The co-extrusion blow molding process is to plasticize a variety of plastic raw materials with different functions through multiple extruders, and then extrude them into tubes after merging at the die head. Then, the tubes are blown into film bubbles by compressed air, and the modified atmosphere packaging film is obtained after cooling and shaping. This process achieves functional superposition through multi-layer co-extrusion, such as enhancing the wear resistance of the outer layer and improving the gas barrier property of the inner layer. The cast film process is to extrude the molten plastic raw materials through the extruder, and then directly cast them onto the cooling roller through the T-die head, and quickly cool and shape them. It is usually used to produce single-layer or small-layer films. Its production process pays more attention to the uniform spreading and rapid cooling of the melt. The two processes have different molding methods and material flow paths, which determine the difference in film performance from the source.
During the co-extrusion blow molding process, the film is stretched in the transverse and longitudinal directions to form a bidirectional oriented structure, which significantly improves the tensile strength and tear resistance of the film. Especially for atmosphere packaging scenarios that require load-bearing or puncture resistance, such as meat and large fruit and vegetable packaging, the mechanical properties of co-extrusion blow molding films are obvious. In contrast, the film formed by the cast film process is mainly stretched in the longitudinal direction, and the transverse orientation is low, resulting in weak transverse tensile strength and tear resistance. It has limitations in packaging applications that require high flexibility and impact resistance, but has high longitudinal strength and is suitable for packaging forms that require high longitudinal stretching.
The coextrusion blow molding process can compound high barrier materials (such as EVOH, PA) with other functional materials through multi-layer coextrusion to form a modified atmosphere packaging film with excellent gas barrier properties. The synergistic effect between different layers can effectively block the penetration of gases such as oxygen and carbon dioxide, and extend the shelf life of food. The film produced by the cast film process usually has lower gas barrier properties than the coextrusion blown film due to the small number of layers and the lack of molecular densification brought by stretching orientation. However, by optimizing the cast film process parameters, such as adjusting the cooling rate and melt temperature, the barrier properties can be improved to a certain extent, but it is still difficult to reach the level of multi-layer coextrusion blown film.
The cast film process has a smooth surface, excellent optical transparency and glossiness due to rapid cooling and uniform melt spreading, and is suitable for food packaging with high requirements for appearance display, such as baked products, candies, etc. Due to the uneven cooling of the film bubble during the blowing process and the interface effect of the multi-layer structure, the optical performance of the co-extrusion blown film is relatively poor, with high haze and insufficient transparency, which may affect the visual presentation of the food. However, with the improvement of die head design and cooling technology in the co-extrusion blow molding process, the optical performance of some high-end products has approached the level of cast film.
The casting process is direct and fast, and the production speed is usually higher than the co-extrusion blow molding process, with higher output per unit time, and relatively low equipment investment and energy consumption. It has cost advantages in the production of single-layer or simple structure modified atmosphere packaging film. The co-extrusion blow molding process involves complex links such as multi-layer co-extrusion, blowing, and cooling, and the equipment debugging and process control are difficult, the production efficiency is low, and the raw material loss rate is relatively high. However, co-extrusion blow molding achieves multifunctional compounding through one-time molding, reducing the cost of subsequent secondary processing. In the packaging scenario with high requirements for comprehensive performance, the cost-effectiveness is more significant in the long run.
The cast film process relies on the precise control of the T-die and the cooling roller to produce a film with excellent thickness uniformity. The thickness tolerance can be controlled within a very small range, which is very important for modified atmosphere packaging with strict requirements on packaging sealing. During the molding process, the co-extrusion blown film is affected by factors such as film bubble expansion and uneven cooling wind speed, and the thickness uniformity is poor, especially the thickness deviation is prone to occur at the shoulder and bottom of the film bubble, which may lead to a decrease in local gas barrier performance or insufficient packaging strength. Although the thickness uniformity can be improved by optimizing the air ring design and adjusting the traction speed, it is still difficult to achieve the accuracy level of cast film.
Based on the above performance characteristics, co-extrusion blown film is more suitable for scenarios with high requirements for mechanical properties and gas barrier properties and relatively low requirements for optical properties, such as fresh meat and frozen food packaging. Cast film, with its excellent optical properties and thickness uniformity, is often used for products that require high appearance display and sealing precision, such as fresh-cut fruit and vegetable packaging and snack food packaging. In actual production, companies need to comprehensively select the appropriate production process based on food characteristics, packaging requirements and cost budget, or develop a composite process combining the advantages of the two processes to meet diverse market needs.
