As the plastic flows through different parts of theinjection molding machine and mold, the pressure acting on the plastic flow front will be lost due to the influence of resistance and friction. In addition, as the plastic comes into contact with the mold wall, it begins to cool, increasing the viscosity of the plastic, thus requiring extra pressure to push the plastic forward.
The plastic skin formed on the mold wall will reduce the cross-sectional area of plastic flow, resulting in pressure drop. There is a maximum limit to the pressure available on the injection molding machine for pushing 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.
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For example, considering that the maximum available hydraulic pressure of the injection molding machine is 2200psi, the required screw speed is 5 inches/second. If it needs 2400psi in order for the screw to advance at a speed of 5 inches/second, 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. In the process of process development, understanding the pressure loss on each part is helpful to determine the overall pressure loss and where the large pressure drop occurred. Then, the mold can be modified to reduce the pressure drop and obtain better continuous flow. It is important to ensure that the maximum pressure is not reached.
In the process of mold test, the following points should be paid attention to:
In order to reach the end of filling, plastics need to obtain an overall pressure of 2200psi.
In order to reach the middle part of the product, the plastic almost needs to obtain an overall pressure of 2200psi.
Based on the above two points, the process is limited by pressure.
Plastic needs a pressure of 1379–983 = 396 psi in order to reach the end of the tertiary shunt from the end of the secondary shunt. The pressure of 1897–1379 = 518 psi is needed for the plastic to flow through the gate.
Therefore, the tertiary runner and gate seem to have a relatively large pressure drop, so the tertiary runner and gate should be enlarged. This will reduce the final pressure at the filling end to 1901psi. Now the process is no longer limited by pressure. Ensuring sufficient injection pressure will help achieve the goal of consistent filling of mold. In the process of mold certification, it is one of the most important steps to study the pressure drop.
Finally, remember the rule of thumb: the required maximum injection pressure should not exceed 80% of the maximum pressure that can be obtained by the injection molding machine.
Is higher than the numerical value-80%, which should be lower for thicker products requiring higher packing pressure. To avoid sudden pressure rise between different parts, the transition should be as smooth as possible.
