An extruder is a mechanical apparatus used to produce objects with a cross-sectional profile. The main parts of an extruder are the feed hopper, a barrel containing the screw auger, and a cross-sectional die. Through changes in temperature, pressure, and shear within the barrel, the raw material is forcibly homogenized before it assumes the cross-sectional shape of the die opening. Food, plastics, and metals are cast into different shapes by using an extruder.
The raw material is fed into a conical-shaped hopper that conveys the material into the barrel section. The hopper is located above the barrel, and part of the screw is also present at the lower end of the hopper. The feed rate is adjusted in accordance with the flow characteristics of the material, the screw speed, and the cross-sectional area of the die.
Barrel with Screw Auger
The barrel is a hollow cylindrical steel body containing the screw auger that propels the material fed through the feed hopper. The barrel comprises the feed section, compression section, and metering section. Compression of the material inside the barrel turns it into a homogeneous mass before entering the die. Compression and shearing action inside a constant diameter barrel is achieved by adopting a tapered screw, or by decreasing the screw pitch. The metering section allows uniform mixing of the compressed mass through the application of uniform pressure. The metering section is also called the cooking section.
The die is the restricted opening at the end of the barrel which gives the desired cross-sectional shape to the extruded product. After exiting the metering section of the barrel, the homogenized mass undergoes a pressure drop upon entering the die, which prevents clogging of the extruded product. The die can have a single opening or multiple openings. To have a good quality of extruded product, it is important to have a die without defects and scratches.