An extruder is a rotating screw cylinder or extrusion barrel. There is a feed chute at one end of the barrel and a specially shaped hole or die at the other, where the product exits.
Extrusion plastics are thermoplastics that soften when heated and harden again when cooled. When said material is fed into the hopper, it is caught by the screw and pushed through the barrel where it is heated and softened enough to continue through the die. Heat is generated by friction as the screw rotates in the plastic mass. The energy for melting actually comes from the motor as it turns the screw. Sometimes the external barrel heaters and preheated feed throat provide more heat.
As the hot soft plastic exits the die, it takes the shape of the hole through which it passes through a long slit that forms a film or sheet, a circular opening that forms a tube, many small holes that form filaments, etc. Once out of the die, the plastic must be rapidly cooled with air, water, or contact with metal and removed for rolling or cutting to desired dimensions.
You can imagine the rotating screw trying to unscrew backwards from the barrel filled with material. It can’t go back because a bearing holds it in place, but when you push it against the material, that material comes out the other end.
The die on the other end acts as a resistor. The longer and smaller it is, the more the screw must work to push the material (horsepower required from the motor that turns the screw).
The extruder operator controls the temperatures of the screw and the incoming material. The canyon is divided into zones. each with their own heating and cooling controls. Barrel temperatures have to match material temperatures, but are selected as needed for each zone. The rear feed zone is especially important as it affects the feed rate and can control the production rate. There are heaters and controls in the head and die as well, and metal temperatures are often close to material temperatures.
The melt temperature of the material is measured at the exit end of the screw just before the plastic enters the die. It’s usually between 350 and 450 degrees F, but it can be higher for certain plastics. If it gets to a high level, there may be material cooling issues or perhaps chemical breakdown of the plastic.
Fusion pressure is also measured at the outlet y of the screw and reflects the resistance of the head and die assembly. Typical pressure is between 500 psi and 5000 psi and for small dies or very viscous material it can be even higher.
Screw speed is generally selected high as long as it produces a good product. Typical screw speeds are between 50 and 150 RPM.
Motor amps are measured as a warning to stay below dangerous limits and to show sudden increases that cause variations in material thickness and unexpected changes in material viscosity (ease of flow). Amps and horsepower vary greatly with the size of the machine, from small 5 to 50 horsepower machines to large compound lines using 1000 horsepower or more.