I teach a one-day Intro to Extrusion seminar, and many of my participants are new to extrusion. Therefore, I explain the basic principles of screws for thermoplastics. That’s good for the newbies, but sometimes I get more experienced extrusion people thanking me for these basics, which they had never learned way back when. So here goes. I hope you all find it useful.
The screw is a conveyor. As it turns, it tries to screw itself backward out of the barrel, but a bearing keeps it from going out the back. Because every action has a reaction—remember Isaac Newton?—it pushes the other way, too, in the forward direction, and that’s what pushes the material out of the die.
Material needs to get soft to go through the die. Any thermoplastic will get soft and moldable (plastic) with heat (thermo). The feed is sometimes preheated (usually for drying), but it gets most of the heat from internal friction as it moves against the barrel walls and screw surfaces. The clearances from flights to barrel are where most heat is generated. Exceptions: Some twin screws, small machines, high-temp resins and PE coatings, where barrel heat is important, too.