Overpacking in Injection Molding Raises Mold Burst Risks

While shrinkage is a well-known issue in injection molding, overpacking remains a less understood but equally significant problem that can dramatically impact product quality. This phenomenon occurs when excessive material is forced into the mold cavity, creating a range of production challenges.

Imagine overfilling a dumpling - the skin stretches thin and may rupture. Similarly, overpacking in injection molding happens when too much molten plastic is injected into the mold cavity under high pressure, creating excessive internal stresses.

This condition can be visualized as wearing pants that are too tight - the part becomes constrained by the mold, potentially leading to ejection difficulties (commonly called "sticking").

Comparing normal injection molding with overpacking reveals key differences in material behavior:

• Normal Process: Plastic molecules fill the cavity with appropriate spacing, allowing for proper packing during the holding phase and stable cooling. This creates parts with uniform density and minimal stress.
• Overpacking: The cavity becomes overcrowded with material molecules under extreme pressure. These constrained molecules create excessive internal stresses that can lead to various defects.

In severe cases, parts that successfully eject may continue to expand beyond intended dimensions due to residual internal pressure - similar to how a compressed spring rebounds when released.

Overpacking doesn't always affect the entire part uniformly. With single-gate designs in elongated parts, areas near the gate may experience overpacking while distant regions show shrinkage. This imbalance creates inconsistent part quality.

Overpacking triggers multiple quality issues that compound production challenges:

• Sticking/Mold Release Issues: Parts adhere tightly to mold surfaces
• Cracking: High internal stresses cause part failure
• Surface Defects: Increased friction creates scratches
• Warpage: Uneven stresses lead to dimensional distortion
• Dimensional Inaccuracy: Post-ejection expansion alters sizes
• Optical Defects: Affects light transmission in clear parts
• Stress Concentration: Reduces part durability and lifespan

Overpacking results from the interplay between mold design and processing parameters:

• Gate Design: Improper gate location/size creates flow imbalances
• Venting: Inadequate air evacuation increases cavity pressure
• Cooling: Non-uniform cooling exacerbates stress issues

• Excessive injection/holding pressures
• Overly long holding times
• Elevated melt/mold temperatures
• Excessive injection speeds

• Implement balanced gate systems
• Ensure proper venting channels
• Design uniform cooling circuits

• Reduce injection/holding pressures appropriately
• Optimize holding time based on material/part geometry
• Maintain proper temperature controls
• Use moderate injection speeds

Choose resins with appropriate flow characteristics and shrinkage properties for the application.

Utilize mold flow simulation to predict and prevent potential overpacking issues during design.

Other factors that influence overpacking include:

• Mold precision and maintenance
• Material drying procedures
• Machine performance consistency

A manufacturer experienced sticking and cracking in plastic housings due to overpacking near the gate. Solutions included:

• Redesigning to multiple gates
• Reducing process pressures
• Switching to higher-flow material

These changes resolved the quality issues and improved production efficiency.

Overpacking presents significant but preventable challenges in injection molding. Through comprehensive mold design, precise process control, proper material selection, and advanced simulation, manufacturers can avoid this hidden quality threat and achieve consistent production results.