How to determine plastic pressure drop in injection molding

As the plastic flows through the injection molding machine and different parts of the mold, there is a loss of pressure acting on the plastic flow front due to resistance and friction. In addition, as the plastic touches the mold wall, it begins to cool, increasing the viscosity of the plastic and thus requiring additional pressure to push the plastic forward.

The plastic skin that forms at the mold wall reduces the cross-sectional area of the plastic flow, resulting in a pressure drop. There is a maximum limit to the amount of pressure that can be obtained on an injection molding machine to push the screw at a set injection speed.

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

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

In this case, the process is limited by pressure. During process development, knowing the pressure loss on each part helps determine the overall pressure loss and where large pressure drops are occurring. The mold can then be modified to reduce the pressure drop and obtain a better-sustained flow. It is important to make sure that the maximum pressure is not reached.

During the mold test, the following points need to be noted.

1. The plastic is required to obtain an overall pressure of 2200psi in order to reach the filled end.

2. In order for the plastic to reach the middle part of the product, and overall pressure of 2200 psi is almost required to be available.

Based on these two points, the process is pressure-limited.

In order to get from the end of the secondary manifold to the end of the tertiary manifold, the plastic requires a pressure of 1379 - 983 = 396 psi. In order for the plastic to flow through the gate, a pressure of 1897 - 1379 = 518 psi is required.

Therefore, the tertiary manifold and gate appear to have a relatively large pressure drop, so both the tertiary manifold and gate should be enlarged. This will reduce the final pressure at the end of the fill to 1901psi, now that the process is no longer pressure limited. Ensuring adequate injection pressure will help achieve consistent filling of the mold. Conducting a pressure drop study is an important step in the mold certification process.

Finally, remember the rule of thumb: the maximum injection pressure required should not exceed 80% of the maximum pressure available to the injection molding machine.

Higher values - 80% - should be lower for thicker products that require higher holding pressures. It is important to avoid sudden pressure rises between sections and to make the transition as smooth as possible.