The balance between breathability and moisture-proof requirements in square-bottomed plastic bags needs to be considered comprehensively from four dimensions: material properties, structural design, process control, and usage scenarios. The core lies in using material selection and structural optimization to ensure moisture-proof performance while providing a certain degree of breathability to meet the storage needs of different items.
PO (polyolefin) material itself has excellent moisture-proof properties; its dense molecular structure effectively blocks water vapor penetration. However, a completely sealed design can lead to moisture accumulation inside the bag, especially for food, agricultural products, and other items susceptible to moisture and spoilage, where a lack of breathability accelerates quality deterioration. Therefore, breathable structures need to be introduced into the PO film, such as creating micropores on the bag surface through laser perforation or molding processes. The pore size of these micropores needs to be precisely controlled, typically at the micrometer level, allowing gas exchange while preventing water molecules from passing through directly, thus establishing an initial balance between moisture-proofing and breathability.
Structural design is key to balancing these two aspects. The square bottom design increases the bag's stability, providing more space for the layout of the breathable structure. For example, breathable areas can be incorporated into the sides or bottom of the bag, employing a zoned design: the main body maintains high moisture resistance, while specific areas achieve localized ventilation through micropores or breathable membranes. This design avoids the degradation of moisture resistance caused by overall ventilation while providing targeted protection for hygroscopic areas (such as the top of food packaging). Furthermore, the shape and distribution of the breathable areas need optimization; for example, using a strip or grid layout ensures uniform ventilation while maintaining the bag's structural strength.
Process control directly impacts the balance between breathability and moisture resistance. Micropore processing must balance precision and efficiency. Laser perforation technology can precisely control pore size and density by adjusting power and frequency, preventing moisture-proof failure due to excessively large pores. Simultaneously, multi-layer composite processes can further enhance performance: a high-barrier material (such as aluminum foil or EVOH) is laminated into the inner layer of the PO film, while the outer layer uses a breathable PO film, achieving a gradient design of "barrier-ventilation" through the interlayer structure. This design prevents external moisture from penetrating while allowing internal moisture to escape through the outer breathable membrane, making it particularly suitable for items requiring long-term storage.
The intended use case is the starting point for a balanced design. For dry foods (such as nuts and tea), moisture protection is paramount, and the breathable design primarily uses micropores with small pore sizes and low density, solely for releasing residual air from the packaging process. For fresh produce (such as vegetables and fruits), which generate moisture and heat through respiration, a design with higher breathability is required, such as a large-area breathable membrane or an adjustable breathable valve. The breathable valve, through a semi-permeable membrane or one-way valve structure, allows gas to escape but prevents external moisture from entering, further enhancing the dynamic balance between moisture protection and breathability.
Environmental factors must also be considered. In humid regions or during high-humidity seasons, the need for moisture protection is more pronounced, requiring a more cautious breathable design, such as reducing the number of micropores or using higher-barrier PO materials; while in dry regions, the breathable area can be appropriately increased to improve the packaging's adaptability. Furthermore, transportation and storage conditions (such as temperature fluctuations and stacking pressure) also affect the balance between breathability and moisture resistance, requiring optimization of design parameters through simulation testing.
The breathability design of square-bottom plastic bags must be based on material properties, achieving a dynamic balance between moisture resistance and breathability through structural optimization and process control, combined with specific usage scenarios and environmental conditions. This balance not only concerns the functionality of the packaging but also directly affects the quality and shelf life of the packaged goods, making it an indispensable core consideration in modern packaging design.
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