Venting in Blow Moulds: Why It’s More Important Than You Think
In extrusion blow moulding, venting is a vital part of tool design. It ensures the expanding parison forms cleanly into the cavity, allows trapped air to escape, and plays a direct role in maintaining both the surface finish and mechanical strength of the final bottle.
At Beaubury, venting is designed into every tool with precision — from parting lines and inserts to surface texturing and interlocking features. It’s not just about removing air; it’s about helping the plastic form consistently, cool evenly, and produce strong, repeatable results.
What Does Venting Do in a Blow Mould?
When the parison is inflated, the displaced air must exit the mould efficiently. If air remains trapped, it can prevent the plastic from making full contact with the cavity surface — leading to incomplete formation, visual imperfections, or internal stress.
Effective venting supports:
Accurate cavity replication
Even wall thickness and detail
Clean surface finish
Strong, uniform weld seams at the parting line
That last point is especially critical. Poor venting can cause incomplete welds at the seam, particularly at the pinch-off, leading to weaker bottles that are more prone to splitting under pressure or during drop tests. Ensuring air is fully evacuated helps create clean, continuous welds with maximum structural integrity.
Types of Venting and Their Applications
We apply a range of venting strategies depending on tool geometry and production requirements:
1. Parting Line Venting
The most common method, where the split between the mould halves allows air to escape during inflation. Tolerances must be tightly controlled to allow airflow without excess flash.
2. Cavity Venting
In areas with complex shapes or deep detail, we machine fine vent paths directly into the cavity. These must be carefully positioned to avoid leaving visible marks while ensuring full formation of features.
3. Vented Inserts
Detailed areas such as logos, textures or tight radii are often formed using vented inserts. These allow trapped air to escape behind intricate features, enabling the plastic to fully fill and replicate the design.
4. Venting Tracks in Interlocking Parts
We incorporate venting tracks around interlocking or high-pressure zones — such as necks, handles or punts — where trapped air can otherwise compromise both form and seam integrity.
5. Surface Venting via Shot Blasting
We regularly shot blast cavity surfaces to create a fine, consistent texture. This micro-textured finish allows air to escape naturally from the centre of the part outward during inflation — reducing the likelihood of internal bubbles and supporting cleaner weld seams.
Shot blasting also improves the visual quality of the part by helping prevent splay and swirl marks that can arise from trapped air or uneven forming.
Venting and Weld Strength
It’s easy to think of venting as purely a visual concern — helping air escape to avoid blemishes — but its role in structural performance is just as important.
At the pinch-off and along the seam line, the quality of the weld is directly affected by venting. If air is trapped in these areas, it can cause incomplete fusion between the two sides of the parison, resulting in:
Weak or brittle seams
Increased risk of splitting during use or testing
Inconsistent burst or drop-test performance
By combining good venting geometry with controlled surface texture, we improve the strength and reliability of every weld — without compromising part aesthetics.
In Summary
Venting is an essential part of blow mould design — not just for form, but for function. From visual quality to mechanical integrity, properly vented tools:
Help the plastic flow and settle correctly
Ensure strong weld seams at the parting line
Prevent surface defects and air traps
Contribute to longer-lasting tools and more reliable production
At Beaubury, we integrate venting at every level — from insert machining and cavity design to surface finishing. It’s a quiet detail, but one that makes a visible and measurable difference on the production line.
