The most complete analysis of heat sealing failures of flexible packaging composite bags


Release time:

2018-12-25

Commonly used heat-sealing materials for composite packaging include CEP, LPPE, CPP, OPP, EVA, hot melt adhesives and other ionic resin co-extruded or blended modified films. The thickness of the heat-sealing layer material generally floats between 20-80μm. In special cases, there are 100-200μm, the same heat-sealing material, the heat-sealing strength increases with the increase of the heat-sealing thickness. For example, the heat-sealing strength of cooking bags is generally required to reach 40-50 Newtons, so the heat-sealing thickness should be above 60-80μm. The temperature is involved in the process of heat-sealing bag making, and how much temperature is controlled is displayed by a temperature meter. In the process of heat-sealing composite bag processing, the more precise the temperature gauge is, the better, and the error range and setting value should not exceed ±5℃. The heat sealing temperature has the most direct influence on the heat sealing strength, and the melting temperature of various materials directly determines the minimum heat sealing temperature of the composite bag.

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

Commonly used heat-sealing materials for composite packaging include CEP, LPPE, CPP, OPP, EVA, hot melt adhesives and other ionic resin co-extruded or blended modified films. The thickness of the heat-sealing layer material generally floats between 20-80μm. In special cases, there are 100-200μm, the same heat-sealing material, the heat-sealing strength increases with the increase of the heat-sealing thickness. For example, the heat-sealing strength of cooking bags is generally required to reach 40-50 Newtons, so the heat-sealing thickness should be above 60-80μm. The temperature is involved in the process of heat-sealing bag making, and how much temperature is controlled is displayed by a temperature meter. In the process of heat-sealing composite bag processing, the more precise the temperature gauge is, the better, and the error range and setting value should not exceed ±5℃. The heat sealing temperature has the most direct influence on the heat sealing strength, and the melting temperature of various materials directly determines the minimum heat sealing temperature of the composite bag.

In the actual production process, due to the influence of the heat-sealing pressure, the speed of the bag making machine, 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 layer material of the composite film, and the higher the heat-sealing temperature required. If the heat-sealing temperature is lower than the softening point of the heat-sealing material, no matter how the pressure is increased or the heat-sealing time is extended, the heat-sealing layer cannot be truly sealed. However, if the heat-sealing temperature is too high, it is easy to damage the heat-sealing material at the welding edge, and melt extrusion will produce "undercutting", which greatly reduces the heat-sealing strength of the seal and the impact resistance of the composite bag. In the actual bag-making heat-sealing process, the pressure of the heat-sealing tool is often adjusted by a rotatable spring. The adjustment process generally consists of two springs, the left and right sides. If the pressure affects the strength of the heat seal, the detection method is: take a composite bag that is being processed and carefully observe the seam. If the pressure is uniform, no bubbles will be generated. Phenomenon; another method is to use a special smooth bamboo block with length of 500px, width of 75px, and thickness of 5000px for testing. Due to insufficient pressure and low strength, leakage often occurs. Therefore, uniform pressure and temperature reduce the strength and layering. One of the basics of the phenomenon.

2. To achieve the ideal heat-sealing strength, heat-sealing pressure is indispensable.

For thin and light packaging bags, the heat sealing pressure must reach at least 2kg/cm2, and it will increase correspondingly 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, resulting in partial heat sealing failure It is good or difficult to eliminate the air bubbles trapped in the welding seam, resulting in false welding. But the heat sealing pressure is not as big as possible. 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-melted state, and too much pressure is easy to squeeze part of the heat seal away. Material, so that the edge of the weld is in a semi-cut state. The weld is brittle and the heat seal strength is reduced. Therefore, the regulation of pressure is very important.

3. The heat sealing time is mainly determined by the speed of the bag making machine.

Fast speed means shorter heat sealing time; slow speed means longer heat sealing time. The heat-sealing time is also a key factor that affects the sealing strength and appearance of the weld. Under the same heat-sealing temperature and pressure, if the heat-sealing time is longer, the heat-sealing layer will be more fully fused and the bond will be stronger; but if the heat-sealing time is too long, it is easy to cause wrinkling and deformation of the weld, which affects the flatness and appearance. If the heat-sealed weld is not cooled well, it will not only affect the appearance of the weld, but also have a certain impact on the strength of the heat seal. The cooling process is to apply a lower temperature to the freshly melted heat under a certain pressure. The sealed weld is shaped. Therefore, insufficient pressure, poor cooling and circulation, insufficient circulation, too high water temperature or untimely cooling will result in poor cooling and lower heat seal strength.

4. The more heat-sealing times, the higher the heat-sealing strength. The number of longitudinal heat-sealing depends on the ratio between the effective length of the longitudinal welding rod and the bag length.

The number of transverse heat sealing is determined by the number of sets of transverse heat sealing device of the machine. Good heat sealing requires at least two times of heat sealing. For a composite film with the same structure and thickness, the higher the degree of peeling between composite layers, the greater the heat seal strength; for products with low composite peel strength, the weld failure is often due to the first interlayer peeling of the composite film at the weld, resulting in heat from the inner surface. The seal layer independently bears the destructive tensile force, and the surface layer material loses its reinforcing effect, which greatly reduces the heat seal strength of the weld; if the degree of peeling between the composite layers is strong, there will be no delamination between the weld edges. The actual heat-sealing strength obtained is much better; when the inner heat-sealing layer is PE or OPP, the heat-sealing strength is much better than BOPP of the same thickness.

5. The influence of the contents of the composite bag.

Some products are powder-packed, which is easy to contaminate the seal during filling. For example, when LDPE material is used as the inner layer material, it is found that the seal is easy to break. This is because the heat-sealability of LDPE to inclusions is not very good. At this time, it is necessary to replace the inner film material or increase the thickness of the material to increase the heat-seal strength.

6. The influence of composite additives.

In the process of compounding the polyethylene film, the polyethylene precipitates after being squeezed by the hot pressing roller, and a layer of white is like crushed powder and white. This phenomenon is that a certain amount of lubricant is added during the production process of polyethylene. Some low-melting waxes are easy to precipitate on the surface of the film. The most direct hazard after the precipitation of this layer of low-melting wax is to greatly weaken the composite strength, and also greatly weaken the heat sealing strength, especially at the edge sealing position, causing easy opening and layer separation.

The solution is: 1) pre-treat the polyethylene again to achieve the desired surface tension; 2) select a suitable adhesive to enhance its composite fastness; 3) reduce the curing temperature to minimize the precipitation of substances, thereby increasing the composite fastness Degree and heat seal strength.

7. The delamination of the flexible packaging composite bag after heat sealing is related to the quality of the printing ink layer and the corona surface.

In the actual production process, in order to achieve the true reproduction of colors, it is inevitable that the inner printing and the surface printing ink are mixed printing. From a theoretical analysis, the inner printing and surface printing inks are not compatible. If the ink layer of the printing film is mixed between the inner and the surface, the fastness between the ink layers is bound to be poor, easy to delamination, and easy to heat seal the weld. This causes delamination, and the strength of the heat seal deteriorates.

Solution: Try to avoid the mixing of surface printing ink and back printing ink, so as to increase the strength of heat sealing and reduce the phenomenon of delamination.