The milk film products currently on the market are milky white film, black and white film, pearl film, high barrier film, etc. The printing ink generally uses polyamide ink, which is a kind of more affinity with polyethylene in many types of ink. Printing does not fade must be strictly controlled and tested on the various production processes of the liquid film.
First, the film production precautions
1. The raw material used for film blowing must be a new material. Absolutely no recycled materials can be used.
The solution of the recycled material has been destroyed, and when the film is mixed with a new material, the heat-sealing property of the film will be reduced, sand holes will be generated, the number of broken packages will increase, and the smoothness of the film surface will decrease. In order to improve the smoothness of the film surface, some manufacturers add an excess amount of oleic amide type slip agent to the blown film raw material, which will cause the surface tension of the film to decrease, and the binding fastness of the ink is naturally difficult to guarantee.
2, blown film temperature control
The optimum extrusion temperature of blown film raw materials is between 160 and 180 °C . Excessively high temperatures can cause the material contents to seep out and affect the surface tension of the film. Too low a temperature will make the film processing unstable and reduce the yield.
3, film surface corona treatment
The principle of corona treatment is that the film passes between two electrodes with high pressure. The high pressure causes the air between the electrodes to ionize, causing electron flow between the electrodes. The electrons are marked on the surface of the film and become small pits that cannot be seen by the naked eye, so that the film surface is formed. Polarity for easy printing ink adsorption.
The surface polarity (ie, surface tension) of the film can be tested with dyne. Generally, the surface tension of the film just after corona treatment should reach 40 dynes or more. After the film is placed for one week, it must reach 38 dynes or more, lower than 36. Dyne's film has no sorption fastness to the ink.
4, the film storage
In general, the film is best used within one week after the completion of the blowing process. It must be stored for a long time. It must also be stored in a dry, cool and ventilated warehouse. The maximum length cannot exceed 3 months. Before long-term stacking, the film must be used. After the dyne liquid test, it can be used only after passing the test, otherwise it will cause losses.
Second, some of the printing process precautions
Poor printing process is the key to ink discoloration. Generally, following the use of suitable table printing inks and appropriate solvents, the following points must be noted when printing:
1. The proportion of ink must be changed according to changes in the environment
Inappropriate solvent ratios can cause ink drying difficulties and surface “sham dryâ€. False dryness means that the solvent with high volatility in the solvent ratio of the ink has volatilized, the volatile substance is not completely volatilized, the surface of the ink is dry and the ink is left for a period of time after printing, the residual solvent continues to infiltrate, and ink adhesion and discoloration occur. .
2, printing speed and drying oven temperature
The printing speed should not be too fast. The temperature of the drying oven is generally 30 to 40 °C . This is to allow the ink solvent to cure and volatilize, and to enhance the ink adhesion.
3, the printing shop environment
The printing shop environment must ensure a certain degree of humidity (between 60% and 70% is better), requires good ventilation, and the temperature is better than 25 to 30 °C . A too dry environment can cause the static electricity of the film to be difficult to release, and it is too humid. The environment makes it difficult to volatilize the ink and the fastness decreases.
Third, the latter process
It is better to place the printed product after it is placed for one day, which is beneficial to the full curing of the ink and the improvement of the fastness of the ink.
After the above processes in the production have been strictly controlled after the detection, the fastness of the ink can generally meet the requirements, but due to the printing of the film, the ink on the surface of the film is vulnerable to various damage, water, acid , mechanical, friction more or less can cause the ink to fade.
Moreover, after the milk is filled, it is generally required to be placed in a refrigerator at 4 °C . After the milk is taken out of the refrigerator, the surface of the film will alternate due to hot and cold, and condensation will occur. During transportation, bags and bags Between the friction due to vibration, to the hands of consumers, and some still need to drink milk heating.
Such a process requires the ink to have very good resistance to freezing, water resistance, heat resistance, and abrasion resistance at the same time. Such an ink product is very difficult to produce. So far, none of the inks has been completely Have the above four characteristics.
To completely solve this phenomenon, only the surface printing into India, the ink is not on the surface there is no need to worry about color, there are three ways: The first is the composite milk film structure outside the BOPP / printing / PE, such a structure required There is a special filling equipment, the cost is also higher), the structure of the second film is PE / printing / PE, such a structure of ordinary filling equipment can also be filled, but the cost is still higher, the third method is casting PE/printing/PE structural film, that is, after the original milk film is printed, an additional process is performed to flow a thin layer of transparent thin film on the printing surface.
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