Venting Design in Injection Moulds

1. Introduction

Venting in injection moulds is a critical aspect of mould design that ensures proper air evacuation from the cavity during the injection process. Proper venting prevents defects such as burn marks, short shots, and poor surface finish, and improves part quality and consistency.

2. Purpose of Venting

When molten plastic is injected into a mould cavity, it displaces the air present inside. This air must be allowed to escape to enable complete and defect-free filling of the cavity. Venting provides a pathway for this trapped air, gases, and volatiles to exit the mould safely.

Key Purposes:

• Preventing gas entrapment

• Avoiding burn marks and flow lines

• Improving part surface quality

• Enabling complete cavity filling

• Enhancing weld line strength

3. Common Venting Issues (When Venting is Inadequate)

• Burn Marks: Caused by compressed air igniting at high temperature.

• Short Shots: Incomplete filling due to back pressure from trapped air.

• Weld Lines: Weak, visible lines where molten flows meet air pockets.

• Surface Defects: Blistering or pitting due to gas or volatiles trapped.

• Dimensional Inaccuracies: From inconsistent packing and shrinkage.

4. Types of Vents

a. Parting Line Vents

• Placed along the parting surface of the mould.

• Most common and cost-effective method.

• Typical vent depth: 0.01 to 0.05 mm depending on material.

• Typical vent width: 3 to 6 mm.

b. Ejector Pin Vents

• Air escapes around the ejector pins.

• Secondary venting option.

• Can be designed to act as a controlled escape route.

c. Peripheral Vents

• Located around the edge of the cavity.

• Used in multi-cavity moulds or complex geometries.

d. Vacuum Vents

• Utilized in high-precision moulds.

• External vacuum applied to remove air before injection.

e. Porous Inserts

• Made of sintered metal or porous steel.

• Allow gases to escape through micro-channels.

• Useful in areas hard to vent traditionally.

5. Design Guidelines for Vents