Hold Pressure in Plastic Injection Molding

I. The Core Function and Impact of Hold Pressure

1.1 Primary Objectives of Hold Pressure

The hold pressure is set to prevent resin backflow and compensate for shrinkage during cooling, ensuring optimal molding results. If set too high, it may cause flash, overfilling, or stress concentration near the gate. Conversely, insufficient hold pressure leads to excessive shrinkage and dimensional instability. The initial injection pressure facilitates mold filling, while hold pressure is critical for maintaining part quality and minimizing shrinkage.

1.2 Relationship with Back Pressure and Process Parameters

Back pressure ensures consistent resin mixing, part quality, and control over the injection screw’s movement. Hold pressure works in tandem with the pressure switchover point and holding time. Inadequate hold pressure results in dents, bubbles, increased shrinkage, dimensional deviations, and inner-layer orientation from melt backflow. Excessive pressure causes sprue stress, difficult mold release, and outer-layer tensile stress.

II. Key Factors in Hold Pressure Optimization

2.1 Multistage Hold Pressure Strategies

Gradually decreasing hold pressure (multistage control) can:

• Reduce warpage and shrinkage differences between the gate and far-end areas.
• Alleviate internal stress and lower energy consumption.
• Optimize filling by transitioning from high-pressure slow speed to high-pressure fast speed phases.

2.2 Critical Role of Holding Time

Holding time controls the duration of hold pressure effect. Insufficient time causes dimensional/weight instability, while excessive time reduces molding efficiency. An optimal holding time should last until the gate solidifies, typically around 30% of the cooling time. Coordinating hold pressure value and time maximizes its effect in sealing the sprue and compensating for shrinkage.

III. Impact on Product Quality and Process Efficiency

3.1 Dimensional Control and Shrinkage Management

The specific volume of a molded product depends on melt pressure and temperature during holding time when the gate closes. Consistent hold pressure (often 50–65% of peak injection pressure) and temperature ensure stable dimensions. For example, adjusting hold pressure in thick-walled parts (e.g., decreasing pressure after injection and re-raising it when the surface layer thickens) can eliminate dents and flash with low clamping force.

3.2 Defect Prevention and Process Benefits

Hold pressure minimizes defects like weld lines, dents, flash, and warpage, benefiting various parts—thin-walled, multi-headed small parts, large long-cycle components, and those with unbalanced cavities or insufficient clamping force. Additionally, lower hold pressure (0.6–0.8 MPa below injection pressure) extends hydraulic pump lifespan and reduces power consumption during prolonged holding.

IV. Distinctions Between Hold Pressure and Injection Pressure

4.1 Operational Mechanisms

Both involve pushing the screw forward, but:

• Injection: Screw moves at a set speed with max injection pressure.
• Hold pressure: Screw moves at a set pressure with max holding speed.
  Multistage injection molding adjusts flow speed and pressure to reduce defects in complex structures and protect small inserts. Multistage hold pressure further minimizes dents by optimizing pressure profiles during cooling.

V. Summary of Hold Pressure’s Core Functions

During cooling, the screw slowly advances to feed molten plastic into the cavity, compensating for shrinkage—a process known as pressure holding. Its key roles include:

• Compensating for product shrinkage and stabilizing production.
• Adjusting dimensions and eliminating defects like weld lines and warpage.
• Enhancing efficiency through optimized pressure and time settings, making it indispensable for high-quality injection molding.
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