Cause Analysis of Poor Heat Seal Strength
1. Impact of Heat Seal Materials
The type of heat seal material affects its seal strength. For example, PP, PE, and EVA have different heat seal strengths. Even within the same type of film, products made with different polymerization methods exhibit varying heat seal strengths. Factors such as material thickness and uniformity also influence the seal strength. Different processing methods for heat seal layers, such as blow molding and casting, can further impact the seal strength. When films are stored for too long, the accumulation of precipitates can decrease heat seal strength, especially after corona treatment. Additionally, moisture-absorbing materials may absorb humidity, leading to delamination of the seal surface and a decrease in bond strength.
2. Impact of Lamination Process on Heat Seal Strength
The poor temperature resistance of printing inks or the use of inappropriate additives in the ink can cause the printed layer to delaminate at high temperatures during the pouch-making process, leading to a decrease in heat seal strength. The heat seal area should be designed without patterns whenever possible. The choice of adhesive in the lamination process is also crucial. Factors such as the adhesive's ratio, viscosity, and other parameters must be optimized, as they can directly affect seal strength. In extrusion lamination or coating processes, high processing temperatures can increase the degree of material surface oxidation and carboxy group formation, which can negatively affect the heat seal ability of the film.
3. Impact of Pouch-making Process on Heat Seal Strength
Insufficient cooling after heat sealing can prevent proper shaping and fail to eliminate internal stresses, negatively affecting both the seal strength and appearance. Regular inspection and enhancement of the cooling water circulation system are crucial. The more heat seal applications, the stronger the seal. The number of longitudinal heat seals is determined by the ratio of the effective length of the longitudinal sealing blade to the pouch length, while the number of transverse seals depends on the combination of transverse sealing units in the machine. For optimal results, at least two heat seal applications are recommended.
