Compound bag of white spots and bubbles


The white spots and bubbles of composite packaging bags have different shapes, and people have different concepts of bubbles and white spots. Although they are called white spots and bubbles, most of them are not the same phenomenon, and the formation mechanism is very different. So white spot and bubble is a general name, worthy of classification and research. Now the reasons for the formation of white spots and bubbles are listed as follows:


White spot bubbles: ① insufficient drying

② Insufficient amount of glue: supervision, poor resolubility, transfer rate decline. Film too thick

(3) The composite glue is not flat: the molecular weight of the glue is large, the aging is not enough, the temperature of the composite hot roller is too low, the temperature is low, and the preheating roller is useless


1. Aluminum coated composite printing substrate, such as bubbles, looks white in the light-colored part of the printing film. The bubble part of the two layers of substrate is not closely fitted, not necessarily round bulge. But the bubbles formed by severe drying are bulging, as big as sesame seeds. The reason for such bubbles is that the adhesive is not dry, and some are caused by insufficient drying of the ink solvent, which often appears in the large-area downprint part. Some small presses still use infrared drying chambers. Minor dryness, though not large bubbles, is minor enough to release solvents through certain films. Such as the same process conditions, PET aluminum foil has bubbles, and BOPP aluminum foil after thermal bubbles will disappear.


Second, there is little difference between the small bubbles formed by poor drying and the bubbles produced by insufficient glue with a magnifying glass, and rational analysis is needed at this time. Any surface that looks smooth will be uneven when viewed with a magnifying glass. Adhesive to smooth the surface needs a certain thickness - that is, the amount of glue. Low viscosity adhesive for ordinary transparent composite film. The printing film should be coated with thick glue, such as white film, and the white film thickness under the color block is larger. The resin content of the inner printing composite ink is less than that of the surface printing white film, so the surface of the composite white ink is rough. Rough benzene is the advantage of composite white ink, which is conducive to the adhesive penetration to produce enough peel strength, but in turn requires more gluing amount. In order to ensure that the ink part has enough adhesive thickness, the glue amount needs to be 3-3.5G, less than this standard is also insufficient glue amount. It is easy to understand the insufficient amount of glue caused by blocking plate. Blocking plate is divided into two types: temporary sudden, due to the poor resolubility of the paint and the continuous decline in the transfer rate during operation. If this kind of blocking plate is not found out the cause of timely elimination, only by stopping the plate can only maintain a temporary. The other type of blocking is gradual, strictly speaking, can not be considered a fault, because it is impossible to completely clean the residual glue in the mesh roller after each composite, and the residual glue is deposited and cured at the bottom of the mesh day after day. The amount of glue gradually becomes smaller, and small bubbles are gradually produced, from less to more, from small to large. Eventually, as a result of making demanding products and compounding thicker films one day, small bubbles become intolerable. At this time to use a special cleaning solution. The problem is solved, but it is not a good production habit, because there are a lot of products between qualified and unqualified before being produced. We advocate regularly measuring the amount of glue, and cleaning when the amount of glue decreases to a predetermined value.


Third, sometimes the drying is very full, the amount of glue is also a lot, and the composite film just got off the machine still has bubbles. In fact, it is unlikely that the just-compounded product does not have a little bubble. Small bubbles can be eliminated by aging, because the solvent-free but not fully cured polyurethane adhesive has a certain fluidity. The higher the temperature, the greater the flow, the greater the molecular weight, the smaller the fluidity, and the essence of curing is the process of increasing molecular weight. In the composite film in the curing chamber, the adhesive slowly flattens 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 much white spots can disappear, if not sure it is best to put the sample into the 80 degree oven for 30 minutes to check whether the bubble is removed. The same size bubble, low viscosity glue can be eliminated, and high viscosity glue may not be able to, this is because 50% high viscosity glue molecular weight, poor leveling.


In the same way, the temperature of the curing chamber is not enough to eliminate bubbles, and the temperature of the composite hot roller is not enough to produce bubbles. The purpose of hot roll heating is to melt and flow the dry solid adhesive, eliminate small holes and gaps, remove air under pressure, and make the two layers tightly ironed. General composite equipment coated rubber substrate close to the hot roller, heat transfer from the rubber layer to CPP or PE. This kind of equipment has a preheating roller between the second uncoiling substrate guide roller, but most customers do not use - that is, no heating, when the temperature is low, PE or CPP is thick, although the temperature of the composite hot roller is not low, but the thicker PE or CPP heat capacity is large, the short time of the hot roll pressing process can not make the substrate temperature rise rapidly. The two layers of film are hot and cold on one side, and the fluidity of the adhesive in the middle is blocked, and it is not surprising that bubbles are produced.

Hunan Kexin Packaging Co., Ltd.
RM1124,Bld 1, Jinqiao International Weilai Town, Changsha, China
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