|Hot runner systems play a critical role in optimizing the thermal profile of a production mold, providing consistency in material flow and fill from part to part. Hot runner molds are two plate molds with a heated runner system inside one half of the mold. They are injection molds in which the runners are kept hot and insulated from the chilled cavities. Plastic freeze-off occurs at gate of cavity; runners are in a separate plate so they do not usually get ejected with the piece. A hot runner system is divided into two parts: the manifold and the drops. There are many variations of hot runner systems:
Externally heated hot runners: Externally heated hot runner channels have the lowest pressure drop of any runner system (because there is no heater obstructing flow and all the plastic is molten), and they are better for color changes, none of the plastic in the runner system freezes. There are no places for material to hang up and degrade, so externally heated systems are good for thermally sensitive materials.
Internally heated hot runners: Internally heated runner systems require higher molding pressures and color changes are very difficult. There are many places for material to hang up and degrade, so thermally sensitive materials should not be used. Internally heated drops offer better gate tip control, and also separate runner heat better from the mold because an insulating frozen layer is formed against the steel wall on the inside of the flow channels.
Insulated hot runners : A special type of hot runner system that is not heated. The runner channels are extremely thick and stay molten during constant cycling. This system is very inexpensive, and offers the flexible gating advantages of other hot runners and the elimination of gates without the added cost of the manifold and drops of a heated hot runner system. Color changes are very easy. Unfortunately, these runner systems offer no control, and only commodity plastics like PP and PE can be used. If the mold stops cycling for some reason, the runner system will freeze and the mold has to be split to remove it. Insulated runners are usually used to make low tolerance parts like cups and frisbees.
Externally heated runners have been in use for a long time, while internally heated runner system is relatively newer, developed mainly with an aim to reduce maintenance cost. Since an internally heated nozzle is 100% sealed, spare parts are rarely needed.
Externally heated hot runner nozzle has heating element (thermocouple, heating coil, sleeve, etc.) located on the outside of the nozzle body. When the coil is heated it expands, pulling away from the nozzle body so that there is a gap and the coil no longer touches the nozzle body uniformly. This means that a higher temperature is needed to properly heat the plastic and that it may not be heating evenly. In addition, higher temperatures mean a higher risk of overheating and burning out the coil. Internally heated nozzles resist burning out because the heater is inside the nozzle body and is an integral part of the body. The complete sealing contributes to efficient heating. The insulation around the heating wire is compacted to maximum density, which eliminates any air voids around the resistance wire and surrounding nozzle bore. When the heater is energized, the heat produced on the surface of the resistance wire travels away immediately through the highly dense insulator, and thus the heater element runs cooler, helping prolong its life.
With internally heated nozzles, the thermocouple and heating element are located much closer to the gate, at the end of the nozzle, delivering heat at a very precise temperature. The result is a better heat profile and more controlled melt temperature leading to better molded part quality. Externally heated nozzles stop short of the gate, and with the structure of the external coil, it is possible that while the nozzle body is being properly heated, the nozzle tip is not. The internally heated runner has optimum control over heat profiles, in addition to significantly lowering the risks of overheating and leaking and provides better economy over externally heated runner systems.
If the thermocouple fails on an externally heated nozzle, many times the entire coil must also be replaced with it. On an internally heated nozzle, the internally located thermocouple can be changed, and the heating element remains in place, adding to the time and cost savings. As long as there is no leaking involved, a burned-out externally heated nozzle coil can easily be replaced without removing the mold from the press.
(Author Panos Trakas)