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Critical Venting Design Rules for Compression Molding to Eliminate Bubbles & Delamination

Created on 05.21

Meta Description: Learn the critical venting design rules for compression molding, including venting slot size, position, and overflow groove design, to eliminate bubbles and delamination defects. #compression molding venting #compression molding bubbles #compression molding delamination

Venting is one of the most critical but often overlooked aspects of compression molding. Unlike injection molding, which uses high pressure to force material into the mold, compression molding relies on slow, uniform pressure to form parts—this makes air trapping a common and costly problem. Trapped air in the mold cavity leads to bubbles, delamination, incomplete filling, and surface defects, which increase scrap rates and rework costs. As an OEM compression molding manufacturer, we’ve refined venting design rules through thousands of projects, helping customers eliminate these defects and improve production stability.

Why Venting Is More Critical for Compression Molding Than Injection Molding

Compression molding uses thermoset plastics, BMC, SMC, and composites—materials with poor flowability compared to injection molding plastics. These materials cannot "push" air out of the mold as effectively, so proper venting is essential to:

Eliminate air bubbles and delamination (the most common defects caused by poor venting).
Ensure complete filling of the mold cavity, avoiding short-shot defects.
Reduce mold pressure, preventing mold damage and flash.
Improve part surface quality and dimensional stability.

Critical Venting Design Rules for Compression Molding

Follow these rules to design effective venting for your compression molding project—tailored to different materials and part shapes.

1. Venting Slot Size: Match to Material Flowability

Venting slot size depends on the material’s flowability—materials with lower flowability require larger venting slots to allow air to escape. Below are standard venting slot sizes for common compression molding materials:

• BMC/SMC: 0.03-0.05mm thick, 8-12mm wide—these materials have moderate flowability, requiring slightly larger venting slots.

• Phenolic Resin: 0.02-0.04mm thick, 6-10mm wide—phenolic resin has good flowability, so smaller slots are sufficient.

• Silicone/Rubber: 0.04-0.06mm thick, 10-15mm wide—elastomers have poor flowability, requiring larger slots.

Note: Venting slots that are too thick will cause flash; slots that are too thin will not allow air to escape effectively, which varies slightly based on part size and material viscosity. #compression molding venting size

2. Venting Slot Position: Focus on Air Trapping Zones

Place venting slots in areas where air is most likely to be trapped—these are typically the last areas to be filled by the material:

• At the end of the material flow path (opposite the material feeding point).

• Around the mold parting line (especially for complex-shaped parts).

• In deep cavities, corners, and thick-walled areas—air is easily trapped in these areas.

• Near insert positions (if using insert compression molding)—air can be trapped around the insert.

3. Overflow Groove Design: Collect Excess Material & Air

For BMC/SMC and thick-walled parts, add overflow grooves (0.5-1mm deep, 10-15mm wide) connected to venting slots. Overflow grooves collect excess material and trapped air, preventing flash on the part and ensuring complete filling. Position overflow grooves in non-critical surface areas to avoid affecting part appearance.

4. Venting for Insert Compression Molding

Insert compression molding requires additional venting around the insert to avoid air trapping between the insert and plastic. Add small venting slots (0.02-0.03mm thick) around the insert’s perimeter, ensuring air can escape during molding.

Common Venting Mistakes to Avoid

• Ignoring Venting Altogether: This is the most costly mistake—leading to high scrap rates and mold damage.

• Designing Venting Slots in Critical Surface Areas: Venting slots can leave marks on the part, so avoid placing them on visible or functional surfaces.

• Using the Same Venting Size for All Materials: Adjust slot size based on material flowability to avoid flash or air trapping.

Our engineering team can help you design optimal venting for your compression molding project, ensuring no bubbles or delamination defects. Contact us for personalized venting design advice. #compression molding venting tips