Analysis of quality failure caused by improper use

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Analysis of quality faults caused by improper use of printing

the phenomenon of text whitening is a common quality problem. It is mainly the whiteness, poor reproducibility and lusterless of printed graphics and texts, which can be divided into solvent whiteness and resin whiteness

1. Solvent whiteness - this kind of whiteness is more common in alcohol based inks. When the humidity is high, due to the evaporation latent heat of the solvent, the ink film nearby will be cooled, making water droplets mixed into it. 2. The force values measured by the single column tensile tester and the double column tensile tester are also different, causing the ink film to whiten. We found that this phenomenon is more obvious when using solvents with high latent heat. The solution is: try to reduce the humidity, reduce the drying speed, and heat and dry during printing

2. resin bleaching - when the solvent balance in the ink is poor, the real solvent volatilizes first, and the remaining diluent in the ink will precipitate the resin in the ink, causing precipitation and whitening. In the printing process, before the ink dries, if the heat supplied to the solvent evaporation is sufficient, the situation will be better. The heat provided by drying is insufficient, and the place far from the drying path or the printing speed is too slow, so that the printed part of the material enters the drying path for too long, it is easy to appear albinism, which seriously affects the gloss, poor transparency and low adhesion of the product. The solution is to increase the printing speed or add slow drying solvent appropriately, and slow drying time is also an effective method

3. adhesion failure - during the plum rain season with high temperature and humidity, after the plastic film is rolled up through gravure printing, or after being cut into packaging bags and stacked, the ink between the printing surfaces or the ink and the unprinted blank surface are adhered together, which cannot be separated, and finally the product is scrapped

internal factors causing adhesion: the ink itself has poor heat resistance and a certain viscosity

a. too much residual solvent. When a large amount of solvent remains in the printing ink, it is like the paint is not dry. Although it looks dry, it is not dry and sticky. As the printed matter is rolled up, the solvent is difficult to evaporate. The resin of the ink cannot be dried and solidified, which will cause serious adhesion of the product. For products with adhesion, the content of residual solvent is often detected by meteorological chromatography. Therefore, the volume and weight of lithium titanate batteries are large, often up to tens of thousands of ppm. In addition, the residual solvent makes the product contain odor, which also affects the flavor of food, and also affects the peel strength on the composite packaging. The solution is to solve it from the performance or drying conditions of the dryer. In addition, the slow drying solvent should be used as little as possible, and the temperature of the gravure printing machine should be controlled. It is often possible to adjust it with an electric heating tube. The volatilization rate of solvent is often closely related to the barrier property of the material itself

b. the additives in the printed material cause the ink to stick: for example, after the material containing plasticizer is printed, the material plasticizer will slowly penetrate into the ink, causing the colorant in the ink to soften. Especially soft PVC, this phenomenon is more obvious. Therefore, the ink for printing rigid PVC cannot be used for printing soft PVC. In addition, the moisture in the raw materials will cause the resin in the ink to expand and produce viscosity, so the printing materials should be kept moisture-proof

c. the resin has poor heat resistance. In the preparation of gravure ink with synthetic resin, the resin with high softening point should be selected. Generally, resins with softening point between 105 ℃ - 110 ℃ are selected. The softening point is too low to resist heat

high external temperature causes adhesion

a. during the operation and use of printed matter or at room temperature, when the finished product without quality problems encounters high temperature, the ink will also soften and produce adhesion. Therefore, it is required that the operating temperature of printed matter should not exceed 40 ℃ - 50 ℃ (except for some special requirements)

b. in order to reduce the residual solvent as much as possible in the gravure printing process, heating and drying must be carried out, and the film temperature will rise. At this time, the heat will accumulate when winding, and the temperature will rise to 50 ℃ - 60 ℃ to produce adhesion. Therefore, after heating and drying, the film must be cooled by the cooling device. Now most printing machines are equipped with cooling equipment

c. the processing causes the temperature to rise, and the printed matter has friction during trimming or winding, resulting in a certain amount of heat. Especially in high-speed slitting and rewinding inspection, the friction heat increases the temperature of the film, which is often in the middle of the drum, and it is easy to adhere. Therefore, the tension should not be too large during operation

when storing products, try to avoid high ambient temperature or high humidity. It is recommended to place them in an environment with a temperature of 2l ℃ - 23 ℃ and a humidity of no more than 60%

the pressure of printing is high, resulting in adhesion

a. the winding is too tight. In the printing process, the film should not be too tensioned and rolled too tightly. Such as nylon 15 μ M printing film, if the length is more than 4000 meters, it is easy to adhere, which is determined by the nature of nylon itself. After adhesion, the printing ink layer of the product is firmly adhered to the material, and the product is scrapped

b. it is also easy to cause adhesion due to the unreasonable design of printed pictures and texts. The thicker the picture and text, the easier it is to adhere, because the thicker the ink layer, the solvent volatility is affected, and it is difficult to evaporate. It is suggested to rationalize the design of pictures and texts to avoid the phenomenon of ribs. In addition, loose tension winding can also avoid adhesion, and only people who can discover, utilize and integrate various new principles, new concepts, new technologies, new materials, new processes and other latest scientific and technological achievements as soon as possible

VII. The phenomenon of adhesion is also related to the surface tension of printing materials. If the surface tension of the material does not meet the requirements, the poor adhesion of the ink layer will also adhere. The solution is to improve the surface tension of materials, often by corona treatment

author (Quandong)

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