A cartridge heater consists of a resistance wire, coiled inside a straight, rigid tube. They can be used in internally heated drops and manifolds, as well as externally heated manifolds.
Cartridge heaters come in two basic forms - high watt density and low watt density. High watt density are used when space is limited, but have a relatively short lifetime, so low watt density cartridges are used whenever space allows.
- Hot manifolds with indirectly heated drops
- Indirectly heated drops, or nozzles, are heated by conduction of heat from the manifold. Hot manifolds with indirectly heated nozzles are typically heated with cartridge heaters. The cartridges are inserted into drilled holes on either side of the runners, at a distance from the runner about equal to the diameter of the cartridge. The positioning in the longitudinal direction is optimized by actually measuring the temperature distribution. It is important that the drilled hole matches the diameter of the heater or the heater may over-heat resulting in a short life-span.
- Internally heated manifolds and drops
- For internally heated manifolds and drops, the cartridge heaters are placed inside the channel, in contact with the melt. The diameter and length of the cartridge heaters, therefore, are determined by the dimensions of the nozzle and manifold. To use low watt density heaters, the cartridge diameter should be as large as possible. The heater cartridges are designed to withstand direct contact with the plastic.
- Externally heated manifolds and drops
- In a hot runner system with external heating, the flow channels are located in a heat distributor in the form of a plate suspended in the mold. Electric heaters, often in the form of cartridge heaters, are located outside the channels. The distributor is thermally insulated from the rest of the mold by an air gap and rests on shaped washers with limited heat conduction.
- Thermocouples
- Placement of thermocouples is crucial to providing the desired control of the melt temperature. Some thermocouples can be placed within the cartridge heater and this is often done for internally heated systems. In these cases, the thermocouple should be placed at the end of the cartridge close to the tip.
When used for a hot drop, the thermocouple should be placed near the gate tip where temperature control is most critical. When used with externally heated systems, it should be placed where it can measure the temperature of the steel near the flow channel, rather than near the heater.