Hot runner systems in injection moulding keep the plastic melt in the runner channel at processing temperature between the machine nozzle and the mould cavities. The coil heaters wrapped around each nozzle are critical to this: if a coil heater fails or delivers uneven heat, the nozzle freezes, the cycle stops and production halts until the heater is replaced.
Procuring replacement coil heaters, or specifying them for a new hot runner system, requires precise dimensions and clear communication with your supplier. This guide covers what you need to know and what to include in your enquiry.
What a Hot Runner Coil Heater Does
A coil heater is a helically wound resistance element. The wire cross-section may be round, square or rectangular, depending on the design. The coil wraps tightly around a nozzle body. It delivers heat along the length of the nozzle to keep the melt channel at a uniform temperature. Unlike band heaters, which clamp around large barrels, coil heaters are wound to fit closely around smaller cylindrical nozzle bodies where space is constrained.
The coil heater must deliver the right amount of heat along the nozzle length, at the correct voltage. The thermocouple must be integrated or positioned to give the controller accurate temperature feedback; the junction location varies by nozzle design and should be confirmed from the manufacturer’s drawing.
Critical Dimensions
Hot runner coil heaters are specified by three key dimensions:
Inside diameter (ID) of the coil: must match the nozzle outside diameter. This is the most critical dimension. Measure the nozzle outside diameter (not the nozzle bore), as the coil wraps around the nozzle body. A coil that is too loose will have poor thermal contact. A coil that is too tight cannot be installed.
Outside diameter (OD) of the wound coil: the overall diameter including the coil winding. In a hot runner system, space around the nozzle is often limited by the mould cavity spacing (pitch). The OD must fit within the available bore in the mould. Note: the wound OD depends on the coil ID and the wire cross-section dimensions, not just a single “diameter” figure.
Heated length and overall length: the heated length of the coil along the nozzle determines which portion of the nozzle is actively heated. Some nozzle designs also have an unheated section. Specify both heated length and overall length if they differ, or provide the manufacturer’s nozzle drawing.
Measure the nozzle, not the old coil (which may be distorted from removal). If you are replacing a coil heater, measure the nozzle outside diameter carefully with a micrometer or caliper.
Wattage and Voltage
The wattage must match the thermal demand of the nozzle. Too little wattage means the controller cannot maintain the setpoint, especially during startup or high-cycle-rate production. Too much wattage means the controller overshoots between cycles, which can degrade the resin.
If you are replacing an existing coil heater, match the original wattage unless there were performance issues. If you are specifying for a new system, the nozzle designer or hot runner system manufacturer will usually specify the required wattage per nozzle.
Standard voltages are typically 230V for hot runner heaters in India. Confirm the voltage of your hot runner controller output.
Thermocouple Integration
Most hot runner nozzles require a thermocouple for closed-loop temperature control. The thermocouple may be:
- Built into the coil heater: a thermocouple wire runs along the coil and emerges at a specific point determined by the nozzle design. The measurement position varies between designs; confirm the required location from the manufacturer’s drawing.
- Separate from the coil: a standalone thermocouple is mounted in a separate channel or groove in the nozzle body.
If the thermocouple is integrated into the coil, specify the thermocouple type (J and K are most common in hot runner systems), the measurement position along the nozzle, and the lead or connector type.
Lead and Connector
Hot runner systems use connectors to make heater and thermocouple replacement faster. Common connector types include miniature round connectors and flat pin connectors. Specify the connector type and pinout to match your existing system wiring.
If leads (bare wires) are used instead of connectors, specify the lead length, the wire type and the exit direction from the coil.
Matching to Your System
Hot runner systems from different manufacturers use different nozzle designs, and the coil heaters are not interchangeable without checking dimensions. Even nozzles from the same manufacturer may have different heater specifications across different nozzle sizes.
When ordering replacement coil heaters, provide:
- The hot runner system manufacturer and model (if known)
- The nozzle part number or designation (if available)
- Physical measurements of the nozzle (ID, length, available space)
- Photos of the nozzle and the old coil heater
This information helps the supplier match the replacement to your system, even if the original heater came from a different source.
What to Include in Your RFQ
- Coil inside diameter (nozzle outside diameter)
- Coil outside diameter (maximum wound OD)
- Heated length and overall length (if different)
- Wattage and voltage
- Thermocouple type and position (if integrated)
- Connector type or lead details
- Hot runner system manufacturer and nozzle model (if known)
- Quantity
- Photos of the nozzle and existing coil (helpful)
Related Reading
- Coil Heaters product page
- Hot Runner Coil Heaters product page
- Band Heater Selection: barrel heating for extruders and moulding machines
- Manufacturing industry