The white dots and bubbles of composite packaging bags have different shapes, 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, and the formation mechanisms are even more different. Therefore, white dots and bubbles are general names, which are worthy of classification, discussion and research. Now the reasons for the formation of white spots and bubbles are listed as follows:White bubbles: ①Insufficient drying②Insufficient amount of glue: Supervision, poor resolubility, and decreased transfer rate. .Film is too thick③The glue is not leveled after compounding: the molecular weight of the glue is large, the curing is not enough, the temperature of the compound hot roller is too low, the temperature is low, and the preheating roller is useless1. Aluminized film composite printing substrate, if bubbles are generated, it will appear as white spots in the light-colored part of the printing film. In the bubble part, the two layers of substrates are not closely attached, and they are not necessarily bulging and bulging. However, the air bubbles formed by serious drying problems are bulging, as large as sesame seeds. Some of the reasons for this type of bubbles are that the adhesive is not dry, and some are caused by insufficient drying of the ink solvent, which often occurs in large-area drop marks. Some small printing presses are still using infrared drying ovens. Slight poor drying while not developing large air bubbles is small enough to release solvent through some films. Under the same process conditions,PETThe aluminum foil has bubbles, whileBOPPThe bubbles will disappear after the aluminum foil is heated.2. There is little difference between the small air bubbles formed by poor drying and the air bubbles produced by insufficient gluing amount with a magnifying glass. At this time, a rational analysis is required. Any surface that looks smooth will be 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 common transparent composite films. The printing film glue should be thicker, such as white film, and the thickness of the white film under the color block is larger. The resin content of the inner printed composite ink is less than that of the surface printed 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, but in turn requires more glue. In order to ensure that the ink part has enough adhesive thickness, the amount of glue needs to be3-3.5G, less than this standard is also insufficient glue. It is easy to understand that the amount of glue is insufficient due to the blocked version. Blocking score2Type: sudden, caused by poor paint resolubilization and continuous drop in transfer rate during operation. If the cause of this type of blockage is not found out and eliminated in time, it can only be maintained for a while only by parking and wiping the plate. Another type of blockage is a gradual type, which cannot be regarded as a failure strictly, because it is impossible to completely clean the residual glue in the anilox roller after each compounding, and the residual glue is deposited and solidified at the bottom of the mesh hole day after day. The amount of glue is gradually reduced, and small bubbles are gradually formed, from less to more, from small to large. Eventually, small bubbles become intolerable due to the production of demanding products and thicker films one day. At this time, use a special cleaning solution. The problem is solved, but it is not a good production habit, because a large number of products between qualified and unqualified products have been produced before. We advocate measuring the amount of glue on a regular basis, and clean it when the amount of glue decreases to a predetermined value.3. Sometimes the drying is sufficient, and the amount of glue applied is quite large, and there are still bubbles in the composite film just off the machine. In fact, it is unlikely that the newly compounded product will not have any air bubbles. Small air bubbles can be eliminated after curing, because polyurethane adhesives that do not contain solvents but are not fully cured have a certain fluidity. The higher the temperature, the greater the flow, the greater the molecular weight and the less fluidity. The essence of curing is the process of increasing molecular weight. For the composite film in the curing chamber, the adhesive is 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 where the glue cannot flow. Experienced operators can see how big the white spot can disappear. If you are not sure, it is best to put the sample into the80degree in the oven30minutes to check that air bubbles are removed. The same size of bubble, low viscosity glue can be eliminated, but high viscosity glue may not, this is because50%High-viscosity glue has large molecular weight and poor leveling.In the same way, if the temperature of the curing chamber is not enough, the air bubbles cannot be eliminated, and if the temperature of the composite heat roller is not enough, air bubbles will also be generated. The purpose of heating the hot roller is to melt and flow the dry and solid adhesive, eliminate small holes and small gaps, remove air under pressure, and make the two-layer film tightly ironed. In general lamination equipment, the glued substrate is close to the hot roller, and the heat is transferred from the glue layer to theCPPorPE. This type of equipment has preheat rolls between the second unwind substrate guide rolls, but is not used by most customers——That is, without heating, when the temperature is low,PEorCPPWhen it is thicker, although the temperature of the composite hot roller is not low, the thickerPEorCPPThe heat capacity is large, and the temperature of the substrate cannot rise rapidly in a short time during the hot roll lamination process. The two layers of film are hot and cold, and the fluidity of the adhesive in the middle is greatly reduced, and it is not surprising that air bubbles are generated.