Screw and Barrel Heating and Cooling
The screw and barrel is where polymer is fed, melted, conveyed, mixed, devolatilized, and pumped to the die. Both the screws and barrel sections are modular and can be arranged in any configuration necessary to accomplish a particular extrusion objective. Barrel sections normally have either rectangular or circular outside dimensions, depending on the manufacturer. They are
assembled with either a rod through all the sections or with bolts holding the sections together. The barrel is supported at different locations along its length to prevent it from sagging. Each barrel contains a thermocouple to control the heating and cooling input.
The heating and cooling elements, different barrel sections, vent caps, insulated barrel covers, and a barrel support are identified. Barrel sections have flanges on each end for alignment and connection to the next barrel. While it is a sizable task, barrel sections can be disassembled, reconfigured, and reassembled to move feed and vacuum sections. Some machines (particularly smaller ones) may have a clamshell barrel design where the barrel separates in the middle and the entire screw length is exposed. Clamshell barrels are one-piece construction rather than modular sections that bolt together. Individual heating and cooling zones along the barrel provide temperature control similar to a water-cooled single screw extruder.
Barrel sections, like extruders, come in different length to diameter ratios (L/Ds). Typical lengths depend on the screw diameters and the manufacturer. Some common L/Ds are 2.5, 3, 4, 5, 6, 8, 10, and 12. There are many different barrel sections:
• one used for feeding with an opening on top (vent barrel)
• feeding into the side with a vent on top (combi barrel)
• a solid barrel (closed barrel)
The vent barrel has an opening on top that may be either circular or rectangular and is used to vent volatiles from the barrel or to feed different formulation components. The first barrel section in the extruder is opened on top for feeding all or part of the formulation into the extruder. It is cooled and normally has no side heaters. Other vent or feed barrel sections have an opening on top with cast heaters contacting the barrel on the other three sides. Barrel section 2 (the one after the feed throat) contains a vent plug and is heated on three sides with cooling (these barrel sections have the capability to be either heated or cooled as the process requires).