The shape of white spots and bubbles in composite packaging bags is different, and the concepts of bubbles and white spots are also different. Although they are called white spots and bubbles, most of them are not the same phenomena and their formation mechanisms are quite different. So white spots and bubbles are a general term, which deserve to be classified and studied. The reasons for the occurrence of white spots and bubbles are as follows:
White spot blistering: (1) dry thoroughly
(2) Glue loading: supervision, poor solubility, fast transfer speed.
(3) No leveling after leveling: the molecular weight of rubber is not enough, the temperature of COTS is too low, the preheating temperature of COTS is too low and ineffective.
Aluminum-coated composite printing substrates, such as bubbles, appear white spots in the light-colored part of the printing film. The two layers of base material in the bubble part are not bonded tightly, and are not necessarily circular bulges. But the bubbles formed by extreme drying are as big as sesame. Some of the reasons for this bubbles are that the adhesives are not completely dry, some are that the ink solvent is not dry enough, and large-scale printing often occurs. Some small printers still use infrared dryers. Drying is slightly worse, although no bubbles appear, but small enough to release solvents through some thin films. Under the same process conditions, PET and BOPP foil bubbles will disappear after heating.
Secondly, the small bubbles formed by poor drying are not very different from the bubbles produced by inadequate amount of bubbles produced by magnifying glass, so reasonable analysis is needed at this time. Using a magnifying glass to see any smooth surface is bumpy. It requires a certain thickness, that is, the amount of glue, to flatten the surface. Low viscosity adhesives for common transparent composite membranes. Printing film should be coated with thick glue, such as white film, color block thickness under white film. The resin content of composite printing ink is less than the white film of printing surface, so the surface of composite printing ink is rough. Crude benzene is the advantage of composite white ink, it is conducive to the penetration of adhesives, producing enough peeling strength, but in turn requires more glue.
In order to ensure that the ink has enough adhesive thickness, the glue dosage needs 3-3.5g, and the glue dosage below this standard is not enough. It is easy to understand the lack of glue due to blockage. Block 2: Suddenly, the solubility and Operational conversion of the paint are declining. If the reason for eliminating the baffle is not found in time, it can only be maintained by parking eraser for a period of time. Another kind of block is progressive. Strictly speaking, it can not be regarded as a fault, because it is impossible to clean the residual glue after each cot is compounded, and the residual glue deposits and solidifies day after day at the bottom of the mesh hole. The amount of glue decreases gradually, and small bubbles are produced gradually, from less to more, from small to large. Ultimately, small bubbles become intolerable due to the high requirements of products and composite membranes made in one day. A special detergent should be used. The problem has been solved, but this is not a good production habit, because there are many products between qualified and unqualified. We advocate regular measurement of glue volume and cleaning when the glue volume is reduced to a predetermined value.
3. Sometimes the drying is full, and the glue content is not very large, but the composite film or bubbles are removed. In fact, it is impossible to mix a good product without a single bubble. Small bubbles can be eliminated by solidification because polyurethane adhesives without solvents but not fully solidified have certain fluidity. The higher the temperature, the larger the flow rate and the smaller the molecular weight. The essence of curing is the process of increasing molecular weight. In the composite film of curing chamber, the adhesives flow slowly under the action of temperature, the bubbles shrink, and the small bubbles disappear until the molecular weight of the adhesives increases to the extent that it can not flow. Experienced operators can see how much white spots can disappear. If they are not sure, it is best to bake the sample in an 80 degree oven for 30 minutes to check whether the bubbles have been removed. With the same bubble size, low viscosity gum can be eliminated, but high viscosity gum can not. This is because the molecular weight of high viscosity gum is 50%, and its leveling is poor.