Reasons and solutions for the fading problem of pl

Reasons and solutions for the printing discoloration of dairy plastic packaging

a good packaging should be matched by good printing, so as to play a good role in packaging and arouse people's desire to buy. Products with vague printing and lack of words and patterns (fading) will not be recognized by consumers

ultra high strength liquid film is made of high-pressure polyethylene (LDPE) and linear polyethylene (LLDPE) combined with a certain proportion of milky masterbatch through film blowing equipment. Milk film products on the market now include milky white film, black-and-white film, pearlescent film, high barrier film, etc. printing ink generally adopts polyamide ink, which is a kind of ink that is more compatible with polyethylene among many types of inks, In order to prevent printing from fading, each production process of liquid film must be strictly controlled and tested

I. precautions for film production

1. The raw materials used before film blowing must be new materials, and recycled materials must not be used

the solution of recycled material has been destroyed, and when mixed with new material for film blowing, the heat sealing performance of the film will be reduced, resulting in sand holes, which will increase the number of broken packages and reduce the smoothness of the film surface. In order to improve the smoothness of the film surface, some manufacturers add an excessive amount of oleic amide smoothing agent to the film blowing raw materials, which will reduce the surface tension of the film and naturally make it difficult to guarantee the binding fastness of the ink

2. Temperature control of film blowing

the optimal extrusion temperature of raw materials for film blowing is 160 ~ 180 ℃. Excessive temperature will cause the content of raw materials to exude and affect the surface tension of the film; If the temperature is too low, the film processing will be unstable and the output will be reduced

3. Corona treatment of film surface

the principle of corona treatment is that the film passes between two electrodes with high voltage. The high voltage ionizes the air between the electrodes and generates electron flow between the electrodes. Electrons form small pits on the surface of the film that cannot be seen by the naked eye, forming polarity on the surface of the film, which is convenient for the adsorption of printing ink

the surface polarity (i.e. surface tension) of the film can be tested by dyne solution. Generally, the surface tension of the film just after corona treatment should reach more than 40 dyne. After the film is placed for a week, it must also reach more than 38 dyne. The film less than 36 dyne has no adsorption fastness to ink

4. Storage of film

generally speaking, the film is best used within a week after blowing. It needs to be stored for a long time. It must also be placed in a dry, cool and ventilated warehouse for a maximum of 3 months. The films stacked for a long time must be tested with dyne solution before use, and can be used only after passing the test, otherwise it is easy to cause losses

II. Some precautions in the printing process

the poor printing process is the key to the ink fading. Generally, after using the applicable surface printing ink and appropriate solvent, the printing still needs to pay attention to the following points:

1. The proportion of ink must change according to the change of environment and climate

improper solvent ratio will cause ink drying difficulties and surface "false drying". False drying refers to that the solvent with high volatility in the ink solvent ratio has volatilized, and the solvent with low volatility has not completely volatilized, and the ink surface has been dried and crusted. After being placed for a period of time after printing, the residual solvent continues to penetrate, resulting in ink adhesion and decolorization

2. Printing speed and drying oven temperature

the printing speed should not be too fast. It is generally better to control it at 30 meters per minute, and the temperature of the drying oven is generally 30 ~ 40 ℃. This is to allow the ink solvent to complete curing and volatilization, and strengthen the ink adhesion fastness

3. The environment of the printing workshop

the environment of the printing workshop must ensure a certain humidity (preferably between 60% and 70%), and it is required to have good ventilation. The temperature is preferably 25 ~ 30 ℃. Too dry environment will cause film static electricity to be difficult to release, and too wet environment will make it difficult for oil and ink to volatilize, and the fastness will be reduced

third, the next process

the finished printing product should be placed for one day before cutting, which is conducive to the full curing of the ink and the improvement of the adhesion fastness of the ink

the zigzag testing machine of Jinan new era Gold Testing Instrument Co., Ltd. is equipped with the operation manual. After strictly controlled testing of each production process, the ink adhesion test box is able to complete the experiment according to the preset curve, and the degree of progress can generally meet the use requirements. However, because the printing of milk film is surface printing, that is, the ink on the surface of the film is vulnerable to various damages, such as water, acid, machinery Friction will more or less cause the ink to fade. Moreover, after the filling of milk is completed, it generally needs to be put into a 4 ℃ cold storage room for refrigeration. After the milk is taken out of the cold storage room, the film surface will produce condensation due to the alternation of cold and heat. During transportation, the friction between bags will be caused by vibration. When it comes to the hands of consumers, some need to heat and drink the milk. This is a 3 The process of adjusting the mirror requires the ink to have very good freeze resistance, water resistance, heat resistance and abrasion resistance at the same time, and such ink products are very difficult in production. So far, no ink can completely have the above four characteristics

to completely solve this phenomenon, we can only change the surface printing to the inside printing, and there is no need to worry about fading if the ink is not on the surface. There are three methods: the first is that the composite milk film structure is BOPP outer/printing/PE inner. Such a structure requires special filling equipment, and the cost is also high. The cost of each bag should reach about 0.10 yuan/bag (see Figure 1), and the second film structure is PE/printing/PE, Ordinary filling equipment with such a structure can also fill, but the cost is still high. Each bag needs about 0.08 yuan/piece (see Figure 2). The third method is to cast PE/printing/PE structural film (see Figure 3), that is, after the printing of raw milk film is completed, another process is added, and a layer of transparent thin film is salivated on the printing surface. This is the product structure specially developed by our company according to the actual use of customers. Its advantages are: 1. The ink layer is protected by lamination, and there is no worry of color fading. Moreover, due to the high transparency of lamination, the ink brightness is very good; 2. Because the lamination isolates the pollution of toxic substances such as ink and residual solvent on milk, it eliminates the peculiar smell and greatly enhances the hygiene; 3. Due to the coating, some sand holes on the original film are also covered, and the leakage rate is almost zero. Its only disadvantage is that the cost is 1500 yuan/ton higher than that of ordinary film, but the cost of each bag is only 0.005 yuan higher. Compared with the films of the first two structures, its advantages are obvious

if the milk manufacturer pays more attention to the problem of ink fading, it is more feasible to use composite film packaging than QS certified recommended products