During the PVC conduit extrusion process, internal wall stress concentration is primarily caused by uneven material orientation, differential cooling shrinkage, and mold design flaws. This requires coordinated improvements across multiple dimensions, including temperature control, mold optimization, process parameter adjustment, and post-processing.
Temperature control is crucial for reducing internal stress in PVC conduits. As a heat-sensitive polymer, PVC resin requires strict processing temperature control between 160°C and 190°C. Temperatures too low lead to insufficient plasticization, resulting in distinct primary particle boundaries and insufficient molecular chain relaxation. Excessive temperatures accelerate PVC degradation, releasing hydrogen chloride gas and forming carbides, which can lead to internal stress concentration. A "stepped" temperature ramp strategy is essential during production: the feed zone temperature is maintained slightly above the glass transition temperature to promote vitrification, the compression and melting zones are gradually heated to the optimal plasticization range, the metering zone temperature is appropriately lowered to reduce shear heat accumulation, and the die zone temperature is raised back to 190°C to 200°C to ensure sufficient melt plasticization and rapid molding.
Mold design has a decisive influence on internal stress distribution. The die head flow channel should be streamlined to avoid right angles or sudden changes in cross-section that could obstruct material flow. The expansion angle of the diverter comb should be kept within a reasonable range to prevent localized overheating and decomposition due to excessive material flow resistance. The mold compression ratio should match the PVC conduit wall thickness to ensure uniform melt filling within the mold cavity. For conduits with complex cross-sections, the cooling water channel layout can be optimized, with additional cooling holes added in thick-walled areas or a combined dry-wet shaping method employed to reduce shrinkage differences by controlling the cooling rate in stages.
Proper adjustment of process parameters can significantly reduce internal stress generation. The screw speed should be matched to the extrusion rate to avoid excessive molecular chain orientation caused by excessive shear rates. The pull-out speed should be slightly higher than the extrusion speed to achieve uniform molecular chain orientation through moderate stretching, but excessive stretching should be avoided to prevent localized stress concentrations. For thick-walled PVC conduits, a variable speed extrusion process can be used, reducing the speed during the melt filling phase to reduce flow resistance differences, then gradually increasing the speed after the material has initially settled. Furthermore, temperature sensors and pressure gauges should be regularly calibrated to ensure the accuracy of process parameters.
Optimizing material formulation is an essential guarantee for reducing internal stress. Adding heat stabilizers can enhance the heat resistance of PVC resin, preventing degradation and internal stress during processing. Selecting low-volatility plasticizers can reduce material shrinkage variations and avoid stress concentration caused by localized shrinkage. Furthermore, adding impact modifiers (such as CPE) can form a network structure, dissipate stress, and prevent crack propagation, thereby reducing the risk of brittle fracture in the finished product. For high-precision PVC conduits, adding rigid particles such as nano-calcium carbonate can reduce internal stress by grain refinement.
Post-processing is crucial for eliminating residual internal stress. Annealing, by controlling the heating and cooling rates, allows sufficient time for molecular chains to relax, thereby releasing orientation and cooling stresses generated during processing. For precision conduits, a staged annealing process can be used: holding the material near the glass transition temperature for several hours, followed by slow cooling to room temperature, to ensure sufficient internal stress release. Furthermore, vibration aging treatment can stimulate plastic deformation in micro-regions within the material, further reducing residual stress.
Equipment maintenance and process monitoring are key to ensuring extrusion stability. Regularly inspect the screw and barrel for wear to prevent uneven shear heat distribution caused by increased clearance. Calibrate the temperature control modules in each section of the extruder to prevent internal stress fluctuations caused by temperature fluctuations. Online monitoring systems monitor the outer diameter, wall thickness, and eccentricity of the conduit in real time, enabling timely detection of potential stress concentration risks and adjustment of process parameters. For example, if the wall thickness deviation exceeds the allowable range, the system automatically adjusts the pull-out speed or die temperature to ensure dimensional stability.
Optimizing the PVC conduit extrusion process requires systematic improvements in temperature control, mold design, process parameters, material formulation, post-processing, and equipment maintenance. By rationally setting process temperatures, optimizing mold structure, adjusting extrusion parameters, improving material formulation, and implementing post-processing, stress concentration on the conduit's inner wall can be significantly reduced, improving the dimensional stability and service life of the product, and meeting the stringent requirements of PVC conduits in medical and construction applications.
