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White spots and bubbles in composite packaging bags

by:Supouches Packaging     2021-12-29
The white dots and bubble shapes of composite packaging bags are different, and people have different concepts of bubbles and white dots. Although they are both called white spots and bubbles, most of them are not the same phenomenon when detailed, and the formation mechanism is extremely different. Therefore, white dots and bubbles are general names, which are worthy of discussion and research. Now the reasons for the formation of white spots and bubbles are listed as follows: White spots and bubbles: ①Insufficient drying ②Insufficient amount of glue: supervision, poor resolubility, and decreased transfer rate. . The film is too thick ③The glue is not leveled after compounding: the glue has a large molecular weight, insufficient maturation, the temperature of the composite heat roller is too low, the temperature is low, and the preheat roller is useless. The light-colored parts of the film appear to be white spots. The two substrates of the bubble part are not closely attached, and are not necessarily round and convex. But the air bubbles formed by bad drying are bulging, and the big ones are like sesame seeds. Some of the reasons for this kind of bubbles are that the adhesive is not completely dried, and some are caused by insufficient drying of the ink solvent, which often occurs in large areas of printing. Some small printing presses are still using infrared drying ovens. Although the slight poor drying does not create large bubbles, it is only small enough to release the solvent through certain films. As with the same process conditions, the PET aluminum foil has bubbles, but the bubbles disappear after the BOPP aluminum foil is heated. 2. There is little difference between the small bubbles formed by poor drying and the bubbles caused by insufficient amount of glue to observe with a magnifying glass. At this time, a rational analysis is required. Any surface that looks smooth is uneven when viewed with a magnifying glass. The adhesive needs a certain thickness to smooth the surface-that is, the amount of glue. Low-viscosity adhesive for ordinary transparent composite film. The printing film should be thicker, such as white film, the thickness of the white film under the color block is larger. The resin content of the back-printing composite ink is less than that of the surface-printing white film, so the surface of the composite white ink is relatively rough. Rough benzene is the advantage of composite white ink, which is conducive to the penetration of the adhesive to produce sufficient peel strength. In turn, it requires a larger amount of glue. In order to ensure that the ink part has sufficient adhesive thickness, the amount of glue needs to be 3-3.5G, and the amount of glue is insufficient if it is less than this standard. It is easy to understand the insufficient amount of glue due to blockage. There are two types of blockage: temporary and sudden, caused by poor paint resolubility and continuous decline in transfer rate during operation. If this kind of blockage is not found out and eliminated in time, it can only be maintained for a while only by stopping the machine to wipe the board. The other type of blockage is gradual and cannot be regarded as a fault strictly speaking, because it is impossible to completely clean the residual glue in the anilox roller after each compounding, and the residual glue deposits and solidifies on the bottom of the mesh day after day. The amount of glue applied gradually decreases, and small bubbles are gradually generated, from less to more, from small to large. Eventually, due to the production of demanding products and thicker composite films one day, small bubbles became intolerable. At this time, use a special washing solution. The problem is solved, but it is not a good production habit, because a large number of products between qualified and unqualified have been produced before. We advocate regular measurement of the amount of glue applied, and cleaning when the amount of glue drops to a predetermined value. 3. Sometimes the drying is very sufficient and the amount of glue is a lot, and there are still bubbles in the composite film just after the machine. In fact, it is unlikely that the newly compounded product will not have any bubbles. Small bubbles can be eliminated after aging, because polyurethane adhesives that do not contain solvents but are not fully cured have a certain degree of fluidity. The higher the temperature, the greater the flow, and the greater the molecular weight, the lower the fluidity. The essence of curing is the process of increasing molecular weight. For the composite film in the maturing chamber, the adhesive slowly leveled under the action of temperature, the large bubbles shrink and the small bubbles disappear until the molecular weight of the adhesive increases to the point that the glue cannot flow. Experienced operators can see how big the white spots can disappear. If they are not sure, it is best to put the sample in an 80 degree oven for 30 minutes to check whether the bubbles are removed. Bubbles of the same size can be eliminated by low-viscosity glue, but high-viscosity glue may not work. This is because 50% high-viscosity glue has a large molecular weight and poor leveling. In the same way, air bubbles cannot be eliminated if the temperature of the maturing chamber is not enough, and air bubbles will also be generated if the temperature of the composite hot roller is not enough. The purpose of heating by the hot roller is to melt and flow the dry adhesive, eliminate small holes and gaps, remove air under pressure, and make the two-layer film tightly ironed. Generally, the glued substrate of the composite equipment is close to the heat roller, and the heat is transferred from the glue layer to the CPP or PE. This type of equipment is equipped with a preheating roller between the second unwinding substrate guide rollers, but most customers do not use it—that is, without heating, when the temperature is low and the PE or CPP is thicker, although the temperature of the composite hot roller is Not low, but thicker PE or CPP has a large heat capacity, and the short time of the hot roll pressing process cannot make the temperature of the substrate rise rapidly. The two layers of film are hot and cold at the same time, and the fluidity of the adhesive in the middle is greatly reduced, and it is not surprising that bubbles are generated.
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