Because the operating temperature at this stage has already exceeded the plastic’s rheological transition temperature, and the residence time is relatively long, the plastic undergoes a phase transformation. The material in contact with the heated barrel begins to melt, forming a thin layer of molten polymer on the inner surface of the barrel. When the thickness of this melt film exceeds the clearance between the screw flight tips and the barrel wall, it is scraped off by the rotating flights and accumulates ahead of the conveying flights, creating a melt pool. Due to the relative motion between the barrel and the screw root, the melt pool induces a circulating flow of the material. Behind the screw flights lies the solid bed—solid plastic. As the material advances along the screw channels, the groove depth gradually decreases from the melting section to the homogenizing section, forcing the solid bed toward the barrel wall and accelerating heat transfer from the barrel to the solid bed. Simultaneously, the screw’s rotation exerts shear on the melt film adhering to the barrel wall, causing the material at the interface between the melt film and the solid bed to melt. Consequently, the width of the solid bed diminishes until it completely disappears, marking the transition from the solid state to the viscous‑flow state. At this point, the molecular structure of the plastic undergoes fundamental changes: intermolecular stresses are relieved, and for crystalline polymers, the crystalline regions begin to shrink while the amorphous regions increase. With the exception of the largest macromolecules, the material has achieved preliminary plasticization—often referred to as “initial plasticization”—and, under pressure, any entrapped gases are expelled, resulting in initial compaction.

In the homogenization section, several distinctive process characteristics prevail: the screw threads here have a shallow pitch, resulting in a small groove volume; consequently, this is the section where substantial pressure develops between the screw and the barrel. Moreover, the screw’s thrust, combined with the reaction forces from the screen pack and other components, creates a zone of direct mechanical interaction with the polymer. This section also operates at elevated temperatures, subjecting the polymer to significant radial and axial stresses. Such high pressures are sufficient to expel all entrained gases and to compact and densify the melt. It is from these features that this section derives its designation as the “pressure‑equalizing section.” Furthermore, the elevated temperature ensures that any macromolecules that remained unmelting in the melting section achieve full plasticization here, thereby eliminating any residual particulates and yielding a thoroughly and uniformly plasticized material. The fully molten polymer is then discharged through the die at a controlled rate and pressure, with uniformity maintained along the entire extrudate.

The flow state of plastic during extrusion

During the extrusion process, the rotation of the screw pushes the plastic forward while the barrel remains stationary, creating relative motion between the barrel and the screw. This relative motion generates friction, which drags the plastic along as it moves. Additionally, the resistance offered by the die, the perforated screen plate, and the filter mesh at the die head produces a back‑pressure that further complicates the flow of the plastic within the screw and the barrel. Typically, the flow behavior of plastics is regarded as comprising the following four flow regimes:

1. Forward flow refers to the movement of plastic along the screw channels toward the die. It is generated by the pushing force of the rotating screw and constitutes one of the four primary flow modes. The magnitude of forward flow directly determines the extrusion rate.

2. Backflow: Also known as reverse flow, it occurs in the opposite direction to the forward flow. It arises from obstructions—such as the die, screen pack, and filter screen—in the die head that impede the plastic’s forward movement. The pressure generated in the die‑head region (the reaction force opposing the plastic’s advance) gives rise to this phenomenon. From the die head toward the feed port, a “pressure‑driven backflow” is established, also referred to as “backpressure flow.” This can lead to a reduction in production capacity.

3. Crossflow: This refers to the plastic flow along the axial direction, i.e., perpendicular to the screw grooves. It is also generated by the pushing action of the rotating screw. The flow encounters resistance from the groove sidewalls, and due to the mutual friction between the two sides of the threads. As the screw rotates, the plastic within the grooves undergoes a tumbling motion, forming a circumferential flow; thus, crossflow is essentially a type of circumferential flow. This circumferential flow plays an indispensable role in achieving uniform mixing and plasticizing the material into a molten state within the barrel. It facilitates heat exchange between the barrel and the material and is crucial for enhancing extrusion quality, although its impact on the extrusion rate is relatively minor.

4. Leakage Flow: This is also caused by the resistance offered by the die, screen plate, and filter mesh at the die head. However, it does not constitute flow within the screw channels; rather, it arises as backflow in the clearance between the screw and the barrel. It too can lead to a reduction in production capacity. Because the clearance between the screw and the barrel is typically very small, under normal operating conditions, the leakage flow rate is much lower than both forward flow and reverse flow. During extrusion, leakage reduces the effective extrudate output; as the leakage flow increases, the extrudate output decreases. The four types of flow regimes in plastics do not occur in isolation; for any given plastic particle, there is neither true reverse flow nor fully closed recirculating flow. In reality, the molten polymer’s motion within the screw channels represents a composite of these four flow states, advancing along a helical path.

5. Extrusion quality. This refers to whether the plastic is adequately plasticized and whether its geometric dimensions are uniform—specifically, whether the radial wall thickness is consistent and the axial outer diameter is even. In addition to the material itself, factors influencing plasticization include temperature, shear strain rate, and residence time. Excessive extrusion temperature not only causes fluctuations in extrusion pressure but can also lead to thermal degradation of the polymer, potentially resulting in equipment failure. Conversely, reducing the screw groove depth and increasing the screw’s length-to-diameter ratio, while promoting better heat exchange and extending the residence time to achieve uniform plasticization, may reduce extrusion output and complicate screw manufacturing and assembly. Therefore, the key factor for enhancing plasticization is to increase the shear strain rate generated by screw rotation, thereby ensuring thorough mechanical mixing, balanced heat transfer during extrusion, and ultimately guaranteeing uniform plasticization.