Screw and Barrel Structures and Characteristics

Screw and barrels are the core components of plastic extruders. They are responsible for melting, plasticizing, and homogenizing the plastic before it is extruded into the desired shape. The structure and characteristics of the screw and barrel have a significant impact on the quality and efficiency of the plastic product.

Screw Structure

The screw is a helical shaft with a flight pattern that runs along its length. The flight pattern is responsible for transporting the plastic through the barrel and for providing the shear and mixing forces that are necessary for plasticization.

There are many different types of screw designs, each of which is optimized for a particular application. Some common screw designs include:

Conventional screws: These screws are general-purpose screws that can be used with a wide variety of plastics.
Low-filler screws: These screws are designed for use with plastics that have a low filler content, such as CaCO3. They have a shallower flight pattern and a longer metering section than conventional screws.
High-filler screws: These screws are designed for use with plastics that have a high filler content, such as CaCO3. They have a deeper flight pattern and a shorter metering section than conventional screws.

Barrel Structure

The barrel is a cylindrical tube that surrounds the screw. It is responsible for providing the heating and cooling that is necessary for plasticization and for maintaining the pressure that is necessary for extrusion.

The barrel is typically made of steel and is lined with a wear-resistant material, such as nitrided steel or bimetallic steel. The barrel is also equipped with heating and cooling jackets, which are used to control the temperature of the plastic.

There are two main types of barrel designs:

Open barrels: These barrels have a feeding port at one end and an exhaust port at the other end. They are typically used for processes that do not require a high degree of mixing or devolatilization.
Closed barrels: These barrels do not have a feeding port or an exhaust port. They are typically used for processes that require a high degree of mixing or devolatilization.
Screw and Barrel Temperature Control
The temperature of the screw and barrel is an important factor in the plastic extrusion process. The temperature must be high enough to melt and plasticize the plastic, but not so high that it degrades the plastic.

Screw and barrel temperature control is typically achieved using a combination of internal and external heating and cooling systems. Internal heating systems use circulating oil or water to heat the screw and barrel. External heating systems use heaters, such as band heaters or air heaters, to heat the screw and barrel.