Putting one or two vents between the feed throat and die is a good way to remove moisture, trapped air, and other volatiles from melted plastic as it moves through an extruder. But openings in the barrel can also make for problems. One of the most common difficulties is plastic flowing out of the vent. Even a small amount of vent flow hurts removal of volatiles, so product quality suffers. A lot of vent flow can plug the vent and force a shutdown of the extruder.
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