1. Achieving Efficient Solid Conveyance and Melting
Solid Conveyance: The feed section of the screw conveys the PPR pellets (solid particles) supplied from the hopper forward and gradually compacts them. The geometric parameters of the screw (such as groove depth and helix angle) directly determine the conveyance efficiency.
Forced Melting: PPR is a crystalline polymer with a distinct melting point (approximately 140–160°C). Through screw rotation and external heating, the material undergoes intense shearing, friction, and compression in the compression zone, forming a stable melt. The screw compression ratio design (the ratio of feed section groove depth to metering section groove depth) is particularly critical-an appropriate compression ratio (typically 2.5–3.5 for PPR) ensures complete and uniform melting of the PPR pellets, preventing the formation of unmelted "fish eyes" or crystalline spots.
Translated with DeepL.com (free version)
2. Providing Adequate Mixing and Homogenization
PPR materials require the addition of necessary additives (such as antioxidants and color masterbatches), and their molecular weight distribution significantly affects their performance. The screw design achieves homogenization through the following methods:
Distribution mixing: Uniformly dispersing additives and color masterbatches throughout the melt to prevent color variations or additive agglomeration.
Dispersive mixing: Generates high shear forces through special screw features (such as barrier sections, pins, and mixing elements) to break up agglomerated additive particles and disperse them uniformly, while simultaneously promoting the orientation and rearrangement of PPR molecular chains to enhance melt strength.
Temperature homogenization: Minimizes temperature differences in the melt in both the circumferential and axial directions (typically controlled within ±1–2°C), preventing material degradation caused by localized overheating or difficulty in extrusion caused by localized undercooling.
3. Establishing Stable and Controllable Melt Pressure
Creating back pressure: The metering section of the screw, through specific groove depths and lengths, generates sufficient pressure at the die (PPR pipe extrusion typically requires 15–30 MPa). This pressure is necessary to overcome resistance from the die, filter screen, and mold, ensuring that the melt fills the mold densely and continuously.
Suppressing Pressure Fluctuations: Stable pressure is key to ensuring uniform pipe outer diameter and wall thickness. High-performance screw designs (such as split screws and BM-type screws) can significantly reduce pressure pulsations, typically keeping fluctuations within ±1%.
4. Adapting to the Unique Rheological Properties of PPR Material
PPR melt exhibits shear thinning behavior (i.e., viscosity decreases as shear rate increases) and a broad molecular weight distribution. Targeted screw designs include:
Lower compression ratio: Lower than that of PE or PPH, to prevent excessive shear from causing PPR molecular chain breakage or degradation.
Longer melting and metering zones: To accommodate PPR's wide melting temperature range, ensuring sufficient plasticization while preventing yellowing caused by prolonged heat exposure.
Appropriate length-to-diameter ratio (L/D): PPR pipe extrusion typically uses an L/D ratio of 30:1 to 36:1. A sufficient L/D ratio ensures low-temperature extrusion (reducing internal stress) and high output, but an excessively long ratio can lead to material degradation.
5. Balancing Output and Energy Consumption
Geometric dimensions determine the theoretical conveying capacity: parameters such as screw groove depth, pitch, and root diameter collectively determine the volume of melt that can be conveyed per unit of time. Deep grooves increase output but sacrifice mixing efficiency; shallow grooves have the opposite effect.
Optimizing Power Efficiency: An excellent screw design ensures that, while maintaining plastication quality, a greater proportion of the motor's input energy is converted into the material's internal energy and pressure energy, rather than being wasted on ineffective friction or backflow. Typically, high-efficiency screws can reduce energy consumption by 15–25%.