How to Control Holding Pressure in Injection Molding & Its Effect on Product Quality
2026-04-14 11:34:49
Injection Molding
Holding pressure is one of the most critical parameters in injection molding, which directly affects product weight, dimensional accuracy, shrinkage, warpage, surface quality, and internal stress. After the melt fills the cavity, holding pressure is applied to compensate for volume shrinkage caused by cooling and solidification. Improper holding pressure settings will lead to a series of defects such as sink marks, warpage, dimensional instability, flash, and brittle fracture. This article explains the function of holding pressure, its influence on product quality, and practical setting methods for stable production.
I. The Role of Holding Pressure in Injection Molding
After the injection stage, the melt begins to cool and shrink, resulting in volume reduction. Holding pressure pushes additional melt into the cavity to compensate for shrinkage, ensuring the product is fully formed and dense. It also helps compress the melt, reduce porosity, improve dimensional stability, and enhance mechanical properties. Meanwhile, holding pressure affects the pressure distribution inside the cavity, which determines the uniformity of shrinkage and the appearance quality of the product. Insufficient holding pressure cannot compensate for shrinkage, while excessive holding pressure causes excessive stress and deformation.
II. Effects of Holding Pressure on Product Quality
Holding pressure significantly affects dimensional accuracy. Proper holding pressure ensures consistent product weight and dimensions. Low holding pressure leads to large shrinkage and small dimensions; high holding pressure results in oversized dimensions and high internal stress. For precision parts, holding pressure is the most important parameter to control dimensional fluctuation.
Sink marks and voids are mainly caused by insufficient holding pressure, especially in thick wall areas. Increasing holding pressure can effectively eliminate sink marks by compacting the melt. However, excessive pressure may cause stress concentration and warpage.
Surface quality is also affected by holding pressure. Appropriate holding pressure improves surface smoothness, reduces flow marks, and enhances replication of mold texture. Insufficient pressure leads to rough surfaces and poor gloss.
Warpage and deformation are closely related to holding pressure distribution. Unbalanced holding pressure causes uneven shrinkage, resulting in bending and twisting. Reasonable pressure settings help reduce internal stress and improve flatness.
III. Setting Principles of Holding Pressure and TimeHolding pressure is generally 30%–80% of the injection pressure, depending on material type, product structure, and mold design. Crystalline plastics require higher holding pressure due to large shrinkage; amorphous plastics require relatively low pressure. Thick wall products need higher pressure than thin wall products.
Holding time is determined by gate solidification time. The holding stage must continue until the gate is completely frozen to prevent melt backflow. If the holding time is too short, shrinkage compensation is insufficient; if too long, it will reduce production efficiency and increase internal stress. The gate size, mold temperature, and material properties determine the solidification time.
Multi-stage holding pressure can be used for complex products. High pressure is applied at the initial stage to compensate for rapid shrinkage, and low pressure is used later to reduce stress. This method effectively balances shrinkage and stress control.
IV. Interaction Between Holding Pressure and Other ParametersMold temperature affects cooling rate and holding effect. High mold temperature prolongs gate solidification time and requires longer holding time. Low mold temperature leads to fast solidification and shortens the effective holding period.
Injection speed influences the pressure transfer efficiency. High-speed injection may cause turbulence and affect holding uniformity. Moderate injection speed helps stable pressure transmission.
Barrel temperature affects melt viscosity. High temperature reduces viscosity and makes pressure transmission easier, allowing relatively low holding pressure. Low temperature increases viscosity and requires higher holding pressure.
V. Common Defects and Holding Pressure Adjustment
Sink marks and shrinkage cavities: increase holding pressure and extend holding time.Dimensional instability: stabilize holding pressure and optimize holding time.Warpage and deformation: reduce holding pressure or adopt multi-stage pressure.Flash and burrs: decrease holding pressure and check clamping force.High internal stress and brittle fracture: lower holding pressure and shorten holding time.
SummaryHolding pressure is the core parameter for controlling shrinkage, dimensions, and quality of injection molded products. Reasonable settings can eliminate defects, improve stability, and enhance mechanical properties. The key is to select appropriate pressure and time based on material, structure, and mold conditions. Combined with multi-stage holding and coordinated parameter adjustment, manufacturers can achieve high-quality, stable, and efficient injection molding production.
