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How to determine the plastic pressure drop in injection molding

Update:08-07-2020
Summary:

As the plastic flows through different parts of the inj […]

As the plastic flows through different parts of the injection molding machine and the mold, due to the influence of resistance and friction, the pressure on the plastic flow front will be lost. 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 formed on the mold wall of the injection molding machine will reduce the cross-sectional area of ​​the plastic flow, resulting in a pressure drop. There is a major limitation on the pressure available on the injection molding machine to push the screw at the set injection speed.

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

For example, considering that the available hydraulic pressure of the injection molding machine 1 is 220 Psi, the required screw speed is 5 inches/second. To allow the screw to advance at 5 inches/second, if it requires 2400 psi, the machine will not be able to provide such pressure and the screw will not travel at 5 inches/second.

In this case, the process is limited by pressure. In the process of process development, understanding the pressure loss on each part helps to determine the overall pressure loss and where a large pressure drop occurs. Then, the mold can be modified to reduce the pressure drop and obtain better continuous flow. It is important not to reach the pressure of 1 big.

During the trial process, you need to pay attention to the following points:

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

2. In order for the plastic to reach the middle part of the product, it is necessary to obtain 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=396psi in order to reach the end of the third-stage shunt from the end of the second-stage shunt. In order for the plastic to flow through the gate, a pressure of 1897-1379=518 psi is required.

 

Therefore, the three-stage runner and gate seem to have a relatively large pressure drop, so both the three-stage runner and gate should be enlarged. This will reduce the final pressure at the filling end 1 to 1901 psi. Now the process is no longer limited by pressure. Sufficient injection pressure will help achieve consistent filling of the mold. In the mold certification process, research on pressure drop is one of the important steps.

1. Remember the rule of thumb: the required 1 injection pressure should not exceed 80% of the 1 injection pressure obtained by the injection molding machine.

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

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