1.Equipment Condition: The Hardware Foundation of Stability
Temperature Control System: The temperature of each section of the extruder (feeding zone, compression zone, metering zone), as well as the head and die, must be precise and stable. Temperature fluctuations directly lead to changes in melt viscosity, resulting in uneven discharge flow rate and fluctuations in the outer diameter and wall thickness of the pipe.
Screw and Barrel Wear: After long-term use, the clearance between the screw and barrel will increase, causing reduced plasticizing capacity, pulsation in melt delivery, unstable pressure, and bamboo‑joint or periodic uneven wall thickness.
Drive and Control System: Stable driving of the main motor and accurate control of screw speed are the core of stable output flow. Response lag in old frequency converters or control systems will cause speed fluctuations.
2.Raw Material Properties: The Intrinsic Foundation of Stability
Raw Material Quality and Formula:
Melt Flow Rate (MFR): Different grades of PE vary greatly in MFR. Unstable MFR or uneven batch mixing will result in inconsistent melt flow.
Impurities and Moisture: Dust, gel particles, or trace moisture contained in raw materials will form crystal spots, bubbles, or holes during extrusion, damaging production continuity.
Recycled Material Ratio and Quality: When adding recycled material, large differences in particle size, plasticization degree, and MFR compared with virgin material, as well as uneven mixing, are among the most common causes of process fluctuations.
Raw Material Pre-treatment: Although PE has low water absorption, drying is still required if the storage environment is humid, especially for high-performance pipes. Preheating can reduce main motor load and promote stable plasticization.
3.Mold and Calibration System: The Key Link in Forming
Die Design: The smoothness of the flow channel design of the die and mandrel, as well as the appropriate length of the calibration section, directly affect the uniformity of pressure and flow velocity distribution of the melt at the outlet.
Die Temperature: Uneven temperature distribution across different parts of the die will result in different cooling rates of the melt at the outlet, causing eccentricity and wall thickness variation.
Sizing and Cooling:
Vacuum Level of the Vacuum Calibration Tank: Fluctuations in vacuum level will directly lead to fluctuations in the outer diameter of the pipe.
Cooling Uniformity: Uneven water pressure, temperature, and flow rate of the spray cooling water will cause inconsistent cooling shrinkage of the pipe, resulting in bending, ovality, or internal stress concentration.
4.Process Parameters: The Core of Stability Control
Temperature Setting: Not fixed; it needs fine-tuning according to raw materials, screw speed, and ambient conditions. The principle is to ensure uniform and stable melt temperature.
Screw Speed: This is the core parameter determining the extrusion output. Without stable screw speed, no stability can be achieved.
Haul-off Speed: The matching between haul-off speed and extrusion line speed (haul-off ratio) is critical for controlling pipe wall thickness and dimensional accuracy. Slight fluctuations in haul-off speed will be directly reflected in the wall thickness.
Back Pressure: Adjusted by changing screen mesh size, formulation, or temperature in the metering zone. Proper back pressure improves homogenization, but excessive back pressure leads to over-shearing and higher melt temperature.