In the production process of PE printed ziplock bags, the blown film process is the core step determining film quality, and the formation of crystal points significantly affects the film's appearance and physical properties. Crystal points are essentially overpolymerized polymer particles with a molecular weight higher than the surrounding PE material, preventing uniform integration with the substrate during blown film production, ultimately forming protrusions or granular defects on the film surface. To reduce the number of crystal points in the blown film process of PE printed ziplock bags, a comprehensive approach is needed, encompassing raw material selection, process control, equipment optimization, additive usage, and production environment management.
The purity and quality of raw materials are fundamental to reducing crystal points. Residual catalysts in PE resin are one of the main causes of crystal point formation. If the catalyst is not completely removed during the polymerization stage, it will continue to catalyze the polymerization reaction during high-temperature blown film production, leading to localized overpolymerization. Therefore, it is necessary to select PE raw materials with low residual catalyst content and high purity, and prioritize cooperation with suppliers with advanced polymerization processes. Furthermore, the storage and transportation environment of raw materials must be strictly controlled to prevent impurities from entering or becoming damp, thus preventing potential defects in the raw materials before they enter the blown film process.
Precise control of blown film process parameters is crucial for suppressing crystal points. Excessive processing temperature accelerates polymer oxidation, generating free radicals. These free radicals act as active centers, initiating chain reactions and forming high concentrations of unstable polymers, ultimately leading to crystal points. Conversely, excessively low temperatures result in poor plasticization, causing incompletely melted raw materials to be extruded, forming unmelted particles. Therefore, a reasonable extrusion temperature range must be set based on the raw material characteristics, ensuring uniform die head temperature and avoiding localized overheating or undercooling. Simultaneously, the matching of the blow-up ratio and draw ratio needs optimization. An excessively large blow-up ratio reduces film bubble stability and easily leads to uneven stretching; an excessively large draw ratio may cause film thickness fluctuations, increasing the risk of crystal points.
Equipment design and maintenance are the physical guarantees for reducing crystal points. Design flaws in the blown film machine die head, such as dead corners in the flow channel or an unreasonable structure, can cause raw materials to remain in the die head for extended periods, leading to over-polymerization or degradation, and ultimately crystal points. Therefore, the die head condition must be checked regularly, residues cleaned, and dies with scientifically designed flow channels and no dead corners should be prioritized. Furthermore, the screw structure and speed also need optimization to avoid raw material degradation due to insufficient shear force or incomplete filling. The replacement cycle of the filter screen is equally crucial. If the filter screen is clogged or has an insufficient mesh size, it cannot effectively filter out tiny crystal points, exacerbating film defects.
The appropriate use of additives is a chemical means to reduce crystal points. Fluoropolymer processing aids (PPA) can form a lubricating layer on the metal surface of the equipment, preventing polymer melt sticking and thus reducing overpolymerization caused by localized overheating. Simultaneously, PPA can improve the flowability of raw materials, allowing for more uniform extrusion of the melt and reducing the risk of crystal points. The addition of antioxidants can inhibit the oxidative degradation of polymers at high temperatures, preventing free radicals from initiating chain reactions, thereby reducing the formation of cross-linked crystal points. It is necessary to select additive combinations with good compatibility and significant effects based on the characteristics of the raw materials and process conditions.
The dispersibility and compatibility of the masterbatch also need attention. If the surface treatment of the pigments used in the masterbatch is unstable, they are prone to "agglomeration" during processing, forming "powder points." Poor compatibility between the carrier resin of the masterbatch and the blown film raw material can also lead to the formation of crystal points. Therefore, it is necessary to select masterbatches with excellent dispersibility and good compatibility, and to avoid mixing granules and powders to prevent poor plasticization due to differences in raw material particle size.
The cleanliness of the production environment and operational procedures are indirect factors in controlling crystal points. Dust, fibers, and other external contaminants in the workshop, if mixed into the raw materials or adhering to the die surface, can become the nuclei for crystal points, causing surrounding polymer aggregation. Therefore, it is necessary to maintain a clean production environment, clean equipment regularly, and standardize operating procedures to prevent contamination of raw materials during handling or storage.
Process monitoring and dynamic adjustment are essential. During production, first-piece inspection and periodic sampling tests are required to monitor the film's hardness, elasticity, and appearance in real time to determine the degree of vulcanization. If under-vulcanization or over-vulcanization is detected, temperature, time, or pressure parameters must be adjusted immediately. Furthermore, using an online temperature recording system and infrared thermal imaging technology can quickly detect abnormal die temperature areas, allowing for timely intervention and ensuring the stable operation of the blown film process.
