Factors Affecting the Heat Sealing Quality of Cooking Bag Packaging

Update:10 Jun 2020

The heat sealing quality of composite Oxygen absorber/Desiccant film packaging bags has always been one of the most important items for packaging manufacturers to control product quality. The following are the influencing factors of the heat sealing process. The type, thickness and quality of the heat sealing layer material have a decisive influence on the heat sealing strength. Commonly used heat sealing materials for composite packaging include  hot melt adhesives and other ionic resin co-extruded or blended modified films. The thickness of the material of the heat seal layer is generally between 20 and 80 μm, and in special cases it may reach 100 to 200 μm. With the same heat seal material, the heat seal strength increases with the increase of the heat seal thickness. The heat-sealing strength of the cooking bag is generally required to be  so the thickness of the heat-sealing material should be above 60-80μm. The effect of heat sealing temperature on the heat sealing strength is the most direct. The melting temperature of various materials directly determines the heat sealing temperature of the composite bag.

Retort bag

During the production process, due to many factors such as heat sealing pressure, bag making speed and the thickness of the composite substrate, the actual heat sealing temperature is often higher than the melting temperature of the heat sealing material. The smaller the heat sealing pressure, the higher the heat sealing temperature; the faster the machine speed, the thicker the surface material of the composite film, and the higher the heat sealing temperature. If the heat sealing temperature is lower than the softening point of the heat sealing material, no matter how to increase the pressure or extend the heat sealing time, it is impossible to make the heat sealing layer truly sealed. However, if the heat sealing temperature is too high, the heat sealing material at the welding edge is easily damaged and melted and squeezed out, resulting in a "root cut" phenomenon, which greatly reduces the heat sealing strength of the seal and the impact resistance of the bag. To achieve the ideal heat seal strength, a certain pressure is necessary. For light and thin packaging bags, the heat sealing pressure should be at least  and it will increase with the increase of the total thickness of the composite film.

If the heat sealing pressure is insufficient, it is difficult to achieve true fusion between the two films, the local heat sealing is not good, or it is difficult to catch up the bubbles sandwiched between the welds, resulting in virtual welding; of course, the heat sealing pressure is not greater Well, it is better not to damage the welding edge, because at a higher heat sealing temperature, the heat sealing material of the welding edge is already in a semi-molten state, too much pressure can easily squeeze part of the heat sealing material, so that the weld edge is formed In the half-cut state, the weld is brittle and the heat seal strength is reduced. The heat sealing time is mainly determined by the speed of the bag making machine. The 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, the heat sealing time is longer, the heat sealing layer is more fused and the bonding is stronger, but the heat sealing time is too long, it is easy to cause wrinkling of the weld and affect the appearance. If the weld seam after heat sealing is not well cooled, it will not only affect the appearance flatness of the weld seam, but also have a certain influence on the heat seal strength.

The cooling process is the process of removing the stress concentration by setting the weld just after melting and heat sealing with a lower temperature under a certain pressure. Therefore, insufficient pressure, poor circulation of cooling water, insufficient circulation volume, too high water temperature, or untimely cooling will cause poor cooling, heat sealing edge warping, and heat sealing strength is reduced.The more heat sealing times, the higher the heat sealing strength. The number of longitudinal heat seals depends on the ratio of the effective length of the longitudinal welding rod and the length of the bag; the number of transverse heat seals is determined by the number of sets of transverse heat seals on the machine. Good heat sealing requires at least two times of heat sealing. There are 2 sets of hot knife for general bag making machine, and the higher the overlap of the hot knife position, the better the heat sealing effect. For composite films with the same structure and thickness, the higher the peel strength between the composite layers, the greater the heat seal strength. For products with low composite peel strength, the welding seam damage is often due to the interlayer peeling of the composite film at the weld seam, which causes the internal heat seal layer to independently bear the breaking tensile force, and the surface layer material loses its reinforcing effect, and the heat seal of the weld seam The strength is thus greatly reduced. If the composite peel strength is large, no interlayer peeling occurs at the weld edge, and the actual heat seal strength measured is much greater.

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