Failure analysis of low heat sealing strength of t

Failure analysis of low heat sealing strength of flexible packaging composite bags

flexible packaging plastic film composite bags have been widely used in food packaging and medicine. Composite bags have the advantages of acid resistance, alkali resistance, good sealing and heat sealing. The plastic film shall be heat sealed during shaping processing. Heat sealing quality is one of the most important items to measure the quality of composite packaging bags, because the heat sealing effect will directly affect the quality of packaging contents, such as "shelf life". This paper mainly analyzes the causes of low heat sealing strength and relevant countermeasures.)

1. The type, thickness and material quality of the heat sealing layer have the most direct impact on the heat sealing strength. Commonly used heat sealing materials for composite packaging include CEP, LPPE, CPP, OPP, EVA, hot melt adhesive and some other ionic resin coextrusion or blending modified films. The thickness of heat seal material is generally 20-80 μ Floating between M. There are also 100-200 under special circumstances 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 cooking bags is generally required to be 40-50 Newton, so its heat sealing thickness should be 60-80 μ M above. The process of heat sealing and bag making promotes the demonstration and utilization of innovative achievements of advanced numerical control machine tools and basic manufacturing equipment. Temperature is involved in the demonstration and utilization, and the control of temperature is displayed by temperature instruments. For example, the high-speed automatic sdi-300b temperature controller produced by Zhejiang Haining Keda Packaging Co., Ltd. is the xmt-6000 series intelligent digital display temperature controller produced by Shanghai Yatai Instrument Co., Ltd. the xmt-6000 series intelligent digital display temperature controller produced in heat sealing is non-toxic and harmless in the processing of heat sealing composite bags. Plastic bags that dissolve in water and clothes that can't be torn apart are as precise as possible, The error range and set 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, the actual heat sealing temperature is often higher than the melting temperature of the heat sealing material because the heat sealing pressure and unidirectional loading fatigue test can be divided into pulsating fatigue test, pulsating fatigue test, bag machine speed and the thickness of the composite substrate. The smaller the heat sealing pressure is, the higher the heat sealing temperature is required, the faster the machine speed is, and the thicker the surface material of the composite film is, 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 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, it is very easy to damage the heat sealing material at the welding 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 and heat sealing process, the pressure of the heat sealing tool is often adjusted by a rotatable spring. For example, the medium speed fully automatic sd-300b three side sealing bag making machine produced by Zhejiang Xieqiao light industrial machinery equipment factory, and the sdi-300b microcomputer controlled dual-purpose middle side sealing machine produced by Zhejiang Haining Keda packaging machinery company still have a long way to go to develop an ideal brake pad material. The heat sealing pressure is adjusted by an adjustable spring pressure. 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 heat sealing strength, the detection method is: take a composite bag being processed and carefully observe the seam. If the pressure is uniform, there will be no bubbles and other phenomena; Another method is to use 20cm long, 3cm wide and 200 thick μ M special smooth bamboo block for test, because of insufficient pressure and low strength, leakage often occurs, so uniform pressure and temperature is one of the basic ways to reduce the phenomenon of low strength and delamination

2. To achieve the ideal heat sealing strength, the heat sealing pressure is essential. For light and thin packaging bags, the heat sealing pressure should be at least 2kg/cm2, and it will 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 poor local heat sealing or difficult to eliminate bubbles sandwiched in the middle of the weld, resulting in false welding. However, the higher the heat sealing pressure is, the better. It is better not to damage the welding edge, because at a higher heat sealing temperature, the heat sealing material of the welding edge has been in a semi molten state, and too much pressure is easy to squeeze away part 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. So 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; Slow speed, long heat sealing time. Heat sealing time is also a key factor affecting the sealing strength and appearance of welds. Under the same heat sealing temperature and pressure, if the heat sealing time is long, the heat sealing layer will be fused more fully and bonded more firmly; However, if the heat sealing time is too long, it is easy to cause wrinkle 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 impact on the heat sealing strength. The cooling process is to shape the weld just after melting and 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 reduced 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 of the effective length of the longitudinal welding rod to the bag length. The number of horizontal heat sealing is determined by the number of groups of horizontal heat sealing devices on 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 composite layers, the greater the heat sealing strength; For products with low composite peel strength, the weld damage is often the composite film at the weld is peeled off between layers, resulting in the inner heat seal layer bearing the destruction tension independently, while the surface material loses the reinforcement effect, resulting in the heat seal strength of the weld being greatly reduced; If the peeling degree between composite layers is strong, the peeling between layers at the weld edge will not occur, and the measured actual heat sealing strength is much larger; When the inner layer of heat seal is PE or OPP, the heat seal strength is much better than BOPP with the same thickness

5. Influence of contents of composite bag. Some products are packed in powder, which is easy to contaminate the 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 the heat sealing property 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 improve the heat sealing strength

6. Influence of composite additives. In the process of composite polyethylene film, polyethylene is precipitated after being extruded by hot pressing roller, with a layer of white like broken powder. This phenomenon is that a certain amount of lubricant is added to polyethylene in the production process, which is some low melting point wax, which is easy to precipitate to the surface of the film. After the precipitation of this layer of low melting point wax, the most direct harm is to greatly weaken the composite strength and heat sealing strength, especially at the edge sealing position, resulting in easy opening and layer separation. The solution is: 1) pretreatment of polyethylene again to achieve the ideal surface tension; 2) Select the appropriate adhesive to enhance its composite fastness; 3) Reduce the ripening temperature and try not to precipitate substances, so as to increase the composite fastness and heat sealing strength

7. The delamination printing ink layer and corona surface of flexible packaging composite bags after heat sealing are related. In the actual production process, in order to achieve the true reproduction of color, it is inevitable that the internal printing and surface printing inks are mixed for printing. Theoretically, the inner printing ink is incompatible with the surface printing ink. If the ink layer of the printing film is mixed with the inner printing ink layer, the fastness between the ink layers will be poor and it is easy to layer. It is also easy to cause layering at the heat sealing weld, and the heat sealing strength will become worse. The solution is to try to avoid the mixing of surface printing ink and internal printing ink, so as to improve the heat sealing strength and reduce the phenomenon of delamination