Analysis and countermeasures of dry compound transparency
Many plastic composite flexible packaging products require high transparency, so it is necessary to use a large area of ​​blank base film to compound, but in the dry composite process, the plastic composite flexible packaging often has poor transparency or declines, which does not meet the quality requirements, such as finished products It appeared as "fog", and appeared "small pits", "wavy fish scales", "silk lines", "mushroom umbrella cloud", and "drip water splashing" and other phenomena, which affected the transparency of the composite product. The finished product is scrapped. Our factory has encountered the above failures, and the quality of the composite product has been affected to varying degrees.
After a long period of observation and exploration, our factory has summarized a set of practical solutions and achieved initial results.
"Fog" phenomenon and countermeasures
When the composite finished product just got off the machine, the blank space appeared "fog" and appeared randomly. Perhaps this one appeared "foggy", but the next one did not, and the "fog" appeared again after a few sheets. The composite finished product did not Appear regularly. If you put the contents in, and look through the 40-watt fluorescent lamp, it is obvious that it is not clean.
We have noticed that the probability of producing this phenomenon when compounding with a single-component hot-melt adhesive is higher than that with two-component reactive adhesives. At the same time, it is compared in the BOPP / OPP and BOPP / CPP structures. easy to appear.
There are several reasons for this phenomenon.
1. The transparency of the composite material or the printed substrate itself is poor
Before the compounding, the film material is generally tested. The detection method is: take a small section of the film and observe it through light. The film material is required to be smooth, without small spots, sticky dirt, uneven unevenness, and without gray impurities. Therefore, only by choosing high-quality materials produced by regular manufacturers can good products be compounded.
2. The effect of adhesive
The color of the adhesive is too dark, too dark yellow or yellowish red. After drying, this part of the darker color will be left on the film accordingly. Therefore, when the composite product is required to have high transparency, it should be slightly yellow or even colorless. Adhesive. Correct selection of the two-component adhesive can reduce the chance of "fog" in the composite product and even eliminate the "fog" phenomenon.
3. Operation error or insufficient drying temperature
Operation errors or insufficient drying temperature will also produce a "fog" phenomenon. For example, the amount of glue is uneven, one side is thicker, and the other side is thinner. Therefore, uniform coating is very important in the dry composite process.
When compounding with a single-component adhesive, it is generally prone to uneven coating at the place where it is just wound up. If this problem occurs, you can find it by observing a piece of composite product through light.
If two-component adhesive is used for compounding, it will be more difficult to judge uneven coating. Our factory generally takes a section of the finished composite product in the rewinding part, puts it in the drying tunnel for 15 minutes and then takes it out. If this "fog" phenomenon still exists, it means that the adhesive is unevenly coated.
In the high-temperature and rainy season, the "fog" phenomenon is most likely to occur, and it often occurs in composite films with BOPP / OPP, BOPP / CPP, and PET / PE structures. Adjusting the temperature of the drying tunnel to the best state can reduce the occurrence of this phenomenon.
"Small pits" phenomenon and countermeasures
Dry composite products often appear as "small pits" similar to bubbles, but careful analysis is not a bubble. Small pits are unevenly distributed, but the size and thickness are almost the same, visible in the blank space of the substrate. This kind of "small pockmarks" has plagued many flexible packaging companies. We have made comparisons. Two-component two-liquid reactive adhesives have a higher probability of producing small pockmarks than single-component hot-melt adhesives. BOPP / The OPP composite structure is higher than BOPP / CPP and PET / PE. The analysis has the following reasons.
(1) Dust and other particles are mixed in the adhesive. There is too much dust in the air of the workshop environment. There is also dust in the hot air blown out in the drying tunnel after gluing. The dust is stuck on the glue layer and is sandwiched between the two base films during compounding. , Resulting in opaque composite products.
The solution is to filter the adhesive with a 180-mesh wire mesh to remove insoluble particles in the adhesive. At the same time, we should pay attention to the cleanliness of the workshop and keep the ground smooth and free of floating dust. In dry compounding, glue buckets and glue trays must be cleaned before leaving work. Especially in summer, the indoor temperature is high and the drying is fast. Before each shift, the residual adhesive should be poured into the barrel and closed and handled separately.
In addition, high-mesh filter nets or other filter materials should be used to remove the dust in the hot air at the air inlet of the drying tunnel.
Two-component reactive adhesives are susceptible to whitening and turbidity due to temperature. "Turbid" adhesives can no longer be used, otherwise uneven distribution of the adhesive layer will occur, resulting in increased "small pits".
(2) The quality of base film and composite film is not enough. If the OPP heat compound is yellow and outdated, it should be carefully checked. First of all, the surface tension value should be checked. The surface tension of BOPP needs to reach 38mN / m, and the surface tension of PET needs to reach 50mN / m. Low surface tension will result in poor wetting effect on the surface of the material, uneven coating of the glue, resulting in voids, and small pits in the finished product after compounding. Therefore, films with surface tension values ​​that do not meet the requirements must be processed before being used.
The second is to use a magnifying glass to detect the smoothness of the base film and the composite film, requiring no unevenness. If the surface of the material is uneven, and the depth is different, after being pressed by the hot pressing roller, it cannot be completely tightly adhered to the film, and "small pits" will appear. Therefore, the selection of materials is critical.
(3) Poor smoothness of the glue roller, insufficient fluidity of the glue, and poor flatness will cause "small pits". The rubber roller is required to be fine and smooth, preferably a rubber roller made of silicone rubber. The fluidity of the glue is good, the flatness is excellent, and the coating is uniform, which can reduce the chance of "small pits" and the transparency of the composite product has also been improved.
"Fish scale" phenomenon and countermeasures
The rate of "fish scales" in the composite product is less than that of "small pits" and "fog". There are two reasons for this.
(1) The phenomenon of "fish scales" appears in the PE film itself, which appears in the blank after compounding, resulting in a decrease in compound transparency. The solution is to replace the material.
(2) Occasional shutdown during the production process, the glue roller is not relaxed in time, or the glue roller is not separated from the glue solution, and the heat roller and the hot drum are not separated. Generally, wavy irregular horizontal indentation will appear in the blank space under 30-minute compaction, resulting in poor transparency. Therefore, in the non-working state, the glue roller and the anilox roller should be isolated and cleaned, and the heat pressing roller and the heat drum should be separated, and the power supply of the heat drum should be turned off.
Other phenomena and countermeasures
The main reasons for the phenomenon of "mushroom umbrella cloud", "drip water splash", "silk pattern" in the composite product are as follows.
(1) The material itself is bad, with irregular patterns. Just replace it.
(2) The influence of adhesive and drying tunnel. When entering the drying tunnel, the temperature is too high, and the drying speed is too fast, so that the solvent on the surface of the adhesive layer quickly evaporates, and the surface forms a skin. When the heat penetrates into the interior of the adhesive layer, the solvent under the film vaporizes and breaks through the film surface, forming a similar volcano The ring of the mouth, like "mushroom cloud", is distributed on the surface of the adhesive film, resulting in an uneven and opaque adhesive layer. Therefore, the temperature of the drying tunnel should be increased from low to high in order to allow the solvent to be completely and volatile from the inside to the outside in an orderly and uniform manner without leaving any odor. In addition, solvents with different boiling points can be mixed and used, so that when heated and evaporated, the high boiling point solvent on the surface of the adhesive layer will remain for a long time, and the surface film layer will not be skinned, but the amount of residual solvent must meet the requirements and does not exist. Odor and odor problems.
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