In the injection molding process, pressure holding is the process of maintaining a certain pressure to continue to replenish the melt into the mold cavity after the injection stage. Its core function is to compensate for plastic shrinkage and improve product density and dimensional stability.
1. Function
Compensate for shrinkage: The volume shrinks when the melt cools, and the pressure holding pushes the melt to continue filling to avoid shrinkage or voids.
Stabilize the size: Reduce deformation caused by uneven shrinkage.
Improve surface quality: Reduce weld line marks and improve gloss.
Control weight: Insufficient pressure holding will cause the product to be light, and excessive pressure holding may cause flash or internal stress.
2. How to set parameters
Press holding pressure: Usually 30%-60% of the injection pressure. If the plastic is ABS, it is usually 40%-50%, and PP is about 30%.
Too high will cause flash, increased internal stress, and difficulty in demolding.
Too low will cause shrinkage, insufficient size, etc.
Pressure holding time: depends on the gate freezing time, which can be determined by weighing method, and gradually extend the pressure holding time until the product weight no longer increases.
Generally, products with a wall thickness of 2mm take about 5-15 seconds.
Switching point: the timing of switching from injection to pressure holding, usually triggered by screw position or cavity pressure.
3. Common problems and solutions
A. Shrinkage or voids may be caused by insufficient pressure holding or too short time. You can increase pressure or time to optimize cooling.
B. Flash (burrs) may be caused by too high pressure holding or too late switching. You can reduce pressure and switch points in advance.
C. Unstable product size may be caused by insufficient or fluctuating pressure holding time. You can extend the pressure holding time and check pressure stability.
D. Large internal stress or easy cracking may be caused by too high pressure holding or uneven cooling. You can adjust it by reducing pressure and optimizing mold temperature.
4. Notes:
Material differences: Crystalline materials, such as PP, PA, etc., have large shrinkage and require higher holding pressure; non-crystalline materials, such as ABS, PC, etc., can be appropriately reduced.
Mold design: Small gate size requires higher holding pressure, but may cause shear overheating.
Process linkage: Holding pressure and cooling time need to be coordinated to avoid premature pressure release and reflux.
