In a column I wrote for the July 2011 issue, I discussed the value of conducting “forensic” examination of worn screws when they are removed for clues to design flaws.
The main purposes of the heaters are to melt the polymer that remains in the barrel at cold startup, to assist in forming the initial melt, and to “trim” the barrel temperatures for specific purposes such as improving feed rate.
One of the least understood yet most important concepts is viscous dissipation, which is the shearing or stretching of the polymer between the rotating screw and stationary barrel, causing heat to develop in the material.
This type of surge happens most frequently with two-stage screws when the second stage has far more capacity than the first stage. It can also happen when a screw is limited in feeding or melting, causing a partially filled metering section.