In the processing of polypropylene composite film, controlling thickness uniformity and flatness requires systematic optimization of multiple links from equipment debugging, raw material processing to process parameter control. First of all, the stable operation of the extruder is the basis. The screw speed, temperature distribution and melt pressure of the extruder directly affect the plasticization uniformity of the polypropylene melt. Too fast screw speed may lead to increased melt shear heating, causing degradation or uneven plasticization, resulting in thickness fluctuations after film forming; too slow speed will affect the output, and thermal oxidation may occur due to long melt retention time. Therefore, it is necessary to accurately adjust the temperature of each zone of the screw according to the melt index and formula characteristics of the raw materials, so that the melt maintains stable viscosity and fluidity during extrusion, laying the foundation for subsequent uniform molding.
The design and debugging of the die head of polypropylene composite film is crucial to the uniformity of film thickness. The flow channel structure of the die head needs to ensure uniform distribution of the melt to avoid stagnation or flow rate differences. For example, the branch pipe diameter and die lip gap of the coat hanger die head need to be finely adjusted according to the film width and thickness requirements to make the melt flow rate at each point of the die head outlet consistent. Before production, the die lip gap needs to be partially corrected through the trial mold and die head fine-tuning device to compensate for the uneven thickness caused by die head manufacturing errors or melt flow differences. In addition, the uniformity of the die head temperature also needs to be strictly controlled. If the local temperature of the die head is too high or too low, the viscosity of the melt at that location will change, which will in turn affect the discharge amount and film thickness.
The cooling and shaping process is a key link in controlling the flatness of the film. After the polypropylene melt is extruded from the die head, it needs to be quickly cooled and solidified by the cooling roller. The temperature, surface roughness and cooling rate of the cooling roller will directly affect the crystal structure and internal stress distribution of the film. If the temperature of the cooling roller is too low, the melt may be suddenly cooled to form fine crystalline particles. Although it can improve transparency, it may increase the internal stress of the film and cause shrinkage or flatness problems; if the temperature is too high, the crystallization process will be slow, forming larger crystals, resulting in a rough film surface and uneven thickness. Therefore, the temperature of the cooling roller should be reasonably set according to the purpose and performance requirements of the film. Usually, the segmented cooling method is adopted to make the temperature of the film drop evenly during the cooling process and reduce the generation of internal stress.
The stability of the traction and winding system has a significant impact on the thickness uniformity and flatness of the film. The matching of the traction speed and the extrusion speed is the key. If the traction speed is too fast, the film will be stretched to cause the longitudinal thickness to become thinner, and the lateral thickness fluctuation may be caused by uneven tensile stress; if the speed is too slow, the film will accumulate, causing uneven thickness or wrinkles. The parallelism and surface flatness of the traction roller must also be strictly guaranteed. If the traction roller has jumps or surface defects, it will cause uneven stretching or friction on the film during the traction process, resulting in deterioration of the film flatness. Tension control during winding is also important. Excessive tension may cause the film to be stretched and deformed, or even broken; too little tension will cause the winding to be loose, and the film will be wrinkled or deviated during the winding process, affecting the overall flatness.
The pretreatment and mixing uniformity of raw materials are links that cannot be ignored. Uneven dispersion of moisture, impurities or additives in polypropylene resin may lead to local viscosity differences during the melt plasticization process, which in turn affects the uniformity of film molding. Therefore, the raw materials need to be fully dried before production to remove moisture and volatiles, and efficient mixing equipment should be used to ensure that additives such as antioxidants and lubricants are evenly dispersed in the polypropylene matrix. For multi-layer composite films, the melt index and processing performance of each layer of raw materials need to match. If the fluidity difference of each layer of melt is too large, the interface instability will occur during the co-extrusion process, resulting in poor thickness uniformity of the composite film.
Temperature and humidity control of the production environment will also affect the quality of the film. Excessive ambient temperature may reduce the cooling efficiency of the cooling roller, resulting in insufficient cooling of the film and easy deformation during traction; excessive humidity may cause the raw materials to absorb moisture or form condensed water on the surface of the cooling roller, affecting the cooling effect and surface quality of the film. Therefore, the temperature and humidity of the production workshop should be controlled within an appropriate range. Usually, the temperature is maintained at 20-25℃, and the relative humidity is controlled at 50%-60%, so as to provide a stable environmental condition for the processing process and reduce the interference of external factors on the thickness and flatness of the film.
Closed-loop management of quality inspection and process adjustment is the key to continuous optimization. During the production process, it is necessary to monitor the thickness distribution and transverse flatness of the film in real time through online inspection equipment such as β-ray thickness gauge and laser thickness gauge, and timely find abnormal thickness areas. When uneven thickness or excessive flatness is detected, it is necessary to quickly trace back to the extruder, die head, cooling roller or traction system, analyze the specific reasons and make targeted adjustments. For example, if the uneven transverse thickness is caused by uneven die lip gap, the local gap can be corrected by the die head fine-tuning device; if the longitudinal thickness fluctuation is related to the instability of the traction speed, it is necessary to check the speed control of the traction motor and the stability of the transmission system. By continuously accumulating production data, establishing the corresponding relationship between process parameters and film quality, and forming a traceable process optimization mechanism, the thickness uniformity and flatness of polypropylene composite film can be precisely controlled.
