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How to determine the plastic pressure drop in injection molding

As the plastic flows through the injection molding machine and the different parts of the mold, the pressure acting on the leading edge of the plastic flow is lost due to the effects of drag and friction. In addition, as the plastic contacts the mold wall, it begins to cool, increasing the viscosity of the plastic, requiring additional pressure to push the plastic forward.

How to determine the plastic pressure drop in injection molding

The plastic skin layer formed on the mold wall reduces the cross-sectional area of the plastic flow, resulting in a pressure drop. The pressure at the set injection speed for pushing the screw at the set injection speed is maximally limited.

The pressure required to advance the screw at a set injection speed should never exceed the maximum pressure available.

For example, considering that the maximum available hydraulic pressure for an injection molding machine is 2200 psi, the required screw speed is 5 inches/second. In order for the screw to advance at 5 inches per second, if it requires 2400 psi, the machine will not be able to provide such pressure so that the screw will not travel at 5 inches per second.

In this case, the process is limited by pressure. In the process of process development, understanding the pressure loss on each part helps determine the overall pressure loss and where a large pressure drop occurs. The mold can then be modified to reduce the pressure drop for better continuous flow. It is important to ensure that the maximum pressure is not reached.

During the test mode, you need to pay attention to the following points:

1. In order to reach the end of the fill, the plastic requires an overall pressure of 2200 psi.

2. In order to reach the middle part of the product, plastic almost requires an overall pressure of 2200 psi.

Based on the above two points, the process is limited by pressure.

The plastic requires a pressure of 1379–983 = 396 psi to reach the end of the tertiary shunt from the end of the secondary shunt. In order to flow through the gate, plastic requires a pressure of 1897–1379 = 518 psi.

Therefore, the three-stage runner and gate appear to have a relatively large pressure drop, so the three-stage runner and gate should be enlarged. This will reduce the final pressure at the end of the filling to 1901 psi. The process is no longer limited by pressure. Ensuring adequate injection pressure will help achieve consistent mold filling. In the mold certification process, the study of pressure drop is one of the most important steps.

Finally, remember the rule of thumb: the maximum injection pressure required should not exceed 80% of the maximum pressure that can be obtained by the injection molding machine.

Above the value - 80%, should be lower for thicker products that require higher holding pressure. To avoid sudden increases in pressure between different parts, the transition should be as smooth as possible.

 How to determine the plastic pressure drop in injection molding

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