In the manufacturing of polyvinyl chloride glass fiber sleeves, product shrinkage fluctuations are a key factor affecting dimensional stability and application performance. Shrinkage fluctuations mainly stem from the combined effects of material properties, process parameters, and mold design, requiring precise control through systematic optimization. As a thermoplastic resin, polyvinyl chloride (PVC) inevitably shrinks due to the characteristic of its molecular chains untangling upon heating and rearranging upon cooling. While the addition of glass fibers can enhance rigidity, uneven dispersion or weak interfacial bonding can exacerbate localized shrinkage differences. Therefore, synergistic optimization of material formulation, process control, and mold design is necessary to reduce shrinkage fluctuations.
Material formulation optimization is fundamental to controlling shrinkage. The molecular weight distribution and degree of polymerization of PVC resin directly affect melt flowability and crystallization behavior. Selecting resins with a narrow molecular weight distribution and moderate degree of polymerization can reduce differences in molecular chain rearrangement during cooling, thereby reducing shrinkage fluctuations. Simultaneously, the surface treatment of the glass fibers is crucial. Untreated glass fibers have smooth surfaces and weak interfacial adhesion to the PVC matrix, easily leading to stress concentration and uneven shrinkage. Surface modification of glass fibers using silane or titanate coupling agents can enhance their compatibility with PVC, forming a uniform stress transfer network and effectively suppressing shrinkage fluctuations.
Furthermore, adding appropriate amounts of inorganic fillers such as talc or calcium carbonate can fill the gaps between PVC molecular chains, reducing shrinkage space. However, attention must be paid to the particle size distribution and surface treatment of the fillers to avoid localized shrinkage abnormalities caused by agglomeration.
Precise control of process parameters is key to reducing shrinkage fluctuations. Extrusion temperature significantly affects the plasticization effect of PVC and the dispersion state of glass fibers. Too low a temperature results in incomplete PVC plasticization and easy agglomeration of glass fibers; too high a temperature increases the risk of PVC degradation, and glass fibers may break due to excessive shearing, both leading to shrinkage fluctuations. Therefore, a reasonable extrusion temperature gradient must be set according to the PVC type and glass fiber content to ensure uniform melt plasticization. Matching the traction speed and cooling rate is equally crucial. Excessive traction speed, causing the sleeve to be stretched before sufficient cooling, easily generates internal stress, leading to subsequent shrinkage. Insufficient cooling rate increases PVC crystallinity and shrinkage. Air or water cooling devices, combined with traction speed adjustments, are needed to achieve uniform cooling of the sleeve and reduce shrinkage differences.
Mold design is crucial for controlling shrinkage fluctuations. The mold's flow channel structure directly affects the melt filling behavior and pressure distribution. An unreasonable flow channel design results in uneven melt flow resistance within the mold, easily leading to inconsistent sleeve wall thickness and subsequently shrinkage differences. Therefore, the mold's flow channel size and shape must be optimized to ensure uniform melt filling and reduce wall thickness fluctuations. Furthermore, the length of the mold's setting section and the design of the cooling system also require close attention. A setting section that is too short will cause the sleeve to detach from the mold before complete setting, making it prone to deformation; an uneven cooling system, with excessively rapid or slow cooling in certain areas, will exacerbate shrinkage fluctuations. Simulation analysis and actual trial molding are needed to optimize the length of the shaping section and the layout of the cooling channels to achieve uniform cooling and stable shrinkage of the sleeve.
Optimization of post-processing can further reduce shrinkage fluctuations. After extrusion, residual stress may exist inside the sleeve. If used directly, it is prone to dimensional changes during storage or use. Heat setting treatment, i.e., short-term heating and holding at a certain temperature, can eliminate residual stress and stabilize dimensions. The heat setting temperature and time need to be set reasonably according to the glass transition temperature of PVC and the sleeve thickness to avoid excessively high temperatures leading to PVC degradation or excessively low temperatures resulting in incomplete stress elimination.
Establishing a quality inspection and feedback mechanism is essential for continuous optimization of shrinkage fluctuations. Real-time monitoring of sleeve dimensional changes using online inspection equipment, combined with statistical process control (SPC) methods, allows for analysis of the patterns and root causes of shrinkage fluctuations, enabling timely adjustments to process parameters or mold design. Simultaneously, establishing a database of raw materials, process parameters, and shrinkage rates provides data support for subsequent production, achieving closed-loop control of shrinkage fluctuations. By optimizing material formulations, precisely controlling process parameters, improving mold design, optimizing post-processing, and establishing a quality inspection feedback mechanism, the shrinkage fluctuation of polyvinyl chloride glass fiber sleeves can be systematically reduced,
improving the dimensional stability and application performance of the product and meeting the needs of high-end fields for precision sleeves.
