The molding cycle refers to the total time required for the injection molding machine to complete a plastic part. In a molding cycle, the moving parts of the injection device and the clamping device of the injection molding machine are moved once in sequence according to the predetermined order. In order to facilitate the analysis of the injection molding process, the molding cycle can be divided into two parts: the molding operation stage and the auxiliary operation stage.

(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 and condense in the mold cavity. Since the molding and setting are 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, such as filling mold, holding pressure, reversing flow and releasing mold.

• Mold filling stage. This moves from the screw (or plunger) and the molten material enters the mold cavity until it is filled. At the beginning of the mold filling, there is no pressure in the mold cavity, and after the mold cavity is filled, the material flow pressure quickly rises to the maximum value. The filling time is related to the molding pressure. When the mold is filled slowly, the molten material entering the mold first will cool and the viscosity will increase, and the subsequent molten material will enter the mold cavity under higher pressure. Because the molten material is subjected to higher shear stress and the molecular orientation degree is higher, the quality will decrease. In the case of rapid mold filling, more friction heat will be generated when the molten material passes through the pouring system quickly, so that the temperature of the molten material can be maintained at a higher value, the degree of molecular orientation can be reduced, and the degree of welding of the plastic part is also higher. However, when the mold is filled too fast, the welding of the rear part with inserts is often not good, resulting in poor strength of the plastic part.
• Pressure holding stage. This means that the self-melting material fills the mold cavity until the screw (or plunger) recedes. At this time, the melt pressure is basically stable, and the holding pressure is relatively stable. However, the molten material shrinks due to cooling, because the melt is still under constant pressure, the molten material in the cylinder will flow into the mold cavity to make up the gap caused by shrinkage, which is conducive to improving the density of plastic parts, reducing the shrinkage rate and overcoming surface defects. In addition, because the molten material is still flowing, the temperature is also falling, and the orientation molecules are easy to freeze, so this stage is the main stage of the formation of macromolecular orientation. The longer this stage, the greater the degree of molecular orientation.
• The reverse flow phase. This is from the time the screw (or plunger) recedes to the time the molten material solidifies at the gate. At this time, the pressure in the mold cavity is higher than that in the flow channel, so the molten material will flow back from the mold cavity, so that the pressure in the mold cavity will drop rapidly, when the molten material at the gate solidifies, the backflow will stop. If the molten material at the gate has solidified when the screw (or plunger) is backing up, there will be no backflow. Or if a check valve is installed in the nozzle, the reverse flow will not occur. The pressure and temperature of the gate solidification have an important effect on the shrinkage rate of the plastic parts, that is, the pressure holding time is longer, the sealing pressure is high, and the shrinkage rate is small. The flow of the existing molten material in the reverse flow stage will increase the orientation of the molecules, but this orientation is relatively small, and the affected area is not large.
• Demoulding stage. This means that the molten material from the gate is completely solidified until the plastic part is pushed out of the mold cavity. At this stage, the plastic in the inner mold continues to cool, but there is a small amount of flow in the mold. The temperature pressure and volume of the plastic in the mold have changed at this stage, and when the plastic part is demoulded, the pressure in the mold is not necessarily equal to the external pressure, and this pressure difference is called residual stress. If the residual pressure is positive, it is more difficult to release the mold, if the residual stress is negative, the plastic part will shrink inward to produce wrinkles and depressions. Only when the residual stress is close to zero, the demoulding is easy and the quality is better.

(2) Auxiliary operation stage

The auxiliary operation stage is all operations other than the molding operation stage, usually including the opening and closing of the mold, placing the insert, applying the release agent, and taking out the product. Since the molding and setting of the plastic parts are carried out in the closed mold cavity, the molding time should be included in the time of the mold locking, and the action time of the moving parts and the auxiliary operation time should be included in the time of the mold opening.