The so-called molding cycle refers to the total time required for an injection molding machine to complete a plastic part. In one molding cycle, the various moving parts of the injection device and the clamping device of the injection molding machine move once in a predetermined order.

Process

1. Molding operation stage

The molding operation stage refers to the total time required for the molten material to enter the mold, fill the mold cavity for molding, and condense and shape in the mold cavity. Since molding and shaping are both carried out in a closed mold cavity, the molding time should be included in the time when the mold is locked. The molding operation mainly includes four stages: mold filling, pressure holding, backflow and demolding.

1) Mold filling stage. This is from the beginning of the screw (or plunger) to the time when the molten material enters the mold cavity until it fills the mold cavity. When the mold filling begins, there is no pressure in the mold cavity. After the mold cavity is filled, the material flow pressure rises rapidly to the maximum value. The filling time is related to the molding pressure. When the mold is filled slowly, the melt entering the mold first will cool down and the viscosity will increase. The subsequent melt will enter the mold cavity under a higher pressure. Since the melt is subjected to a higher shear stress, the molecular orientation is higher, which reduces the quality. When the mold is filled quickly, the melt will generate more friction heat when passing through the pouring system quickly, which will increase the material temperature. In this way, the temperature of the melt can be kept at a higher value, the molecular orientation can be reduced, and the degree of welding of the plastic part is also higher. However, when the mold is filled too quickly, the welding of the rear part with the insert is often not good, resulting in poor strength of the plastic part.

2) Holding pressure stage. This refers to the period from when the melt fills the mold cavity to when the screw (or plunger) retreats. At this time, the melt pressure is basically stable and the holding pressure is relatively stable. However, the melt shrinks due to cooling. Since the melt is still under constant pressure, the melt in the barrel will flow into the mold cavity to make up for the gap caused by shrinkage, which is beneficial to increase the density of the plastic part, reduce the shrinkage rate and overcome surface defects. In addition, since the melt is still flowing and the temperature is constantly decreasing, the oriented molecules are easily frozen. Therefore, this stage is the main stage for the formation of macromolecular orientation. The longer this stage is, the greater the degree of molecular orientation.

3) Backflow stage. This refers to the period from when the screw (or plunger) retreats to when the melt at the gate solidifies. At this time, the pressure in the mold cavity is higher than that in the runner, so the melt will flow back from the mold cavity, causing the pressure in the mold cavity to drop rapidly. When the melt at the gate solidifies, the backflow will stop. If the melt at the gate has solidified when the screw (or plunger) retreats, backflow will not occur. Or if a check valve is installed in the nozzle, backflow will not occur. The pressure and temperature when the gate solidifies have an important influence on the shrinkage rate of the plastic part, that is, the longer the holding time, the higher the sealing pressure, and the less backflow, the smaller the shrinkage rate. In the backflow stage, since there is flow of melt, the orientation of molecules will increase, but this orientation is relatively small and the affected area is not large.

4) Demolding stage. This refers to the period from when the melt from the gate is completely solidified to when the plastic part is ejected from the mold cavity. During this stage, the plastic in the mold continues to cool, but there is still a small amount of flow in the mold. The temperature, pressure and volume of the plastic in the mold all change during this stage. When the plastic part is demolded, the pressure in the mold is not necessarily equal to the external pressure. This pressure difference is called residual stress. If the residual pressure is positive, demolding is more difficult. If the residual stress is negative, the plastic part will shrink inward and produce wrinkles and depressions. Only when the residual stress is close to zero is demolding easy and the quality is better.

2. Auxiliary operation stage

The auxiliary operation stage is all operations except the molding operation stage, usually including the opening and closing of the mold, the placement of inserts, the application of mold release agents, and the removal of products. Since the shaping and shaping of plastic parts are carried out in a closed mold cavity, the molding time should be included in the time when the mold is locked, and the action time of the moving parts and the auxiliary operation time should be included in the time when the mold is opened.

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