Square-bottom plastic bags, with their square bottom structure, offer stable stacking and high space utilization, making them widely used in the chemical, food, and building materials industries. During the production process, the heat-sealing process is a critical step in determining the effectiveness of corner seals. Key control points include temperature, pressure, speed, cooling, and material compatibility.
Heat-sealing temperature is the primary factor influencing sealing performance. The heat-sealing temperature range varies significantly between materials. For example, PE (polyethylene) has good heat resistance, so a higher heat-sealing temperature can be used to improve production efficiency. However, materials with poor heat resistance, such as BOPP (biaxially oriented polypropylene), require a lower heat-sealing temperature to prevent shrinkage and wrinkling at the seal area, which can weaken the seal. If the heat-sealing temperature is too high, the heat-sealing material may melt and extrude, reducing the thickness of the heat-sealing layer and resulting in a weak seal at the corners. If the temperature is too low, the material may not fully melt, resulting in a weak seal. Therefore, the heat-sealing temperature range must be precisely set according to the material characteristics to ensure a uniform and dense seal at the corners.
Coordinated control of heat-sealing pressure and speed is crucial for corner seal quality. Heat sealing pressure must be adjusted based on the thickness of the composite film, the seal width, and the material properties. Excessive pressure can lead to a loose seal, while excessive pressure can penetrate the material or cause edge deformation. Heat sealing speed directly impacts production efficiency and sealing quality. Excessive speed may result in insufficient heat sealing time and incomplete fusion of edges and corners; excessively slow speed may cause material degradation due to prolonged heat exposure. In actual production, the optimal pressure-speed combination must be determined through experimentation. For example, independent servo motors can be used to control the heat sealing blade's lift and feed, enabling independent adjustment of pressure and speed to avoid speed adjustments affecting heat sealing quality.
Cooling is a critical step in consolidating heat sealing results. Rapid cooling is required after heat sealing to stabilize the seal structure. Excessively high cooling water temperature, poor cooling water circulation, or insufficient cold sealing blade pressure can lead to poor cooling of the heat-sealed area, reducing seal strength. For example, the cooling water temperature should be controlled at around 20°C to ensure rapid curing of the heat seal layer and avoid material shrinkage or edge warping caused by slow cooling. Furthermore, sufficient cooling water circulation is required to maintain stable cooling efficiency and prevent localized overheating that can cause sealing defects.
Material selection and structural compatibility are essential for ensuring corner seals. The bottom of a square-bottom plastic bag is typically constructed from multiple layers, up to eight. Excessive heat-sealing layers can lead to uneven sealing in the center molding area. Therefore, the appropriate composite structure must be selected based on the intended use. For example, high-barrier materials can be used to prevent leakage, or interlayer adhesive properties can be adjusted to improve corner seals. Furthermore, material thickness must be compatible with the heat-sealing process. Too thin materials may break due to concentrated heat-sealing pressure, while too thick materials require higher heat-sealing temperatures, increasing the process complexity.
The bag-making machine's tension control significantly impacts the smoothness of the corner seal. During production, the tension between the main film, accordion material, and base material must be maintained stable to prevent uneven tension from causing film shrinkage or warping, which could lead to misaligned corner seals. Typically, the bag-making machine's tension should be controlled between 0.1 and 0.15 MPa. An automatic tension adjustment system compensates for material elongation variations in real time to ensure a smooth, wrinkle-free heat-sealed area.
Optimizing the number of heat-sealing cycles and blade gap can further improve corner seal reliability. The bottom of a square-bottom plastic bag requires multiple heat seals to form, but over-heat sealing can lead to root breakage (i.e., material fracture at the seal). A typical transverse heat-sealing system uses five sets of heat-sealing blades, performing two or more heat seals to ensure corner seal strength. The gap between the heat-sealing blades should be set based on the film thickness, typically 1.0 to 1.5 mm. A gap that is too small shortens the heat-sealing time and reduces seal strength, while a gap that is too large can result in a loose seal.
Process validation and continuous improvement are long-term strategies for ensuring corner seal quality. Before production, heat-sealing parameters should be verified through sample testing. For example, check that the corner seal layer is uniform and free of bubbles or damage. Regular spot checks should be conducted during production, using a seal tester to measure corner leakage rates. Customer feedback on sealing issues should prompt adjustments to process parameters or optimization of material structure. Through continuous improvement, the stability and consistency of the corner seals of square-bottom plastic bags can be gradually improved.
