Hollows, shrinkage cavities and vacuum bubbles at injection molding gates are mainly caused by insufficient packing, premature gate solidification, uneven cooling, trapped air or excessive moisture in raw materials. These issues occur because the molten plastic cannot be fully replenished during shrinkage. Corresponding solutions are sorted into four categories: process parameters, mold structure, material treatment and temperature control.

1. Main Causes

1. Inadequate packing pressure and short packing time: Material feeding stops before the gate fully solidifies, forming internal vacuum cavities as the gate shrinks.

2. Improper gate design: Excessively thick or long gates cool slowly. Undersized or poorly positioned gates hinder pressure transmission and material replenishment.

3. Excessively fast or uneven cooling: The outer layer solidifies first while the inner material continues to shrink, leaving hollows due to insufficient feeding.

4. Poor mold ventilation: Trapped air in runners turns into visible pores after solidification.

5. Insufficient material drying: Moisture inside raw materials vaporizes during heating and creates bubbles, which is commonly seen with PA, ABS and PC.

6. Loose melt and trapped air: Low back pressure and improper injection speed lead to porous structures inside the gate.

2. Process Parameter Adjustment

1. Increase packing pressure and extend packing time

   Set the packing pressure to 60%–80% of the injection pressure. Ensure the packing duration covers the entire gate sealing process to sustain material replenishment.

2. Optimize multi-stage injection speed

   Apply segmented speed control: run fast for gate filling, slow down when passing through the main gate, and speed up again to fill the cavity. This prevents excessive shear and air entrapment.

3. Accurate velocity-to-pressure (V/P) switchover

   Switch from speed control to pressure control when the cavity is 95%–98% filled. Early switchover causes shrinkage, while late switchover leads to flash.

4. Properly raise back pressure (5–15 bar)

   This compacts the melt and expels air, effectively reducing porous hollows.

3. Mold and Structural Optimization

1. Reduce gate dimensions

   Oversized gates tend to produce shrinkage defects. Trim the gate diameter and length appropriately, and adopt tapered transitions.

2. Optimize gate size and position

   Enlarge undersized gates or reposition them to avoid premature freezing and blocked material feeding.

3. Improve ventilation

   Add micro vent slots (0.02–0.03 mm deep) at the gate root to eliminate trapped air.

4. Optimize cooling layout

   Apply forced local cooling on problematic gate areas to balance solidification.

5. Upgrade runner systems (if necessary)

   For thick-walled or high-precision products, adopt hot runners or valve gates to precisely control gate solidification.

4. Temperature Control

1. Graded barrel temperature setting

   Set a slightly lower temperature at the front section to avoid bridging, raise the middle section to reach the melting point, and keep the rear section stable to ensure uniform melt quality.

2. Reasonable mold temperature

   Excessively high mold temperature increases shrinkage rate, especially for crystalline plastics such as PP and PA. Too low mold temperature will cause rapid surface solidification.

3. Targeted cooling for gates

   Strengthen cooling at positions prone to sink marks to accelerate overall solidification.

5. Raw Material Handling

6. Judgment: Normal Shrinkage vs. Defective Voids

• Acceptable condition: Minor hollows only exist at the trimmed gate end without extending to the finished product. This is normal shrinkage.

• Defective condition: Voids run through the gate, extend into the product or impair structural strength. In this case, adjust packing parameters, gate dimensions and ventilation immediately.

Summary