The Plastic Injection (Molding) Cycle

The plastic injection process pass by some phases, in this topic we will discuss all these phases in detail.

Mold Closing Phase
The mold should be closed as quickly as possible, without putting undue strain on the mold or the machine until the mold protection facility is tripped.

Mold Protection Phase
This is applied at the final stage of mold closing just before the two mold halves meet so as to avoid damage to the mold tool due to trapped moldings or the location of delicate shot-off areas on either half of the mold. Both the distance and operating pressure must be set to suit the requirements on the machine control panel.

Injection (Mold Filling) Phase
In general, mold filling takes place rapidly, as previously discussed. However, care must be taken in order not to generate excessive shear stress in the material by excessively short fill times. An ideal fill time should prevent large reductions in melt temperature and avoid the generation of high injection pressure.
The injection speed should be adjusted to produce the required filling time and the injection pressure should be set to enable this injection speed to be achieved. For example, a large cavity may require a lower relative injection speed to give the air in the cavity time to be displaced to prevent compressed air from being developed giving rise to shorts or burning.

Holding Time and Pressurising Phase (Pack Pressure)
During this stage, a little more material is forced into the mold before the gate freezes off. The pressure prevents the material in the mold from flowing back into the injection cylinder. The length of time required for holding depends upon the material being molded, gate size, melt, and mold temperatures. The setting of the hold-on stage is critical to the size and dimensional stability of components.

Cooling and Refill Phase
This starts after the hold-on time finishes. Should the component become cool enough to be ejected immediately after the screw back phase then no further cooling time is necessary and the parts may be ejected. In most cases, however, the moldings will require further cooling before ejection can take place.
The total cooling time required is largely dependent on the wall thickness of the component and the material used and can be calculated accordingly.

Screw Back Phase
During the cooling time, the screw rotates and is displaced backward along the injection cylinder. This backward displacement takes place entirely during the cooling phase of the components as previously described. Once the screw has transported sufficient material to the front of the cylinder, the screw trips a limit switch and stops rotating.

Mold Open Phase
The same principles apply to the mold open phase as to the mold closing phase. The abrupt opening should be avoided, to prevent damage to the machine and mold. The distance of opening should be just enough to allow for the moldings and any runners to be ejected cleanly. This will keep cycle time to a minimum.

Ejection Phase
Adequate time must be allowed for the ejection of the components from the mold.
Ejection can be employed in a semi-auto or automatic mode.
Automatic ejection can be by mechanical, hydraulic, or pneumatic systems, depending on the design of the ejector system. Air ejectors may be used to blow the component clear and can be set on a pulsating or continuous blow sequence. Timers can be used on both hydraulic and air ejectors.
Hydraulic ejectors may be used in a multistroke mode in certain circumstances to eject components that tend to ‘stick’ to the ejector pins. Alternatively, pick-and-place robots may be used where the cost is justified.
Many of these topics are discussed in more detail in the appropriate sections of this book.
The figure below illustrates a summary of the molding cycle.

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