Understanding Melting in Single-Screw Extruders

February 6, 2023

Over the years I have talked to many people with lots of experience in extrusion who do not understand how single screws melt polymer, or how the extruder drive supplies that energy. Many think the barrel heaters supply much of the energy entering the polymer, which is mystifying to me … and completely wrong.

In all fairness, it is difficult to visualize melting with the screw turning in the barrel because of the geometry. But if you flip the observation point—imagine the barrel turning around the screw—you can more clearly visualize the forces involved. It’s not any different than imagining the sun turning around the earth when we know the opposite is true. In an actual extruder the screw typically turns counterclockwise, when viewed from the drive end (or clockwise from the discharge end), and the barrel is stationary. But if we flipped the observation point so that we’re “sitting” on the screw, the barrel would appear to be turning clockwise around a stationary screw.

Barrel heating is initially required before startup to get the screw filled with melted polymer and to obtain a surface temperature where the polymer will stick to the barrel. From then on, almost all the energy entering the polymer comes from the energy required to turn the screw relative to the barrel—or in our “flipped” case, the barrel relative to the screw. Because of the initial heating, the polymer is stuck to the barrel and is pushed forward by the angle of the screw flights. Once screw rotation starts, the polymer is melted almost entirely by shear.

“Shearing” is defined as applying a force that distorts one surface of an object relative to another surface. Shearing an object introduces heat into it. In an extruder the distorting force to shear the polymer requires energy to rotate the screw in the layer of viscous polymer. The rotational energy from the drive is converted from mechanical to thermal energy via shear putting heat into the polymer.

Read more: Understanding Melting in Single-Screw Extruders





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