Optimization of raw materials and formulas
Raw material purity control: If the PVC resin contains impurities, it will affect the fluidity and uniformity of the material during processing, and thus lead to poor roundness of the baseball. Therefore, it is necessary to strictly screen raw material suppliers to ensure that the purity of PVC resin reaches a relatively high level. When raw materials enter the factory, sampling tests are conducted to check indicators such as impurity content and ash content. Raw materials that do not meet the requirements are resolutely rejected.
Formula system adjustment: In the PVC formula, the types and dosages of additives such as plasticizers and stabilizers will affect the processing performance and physical properties of the material. For instance, choosing the right plasticizer can enhance the flexibility and fluidity of PVC, making it easier to fill the mold cavity during injection molding and reducing the roundness error caused by uneven filling. Meanwhile, the dosage of the stabilizer should be reasonably adjusted to ensure that the PVC does not decompose during processing and maintain the stability of the material’s performance. Through multiple experiments, the formula system was optimized to find the best combination and dosage of additives.
Mold design and manufacturing
The precision of the mold cavity: The precision of the mold cavity is one of the key factors determining the roundness of PVC baseball. During the mold manufacturing process, high-precision processing equipment and techniques should be adopted to ensure that the dimensional accuracy and surface roughness of the cavity meet the requirements. For instance, when using a CNC machining center for cavity processing, the dimensional tolerance is controlled within an extremely small range, and the surface roughness is reduced to below Ra0.8μm. Meanwhile, during the mold assembly process, the fit accuracy of each component should be strictly controlled to avoid cavity deformation caused by assembly errors.
Gate and runner design: A reasonable gate and runner design can enable the PVC melt to flow uniformly within the mold, reducing roundness issues caused by uneven flow. The position and size of the gate should be optimized according to the shape and size of the baseball to ensure that the melt can fill the cavity quickly and evenly. The cross-sectional shape and size of the flow channel should also be reasonably designed to reduce pressure loss and melt flow resistance. For example, the adoption of circular flow channels can reduce the shear stress of the melt during the flow process and improve the fluidity of the melt.
Exhaust system optimization: If the gas inside the mold cannot be discharged in time, it will form bubbles or defects on the surface of the PVC baseball, affecting its roundness. Therefore, a reasonable exhaust system should be designed, and exhaust slots or exhaust holes should be set at appropriate positions on the mold. The depth and width of the exhaust groove should be determined based on the characteristics of the PVC material and the size of the baseball to ensure that the gas can be discharged smoothly and at the same time prevent the melt from overflowing.
Injection molding process control
Injection parameter adjustment: Parameters such as injection pressure, injection speed and injection time will affect the filling situation of PVC melt in the mold cavity. If the injection pressure is too small and the melt filling is insufficient, it will lead to problems such as material shortage and poor roundness in the baseball. If the injection pressure is too high, it may cause the melt to overflow the mold, resulting in flash. If the injection speed is too fast, it will cause turbulence in the melt within the cavity, resulting in roundness errors. If the injection speed is too slow, the melt will cool too quickly, affecting the filling effect. Therefore, based on the structure of the mold and the characteristics of the PVC material, the best combination of injection parameters should be found through multiple mold trials.
Pressure holding and cooling control: The pressure holding time and pressure are designed to compensate for the shrinkage of PVC material during the cooling process and ensure the dimensional accuracy and roundness of the baseball. If the pressure-holding time is too short or the pressure is insufficient, the baseball will contract and dent, and its roundness will deteriorate. If the holding time is too long or the pressure is too high, it may cause internal stress in the baseball. The cooling time and temperature also affect the crystallinity and shrinkage rate of PVC materials, and thereby influence the roundness. If the cooling time is too short and the PVC material fails to crystallize and cool sufficiently, it will cause the baseball to continue to shrink and change its roundness after demolding. If the cooling time is too long, it will reduce production efficiency. Therefore, it is necessary to reasonably control the pressure-holding and cooling parameters to ensure that the baseball contracts uniformly during the cooling process and maintains its roundness.
Post-treatment process
Heat treatment correction: After injection molding, PVC baseball may have certain internal stress and deformation. Through heat treatment processes, PVC materials can relax at a certain temperature, eliminate internal stress and improve roundness. For example, put a baseball into a heat treatment furnace, keep it at an appropriate temperature for a period of time, and then cool it slowly. The temperature and time for heat treatment should be determined based on the characteristics of the PVC material and the size of the baseball to avoid deformation or performance degradation of the baseball due to excessively high temperatures or long times.
Surface finishing: For defects such as burrs and flash produced during the injection molding process, they can be removed by mechanical processing or manual finishing. During the trimming process, it is necessary to pay attention to controlling the force and accuracy of the trimming to avoid causing secondary effects on the roundness of the baseball. For example, when using sandpaper or a file for manual finishing, gently grind to ensure the surface is smooth and the roundness meets the requirements.