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How to determine the pressure drop of plastic in injection molding
- Jul 30, 2018 -

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As the plastic flows through different parts of the injection molding machine and dies, the pressure on the plastic flow front will be lost due to the influence of friction and friction. In addition, as the plastic contact die wall, it starts cooling, increasing the viscosity of the plastic, thereby requiring additional pressure to push the plastic forward.

The plastic cortex formed on the die wall reduces the cross-sectional area of the plastic flow, resulting in pressure drop. The injection molding machine can obtain maximum pressure on pushing screw with set injection speed.

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

For example, taking into account that the maximum hydraulic pressure of the injection molding machine is 2200psi, the required screw speed is 5 inches / sec. In order to make the screw forward at a speed of 5 inches / second, if it needs 2400psi, the machine will not provide such pressure so that the screw will not travel at a speed of 5 inches / seconds.

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

In the process of testing, we need to pay attention to the following points:

1, plastics in order to reach the end of filling, the overall pressure of 2200psi can be obtained.

2, plastics in order to reach the middle part of the product, it is almost necessary to obtain the overall pressure of 2200psi.

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

Plastic reaches 1379 to 983 = 396psi pressure in order to reach the end of the three grade runner from the end of the two grade channel. In order to flow through the gate, plastics need 1897 to 1379 = 518psi pressure.

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

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

Higher than the value - 80%, for thicker products requiring higher holding pressure, it should be lower. To avoid sudden increase in pressure between different parts, the transition should be as smooth as possible.