PO plastic bags, combining the rigidity of polypropylene with the flexibility of polyethylene, possess unique advantages in performance and appearance. Their manufacturing requires coordinated control of multiple stages, including raw material selection, blending modification, molding, and post-processing, to ensure stable and optimized material properties and bag quality.
The raw material system typically uses linear low-density polyethylene (LLDPE) and polypropylene (PP) as the matrix, supplemented with metallocene-catalyzed polyolefins, elastomers, or functional masterbatches for blending or co-extrusion. Blending involves high-speed mixing and melt mixing to allow different polyolefin molecular chains to interpenetrate, forming a network structure that combines rigidity and flexibility. Co-extrusion combines different melt flows in layers within a die, molding them into a multi-layered film in a single step, which can maintain the outer layer's rigidity while improving the inner layer's heat-sealing properties. Formulation design must consider melt index matching, crystallinity control, and functional requirements, such as the introduction of antistatic, anti-fogging, or flame-retardant modifiers, ensuring uniform dispersion without disrupting the main chain continuity.
Molding is the key step in giving PO bags their shape and performance. Blow molding is one of the mainstream processes. The blended melt is extruded into a preform through a ring die, stretched bidirectionally by air through a mandrel, cooled and shaped, then slit and wound into film rolls. Films produced using this method have uniform strength in both directions and excellent thickness, suitable for producing ordinary logistics bags and food bags. Casting extrudes the melt into sheets through a flat die, then rapidly cools and shapes them using cooling rollers. This method produces films with superior transparency and surface smoothness, often used for high-transparency display bags and printing substrates. Hot-press welding is mostly used for small or irregularly shaped bags. Heating melts and bonds the film edges; the process is simple, requires low equipment investment, and is suitable for small-batch and customized production.
In the bag-making process, depending on the application, heat sealing, ultrasonic sealing, or high-frequency welding can be used to seal the bag opening and bottom. The heat sealing temperature, pressure, and time must be precisely set according to the material's melting point and thickness to ensure a strong seal without scorch marks or wrinkles. For PO bags requiring printing, a corona treatment is typically applied to the film surface to improve ink adhesion, followed by gravure or flexographic printing to complete the graphic printing before proceeding to the bag manufacturing process.
Quality control is maintained throughout the entire process, including raw material melt flow index and ash content testing, film thickness and tensile property testing, heat seal strength and sealing performance inspection, and online monitoring of appearance defects and dimensional accuracy. Online thickness gauges, tensile testing machines, and sealing performance testing equipment allow for timely feedback and correction of process deviations, ensuring the consistency and reliability of the finished product.
Overall, the manufacturing method of PO plastic bags is a systematic engineering process integrating material blending and modification, precision molding, and process quality control. The synergistic optimization of parameters at each stage enables an ideal balance in terms of strength, heat resistance, heat sealability, and appearance, meeting the stringent application requirements of various fields such as logistics, food, industry, and daily necessities.