The melt flow index (MFI) of PE film, a core parameter for measuring its melt flowability, directly determines the material's flow behavior during processing, thus profoundly impacting its stretch molding performance. This index, reflecting molecular weight and molecular chain structure characteristics, serves as a crucial bridge connecting the material's fundamental properties and the molding process.
Essentially, the melt flow index characterizes the mass or volumetric rate at which the melt of a thermoplastic plastic passes through a standard capillary tube under specific temperature and pressure. For PE film, a high MFI directly corresponds to a difference in molecular weight: a high MFI indicates a lower molecular weight, shorter molecular chains, fewer branches, lower melt viscosity, and stronger flowability; a low MFI reflects a higher molecular weight, longer molecular chains, or more branches, higher melt viscosity, and weaker flowability. This molecular-level difference fundamentally determines the behavior of PE film during stretch molding.
During stretch molding, PE film with a high MFI, due to its excellent flowability, can quickly fill the mold cavity, reducing processing time and energy consumption. However, this characteristic also brings significant drawbacks: shorter molecular chains result in weaker intermolecular forces, making them prone to slippage under tensile stress, hindering the formation of an ordered oriented structure and leading to reduced tensile strength. Simultaneously, low-viscosity melts are prone to uneven local flow during stretching, causing film thickness fluctuations and affecting product uniformity. Furthermore, high melt index PE films typically have poor toughness and are prone to brittle fracture during stretching, limiting their application in high-strength packaging.
Conversely, low melt index PE films, due to their higher molecular weight and tighter molecular chain entanglement, have higher melt viscosity and limited flowability. During stretching, this manifests as higher tensile resistance, requiring greater processing stress to achieve deformation. However, once the initial resistance is overcome, the longer molecular chains can gradually align and form a highly ordered structure along the stretching direction, significantly improving the film's tensile strength and toughness. At the same time, high-viscosity melts exhibit more stable flowability during stretching, which helps maintain film thickness uniformity and improves product quality. Furthermore, low melt index PE films exhibit superior resistance to environmental stress cracking and creep, making them suitable for packaging applications requiring long-term load-bearing or harsh environments.
The melt index also influences the stretching performance of PE films, impacting their adaptability to processing techniques. High melt index materials, due to their high fluidity, are better suited for high-speed blow molding or casting processes, meeting the demands of large-scale production; however, strict control of stretching temperature and speed is necessary to prevent surface defects caused by melt fracture. Low melt index materials require lower stretching rates and higher processing temperatures to reduce melt viscosity and promote molecular chain orientation; while this process is more complex, it fully leverages the material's mechanical properties, resulting in high-value-added products.
In practical applications, the choice of melt index must be balanced based on the specific use of the PE film. For example, food packaging films require a balance between transparency and heat-sealing performance, typically using medium melt index materials to ensure processing efficiency while maintaining sufficient tensile strength; while industrial stretch films require high tensile strength and toughness, prioritizing low melt index materials to improve performance through molecular orientation. Furthermore, the melt flow index and molecular structure of PE film can be adjusted through blending modification or the addition of nanofillers, further optimizing its stretch forming performance.
The melt flow index, as a core parameter for controlling the performance of PE film, profoundly determines the material's stretch forming behavior by influencing its molecular chain structure and melt flowability. High melt flow index materials excel in processing efficiency but require sacrificing some mechanical properties; low melt flow index materials prioritize performance advantages but require optimized process conditions. In actual production, the melt flow index range must be scientifically selected based on product requirements and process conditions, and the optimal balance between performance and efficiency must be achieved through material modification.
