What are the four stages of formation?

1, Filling stage
The filling stage is the first and crucial step in the molding process. At this stage, plastic melt is injected into the mold cavity until the cavity is filled to approximately 95%. The quality of the filling process directly affects the subsequent stages and the performance of the final product.
The length of filling time and the speed of filling are constrained by various factors. In theory, the shorter the filling time, the higher the molding efficiency. However, in practical operation, the filling speed needs to balance multiple factors, such as mold temperature, melt temperature, injection pressure, and melt flowability.
During high-speed mold filling, the molten material generates a significant amount of frictional heat as it rapidly passes through the pouring system, causing an increase in material temperature. This helps to maintain a higher temperature of the molten material, reduce the degree of molecular orientation, and improve the degree of fusion of the plastic part. However, filling the mold too quickly may also lead to poor welding of the rear part with embedded parts, resulting in a decrease in the strength of the plastic part. On the contrary, when filling the mold slowly, the molten material that enters the mold first will cool down and its viscosity will increase. The subsequent molten material needs to enter the mold cavity under higher pressure, resulting in higher shear stress and higher molecular orientation, which leads to a decrease in quality.
Therefore, during the filling stage, it is necessary to control the filling speed and pressure reasonably to ensure that the melt can uniformly and fully fill the mold cavity, while avoiding excessive internal stress and defects.
2, Pressure holding stage
The pressure holding stage is the second critical stage in the molding process. At this stage, the screw (or plunger) slowly advances for feeding and compaction to compensate for the shrinkage behavior of the plastic during the cooling process. The function of the pressure holding stage is to increase the density of the plastic part and reduce the shrinkage rate, thereby improving the dimensional accuracy and surface quality of the plastic part.
During the pressure holding process, the back pressure is high because the mold cavity is already filled with plastic. The screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is also relatively slow. This flow is called pressure holding flow. During the pressure holding process, the plastic has already filled the mold cavity, and the gradually solidified melt serves as a medium for transmitting pressure, transferring the pressure in the mold cavity to the surface of the mold wall. Therefore, it is necessary to apply appropriate locking force to prevent the mold from being stretched and causing defects such as burrs and overflow in the molded product.
The length of holding time and the magnitude of holding pressure have a direct impact on the density, shrinkage rate, and surface quality of plastic parts. If the holding time is too long or the holding pressure is too high, it may cause excessive residual stress inside the plastic part, and even cause deformation or cracking of the plastic part. On the contrary, if the holding time is too short or the holding pressure is insufficient, it may lead to insufficient density of the plastic part, excessive shrinkage rate, and affect the dimensional accuracy and performance of the plastic part.
Therefore, during the compaction stage, it is necessary to control the compaction time and pressure reasonably to ensure that the plastic parts can be fully compacted and compensated, while avoiding excessive residual stress and defects.
3, Cooling (reverse flow/solidification) stage
The cooling stage is the third important stage in the molding process. At this stage, the plastic melt in the mold cavity gradually cools and solidifies, forming the desired shape of the plastic part. The cooling time accounts for 70% to 80% of the entire molding cycle, so the design and optimization of the cooling system are of great significance for improving molding efficiency and reducing costs.
During the cooling process, the heat in the melt is transferred to the mold through thermal conduction, and then transferred to the coolant through the cooling water pipe by the mold. The coolant takes away the heat, gradually cooling the mold and plastic parts to the desired temperature. The speed of cooling and the uniformity of cooling effect directly affect the dimensional accuracy, surface quality, and performance of plastic parts.
Sometimes, the cooling stage is also referred to as the "reverse flow" stage or the "solidification" stage. In the reverse flow stage, when the screw (or plunger) retreats, the pressure inside the mold cavity is higher than that inside the flow channel, and the molten material may flow back from the mold cavity to the reflux channel, causing a rapid decrease in pressure inside the mold cavity. However, when the molten material at the gate solidifies, the backflow will stop. Although the reverse flow stage is relatively short, it has a certain impact on the shrinkage rate and density of the plastic parts.
The solidification stage refers to the cooling of the plastic part in the mold to sufficient rigidity so that it will not deform or be damaged due to external forces during demolding. The length of curing time depends on factors such as the thickness of the plastic part, mold material, and the design of the cooling system.
Therefore, in the cooling stage, it is necessary to design the cooling system reasonably, optimize the parameters such as the position, quantity, and diameter of the cooling water channels, in order to improve the cooling speed and uniformity of the cooling effect. At the same time, it is necessary to set the cooling time reasonably according to the shape and size of the plastic parts to ensure that they can receive sufficient cooling and solidification.
4, Demoulding stage
The demolding stage is the final stage of the molding process. At this stage, the cooled and solidified plastic parts that have reached the required rigidity are removed from the mold. The choice of demolding method and the quality of demolding process directly affect the appearance quality, dimensional accuracy, and performance of plastic parts.
There are mainly two types of demolding methods: top rod demolding and demolding with stripping plate. Top bar demolding is achieved by setting up a top bar or ejector mechanism in the mold to push the plastic part out of the mold. The demolding of the stripping plate is achieved by moving a certain part of the mold (such as a slider or stripping plate) to separate the plastic part from the mold.
During the demolding process, it is necessary to avoid deformation, scratches, or damage to the plastic parts due to excessive external forces. Therefore, it is necessary to design the demolding mechanism and demolding parameters reasonably, such as ejection force, ejection speed, and demolding angle. At the same time, it is necessary to clean and lubricate the mold before demolding to reduce demolding resistance and friction damage.
In addition, attention should be paid to the influence of residual stress during the demolding process. Residual stress refers to the internal stress generated in plastic parts during the forming process due to uneven cooling, temperature differences, or internal defects. Excessive residual stress may cause deformation, cracking, or damage to plastic parts after demolding. Therefore, it is necessary to perform appropriate annealing or stress relief treatment on the plastic parts before demolding to reduce residual stress and improve the stability and durability of the plastic parts.