Functions of the Barrel and Screw
The barrel and screw constitute the core components of equipment such as plastic extruders and injection molding machines. They primarily perform critical functions including material conveyance, compaction, melting, mixing, homogenization, and constant-pressure, quantitative extrusion-factors that directly determine the quality of plasticization and overall production efficiency.
Analysis of Core Functions
The barrel and screw work in tandem to transform solid plastic into a uniform melt through a combination of rotational shear and external heating:
Conveyance: The rotation of the screw drives the material forward from the feed throat. The screw channels in the feeding section are relatively deep, ensuring the stable conveyance of solid particles and providing initial preheating.
Compaction and Venting: Upon entering the compression section, the depth of the screw channels gradually decreases; the material is compressed, and any trapped internal air is forced backward toward the feed throat to be vented, thereby preventing bubble defects in the finished product.
Melting and Plasticization: Under the dual influence of shear friction and external heating from the barrel, the plastic's temperature rises to a molten state; the compression section serves as the primary zone where this melting process occurs.
Mixing and Homogenization: In the homogenization section (also known as the metering section), the screw channels maintain a fixed depth. This section further stirs the melt to ensure uniformity in temperature and viscosity, facilitating a constant-pressure, quantitative, and constant-temperature output.
Pressure Generation and Extrusion: Pressure accumulates at the front end of the screw, overcoming the resistance of the die head and enabling the stable extrusion of the melt into the mold or die for shaping.
Different types of plastics require matching with specific screw designs: Amorphous plastics (such as PVC) are best suited for **gradual-transition screws**, whereas crystalline plastics (such as PE and PP) are ideally processed using **abrupt-transition screws**.
