Precautions for using hot runners in mold

In order to eliminate or reduce the faults in use as much as possible, the following matters should be pay attention to when selecting and applying the hot runner system:

1. Selection of heating method

Internal heating: Internal heating nozzle structure is more complex, high cost, difficult parts replacement, high requirements for electric heating elements. Placing the heater in the middle of the flow channel will produce an annular flow, increase the friction area of the container, and the pressure may be three times as much as that of an externally heated nozzle. However, since the internally heated heating element is arranged in the torpedo type body in the nozzle, all the heat is supplied to the material, so the heat loss is small and the electric energy can be saved. If use pin point gate, the tip of torpedo type body is kept in the gate center, which is beneficial to cut off the gate after injection and lower residual stress of plastic parts due to late condensation.

External heating: the use of external heating nozzles can eliminate cold film and reduce pressure loss.

At the same time, due to its simple structure, convenient processing, and thermocouple installed in the middle of the nozzle so that accurate temperature control and other advantages, has been widely used in production.

But the heat loss of the external heat nozzle is larger than that of the internal heat nozzle.

2. Selection of gate type

The design and selection of gates directly affect the quality of plastic parts. When applying the hot runner system, the appropriate gate form should be selected according to the resin's flow properties, molding temperature and product quality requirements to prevent drooling, dripping, material leakage and poor color change.

3. Temperature control method

When the gate type is determined, the control of melt temperature fluctuations will play a key role in the quality of plastic parts. In many cases, the phenomenon of burnt material, degradation or flow channel blockage is mostly caused by improper temperature control, especially for heat-sensitive plastics, which often require rapid and accurate response to temperature fluctuations.

Therefore, the heating element should be reasonably designed to prevent local overheating, ensure the gap between the heating element and the hot flow plate or nozzle, and minimize the heat loss. At the same time, more advanced electronic temperature controller should be selected as far as possible to meet the temperature control requirements.

4. Calculation of temperature and pressure balance of each branch

The purpose of the hot runner system is to inject the hot plastic from the nozzle of the injection molding machine through the hot runner at the same temperature and distribute the melt to each gate of the mold with a balanced pressure. Therefore, the temperature distribution of the heating zone of each runner and the pressure of the melt flowing into each gate should be calculated. Therefore, the temperature distribution of the heating zone of each runner and the pressure of the melt flowing into each gate should be calculated.

Calculation of the offset of nozzle and sprue bushing center caused by thermal expansion. That is to ensure that the hot (expanded) nozzle and cold (no expansion) sprue bushing center line can be accurately aligned.

5. Heat loss calculation

The internally heated runner is surrounded and supported by the cooling mold sleeve, so the heat loss caused by heat radiation and direct contact (conduction) should be calculated as accurately as possible. The actual passage may be reduced by thickening of the condensing layer on the runner wall.

6. Installation of runner plate

Full consideration should be given to the two aspects of heat insulation and withstand injection pressure. Usually, cushions and supports are set between the runner plate and the template. This aspect can withstand the injection pressure to prevent the hot runner plate from deforming and causing material leakage. On the other hand, It also reduces heat loss.

7. Maintenance of hot runner system

For hot runner molds, it is very important to regularly perform preventive maintenance of hot runner components during use. This work includes electrical testing, inspection of sealing components and connecting wires, and cleaning of component dirt.