Changes in plastic during injection molding
During the injection molding process, plastics undergo a complex series of physical and chemical changes that directly impact the quality and performance of the molded part. The first stage is the plasticization phase in the barrel. After solid plastic particles are added, they are propelled forward by the rotation of the screw. Simultaneously, they are heated by the barrel’s external heat and the shear friction of the screw, gradually transforming from a solid state to a molten state. During this process, the plastic particles first transition from a glassy state to a highly elastic state, and then, as the temperature rises, to a viscous flow state. During the plasticization process, the molecular chains within the plastic gradually expand and diffuse, achieving uniform mixing while simultaneously removing air and volatiles from the plastic, preparing it for subsequent injection molding. Incomplete plasticization, resulting in the presence of unmelted particles, can lead to surface roughness and reduced mechanical properties in the molded part.
After molten plastic is injected into the mold cavity, it undergoes a series of phases: filling and holding. During the filling phase, the high-speed, high-pressure molten plastic rapidly fills the mold cavity. The plastic’s flow is complex, and due to the cooling effect of the mold cavity walls, the plastic near the walls cools and solidifies first, forming a thin solidified layer, while the plastic inside remains molten. As the filling process progresses, the solidified layer gradually thickens, while the flow front of the molten plastic continues to advance until it fills the entire cavity. During the holding phase, to compensate for the volume loss caused by the plastic’s cooling and shrinkage, continued pressure is applied to the cavity to continuously replenish the molten plastic. At this point, the plastic undergoes a certain degree of compression deformation under the action of pressure, and the molecular chains become more tightly arranged, which helps to improve the density and dimensional accuracy of the plastic part.
The cooling stage is the most significant stage in the injection molding process where the plastic undergoes the most significant physical changes. As the molten plastic cools in the mold cavity, its temperature gradually decreases, transforming it from a viscous flow state to a highly elastic state, and finally to a solid state. During the cooling process, the plastic shrinks in volume because the distance between molecular chains decreases as the temperature drops. Different types of plastics shrink at different rates. Crystalline plastics generally shrink more than amorphous plastics because the ordered arrangement of molecular chains during crystallization leads to further volume contraction. The cooling rate significantly affects the crystallinity and molecular orientation of the plastic. Rapid cooling inhibits crystallization and reduces crystallinity, while slow cooling increases it. Molecular orientation, caused by the alignment of molecular chains along the flow direction during plastic flow, is retained after cooling and finalization, resulting in differences in the mechanical properties of the plastic part in different directions.
During the injection molding process, plastics may undergo chemical changes, especially at high temperatures and for extended periods. For example, some plastics can undergo thermal degradation at excessively high temperatures, breaking their molecular chains and causing performance degradation, such as discoloration and reduced mechanical properties. Plastics containing additives may also experience migration, volatilization, or decomposition during the molding process, affecting the properties of the plastic and the quality of the part. For example, plasticizers may volatilize at high temperatures, reducing the flexibility of the part; and stabilizer decomposition can degrade the plastic’s aging resistance. Therefore, during the injection molding process, barrel temperature and molding time must be strictly controlled to avoid excessive chemical changes in the plastic.
The plastic undergoes certain changes during the demolding phase. As the temperature of the molded part continues to decrease, its size and shape gradually stabilize. However, due to the constraints of the mold cavity and uneven cooling shrinkage, internal stresses are generated within the part. After the part is removed from the mold, these internal stresses may cause warping and deformation. Furthermore, friction and collisions between the part and the mold cavity walls during demolding may cause scratches or damage to the part’s surface. To minimize the adverse effects of this demolding phase, it is necessary to rationally design the mold’s demolding structure, control the demolding speed and force, and perform appropriate post-processing, such as annealing, to eliminate internal stresses and stabilize the part’s size and performance.
