As a flexible packaging material that is continuously rolled and easy to slit, the forming process of roll film directly determines its microstructure, mechanical properties, and functional performance. Currently, the mainstream forming methods mainly include extrusion casting, extrusion blow molding, and biaxial stretching. Each process has its own characteristics in terms of equipment configuration, process arrangement, and parameter control, and can be flexibly selected according to the material system and application requirements to achieve efficient production and performance optimization.
Extrusion casting is a widely used continuous forming method in roll film production. The process involves feeding resin granules into an extruder, heating and melting them, and then extruding them into uniform thickness sheets through a high-precision T-die. These sheets are then rapidly cooled and shaped under the high-speed clamping of cooling rollers to form a continuous film. Due to the fast cooling rate and low crystallinity, the film produced by the casting method has high transparency and good thickness uniformity. Furthermore, the production line speed is relatively high, making it suitable for the mass production of roll films made of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and some modified polypropylene (PP). When multi-layer composite functions are required, multiple extruders and co-extrusion dies can be used to feed material simultaneously, achieving integrated molding of the heat-sealing layer, support layer, and barrier layer, reducing process losses and interlayer bonding risks associated with secondary lamination.
The extrusion blow molding process uses a ring die to extrude molten resin into a preform, then injects compressed air to cause lateral expansion, while simultaneously stretching it longitudinally under the action of a traction device. After curing by a cooling air ring, it is wound into a film. This process has relatively low equipment investment, high operational flexibility, and is suitable for producing continuous roll films with large widths and uniform thickness distribution, especially in LLDPE and LDPE single or simple blend systems. The matching of the blow ratio and traction ratio has a significant impact on the transverse strength, stiffness, and transparency of the film, requiring precise control during the process debugging stage to avoid thickness deviations or localized weak points.
For polypropylene continuous roll films, biaxial stretching is a key step in improving performance. This process typically involves longitudinal stretching followed by transverse stretching while the film is in a highly elastic or moderately crystalline state. This causes the PP molecular chains to align regularly in both directions and induce crystallization, significantly improving the film's strength, stiffness, transparency, and barrier properties, forming typical BOPP roll films. Biaxial stretching requires extremely precise control of the temperature field, stretching rate, and setting temperature; fluctuations in any stage can lead to uneven molecular orientation, affecting the final product's mechanical balance and optical quality.
In the post-forming stage, surface treatment and coating processes can be introduced to expand functionality. Corona treatment introduces polar groups by generating micro-discharges on the film surface, improving printability and heat-sealing adhesion. In-line coating can impart functions such as anti-fogging, antistatic, or grease barrier properties to the film, meeting the needs of specific applications.
Overall, the roll film forming process, based on extrusion casting and blow molding, combined with biaxial stretching and post-processing technologies, can achieve comprehensive coverage from single-material to multi-layer composites, and from general packaging to high-performance barrier applications. In production practice, process selection and parameter optimization should be carried out based on resin characteristics, product structure and end requirements to ensure that the film has both excellent molding efficiency and stable performance, providing reliable support for downstream packaging applications.