Pressure in the injection molding process

1. Injection pressure

This is the pressure that causes the flow of plastic and can be measured with a sensor on the injection nozzle or hydraulic line. It has no fixed value, and the more difficult the mold is filled, the injection pressure increases. The injection line pressure and injection pressure are directly related.

First stage pressure and second stage pressure

During the filling phase of the molding cycle, high injection pressure may be required to maintain the molding speed at the required level. After the mold is filled, high pressures are no longer required. However, when molding some semi-crystalline thermoplastics (such as PA and POM), the sudden change in pressure will deteriorate the structure, so sometimes it is not necessary to use the second stage pressure.

2. Clamping pressure

To counteract the injection pressure, it is necessary to use the clamping pressure. Instead of automatically selecting the maximum value available, a suitable value should be calculated by considering the projected area. The projected area of the injected part is the maximum area seen from the direction of application of the clamping force.

For most injection molding situations, it is about 2 tons per square inch or 31 meganewtons per square meter. However, this is a low value and should be taken as a very rough empirical value because, once the injection molded part has any depth, then the sidewalls must be taken into account.

3. Backpressure

This is the pressure that must be generated and exceeded before the screw recedes. Although the use of high back pressure is conducive to the uniform dispersion of color and plastic melting, it also prolongs the return time of the screw, reduces the length of the fibers contained in the filled plastic, and increases the stress of the injection molding machine; therefore, the lower the backpressure, the better, and under no circumstances should it exceed 20% of the injection pressure (maximum quota) of the injection molding machine. 

4. Nozzle pressure

Nozzle pressure is the pressure inside the nozzle. It is about the pressure that causes the flow of plastic. It does not have a fixed value but increases with the difficulty of filling the mold. There is a direct relationship between nozzle pressure, line pressure, and injection pressure.

On screw injection molding machines, the nozzle pressure is about ten percent less than the injection pressure. In the case of piston injection molding machines, the pressure loss can reach about ten percent. In the case of piston injection molding machines, the pressure loss can reach 50 percent.