Mold venting: a little-discussed but highly effective setting

Before injection the cavity is full of air. When the melt enters, this air has to escape somewhere; if it can't, it gets trapped, heats up as it compresses, and disrupts filling. Venting opens an escape path for this air — an easily overlooked mold design topic with a big impact.

Signs of poor venting

• Burn marks / dieseling: Dark brown, black or scorched-looking marks in the region where filling finishes last. Trapped air overheats as it compresses, sometimes reaching a temperature that actually ignites the material (similar to compression ignition in a diesel engine, hence the name "dieseling").
• Short shot: As the pressure of the trapped air rises, it acts like an "air plug" ahead of the melt; the flow slows or stops and the cavity doesn't fill completely. (See the short-shot guide for other possible causes of short shots.)
• Inconsistent fill/part weight: Results that vary cycle to cycle with the same settings are often caused by the air sometimes escaping and sometimes getting trapped.

Where vents go

Vents are placed at the last-filling regions identified by filling analysis or short-shot trials — usually the parting line, ejector pins and the corners farthest from the gate. Locations where weld lines form (as described in weld lines and air traps) are also points that often need venting.

Size: depth and land

A vent is defined by two dimensions: depth (the narrow gap the air passes through) and how far this gap extends (the land). In practice:

• The land is kept short (around 1 mm), after which the air opens into a deeper relief channel — the goal is for the air to actually escape freely.
• Depth depends on the material: narrower (around 0.010–0.020 mm) for free-flowing (low-viscosity) materials, slightly wider (around 0.020–0.030 mm) for general-purpose materials like ABS/PC, and somewhat above that for filled/high-viscosity materials. A vent must be narrow enough to hold back the melt, but not so shallow that it blocks the air.
• Width is kept as wide as practical; making a narrow vent "a bit longer" is not a substitute for setting the depth correctly.

A common mistake: forcing the air out with pressure

When a burn mark or short shot appears, the first instinct is usually to increase injection speed or pressure. This can compress the air faster and make the problem worse. The correct order is to first open an escape path for the air (add/enlarge a vent, move it to the right place), then adjust speed/pressure if still needed.

*This article is adapted from mold venting articles on international sources such as PTonline.com and PlasticsToday.com.*