Factors Affecting the Heat Sealing Process of Composite Packaging Bags

The heat sealing quality of composite packaging bags has always been one of the most important aspects for packaging bag manufacturers to control product quality.

Today, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will analyze the influencing factors of the heat sealing process of composite packaging bags:

1, The type, thickness, and quality of the heat-sealing layer material have a decisive impact on the heat-sealing strength.

Commonly used heat-sealing materials for composite packaging include CPE, CPP, EVA, hot melt adhesive, and other ion-type resin co-extruded or co-blended modified films. The thickness of the heat-sealing layer material is generally between 20 and 80 μm, and in special cases, it can reach 100 to 200 μm. For the same heat-sealing material, the heat-sealing strength increases with the increase in heat-sealing thickness. The heat-sealing strength of retort pouches generally requires 40 to 50 Newtons, so the thickness of the heat-sealing material should be above 60 to 80 μm.

2, Heat sealing temperature has the most direct impact on heat sealing strength.

The melting temperature of various materials directly determines the minimum heat sealing temperature of the composite bag. In the production process, due to the influence of heat sealing pressure, bag making speed, and composite substrate thickness, the actual heat sealing temperature used is often higher than the melting temperature of the heat sealing material. The smaller the heat sealing pressure, the higher the required heat sealing temperature; the faster the machine speed, and the thicker the surface material of the composite film, the higher the required heat sealing temperature. If the heat sealing temperature is lower than the softening point of the heat sealing material, it is impossible to truly seal the heat sealing layer no matter how much pressure is increased or the heat sealing time is extended. However, if the heat sealing temperature is too high, it is easy to damage the heat sealing material at the weld, causing it to melt and extrude, resulting in a "root cutting" phenomenon, greatly reducing the heat sealing strength of the seal and the impact resistance of the bag.

3, A certain amount of pressure is essential to achieve ideal heat sealing strength.

For lightweight packaging bags, the heat sealing pressure must reach at least 2 kg/cm2, and it increases accordingly with the increase in the total thickness of the composite film. If the heat sealing pressure is insufficient, the two layers of film cannot be truly fused, resulting in poor local heat sealing or difficulty in expelling air bubbles trapped in the weld, causing false welding; of course, the heat sealing pressure is not the greater the better, it should not damage the weld, because at a higher heat sealing temperature, the heat sealing material at the weld is in a semi-molten state, and too much pressure easily squeezes out some of the heat sealing material, causing the weld edge to be semi-cut, the weld becomes brittle, and the heat sealing strength is reduced.

4, Heat sealing time is mainly determined by the speed of the bag making machine.

Heat sealing time is also a key factor affecting the sealing strength and appearance of the weld. With the same heat sealing temperature and pressure, a longer heat sealing time means that the heat sealing layer is more fully fused and the bonding is stronger, but if the heat sealing time is too long, it is easy to cause wrinkles in the weld, affecting the appearance.

5, If the weld after heat sealing is not cooled properly, it will not only affect the flatness of the weld appearance, but also have a certain impact on the heat sealing strength.

The cooling process is the process of shaping the weld that has just been heat-sealed under a certain pressure and at a lower temperature to eliminate stress concentration. Therefore, insufficient pressure, poor cooling water circulation, insufficient circulation volume, too high water temperature, or untimely cooling will all lead to poor cooling, warping of the heat-sealed edges, and reduced heat-sealing strength.

6, The more heat sealing times, the higher the heat sealing strength.

The number of longitudinal heat sealing times depends on the ratio of the effective length of the longitudinal welding bar to the bag length; the number of transverse heat sealing times is determined by the number of sets of the transverse heat sealing device of the machine. Good heat sealing requires at least two heat sealing times. General bag making machines have 2 sets of hot knives, and the higher the degree of overlap of the hot knife positions, the better the heat sealing effect.

7, For composite films with the same structure and thickness, the higher the interlayer peel strength of the composite layer, the greater the heat sealing strength.

For products with low composite peel strength, the weld failure is often caused by the interlayer peeling of the composite film at the weld, causing the inner heat sealing layer to independently bear the destructive tensile force, and the surface material loses its reinforcing effect, thus greatly reducing the heat sealing strength of the weld. If the composite peel strength is large, interlayer peeling will not occur at the weld, and the actual heat sealing strength measured will be much greater.

Through understanding, we believe that the reason for the high bag breakage rate is that the two sets of heat sealing hot knives are not completely overlapped, resulting in false welding near the heat sealing edge, the CPP film becomes thinner, and part of the structure is damaged. During retorting, the thinned CPP layer cannot withstand the pressure and causes bag breakage.

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