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Tech Focus Overcoming standard mould venting limitations: A different approach

| Author / Editor: Domiano Balzi / Rosemarie Stahl

There are numerous problems that can be caused by improper venting: Short shots, carbonisations and cracks in the finished part are only a few. Alternative venting valves can help overcome standard mould venting limitations and improve mould performance.

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The body of an external mould venting valve houses a guide for a slide and contains wide venting channels connected to the venting area between the body and slide (Figure 1).
The body of an external mould venting valve houses a guide for a slide and contains wide venting channels connected to the venting area between the body and slide (Figure 1).
(Source: Ermanno Balzi)

Proper mould venting is essential for producing quality parts. During the moulding process, the air contained in the mould needs a way to escape, otherwise the melt will compress and trap that air in the cavity. A combination of melt pressure and high temperature will then ignite the oxygen (called the “diesel effect”), causing burnings (carbonisation), gloss marks and stress cracks in the finished plastic part. Poor venting, along with the diesel effect, can also cause the mould to wear on the mould parting lines, resulting in flash (excess plastic) on the injected part.

Reducing injection speed will allow more time for the air to escape from the cavity through the vents, but this leads to other problems, such as short shots (incomplete parts). This happens because, as injection speed slows, the temperature of the plastic decreases. The lower the temperature of the plastic, the higher the melt viscosity, causing the plastic to begin to harden before the mould is completely full and therefore producing incomplete parts.

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Also, when semi-crystalline polymers are being injection-moulded, it is necessary to apply pack pressure (increased injection pressure) to the material after filling is complete.

This helps the polymer chains to rearrange while freezing and form partly ordered regions, guaranteeing the finished part’s mechanical properties. Once the material reaches the glass transition temperature, the movement of molecular chains is frozen.

Packing is possible as long as a centre-core flow path of melted material can transfer the packing pressure from the injection point to the entire injected part. The longer the filling time, the cooler the material, which can result in poor packing. This poor packing causes warpage of the part and less conformity with cavity design and texture. Knit lines and other aesthetic characteristics of the part are strictly related to the plastic’s temperature during moulding, so efficient filling time is essential for achieving proper welding lines and quality parts.

Improper cavity filling also can trap air between two plastic flow fronts. Software analyses that can predict the critical spots at which air traps are likely to occur can help ensure proper placement of mould venting. Standard mould venting involves machining pathways between the mould’s mechanical elements through which air can escape.

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