Core Principle
The screw is the core component of an extruder. As it rotates within the heated barrel, its gradually varying groove depth-from deep to shallow-performs four consecutive actions on the PP plastic pellets: conveying, compacting, melting, and mixing. This ultimately establishes a stable pressure at the die, allowing the melt to be extruded uniformly into pipe.
Basic Operating Process
Once PP pellets enter the barrel from the hopper, they are propelled forward by the rotating screw threads. As the cross-sectional area of the screw grooves gradually decreases along the direction of material flow, the material is subjected to increasing compressive force. At the same time, intense friction and shear forces occur between the material and the inner wall of the barrel, generating a large amount of heat. Combined with external heating, this causes the PP to gradually transition from a solid state to a viscous melt.
Detailed Description of the Three Functional Zones
Feeding Zone: The screw grooves are deepest here; the primary function is solid material conveyance.
PP pellets are received from the hopper and conveyed forward by the thrust of the screw threads, while being preliminarily preheated by the barrel.
Compression Zone: The depth of the screw grooves gradually decreases; the primary functions are compaction and melting.
As the screw grooves gradually become shallower, the material is subjected to intense compression, increasing internal friction. Shear heat causes the PP to melt completely into a viscous flow state.
Metering Section: The shallowest (constant) screw groove depth; the core function is homogenization and pressure buildup.
In this section, the melt is further agitated and mixed to ensure uniform temperature and viscosity. Simultaneously, it overcomes the resistance of the die head and mold to establish stable pressure, eliminating pressure fluctuations and achieving metered, constant-pressure extrusion.