Venting System Design in Injection Molds

During cavity filling, the plastic melt must efficiently displace and remove two types of gases from the mold: 1) the air present in the runners and cavities, and 2) the low-molecular volatiles released by the plastic itself during heating and shearing. Inadequate venting traps these gases, leading to a range of defects:

• Gas Traps/Air Pockets: Visible bubbles or voids within the part wall.

• Weak Weld Lines: Poorly bonded areas where melt fronts meet, trapped gas preventing complete fusion.

• Short Shots/Incomplete Filling: Gas counter-pressure acting against the injection pressure, preventing the melt from completely filling the cavity.

• Poor Surface Finish: Vague contours, blemishes, or burn marks (diesel effect) from compressed, overheated gas.

When localized filling issues occur due to poor venting, adding overflow wells can be an effective secondary measure. These wells, placed beyond the cavity, capture both cold material (the melt front) and the entrapped gas, facilitating complete cavity fill.

Common Venting Methods in Injection Molds

1. Utilizing Ejection & Fit Clearances
In simple, small molds, the natural microscopic gaps around components like ejector pins, core pins, or inserts can serve as vents. The clearance is typically kept between 0.03–0.05mm, depending on the melt viscosity of the plastic. This method is simple but can be insufficient for complex parts and prone to clogging with time.

2. Venting Channels on the Parting Line
This is the primary and most widely used method. Shallow channels are machined into the parting plane, starting from the cavity edge.

• Design: The initial section (≈0.5-1.5mm long, ≈0.02-0.05mm deep) acts as a seal, preventing flash.

• Relief: This sealed section then opens into a much deeper and wider dovetail-shaped or open channel to allow gas to escape freely into the atmosphere.

• Safety: To prevent molten plastic from spraying toward the operator, the vent channel is often offset or angled after 5-8mm, with its depth increased in this section for easier escape.

3. Venting Pins (Vent Plugs)
When the last area to fill is deep within the cavity, away from the parting line and without moving cores or ejector pins, a vent plug is embedded at that spot. These plugs, often made from sintered metal (so-called "porous steel"), contain a network of microscopic interconnected pores that allow gas to pass through but block the plastic melt.

4. Runner Venting
To prevent gas-related issues like burns or increased stress in the part, it's crucial to vent the runner system as well. Vent channels are often machined at the end of the runners, just before the gate or at dead ends, to ensure gases in the melt stream can escape before entering the cavity.