The causes of layering at the sealing edge of flexible packaging bags and the countermeasures for process improvement

Layering, bulging at the sealing edge, and opening detachment at the sealing part are common quality issues in the post-processing of  flexible packaging. The overall bonding of the finished composite film is normal, but there is local interlayer peeling at the heat-sealing edge, which not only affects the sealing performance of the packaging bag but also causes batch production failure. This problem is more likely to occur under high-temperature heat sealing and high-speed bag production conditions, and is closely related to the heat resistance, stress concentration, and process parameters of the polyurethane laminating adhesive.

 

The main cause of heat sealing is the high-temperature stress impact. The heat sealing temperature of bag production is generally above 150℃, and the instantaneous high temperature will cause thermal shock to the Composite Adhesive layer. The general Polyurethane Adhesive is not heat-resistant and its strength will temporarily decrease and become soft under high temperature. Under the pulling and contraction stress of the sealing pressure, the edge is prone to differential layering, and a clear edge detachment defect will form after cooling.

The cutting and tension factors are also crucial. If the coating amount at the sealing position is too large and the adhesive layer is too thick, the thermal expansion and contraction stress will be greater. The sharpness of the bag cutting knife is insufficient, and the cutting burrs will pull the film layer, causing the interlayer adhesion to be torn. Insufficient curing and incomplete cross-linking of the adhesive layer result in poor heat resistance and are prone to local layering when exposed to heat. In addition, the sealing position is close to the printing ink layer, and the compatibility between the ink and the adhesive is poor, which can also induce local layering.

 

The improvement plan requires the selection of heat-resistant polyurethane laminating adhesive to enhance the heat resistance stability of the adhesive layer. Reasonable control of the coating dry amount to avoid stress concentration in the thick adhesive layer. Optimize the heat sealing temperature, pressure, and speed to reduce the instantaneous heat shock. Ensure that the adhesive is fully cured to enhance the interlayer adhesion strength. At the same time, precisely avoid the sealing position near the printing ink layer or optimize the ink compatibility, which can completely solve the problem of edge layering.