Meaning
Internal structural cavities within injection moulded parts form when the outer skin cools and shrinks faster than the molten core. The creation of vacuum voids occurs during the cooling phase, where the solidifying skin pulls the remaining molten material outward, creating an empty space in the centre of the thick section. These voids weaken the part and can cause mechanical failure under load.
The defect is managed by increasing the pack pressure and hold time to keep the cavity packed with melt.
Void formation
Thick-walled parts are highly susceptible to this defect because the centre of the section remains hot long after the skin has solidified. As the polymer cools, it shrinks, but because the rigid skin cannot collapse, it pulls the material toward the outer walls. This action creates a region of negative pressure in the centre, forming vacuum voids that contain no gas.
This defect is particularly common in amorphous resins where the shrinkage rate is high. This physical separation of the polymer chains reduces the load-bearing capacity of the part.
Defect Distinction
Moulders must distinguish between these shrinkage-induced holes and those caused by trapped gas or moisture. If the hole contains gas, it is a blister or gas pocket, which is caused by inadequate venting or underdried resin. When vacuum voids are cut open, they do not release any gas, and their internal surfaces are smooth.
This distinction is critical because the solutions for gas pockets are opposite to those for shrinkage holes. This analysis prevents wasting time on incorrect process changes. It is a key diagnostic step in defect analysis.
Process Remedy
Increasing the hold time allows the screw to push more melt into the cavity to fill the shrinking core. Moulders can also increase the gate size to delay gate freeze-off, ensuring that the holding pressure remains active for longer. These process changes help to eliminate the internal cavities, making the part stronger.
This adjustment improves part reliability.
