How to solve the problem of color change in milk plastic bags

Excellent packaging design can only be achieved with good printing methods. Good packaging is a silent salesman who can make use of sales functions to stimulate people’s desire to buy. On the contrary, packaging products with fuzzy characters and discolored graphics are hardly recognized by consumers. Plastic bags are commonly used in the packaging of liquid milk. To improve the printing effect of plastic bags, we must start from the selection of materials to the printing process and post-press processing.

The printing effect of plastic packaging is determined by the film itself and the printing process. To get the desired results, we must start with the strict control of the production of the film, and pay attention to every process after printing and printing.

Thin film selection and processing

The ultra-high-strength liquid film is made of high-density polyethylene (LDPE) and linear polyethylene (LLDPE) with a certain proportion of cream masterbatch, which is manufactured by blown film equipment. At present, there are milk film, black and white film, pearl film, and high barrier film on the market. Printing inks generally use polyamine inks because they are more compatible with polyethylene in many types of inks. In addition, in order to make the printing not fade, the production processes of the liquid film must be strictly controlled and tested.

Raw materials

New materials must be used for blown film. Absolutely no recycled materials should be used. Because the solution of the recycled material has been destroyed, the heat sealing performance of the film will be reduced when a new material is blown into the film, and sand holes will be generated to increase the number of broken packets, and the film surface will be less smooth. In order to improve the smoothness of the film surface, some manufacturers add excessive amounts of oleic acid and amine slip agents to the film-blown material. This will cause the surface tension of the film to decrease, and the bond fastness of the ink is naturally difficult to guarantee.

Extrusion temperature

The optimum extrusion temperature for blown film stock is between 160°C and 180°C. Excessive temperature will cause the material content to seep out and affect the surface tension of the film; if the temperature is too low, the film processing will be unstable and the yield will be reduced.

Surface corona treatment

The principle of corona treatment is that the film passes between the two electrodes with high pressure. The high pressure causes the air between the electrodes to ionize and generate electron flow between the electrodes. The electrons are marked on the surface of the film as small pits invisible to the naked eye, so that the film surface Polarity is formed to facilitate printing ink adsorption.

The surface polarity (ie, surface tension) of the film can be tested with dyne, and the surface tension of a typical corona-treated film should be greater than 40 dynes. After the film has been placed for a week, it must also reach 38 dynes or more because it is lower than 36 dyne film, no adsorption fastness to ink.

Film storage

In general, the film is preferably used within one week after the completion of the blowing. If you need to store it for a long time, you must choose a dry, cool and ventilated warehouse. The maximum length cannot exceed 3 months. In addition, the film deposited for a long time must be tested by using the dyne test before it is used. Otherwise, it can easily cause damage.

Printing process control

Poor printing process is the key to ink discoloration. Generally after the proper ink and solvent are used, there are still three aspects to be noted in the printing process.

The ratio of ink

The proportion of ink should be adjusted according to changes in the environment. Inappropriate solvent ratios can cause ink drying difficulties and surface “sham dry”. Counterfeit stem is that the low-order solvent is not completely volatilized, and the ink surface has been dried and crusted. After printing for a period of time, residual solvent continues to infiltrate, and ink adhesion and decoloration occur.

Printing speed and oven temperature

The printing speed should not be too fast. Generally, it is better to control it at 30m/min. The drying oven temperature is generally 30-40°C. This is to allow the ink solvent to cure and volatilize and enhance the ink adhesion fastness.

Printing environment

The printing shop environment must guarantee a certain humidity (60-70%), require good ventilation, the temperature is better at 25-30 °C, too dry environment will cause the film electrostatic is difficult to release, and too humid environment will make ink Volatilization is difficult and the fastness is reduced.

Afterwards

It is best to place the printed product after one day of cutting, which is conducive to the full curing of the ink and the improvement of the fastness of the ink.

After strict control of the production process, the ink fastness can generally meet the use requirements. However, due to the printing of the film, the ink on the surface of the film is vulnerable to various damages. Water, acid, mechanical, and friction may cause the ink to fade more or less. 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 heat and cold and condensation will occur. During transportation, friction occurs between the bag and the bag due to vibration. In the hands of consumers, some need to heat milk. Such a process requires the ink to have excellent antifreeze, water resistance, heat resistance, and abrasion resistance, and such an ink product is very difficult to produce. So far, none of the inks have fully possessed the above four characteristics.

To completely solve this phenomenon, only the surface printing into India, ink is not on the surface there is no need to worry about color, there are three ways.

The first method, the composite milk film structure is BOPP (outside) / printing / PE (inner), this structure requires special filling equipment, the cost is also high, each bag costs up to 0.10 yuan.

That is, after the original milk film is printed, an additional process is performed to cast a thin, transparent film on the printed surface. It has three major advantages: First, the ink layer is protected by lamination and there is no fear of discoloration. Moreover, the brightness of the ink is very good due to its high transparency. Secondly, due to the film's insulation against the contamination of milk with toxic substances such as ink and residual solvents, the odor is eliminated and the sanitation is greatly enhanced. Finally, because the cover film and some of the sand holes on the original film are also covered, the leakage rate is almost zero. Its only drawback is that its cost is increased by 1,500 yuan per ton compared to ordinary films, but the average cost per bag is only increased by 0.005 yuan. Compared with the films of the first two structures, its advantages are still more obvious.

Reprinted from: Ke Yin Network