1. Equipment Factors: Determining the Upper Limit of Conveying Efficiency
Screw Design and Wear
Screw Configuration: Different screw designs (such as conventional full-thread, barrier-type, and split-type) directly affect solid conveying efficiency and melt pressure-building capacity. For shear-sensitive materials like PVC, gradient or step-type screws are typically used, with a compression ratio ranging from 2.5:1 to 3.5:1. If the compression ratio is too low, melt pressure will be insufficient; if it is too high, excessive shear heat may cause PVC decomposition.
Degree of Wear: This is a key consideration when evaluating used equipment. The radial clearance between the screw and the barrel is a critical indicator. For PVC processing, the ideal clearance is typically 0.1–0.3 mm. When this clearance increases due to wear, melt flows back through the gaps between the screw flights, causing a sharp drop in conveying efficiency. For example, for a 65 mm screw, if the gap increases from 0.2 mm to 0.5 mm due to wear, the maximum conveying speed may decrease by 20%–30%.
Barrel Structure
Feed Port Design: The shape, dimensions, and presence of a forced cooling system in the feed port affect the bulk density of PVC powder and feeding efficiency. Poorly designed feed ports can easily lead to "bridging" or uneven feeding.
Die and Filtration System
Die Resistance: A complex die head flow channel design, excessive compression ratio, or an overly long forming section can all increase backpressure on the melt flow, thereby reducing conveying speed.
Filters and Manifold Plates: The higher the mesh count and the greater the number of filter layers, the greater the resistance to the melt, resulting in a decrease in conveying speed. At the same time, filters also serve to remove impurities and enhance mixing efficiency.
2. Material Factors: Factors Affecting Flow Resistance
Formulation: This is the most flexible and commonly used control method in production.
Lubricants: Excessive amounts of external lubricants (such as paraffin or PE wax) can form an over-lubricated film, causing the melt to slip along the barrel wall and reducing conveying efficiency. Insufficient internal lubricants (such as stearic acid or polyethylene oxide wax) increase intermolecular friction within the melt, leading to higher melt viscosity, poorer flowability, and reduced conveying speed.
Fillers: The addition of fillers such as calcium carbonate increases melt viscosity and flow resistance, thereby reducing conveying speed. To improve flow, the amount of lubricant typically needs to be adjusted accordingly.
Impact modifiers: The addition of modifiers such as CPE also increases melt viscosity, which has a certain negative impact on conveying speed.
Resin Properties
Molecular Weight and Distribution: Higher molecular weight results in greater melt viscosity and more difficult flow, leading to a corresponding decrease in conveying speed. Resins with a broad molecular weight distribution have a wider processing window but exhibit relatively poorer flowability.
Particle Morphology: PVC resins with irregular particle shapes and low porosity have lower feeding efficiency in the feed section, which affects initial conveying.
3. Process Factors: Real-Time Control Measures
Temperature Settings
Impact: Temperature is key to regulating the viscosity of the PVC melt. As temperature rises, melt viscosity decreases, flowability improves, and conveying speed increases. However, PVC is heat-sensitive; excessively high temperatures (typically above 200°C) accelerate its decomposition, generating gases and black spots, which in turn disrupt continuous conveying.
Temperature Gradient: A gradually increasing temperature gradient is typically set along the material flow path, from the feed section through the homogenization section to the die. An incorrect gradient (such as a temperature in the homogenization section lower than that in the compression section) can cause pressure backflow, severely hindering conveyance.
Screw Speed
Effect: Within a reasonable range, an increase in screw speed results in a nearly linear increase in conveyance speed. This is the most direct means of adjusting output.
Limitations: However, the speed cannot be increased indefinitely. Excessively high speeds generate intense shear heat, which may lead to:
Material degradation: PVC decomposes due to overheating.
Inadequate Plasticization: The material's residence time in the barrel is too short, preventing sufficient plasticization.
Melt Breakage: The surface of the extrudate becomes rough.
Die Pressure
Effect: An increase in die pressure (e.g., due to die blockage or excessive haul-off speed) reduces the conveying speed. This is because the extruder must overcome greater back pressure to push the material out.
4. Downstream Equipment Factors: Matching Traction and Cooling
Traction Speed
Matching Principle: The traction speed must be precisely matched to the extruder's feed rate.
Excessive traction speed: This will generate tensile stress on the pipe, causing the wall to thin, and may also cause the melt to rupture due to stretching.
Excessively slow draw: Causes material to accumulate at the die and cooling jacket, creating backpressure that acts against the extruder, reducing feed rate, and causing the pipe to sag and deform due to gravity.
Cooling and Shaping
Cooling jacket resistance: Factors such as the chamfer at the cooling jacket inlet and dimensional tolerances create resistance. If resistance is too high, it significantly increases die pressure and reduces feed rate.
Cooling Efficiency: Insufficient cooling prevents the pipe inside the forming sleeve from solidifying quickly, causing it to stretch and thin under the pulling force. To maintain wall thickness, the operator may be forced to reduce the pulling speed, which indirectly affects the speed of the entire production line.