Optimization Design of Holding Pressure in Injection Molds

In the injection molding process, the holding pressure stage has a crucial impact on product dimensional accuracy, surface quality, and mechanical properties. A well-designed holding-pressure strategy not only reduces defects (such as sink marks, warpage, and internal stress) but also improves production efficiency and mold lifespan. This article explores key factors in optimizing holding-pressure for injection molds and provides practical design recommendations.

The Role of Holding Pressure

Holding-pressure refers to maintaining a certain pressure after injection to allow molten plastic to continue filling the cavity, compensating for volume shrinkage during cooling. Its main functions include:

Reducing sink marks and voids: Ensures continuous material feeding to prevent surface depressions or internal porosity caused by shrinkage.
Improving dimensional stability: Guarantees that the product meets design specifications after cooling.
Minimizing internal stress: Proper holding pressure reduces deformation and stress concentration caused by uneven shrinkage.

Optimization of Holding Pressure Parameters

Holding Pressure

Holding-pressure is typically 50%~80% of injection pressure. Excessive pressure may lead to:

Part sticking, making demolding difficult.
Increased internal stress, affecting product strength.
Accelerated mold wear.
Insufficient pressure, however, may result in inadequate material compensation, causing sink marks or dimensional deviations.

Holding Time

Holding time should last until the gate solidifies. Prolonged holding time may cause:

Extended cycle time, reducing production efficiency.
Over-compaction of the product, increasing internal stress.
If too short, insufficient material feeding may occur, affecting product density.

Optimization of Holding Pressure Curve

Multi-stage holding-pressure (e.g., high first, then low) can better control shrinkage:

First stage (high pressure): Rapid compensation to minimize sink marks.
Second stage (low pressure): Reduces internal stress and prevents sticking.

The Influence of Mold Design on Holding Pressure

Gate Design

Gate size: A larger gate extends holding pressure time but must avoid cold slug clogging.
Gate location: Should be placed in thicker sections to facilitate material compensation.

Cooling System Optimization

Uniform cooling reduces uneven shrinkage and prevents holding pressure failure due to localized overheating.

Venting Design

Proper venting prevents short shots or burning caused by trapped air during the holding phase.

Practical Application Recommendations

Trial molding stage: Use mold flow analysis (e.g., Moldflow) to optimize holding pressure parameters.
Monitoring and adjustment: Employ pressure sensors to monitor cavity pressure in real-time and dynamically adjust the holding curve.
Material adaptation: Different plastics (e.g., ABS, PP, PC) have varying shrinkage rates, requiring tailored holding strategies.

Holding-pressure optimization is a critical aspect of injection mold design. Proper holding pressure parameters and mold structure can significantly enhance product quality and production efficiency. Through scientific analysis, parameter adjustment, and mold optimization, manufacturers can ensure precision while reducing defect rates, achieving stable and efficient production.