Comprehensive Summary of Injection Mold Venting System Design

Summary: Injection mold venting is not just a detail; it's a quality guarantee. From 0.025mm slots to complex insert venting, JST Mold masters the science of air exhaustion. Read our full guide to e

Efficient venting is the backbone of precision injection molding. This technical guide explores the six critical dimensions of venting system design—from parting line slots to advanced insert venting. Learn how to calculate optimal vent depths based on material flash limits (ABS, PC, PA) and strategically position vents to eliminate air traps and burn marks, ensuring 100% part quality for your complex projects.

Professional Technical Guide by JST Mold Engineering Department

In the precision world of injection molding, the venting system is a silent hero. Proper venting ensures that as molten resin fills the cavity, displaced air is exhausted rapidly, preventing common defects like Air Traps and Burn Marks.

I. The Core Functions of Venting

When molten plastic is rapidly injected into a mold cavity, it must displace the air currently inside. If this air cannot escape:

It becomes trapped at the end of the flow path.

Compression causes the air to heat up rapidly, scorching the plastic (Burn Marks).

It creates physical obstructions, resulting in incomplete parts (Air Traps).

II. Primary Types of Venting

Depending on the part geometry, JST Mold engineers utilize several methods:

Parting Line Venting: The most common and effective method.

Ejection Mechanism Venting: Utilizing the natural clearance in ejector pins, lifters, and sleeves.

Insert Venting: Employing specialized inserts to guide air out of deep areas.

Venting Valves: High-precision valves designed for critical locations.

Pro Engineering Notes:
Venting slots on the Parting Line should ideally be machined on the Cavity (Female) side.
Always leverage the fit clearance between moving components (pins, sleeves) to allow gas escape without creating flash.

III. Using Venting Valves

Venting valves are powerful but require careful consideration:

They must be placed on non-aesthetic surfaces to avoid visible marks.

They are ideal for high-quality parts where traditional venting is insufficient.

They are best positioned at the end of weld lines.

Cost Note: Due to their complexity, they are typically used only upon client request or when absolutely necessary.

IV. Venting Slot Design & Dimensions

Precision is key when machining venting slots to balance airflow against the risk of flash.

Slot Spacing: Maintain a distance of 30mm to 50mm between vents.

Placement: Avoid placing vents on curved surfaces to ensure accuracy and ease of maintenance.

Venting Depth (X-Value): The depth must be less than the material's Flash Limit.

Recommended Vent Depths (X) for Common Materials:

MATERIAL	Depth X (mm)	MATERIAL	Depth X (mm)
ABS	0.025 - 0.038	PE (Polyethylene)	0.10
POM (Acetal)	0.025 - 0.038	PP / PS	0.013 - 0.025
PC (Polycarbonate)	0.05	PA (Nylon)	0.013 - 0.025
PMMA (Acrylic)	0.013 - 0.038	PVC	0.013 - 0.038

V. Ejection and Insert Venting Strategies

For complex features, standard parting line vents are often not enough.

Deep RIB Areas: Use venting inserts at the bottom of deep ribs where air is easily trapped.

Thin Walls: For sections with large surface areas, place inserts at the junction of weld lines.

Appearance Parts: When dealing with buttons or lenses, ensure the venting insert is positioned where it will not impact visual quality.

Overflow Wells: If the design allows, draw gas into an "overflow well" outside the part area, which can be trimmed off after molding.

VI. Checklist: Strategic Venting Locations

To ensure 100% quality, our engineering team at JST Mold performs a final check at these critical locations:

The end of all thin-walled sections.

Known air entrapment zones identified in Moldflow analysis.

All weld lines and flow junction points.

The furthest point from the gate (flow end).

The bottom of blind holes, RIBs, and BOSSes.

Along the entire perimeter of the Parting Line.

Summary: Injection mold venting is not just a detail; it's a quality guarantee. From 0.025mm slots to complex insert venting, JST Mold masters the science of air exhaustion. Read our full guide to eliminate burn marks once and for all.

Partner with JST Mold

Excellence in precision molding starts with expert engineering. If you need a professional DFM (Design for Manufacturability) analysis or have a challenging project, our team is ready to help.

Contact us: info@jstmold.com

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