Meaning
Extrusion processes designed for high-performance polymers measure the percentage of volatile monomers, solvents, or moisture removed from the melt during vacuum venting. This devolatilization stripping efficiency directly affects the mechanical properties of the finished parts by eliminating pockets of trapped gas that form voids during cooling. High efficiency prevents bubble formation and ensures the structural density of extruded sheets or profiles.
Mechanism of Removal
Molten polymer passes through a low-pressure venting zone in the extruder barrel where volatile compounds evaporate due to the reduced pressure. Injecting a stripping agent like water or nitrogen increases the surface area of the bubbles and accelerates the diffusion of lighter molecules out of the viscous melt. This phase separation requires precise control of screw speed and temperature to maximize the surface area exposed to the vacuum port.
The stripping agent creates a concentration gradient that co-distills the volatile organic compounds out of the polymer matrix, which is essential for achieving high purity levels.
Quality Improvement
Poor removal of volatile components leads to silver streaks, surface blisters, and internal voids in moulded or extruded parts. Improving the devolatilization stripping efficiency reduces the residual monomer content, which is essential for medical and food-contact plastic applications where chemical leaching must be avoided. It also prevents the emission of unpleasant odours from the finished plastic during subsequent processing.
Process Variables
Screw configuration and vacuum level are the primary factors that determine how much gas is extracted from the melt. A deep-flighted screw section at the vent port creates a thin polymer film that facilitates rapid gas release. If the vacuum pressure drops or the vent becomes clogged with polymer build-up, the efficiency declines and causes a spike in defect rates.
Monitoring the vacuum levels during the run maintains consistent material quality.
