Analysis of quality failure due to improper use of printing

Graphic whitening is a common quality problem. The main reason is that the printed images are whitened, poorly reproducible, and dull, and can be divided into two types: solvent whitening and resin whitening.

1. Solvent whitening - This bleaching is more common in alcoholic inks. When the humidity is high, due to the latent heat of vaporization of the solvent, the ink film is cooled near the ink film, causing the water droplets to mix therewith, resulting in whitening of the ink film. We found this phenomenon to be more pronounced when using high latent heat solvents. The solution is: reduce the humidity as much as possible, reduce the drying speed, and heat drying when printing.

2. Resin whitening - When the solvent balance in the ink is poor, the true solvent volatilizes first, and the presence of the diluent in the ink will cause the resin in the ink to precipitate, causing whitening due to precipitation. In the printing process, before the ink is dried, if the supply solvent evaporates enough heat, the situation will be better. The heat provided by the drying is insufficient, and the distance from the drying tunnel or the printing speed is too slow, so that the time for the material to be printed into the drying tunnel is too long, and the whitening phenomenon is liable to occur, which seriously affects the product luster, the transparency is not good, and the adhesion force is poor. not tall. The solution is: to increase the printing speed or add slow-drying solvent properly, so that the drying time is also an effective method.

3. Adhesion failure - During the high-temperature, high-humidity rainy season, after the plastic film is wound by gravure printing, or after divided and cut into bags, the ink or ink between the printing surfaces is adhered to the unprinted blank surface. Together, can not be separated and eventually cause the product to be scrapped.

Intrinsic factors that cause adhesion: The ink itself is poorly heat-resistant and has some tackiness.

a. Too much residual solvent. When a large amount of solvent remains in the printed ink, it looks like the paint is not dry, although it looks dry but it is still not dry and sticky. As the printed matter is rolled up, the solvent is also difficult to evaporate. The resin of the ink cannot be dried, cured, and cause the product to seriously adhere. Viscous products are detected by gas chromatography and residual solvent content often reaches tens of thousands of PPM. In addition, the residual solvent causes the product to have an odor, which also affects the flavor of the food and also affects the peel strength on the composite package. The solution is: to solve the performance of the dryer or drying conditions, the other slow-drying solvent as little as possible, the temperature control gravure printing press, often with electric heating pipe to adjust it. The volatilization rate of the solvent is also often related to the barrier property of the material itself.

b. The auxiliaries in the printed material cause the ink to become sticky: If the material containing the plasticizer is printed, the material plasticizer will slowly penetrate into the ink, causing the colorant in the ink to soften. Especially soft polyvinyl chloride is more obvious. Therefore, the hard PVC printing ink can not be used to print soft polyvinyl chloride. In addition, the moisture in the raw material causes the resin in the ink to swell and become sticky, so the printed material should be stored in a moisture-proof manner.

c. The resin is poor in heat resistance. In a synthetic resin formulation gravure ink, a resin with a high softening point should be used. Resins with softening points between 105°C and 110°C are generally used. The softening point is too low to be heat resistant.

High external temperature causes adhesions.

a. In the course of operation and use of the printed matter or at room temperature, when the finished product without quality problems encounters high temperatures, the ink will also soften and cause blocking. Therefore, the operating temperature of printed matter should not exceed 40°C-50°C (except for some special requirements).

b. In order to minimize the residual solvent in the gravure printing process, it must be heated and dried, and the temperature of the film will increase. At this time, the coil will be accumulated and the heat will be accumulated, and the temperature will increase to 50°C-60°C to cause blocking. Therefore, after the film is heated and dried, it must be cooled by a cooling device. Most of the printing machines are now equipped with cooling devices.

c. The temperature caused by the processing increases, and the print has friction during cutting or winding, which generates a certain amount of heat. Especially at high speed slitting and rewinding inspections, the frictional heat raises the temperature of the film, often in the middle of the reel, and it is very easy to stick. Therefore, the tension during operation cannot be too large.

When the product is stored, try to avoid high ambient temperature or high humidity. It is recommended to place it in a temperature of 2l°C-23°C and a humidity of less than 60%.

The pressure of the printed material is high and the blocking occurs.

a. Rewind too tightly. The film should not be too tensioned and rolled too tightly during printing. For example, a 15μm nylon film can easily stick if it is longer than 4,000 metres. This is due to the nature of nylon itself. The adhered product's printing ink layer is firmly bonded to the material and the product is scrapped.

b. Due to the unreasonable design of printing graphic, it is easy to cause adhesion. The thicker the graphic, the easier it is to adhere because the thicker ink layer is affected by solvent volatility and it is difficult to evaporate. It is advisable to rationalize the graphic design so as to avoid the phenomenon of convex ribs. The other is to loosen the tension and to avoid sticking.

Seven. The phenomenon of adhesion is also related to the surface tension of the printed material. If the material surface tension does not meet the requirements, the adhesion of the ink layer will not adhere as well. The solution is to increase the surface tension of the material, often by corona treatment.

Author (Quandong)

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