This article explains in detail the most comprehensive analysis of the heat sealing failures of flexible packaging composite bags. I hope you will be helpful to the flexible packaging factory !
1. The type, thickness and material quality of the heat seal layer have the most direct influence on the heat seal strength.
Commonly used heat sealing materials for composite packaging include CEP , LPPE , CPP , OPP , EVA , hot melt adhesives and some other ionic resin co-extruded or blended modified films. The thickness of the heat seal layer material generally floats between 20-80 μm . In special cases, there are also 100-200μm , the same kind of heat seal material, the heat seal strength increases with the increase of the thickness of the heat seal. For example, the heat-sealing strength of high-temperature cooking bags generally requires 40-50 Newtons. Therefore, the heat-sealing thickness should be above 60-80 μm . The temperature is involved in the heat-sealing bag-making process, and the temperature control is displayed by the temperature meter. In the process of heat-sealing compound bags, the more precise the temperature table is, the better the error range and setting value should be no more 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 the composite bag.
In the actual production process, due to 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 and the faster the speed. The thicker the surface material of the composite film , 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 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, but also easy to weld at the edge of heat-seal material damage, melt extrusion to produce "undercut" phenomenon, greatly reduces the impact resistance and heat seal strength composite seal bag. In the actual heat-sealing process of bag making, the pressure of the heat-sealing cutter is often adjusted by a rotatable spring. In the adjustment process, the general tool is composed of two springs, which are divided into left and right sides. If the pressure affects the heat seal strength, the detection method is: take a composite bag being processed and observe the seam carefully. If the pressure is even, it will not produce bubbles. Phenomenon; another method is to use special smooth bamboo blocks with a length of 500px , a width of 75px , and a thickness of 5000px for testing. Due to insufficient pressure and low strength, leaking and breaking phenomena often occur, so uniform pressure and temperature reduce the strength and delamination. One of the basics of the phenomenon.
2. To achieve ideal heat seal strength, heat seal pressure is essential.
For light and thin packaging bags, the heat sealing pressure must 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 real fusion between the two films, resulting in partial heat sealing. It is good or difficult to eliminate the air bubbles trapped in the middle of the weld, resulting in virtual welding. However, the heat sealing pressure is not as large as possible. It is advisable 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, and too much pressure can easily squeeze part of the heat seal. Material, so that the edge of the weld is in a half-cut state. The weld is brittle and the heat seal 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.
Faster speed means shorter heat sealing time; slower speed means longer heat sealing time. Heat-sealing time is also a key factor affecting the sealing strength and appearance of the weld. Under the same heat sealing temperature and pressure, if the heat sealing time is long, the heat sealing layer is more fused and the bond is stronger; but if the heat sealing time is too long, it is easy to cause wrinkle deformation of the weld, affecting the flatness and appearance. If the welding seam after heat sealing is not well cooled, it will not only affect the appearance of the welded flatness, but also have a certain effect on the heat sealing strength. The cooling process is to use a lower temperature to heat the fusion just under a certain pressure The weld after sealing is shaped. Therefore, insufficient pressure, insufficient cooling and circulation, insufficient circulation volume, too high water temperature or untimely cooling will result in poor cooling and reduced heat seal strength.
4. The more heat-sealing times, the higher the heat-sealing strength. The length of longitudinal heat-sealing depends on the ratio of the effective length of the longitudinal welding rod and the length of the bag.
The number of lateral heat seals is determined by the number of lateral heat seal devices on 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 peeling degree of the composite layer, the greater the heat seal strength; for products with low composite peel strength, the weld failure is often due to the peeling of the composite film at the weld seam first, resulting in heat from the inner surface The seal layer independently withstands the rupture tensile force, while the surface layer material loses its reinforcing effect, resulting in a significant reduction in the heat seal strength of the weld seam; if the peeling degree between the composite layers is strong, there will be no peeling between the weld edges, measured The actual heat seal strength obtained is much more human; when the inner layer of the heat seal is PE or OPP , the heat seal strength is much better than BOPP of the same thickness .
5. The influence of the contents of the compound bag.
Some products are packed in powder, which is easy to contaminate the seal when filling. For example, when LDPE material is used as the inner layer material, it is found that 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 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 compounding polyethylene film, polyethylene is precipitated after being pressed by a hot press roller. A layer of white is like white powder. This phenomenon is that polyethylene is added with a certain amount of lubricant during the production process. Some low-melting waxes easily precipitate to the film surface. The most direct hazard after precipitation of this layer of low-melting wax is to greatly weaken the composite strength, and also greatly weaken the heat seal strength, especially in the edge sealing position, causing easy opening and delamination.
The solutions are: 1) Pre-treat the polyethylene again to achieve the ideal surface tension; 2) Select the appropriate adhesive to enhance its compound fastness; 3) Reduce the curing temperature as much as possible to prevent the precipitation of substances, thereby increasing the compound fastness Degree and heat seal strength.
7, the heat-sealing of flexible packaging bags and delamination of the printed ink layer faces the corona related quality.
In the actual production process, in order to achieve the true reproduction of colors, it is inevitable that the inner and surface printing inks are mixed and printed. Theoretical analysis shows that the inner and surface printing inks are incompatible. If the ink layer of the printed film is mixed with the inner and outer surfaces, the fastness between the ink layers is inevitable, which is easy to delaminate and easy to heat seal welds. As a result of the delamination phenomenon, the heat seal strength becomes worse.
Solution: Try to avoid the mixture of surface printing ink and internal printing ink, so as to improve the heat sealing strength and reduce the phenomenon of delamination.
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