In order to eliminate or reduce faults in use as much as possible, the following matters should be noted when selecting and applying hot runner systems:
1. Selection of heating mode
Internal heating method: The structure of the internal heating nozzle is more complex, the cost is higher, the parts are difficult to replace, and the requirements for electric heating elements are higher. When the heater is placed in the middle of the flow path, it will produce annular flow, increase the friction area of the volume, and the pressure drop may be three times as much as the external thermal nozzle.
However, because the internal heating element is located in the torpedo body in the nozzle, the heat is all supplied to the material, so the heat loss is small and the electric energy can be saved. If the point gate is used, the tip of the torpedo body is kept in the center of the gate, which is conducive to cutting off the gate after injection and reducing the residual stress of the plastic parts due to late condensation at the gate.
External heating method: The use of external heating nozzle can eliminate cold film and reduce pressure loss. At the same time, because of its simple structure, convenient processing, and the thermocouple installed in the middle of the nozzle to make the temperature control accurate, it has been widely used in production. However, the heat loss of the external heat nozzle is larger, and it is not as energy saving as the internal heat nozzle.
2, the choice of gate form
The design and choice of gate directly affect the quality of plastic parts. When applying the hot runner system, the appropriate gate form should be selected according to the flow performance of the resin, the molding temperature and the product quality requirements to prevent the phenomenon of salivation, drip, leakage and poor color change.
3, temperature control mode
When the gate form is determined, the control of melt temperature fluctuation will play a key role in the quality of plastic parts. Many times the phenomenon of coke, degradation or flow obstruction is mostly caused by improper temperature control, especially heat-sensitive plastics, which often require rapid and accurate reaction to temperature fluctuations.
To this end, the heating element should be reasonably set up to prevent local overheating, to ensure the matching gap between the heating element and the runner plate or nozzle, to minimize heat loss, and to choose a more advanced electronic thermostat as far as possible to meet the temperature control requirements.
4, temperature and pressure balance calculation of each shunt
The purpose of the hot runner system is to pass the hot plastic injected from the injection molding machine nozzle through the hot runner at the same temperature and distribute the melt to each gate of the mold with a balanced pressure, so the temperature distribution in the heating zone of each runner and the melt pressure flowing into each gate should be calculated.
Calculation of center offset of nozzle and gate sleeve caused by thermal expansion. That is to ensure that the center line of the hot (expanding) nozzle and the cold (non-expanding) gate sleeve can be accurately positioned.
5, heat loss calculation
The internally heated flow path is surrounded and supported by the cooled mold sleeve, so the amount of heat loss caused by thermal radiation and direct contact (conduction) should be calculated as accurately as possible, otherwise, the actual flow path diameter will become smaller due to the thickening of the condensing layer of the flow path wall.
6, the installation of the runner plate
Both insulation and the ability to withstand injection pressure should be fully considered. Usually, a pad and a support piece are arranged between the runner plate and the template, which on the one hand can withstand the injection pressure, avoid the runner plate deformation and the material leakage phenomenon, and on the other hand can reduce the heat loss.
7, hot runner system maintenance
For hot runner molds, it is very important to carry out regular preventive maintenance of hot runner components during use, including electrical testing, inspection of sealing components and connecting wires, and cleaning of component dirt.