Injection holding pressure
During the injection molding process, holding pressure is a key parameter determining part quality, directly impacting its dimensional accuracy, density, and mechanical properties. The holding phase is a crucial step following the filling phase. Its primary function is to continue replenishing the mold cavity as the melt contracts during cooling, compensating for the volumetric shrinkage caused by cooling and thus preventing defects such as sink marks, dents, and bubbles in the part. The holding pressure, duration, and pressure profile during this phase all need to be precisely determined based on factors such as the part’s structure, material properties, and mold design. Failure to do so can easily lead to unstable product quality.
The setting of the holding pressure is closely related to the wall thickness and structure of the plastic part. For plastic parts with thicker walls, due to their large cooling shrinkage, higher holding pressure and longer holding time are required to ensure that the melt can be fully replenished to all parts of the cavity and avoid shrinkage holes or depressions in thick-walled areas. On the contrary, for thin-walled plastic parts, the holding pressure should not be too high, otherwise it may cause large internal stresses inside the plastic part, causing deformation or cracking during subsequent use. In addition, when the plastic part has a complex structure, multiple cavities, or features such as ribs and bosses, the distribution of the holding pressure needs to be more uniform to ensure consistent filling and shrinkage compensation effects in all parts. Otherwise, local dimensional deviations or defects are likely to occur.
Material properties significantly influence the choice of holding pressure. Different plastic materials have different melt viscosities, shrinkage rates, and crystallization characteristics, which determine their shrinkage compensation requirements during the holding phase. For example, crystalline plastics such as polyethylene (PE) and polypropylene (PP) experience significant volumetric shrinkage during cooling, necessitating higher holding pressures to compensate for this shrinkage. Amorphous plastics such as polystyrene (PS) and polyvinyl chloride (PVC) experience relatively small shrinkage, and the holding pressure can be appropriately reduced. Furthermore, for plastics with higher viscosities, such as polycarbonate (PC), higher holding pressures are required to promote melt flow and achieve effective shrinkage compensation. For plastics with lower viscosities, such as polyethylene, excessive holding pressures can lead to problems such as overflow.
The matching relationship between holding pressure, injection pressure, and holding time is the key to ensuring injection molding quality. Holding pressure is usually set at 50% – 80% of the injection pressure. If the holding pressure is too high and close to the injection pressure, it may cause increased stress in the plastic part, increase the burden on the mold, and even cause insufficient clamping force and overflow. If the holding pressure is too low, effective shrinkage compensation cannot be achieved, resulting in shrinkage marks or undersized plastic parts. The holding time setting also needs to be coordinated with the holding pressure. If the holding time is too short, shrinkage compensation is insufficient. If the holding time is too long, the production cycle will be extended, production efficiency will be reduced, and it may also cause increased stress in the plastic part. Generally speaking, the holding time should last until the gate solidifies to prevent melt backflow. The specific time needs to be determined based on the thickness of the plastic part and the solidification rate of the material.
In actual production, the adjustment of the holding pressure needs to be dynamically optimized based on the quality feedback of the plastic parts. During the mold trial stage, it is usually necessary to observe the quality changes of the plastic parts by gradually adjusting the holding pressure. For example, the optimal holding pressure parameters can be determined by measuring the dimensions of the plastic parts, checking whether there are shrinkage marks or depressions on the surface, testing the mechanical properties, etc. During mass production, it is also necessary to consider the impact of changes in factors such as ambient temperature, raw material humidity, and equipment status on the holding effect, and to make timely fine-tuning of the holding pressure. For example, when the ambient temperature is low, the melt cools faster, and it may be necessary to appropriately increase the holding pressure or extend the holding time; when the raw material humidity is high, the melt viscosity may change, and the holding pressure needs to be adjusted accordingly to ensure the shrinkage compensation effect. Through continuous optimization and adjustment, it can be ensured that the holding pressure is always in the best state, thereby stably producing high-quality plastic parts.
