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Mould Selection and Application Hot Runner Considerations
Mar 28, 2018

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

1.the choice of heating method

Internal heating method: The structure of the internal thermal nozzle is complicated, the cost is high, parts replacement is difficult, and the requirements for the heating element are high. By placing the heater in the middle of the flow channel, an annular flow will occur, increasing the friction area of the volume, and the pressure drop may be as much as 3 times that of the external thermal nozzle.

However, since the internally heated heating element is disposed in the torpedo in the nozzle, the heat is supplied to the material, so that the heat loss is small and the electric energy can be saved. If the spot gate is used, the tip of the torpedo is kept at the center of the gate, which facilitates the gate cut after injection and causes the residual stress of the plastic part to be lower due to the gate condensation late.

External heating method: The use of external thermal nozzles eliminates cold film and reduces pressure loss. At the same time, due to its simple structure, convenient processing, and the thermocouple is installed in the middle of the nozzle to enable accurate temperature control, it is currently used in production. However, the thermal loss of the external thermal nozzle is larger than that of the internal thermal nozzle.

2. the choice of gate form

The design and selection of the gate directly affects the quality of the plastic parts. When using 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 flow, drooling, leaking, and poor color change.

3. the way of temperature control

When the gate shape is determined, the control of melt temperature fluctuations will play a key role in the quality of plastic parts. Many times the occurrence of coke, degradation, or flow path clogging is caused by improper temperature control. In particular, heat-sensitive plastics often require rapid and accurate response to temperature fluctuations.

For this reason, heating elements should be reasonably installed to prevent local overheating, ensure the matching gap between the heating element and runner plate or nozzle, minimize heat loss, and try to select the more advanced electronic thermostat to meet the temperature control. Claim.

300S 6 zones hot runner controller (2).jpg

4. the temperature and pressure balance calculation of each shunt

The purpose of the hot runner system is to dispense the hot plastic injected 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, so the temperature distribution in the heating zone of each runner is dealt with. And the melt pressure that flows into each gate is calculated.

Calculation of center offset of nozzle and sprue bush caused by thermal expansion. That is, the centerline of the hot (expanded) nozzle and the cold (non-expanding) sprue bushing should be aligned accurately.

5. heat loss calculation

The internally heated flow path is surrounded and supported by the cooled mold case, so the amount of heat loss due to thermal radiation and direct contact (conduction) should be calculated as accurately as possible, otherwise, the actual flow path path will be due to the flow path. The wall condensation layer becomes thicker and smaller.

6. the flow channel board installation

Full consideration should be given to thermal insulation and injection pressure. Generally, a spacer and a support member are arranged between the flow passage plate and the template. On the other hand, the injection pressure can be used to prevent the flow passage plate from being deformed to cause leakage, and on the other hand, the heat loss can also be reduced.

7. hot runner system maintenance

For hot runner molds, it is important to periodically perform preventive maintenance of the hot runner components during use. This includes electrical testing, inspection of sealing components and connecting leads, and cleaning of component contaminants.

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