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

Generally used heat-sealing materials for general composite packaging include CEP, LPPE, CPP, OPP, EVA, hot-melt adhesive, and some other ionic resin co-extrusion or blended modified films. The thickness of the heat-sealing layer material generally floats between 20-80 μm. In special cases, there are also 100-200 μm, the same heat-sealing material, the heat-sealing strength increases with the heat-sealing thickness. For example, the heat-sealing strength of cooking bags is generally required to be 40-50 Newtons, so the heat-sealing thickness should be above 60-80 μm. Temperature is involved in the process of heat-sealing the bag, and how much the temperature is controlled is displayed by the temperature meter. In the process of heat-sealing composite bags, the more precise the thermometer is, the better, the error range and set value should not be greater than ± 5 ° C. The heat seal temperature has the most direct effect on the heat seal strength. The melting temperature of various materials directly determines the minimum heat seal temperature of the composite bag.

In the actual production process, due to various influences such as heat sealing pressure, bag making speed and thickness of the composite substrate, 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 heat-sealing temperature, the faster the machine speed, the thicker the surface 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 to increase the pressure or extend the heat-sealing time, the heat-sealing layer cannot be truly sealed. However, if the heat-sealing temperature is too high, the heat-sealing material at the welding edge will be easily damaged, and the phenomenon of "undercutting" will occur 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 rotatable spring. The adjustment process generally consists of two springs, divided into left and right sides. If the pressure affects the strength of the heat seal, the detection method is: take a compound bag that is being processed to carefully observe the seam. If the pressure is uniform, air bubbles will not be generated. 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 testing. Due to insufficient pressure and low strength, leaks often occur, so uniform pressure and temperature are reduced in strength and layered. One of the basics of the phenomenon.

2. To achieve the desired heat seal strength, heat seal pressure is essential.

For light and thin packaging bags, the heat-sealing pressure must be at least 2kg / cm2, and 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 Good or difficult to eliminate air bubbles trapped in the middle of the weld, causing false welding. But the heat-sealing pressure is not as large as possible, it is better not to damage the welding edge, because at 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 out some heat-sealing Material, so that the edge of the weld seam forms a semi-cut state. The weld is brittle and the heat seal strength is reduced. So the regulation of pressure is very critical.

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

快 Fast speed, short heat sealing time; slow speed, long heat sealing time. Heat sealing time is also a key factor affecting the strength and appearance of the weld seal. Under the same heat-sealing temperature and pressure, the heat-sealing time is longer, the heat-sealing layer is more fully fused and the bonding is stronger; but the heat-sealing time is too long, which may cause wrinkling and deformation of the weld, affecting the flatness and appearance. If the weld after heat sealing is not cooled well, it will not only affect the appearance flatness of the weld, but also have a certain effect on the strength of the heat seal. The cooling process is to use a lower temperature to heat the hot melt just under a certain pressure. The sealed weld is shaped. Therefore, insufficient pressure, insufficient cooling and circulation, insufficient circulation volume, too high water temperature or untimely cooling will cause poor cooling and reduce heat seal strength.

4. The more the number of heat seals, the higher the heat seal strength. The number of longitudinal heat seals depends on the ratio of the effective length of the longitudinal welding rod and the bag length.

次数 The number of transverse heat-seals 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 composite films of the same structure and thickness, the higher the inter-layer peeling degree, the greater the heat seal strength. For products with low composite peel strength, weld failure is often caused by the inter-layer peeling of the composite film at the weld first, resulting in heat from the inner surface. The sealing layer independently bears the breaking tensile force, and the surface layer material loses the reinforcing effect, resulting in a significant reduction in the heat seal strength of the weld. If the peeling between the composite layers is strong, the interlayer peeling at the welding edge will not occur. The actual heat seal strength obtained is much more; when the heat seal inner layer is PE or OPP, the heat seal strength is much better than BOPP of the same thickness.

5, the impact of the contents of the composite bag.

Some products are powder packs, which are easy to stain and seal when filling. For example, when LDPE material is used as the inner layer, the seal is easily broken. This is because the heat sealability of LDPE to inclusions is not very good. At this time, it is necessary to replace the inner layer film material or increase the thickness of the material to increase the heat seal strength.

6. The effect of composite additives.

In the process of composite polyethylene film, the polyethylene is precipitated after being pressed by the hot pressing roller. A layer of white is like white powder. This phenomenon is that during the production process of polyethylene, a certain amount of lubricant is added. Some low-melting waxes are easily deposited on the film surface. The most direct harm after the precipitation of this layer of low-melting wax is that the composite strength is greatly weakened, and the heat-sealing strength is also greatly weakened, especially at the edge sealing position, causing easy opening and delamination.

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

7. The delamination after heat sealing of the flexible packaging composite bag 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 color, it is inevitable to mix printing with back printing and surface printing ink. Theoretically, the back printing and surface printing inks are not compatible. If the printing film ink layer is mixed with the back surface, the fastness between the ink layers will inevitably be poor, and it is easy to delaminate, and it is also easy to seal the weld As a result of the delamination phenomenon, the heat seal strength becomes worse.

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