1. The type, thickness and quality of the heat seal material have the most direct influence on the heat seal strength. Common heat sealing materials for composite packaging include CEP, LPPE, CPP, OPP, EVA, hot melt adhesive and other ion-exchange resin coextrusion or blending modified films. The thickness of heat seal material is generally 20-80 μ M. Under special circumstances, it can reach 100-200 μ M, the heat sealing strength of the same heat sealing material increases with the increase of heat sealing thickness. For example, the heat sealing strength of the cooking bag is generally required to reach 40-50 N, so its heat sealing thickness should be 60-80 μ M above. The temperature is involved in the process of heat sealing bag making, and the temperature control is displayed by the temperature instrument. During the processing of heat sealed composite bags, the more precise the thermometer is required, the better. The error range and setting value should not be greater than ± 5 ℃. The heat sealing temperature has the most direct influence on the heat sealing strength. The melting temperature of various materials directly determines the minimum heat sealing temperature of composite bags. In the actual production process, due to the influence of heat sealing pressure, bag making machine speed and composite substrate thickness, the actual heat sealing temperature is often higher than the melting temperature of the heat sealing material. The smaller the heat sealing pressure is, the higher the heat sealing temperature is required, the faster the machine speed is, the thicker the surface material of the composite film is, and the higher the heat sealing temperature is 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 prolonged, 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 weld edge, resulting in “undercutting” phenomenon during melt extrusion, 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 rotary spring. In the adjustment process, the tool is generally composed of two springs, which are divided into left and right sides. If the pressure affects the strength of the heat seal, the detection method is: take a composite bag being processed to carefully observe the stitch. If the pressure is uniform, there will be no bubbles and other phenomena; Another method is to use a special smooth bamboo block with a length of 500px, a width of 75px, and a thickness of 5000px for the test. Due to insufficient pressure and low strength, leakage often occurs, so even pressure and temperature are one of the basic factors to reduce the low strength and delamination. 2. To achieve ideal heat sealing strength, heat sealing pressure is indispensable. For light and thin packaging bags, the heat sealing pressure should reach at least 2kg/cm2, and 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 two layers of films, resulting in poor local heat sealing or difficult to eliminate bubbles sandwiched in the middle of the weld, resulting in faulty welding. However, the higher the heat sealing pressure is, the better it is. It is better not to damage the weld edge, because at a higher heat sealing temperature, the heat sealing material of the weld edge has been in a semi molten state, and too much pressure is easy to squeeze away some of the heat sealing material, making the weld edge form a semi cut state. The weld is brittle and the heat sealing strength is reduced. Therefore, pressure regulation is very critical. 3. The heat sealing time is mainly determined by the speed of the bag making machine. Fast speed, short heat sealing time; If the speed is slow, the heat sealing time will be long. The heat sealing time is also a key factor affecting the sealing strength and appearance of the weld. At the same heat sealing temperature and pressure, if the heat sealing time is long, the heat sealing layer will be more fully fused and firmly combined; However, if the heat sealing time is too long, it is easy to cause wrinkling and deformation of the weld, affecting the flatness and appearance. If the cooling of the weld after heat sealing is not good, it will not only affect the appearance flatness of the weld, but also have a certain impact on the strength of the heat sealing. The cooling process is to shape the weld after just melting heat sealing with a lower temperature under a certain pressure. Therefore, insufficient pressure, poor cooling and circulation, insufficient circulation, too high water temperature or untimely cooling will lead to poor cooling and lower heat seal strength. 4. The more times of heat sealing, the higher the heat sealing strength. The number of longitudinal heat sealing depends on the ratio of the effective length of the longitudinal welding rod to the bag length. The number of transverse heat sealing times is determined by the number of sets of transverse heat sealing devices of the machine. Good heat sealing requires at least two times of heat sealing. For the composite film with the same structure and thickness, the higher the peeling degree between the composite layers, 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, which causes the internal heat seal layer to independently bear the destructive tension, while the surface material loses its reinforcement, resulting in a significant reduction in the heat seal strength of the weld; If the delamination degree between the composite layers is strong, the delamination between the layers at the weld edge will not occur, and the measured actual heat sealing strength will be much more; When the inner layer of heat sealing is PE or OPP, the heat sealing strength is much better than BOPP with the same thickness. 5. The influence of the contents of the composite bag. Some products are powder packed, which is easy to stain and seal during filling. For example, when LDPE material is used as the inner layer, it is found that the seal is easy to crack. This is because LDPE is not very good at heat sealing of inclusions. At this time, it is necessary to replace the inner film material or increase the thickness of the material to improve the heat sealing strength. 6. The influence of composite additives. In the process of composite polyethylene film, polyethylene will precipitate after being extruded by the hot pressing roller, which is white as powder. This phenomenon is that during the production of polyethylene, a certain amount of lubricant is added, which is some wax with low melting point, and it is easy to precipitate to the film surface. The most direct harm of this layer of wax with low melting point after precipitation is that it greatly weakens the composite strength and heat sealing strength, especially at the edge sealing position, resulting in easy opening and separation. The solution is: (1) pretreat the polyethylene again to achieve the ideal surface tension; (2) Select appropriate adhesive to enhance its composite fastness; (3) Reduce the ripening temperature as much as possible to avoid material precipitation, so as to increase the composite fastness and heat sealing strength. 7. The delamination of flexible packaging compound bags after heat sealing is related to the quality of printing ink layer and corona surface. In the actual production process, in order to achieve the true reproduction of color, it is inevitable to mix printing of inner printing and surface printing ink. Theoretically, the inner printing ink is incompatible with the surface printing ink. If the inner and outer printing ink layers of the printing film are mixed, the fastness between the ink layers will inevitably be poor, which is easy to delaminate. The delamination phenomenon is also easy to occur at the heat sealing weld, and the heat sealing strength will become worse. Solution: Try to avoid the mixing of surface printing ink and interior printing ink, so as to improve the heat sealing strength and reduce delamination.
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