How to Improve Poor Venting in Plastic Injection Molds

Poor mold venting occurs when air or gas generated during melting becomes trapped in the cavity, leading to defects such as burn marks, short shots, weld lines, and gas marks. This issue can be systematically addressed through mold design optimization, process parameter adjustment, material management, and regular maintenance.

1. Mold Venting System Optimization

1.1 Vent Channel Design

Vents must be strategically placed at the last points to fill, such as the end of flow paths, weld line intersections, and deep ribs. The dimensions depend on the material viscosity:

Low viscosity (PE, PP): Depth 0.02~0.05 mm.

High viscosity (PC, PMMA): Depth 0.01~0.03 mm (to prevent flashing).

Glass-filled materials: Slightly deeper (0.05~0.08 mm) but require frequent cleaning.

Follow the "wide and shallow" principle and use multi-stage vents to ensure efficient gas escape.

1.2 Auxiliary Venting Methods

For complex geometries where standard vents are ineffective:

Venting pins/inserts: Use porous metal inserts in hard-to-reach dead spots.

Parting line gaps: Utilize controlled gaps between mold inserts (0.01~0.03 mm).

Vacuum venting: For large or optical parts, actively draw air out using a vacuum pump to eliminate trapped gas entirely.

1.3 Runner & Cavity Refinement

Optimize the runner system for balanced filling to prevent air from being trapped in specific cavities. Polish the cavity surfaces to reduce flow resistance, allowing the melt front to push air out more effectively.

2. Processing Parameter Adjustment

2.1 Injection Speed Profiling

Use a two-stage speed strategy. Fill the mold at a moderate speed initially to avoid turbulent air entrapment. As the melt approaches the vents, increase the speed slightly to ensure the gas is pushed out before the melt seals the vent. Avoid excessive speed which compresses air, causing burns.

2.2 Temperature Control

Increase the melt temperature and mold temperature to reduce viscosity and improve flow, making it easier for gas to escape ahead of the melt front. However, be cautious not to overheat the material, as thermal degradation can generate additional gases that exacerbate the problem.

2.3 Pressure Optimization

Apply sufficient injection pressure to ensure the melt reaches the vents. Adjust the packing pressure carefully; excessive packing can cause the melt to backflow into the vents or compress residual gas further.

3. Material Preparation & Selection

3.1 Drying

Moisture in hygroscopic materials (PA, PC, ABS) vaporizes during injection, creating steam that compounds venting issues. Ensure thorough drying:

PA/PC: Dry to <0.2% moisture (80~120°C for 4~6 hours).

ABS/PS: Dry at 60~80°C for 2~4 hours.

Prevent re-absorption of moisture after drying.

3.2 Material Flow Properties

Select high-flow resin grades to reduce melt resistance. For materials with inherently low flow, consider adding processing aids or lubricants to help the melt displace air more efficiently.

4. Mold Maintenance & Management

4.1 Regular Vent Cleaning

Vents are prone to clogging with degraded plastic or glass fibers. Clean vents regularly using copper brushes or ultrasonic cleaners to maintain the critical gap dimensions.

4.2 Wear Repair

Check parting lines and insert fits for wear. Excessive wear can cause flash, while reduced clearance can block vent paths. Repair or replace worn components to maintain optimal venting efficiency.

4.3 Defect Analysis

Use part defects to diagnose venting issues:

Burn marks: Indicate trapped compressed air (add vents).

Short shots: Indicate insufficient venting or flow (check vent size and location).

Weld lines: Indicate gas trapped at the meeting point of two melt fronts (vent the weld line area).