Whether the difference between the transverse and longitudinal shrinkage rates of plastic shrink film is significant requires comprehensive analysis from three aspects: material properties, processing technology, and application scenarios. This difference mainly stems from the interaction of molecular chain orientation, stretching processes, and heat shrinkage mechanisms; different materials of shrink film exhibit varying transverse and longitudinal shrinkage characteristics.
At the material properties level, polyethylene (PE) shrink film typically exhibits a small difference between its transverse and longitudinal shrinkage rates. Its molecular chain structure aligns simultaneously in both directions during processing, resulting in relatively balanced shrinkage performance. For example, when packaging bottled water or dairy products, PE shrink film can both tightly adhere to the bottle body through longitudinal shrinkage and fix the bottle's position through transverse shrinkage. This balance makes it a commonly used material for bulk packaging. In contrast, polyester (PET) shrink film has a significantly higher transverse shrinkage rate than its longitudinal shrinkage rate. Its molecular chains are more strongly oriented in the transverse direction, resulting in greater transverse shrinkage force. This characteristic makes PET film more suitable for packaging cylindrical, rigid objects, such as metal cans or glass bottles. Transverse shrinkage ensures the film fully conforms to curved surfaces, while longitudinal shrinkage prevents excessive compression and deformation.
The processing technology also significantly impacts the transverse and longitudinal shrinkage rates. During blown film production, parameters such as mandrel rotation speed, blow-up ratio, and traction speed alter the orientation of the molecular chains. For example, increasing the mandrel rotation speed enhances circumferential shear force, increasing the transverse orientation of the molecular chains and thus improving the transverse shrinkage rate. Conversely, excessive longitudinal tensile strength may inhibit transverse shrinkage, widening the difference between the two. Furthermore, the ratio of the cooling and shaping sleeve diameter to the blown film bubble diameter directly affects the transverse shrinkage rate: a smaller ratio results in a larger transverse shrinkage space and a higher shrinkage rate. This process control allows the performance of the shrink film to be adjusted according to specific needs; for example, a lower shrinkage rate can be designed for soft packaging to prevent deformation, while a higher transverse shrinkage rate can be designed for hard objects to ensure a good fit.
Application requirements further amplify the differences in transverse and longitudinal shrinkage rates. On automated production lines, PE shrink film, due to its balanced transverse and longitudinal shrinkage, can quickly adapt to high-speed packaging demands, reducing adjustment time. Meanwhile, the high transverse shrinkage rate of PET shrink film allows for fixation of cylindrical items with only unidirectional heat shrinkage, simplifying the process. For shrink film with printed graphics, a balanced shrinkage rate in both the transverse and longitudinal directions prevents pattern distortion and maintains visual appeal. Furthermore, ambient temperature and heating time also affect the shrinkage rate: increased temperature or prolonged heating time increases both transverse and longitudinal shrinkage rates, but the increases may differ, leading to variations in shrinkage.
Material formulation and the use of additives also alter shrinkage characteristics. Adding compatible resins to PE film improves molecular weight distribution, resulting in more uniform transverse and longitudinal shrinkage; while introducing compatibilizers may reduce shrinkage but enhance heat resistance. PET film has higher crystallinity and a more compact molecular chain arrangement, generally resulting in a higher transverse shrinkage rate, but this difference can be partially balanced by adjusting the stretching process. The transverse shrinkage rate of PVC shrink film is significantly affected by the blow-up ratio; increasing the blow-up ratio increases transverse shrinkage, but the traction speed must be controlled to avoid excessive longitudinal stretching.
In practical applications, the difference in transverse and longitudinal shrinkage rates of the shrink film needs to be adjusted according to the characteristics of the packaged item. For example, when packaging aluminum cans, a PET film with a high transverse shrinkage rate ensures a tight fit between the cans; while when packaging square cartons, a balanced PE film avoids corner gaps. For heat-sensitive foods, shrink film with a lower shrinkage temperature can reduce the impact of heating on the food, and in this case, the stability of the transverse and longitudinal shrinkage rates is more important.
