How much each contributes depends on the material, the screw speed, the back pressure and the cycle time. The front zones often need little heater power, and can even rise above their set point with the band switched off.

Each zone is a closed loop: one or more bands, one thermocouple, and one controller output. The controller compares the thermocouple reading with the set point and switches power to the bands through a solid-state relay or a contactor. The feed throat at the back of the barrel is not heated; it is water-cooled so the pellets stay solid until the screw takes them in (see hopper and feed throat).

How the zones are set from the feed end to the nozzle is explained in barrel temperature zones, and how the controller holds them in temperature control.

The parts of the barrel heating system

Barrel

The steel barrel the screw turns in. Its outside is divided into heating zones from the feed throat to the nozzle, and the bands must sit flat on a clean barrel surface to pass their heat into it. The barrel itself, its bore and its wear are covered in the barrel guide.

Ceramic heater bands

Each band heats one zone, or part of a zone on a long barrel. Ceramic bands are widely used on barrels; mica bands, mineral-insulated bands and cast heaters are also used. Size, power and voltage must match the zone: a band connected to a higher voltage than its rating produces far more power than it was designed for and burns out. See heater band types for how they compare, and the ceramic heater bands we supply.

Ceramic blocks and coil

Inside the steel shell of a ceramic band, a resistance coil runs through rows of ceramic blocks that hold it in place and insulate it from the shell. The blocks conduct and radiate heat into the barrel. A cracked block or a coil that has sagged against the shell can cause a short to earth or an open circuit.

Terminal box

A cover over the band's power terminals, where the heater cable connects. Terminals run hot, so loose connections oxidize and burn, and molten plastic on the terminals or a damaged cover can cause a short. Use heat-resistant cable and keep it off the hot barrel.

Clamp bolt

The clamp bolt tightens the band around the barrel so its heat passes into the steel. A loose band leaves an air gap: the element runs hotter than it should, the zone heats slowly, and the band fails early. A heater band removal tool helps free a band stuck to the barrel by burnt plastic; the full procedure is in replacing heater bands.

Thermocouples

A thermocouple is set into the barrel wall in each zone, usually between the bands, and sends the controller a small voltage that depends on the temperature. If it is the wrong type, damaged, or not seated in its hole, the controller holds the zone at the wrong temperature, often without any alarm. See the thermocouple guide and our article on thermocouple types and mounting styles.

Nozzle heater

A small band, coil or cast heater around the nozzle keeps the melt at temperature between shots. The nozzle touches the cooled sprue bushing on every shot, so this zone loses heat quickly and is the one most likely to freeze off or, when set too hot, to drool. See the nozzle heater page.

Temperature controller

The temperature section of the machine controller, or a separate controller on some older machines. It runs a control loop for every zone, raises alarms when a zone leaves its tolerance band and, on many machines, blocks screw rotation and injection until the barrel is at temperature. The wider control system is covered in the machine controller guide.

Common heating problems and their symptoms

• A zone never reaches its set point while the output shows 100%. A band has burnt open, a cable or terminal is broken, a fuse or breaker has tripped, or the relay is not switching.
• A zone runs above its set point with the output at 0%. Shear heat from the screw, a relay stuck closed, a thermocouple pulled out of its hole, or two zones' thermocouples swapped.
• The temperature swings around the set point. A thermocouple with poor contact, a loose band, or control settings that no longer suit the zone.
• The earth leakage breaker trips when the heaters switch on. Moisture or plastic inside a band, or damaged cable insulation touching metal.
• Unmelted pellets, high screw torque or slow recovery. A cold zone, often the rear or middle one. This loads the screw and its drive.
• Black specks, burnt material or gas. A zone running too hot, or a thermocouple reading lower than the real temperature.
• Nozzle freezing or drooling. A nozzle heater problem; see nozzle problems.

Check before replacing: a clamp meter on the heater cable shows whether the band draws current when the output is on, and a handheld probe on the barrel shows whether the thermocouple reading is plausible. Several of these symptoms also come from a worn screw and barrel, so read the barrel and screw guide if the heating checks out.

What to send for a replacement quote

For heater bands, send:

1. The machine brand and model, and which zone the band is for.
2. The inside diameter and width of the band.
3. The power (W) and voltage from the band's label.
4. The construction: ceramic or mica, one piece or two halves, and the number of clamp bolts.
5. The connection: terminal box, screw terminals or lead wires, and the lead length.
6. Any hole or cut-out for a thermocouple, and a photo of the old band.

For thermocouples, send the type (K or J, as shown on the controller or the old sensor), the mounting style, the thread size, the probe length, the cable length and the plug. See our thermocouple sensors and the heaters category, then request a quote.

Maintenance that keeps the heaters working

• Watch each zone's actual temperature and output. A zone that needs more output than it used to at the same settings has a weakening band or a band that has lost contact.
• Re-tighten the clamp bolts of a new band after its first heat-up, and check all clamps at planned stops.
• Fix nozzle and front flange leaks promptly. Plastic that runs under a band insulates it, burns, and causes the next failure.
• Inspect terminal boxes and cables for discoloration, brittle insulation and loose terminals.
• Check that every thermocouple is fully seated and its cable undamaged, and compare readings with a handheld probe from time to time.
• Label every band cable and thermocouple with its zone, so a replacement is never connected to the wrong output.
• If the barrel has insulation covers, refit them after maintenance: they reduce heat loss and protect people from the hot barrel.