Do you know how to select the hot runner system for your plastic mold

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

1. Selection of heating method

Internal heating method:
The internal heating nozzle with a more complex structure and a higher cost, is difficult to replace parts and has higher requirements for electric heating elements. Place the heater in the middle of the flow channel, which will produce annular flow and increase the friction area of the volume, and the pressure drop may be as much as 3 times that of the external heating nozzle.

However, since the internal heating element is located in the torpedo body of the nozzle, the heat generated is supplied to the material, which can lead to the low thermal losses and electrical energy saving. If adopt the pin gate, keep the torpedo body top in the center of the gate, it is beneficial to cut gate after injection and condense the gate late, resulting in the lower residual stress of the plastic part.

External heating method: It can eliminate cold film and reduce pressure loss by using external heating nozzle. At the same time, it has been widely used in production currently, due to many advantages such as its simple structure, convenient processing, and the thermocouple installed in the middle of the nozzle for accurate temperature control. However, the heat loss of the external heating nozzle is large, which is not as energy-saving as the internal heating nozzle.

2. Selection of gate

The design and selection of gates directly affect the quality of plastic parts. When applying a hot runner system, the gate type should be selected appropriately, based on the flow properties of the resin, molding temperature, and product quality requirements. In this case, it can prevent flowing, dripping, leakage, and
poor color change.etc.

3. Temperature control method

After confirming the gate, the melt temperature fluctuations will play a key role in the quality of plastic parts. In many cases, the phenomenon such as the material burning, degradation or flow blockage, is mostly caused by improper temperature control. Especially for heat-sensitive plastics, it is often required to respond to temperature fluctuations rapidly and accurately.

Therefore, the heating element should be set reasonably, in order to prevent local overheating. In order to minimize heat loss, it needs to ensure the matching gap between the heating element and the runner plate or nozzle. In the meanwhile, it had better to choose a more advanced electronic temperature controller and satisfy the temperature control requirements.

4. Calculation of temperature and pressure balance in each runner

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

The calculation of center offset in nozzle and the sprue bush, caused by thermal expansion, means it should be ensured that the center line of the expanded hot nozzle and the non-expanded cold sprue bush can be accurately positioned and aligned.

5. Heat loss calculation

The runner of internal heating is surrounded and supported by the cooled mold sleeve. Therefore, the heat loss, caused by thermal radiation and direct contact, should be calculated as accurately as possible. Otherwise, the actual runner channel will become smaller due to the thickening of the condensation layer on the runner wall.

6. Installation of runner plate

There should be full consideration about the problem both insulation and injection pressure. Usually, it should be set pads and supports between the runner plate and the mold plate. On the one hand, this can withstand the injection pressure to prevent the runner plate from deforming and causing leakage. On the other hand, it can reduce heat loss.

7. Hot runner system maintenance

As for the hot runner molds, it is very important to keep preventive maintenance regularly of hot runner components during use. This work includes electrical testing, inspection of sealing components and connecting wires, cleaning of component dirt and so on.