Solutions to Heat Seal Failures in Laminated Pouches

As professionals in the flexible packaging industry, you have certainly encountered failures caused by heat sealing of laminated pouches. To help everyone solve similar problems more easily, the editor has compiled a comprehensive analysis of the causes of heat seal failures in flexible laminate packaging pouches, and hopes it will be of help to you!

1. The type, thickness and quality of the heat seal layer material have the most direct influence on heat seal strength.

Common heat seal materials used in general flexible laminate packaging include CEP, LPPE, CPP, OPP, EVA, hot melt adhesive, and other co-extruded or blend-modified films of ionomer resins. The thickness of the heat seal layer material generally fluctuates between 20—80μm. Under special circumstances it can reach 100—200μm; for the same heat seal material, the heat seal strength increases as the heat seal thickness increases. For example, retort pouches generally require a heat seal strength of 40—50 Newtons, so their heat seal thickness should be above 60—80μm. The heat sealing and bag-making process involves temperature, and temperature control is displayed by a temperature instrument. In the processing of heat-sealed laminated pouches, the more precise the requirement for the temperature gauge, the better; the error range should preferably not exceed ±5℃ from the set value. Heat seal temperature has the most direct influence on heat seal strength; the melting temperature of each material directly determines the minimum heat seal temperature of the laminated pouch.

In actual production, due to the influence of heat seal pressure, bag-making machine speed and the thickness of the laminated substrate, the heat seal temperature actually used is often higher than the melting temperature of the heat seal material. The lower the heat seal pressure, the higher the required heat seal temperature; the faster the machine speed and the thicker the surface layer material of the laminated film, the higher the required heat seal temperature. If the heat seal temperature is below the softening point of the heat seal material, no matter how much the pressure is increased or the heat seal time extended, the heat seal layer cannot be truly sealed. However, if the heat seal temperature is too high, it is very easy to damage the heat seal material at the weld edge, and melting extrusion produces an "undercut" phenomenon, greatly reducing the heat seal strength of the seal and the impact resistance of the laminated pouch. In the actual bag-making and heat-sealing process, the pressure of the heat seal knife is usually adjusted by a rotatable spring. During adjustment, the knife generally consists of two springs, on the left and right sides. If pressure affects heat seal strength, the inspection method is: take a laminated pouch being processed and carefully observe the seam; if the pressure is uniform, no bubbles or similar phenomena will occur; another method is to test with a special smooth bamboo block 500px long, 75px wide and 5000px thick. Because the pressure is insufficient, the strength is low and leakage/bursting often occurs. Therefore, uniform pressure and temperature are among the fundamentals for reducing low strength and delamination.

1. To achieve ideal heat seal strength, heat seal pressure is essential.

For lightweight packaging pouches, the heat seal pressure must reach at least 2kg/cm2, and it should increase correspondingly with the total thickness of the laminated film; if the heat seal pressure is insufficient, it is difficult for the two layers of film to achieve true fusion, resulting in poor local heat sealing or difficulty in eliminating bubbles trapped in the weld seam, causing false sealing. However, heat seal pressure is not better the greater it is; it should be set so as not to damage the weld edge, because at higher heat seal temperatures the heat seal material at the weld edge is already in a semi-molten state, and too much pressure easily squeezes away part of the heat seal material, forming a semi-cut state at the edge of the weld seam. The weld seam becomes brittle and heat seal strength decreases. Therefore, pressure adjustment is very critical.

1. Heat seal time is mainly determined by the speed of the bag-making machine.

The faster the speed, the shorter the heat seal time; the slower the speed, the longer the heat seal time. Heat seal time is also a key factor affecting the sealing strength and appearance of the weld seam. Under the same heat seal temperature and pressure, a longer heat seal time makes the heat seal layer fuse more fully and bond more firmly; but if the heat seal time is too long, it easily causes the weld seam to wrinkle and deform, affecting flatness and appearance. If the weld seam after heat sealing is not cooled well, it will not only affect the flatness of the weld appearance but also have a certain impact on heat seal strength. The cooling process is to shape the just-molten heat-sealed weld seam at a lower temperature under a certain pressure. Therefore, insufficient pressure, poor cooling and circulation, insufficient circulation volume, too high water temperature or delayed cooling will all lead to poor cooling and reduced heat seal strength.

1. The more heat seal passes, the higher the heat seal strength; the number of longitudinal heat seal passes depends on the ratio of the effective length of the longitudinal sealing bar to the bag length.

The number of transverse heat seal passes is determined by the number of groups of transverse heat sealing devices on the machine. Good heat sealing requires at least two or more heat seal passes. For laminated films of the same structure and thickness, the higher the peel strength between laminated layers, the greater the heat seal strength; for products with low lamination peel strength, weld failure often occurs when the laminated film at the weld seam delaminates first, causing the inner heat seal layer to bear the destructive tensile force alone while the surface layer material loses its reinforcing effect, so the heat seal strength of the weld seam is greatly reduced; if the peel strength between laminated layers is strong, delamination at the weld edge will not occur, and the actual measured heat seal strength will be much higher; when the heat seal inner layer is PE or OPP, the heat seal strength is much better than that of BOPP of the same thickness.

1. Influence of the contents of the laminated pouch.

Some products are powder-filled and tend to contaminate the seal during filling. For example, when LDPE material is used as the inner layer, the seal is found to break easily. This is because LDPE does not have very good heat sealability against inclusions; in this case, it is necessary to replace the inner film material or increase the material thickness to improve heat seal strength.

1. Influence of additives in composite materials.

In the process of laminating polyethylene film, after the polyethylene is pressed by hot rollers, an exudation phenomenon occurs, appearing as a layer of white, powdery white substance. This phenomenon is caused by the addition of a certain amount of lubricant, some low-melting-point wax, during polyethylene production, which easily exudes to the film surface. The most direct harm after this layer of low-melting-point wax exudes is that it greatly weakens the lamination strength and also greatly reduces heat seal strength, especially at the sealing edge, causing easy opening and delamination.

The solutions are: 1) re-pretreat the polyethylene to achieve the ideal surface tension; 2) select a suitable adhesive to enhance its lamination fastness; 3) reduce the curing temperature to minimize the exudation of substances, thereby increasing lamination fastness and heat seal strength.

1. Delamination of flexible laminate packaging pouches after heat sealing is related to the quality of the printed ink layer and the corona-treated surface.

In actual production, to achieve true color reproduction, it is inevitable to mix reverse printing and surface printing inks. Theoretically, reverse printing and surface printing inks are not compatible; if the ink layer of the printed film is a mixture of reverse and surface printing inks, the fastness between ink layers will inevitably be poor and easy to delaminate, and delamination is also likely to occur at the heat-sealed weld seam, thereby deteriorating heat seal strength.

Solution: try to avoid mixing surface printing ink with reverse printing ink, thereby improving heat seal strength and reducing delamination.