Analysis of common printing problems in plastic bag printing machines

1. Adhesion fastness between printing ink and plastic film

The adhesion fastness of the ink used for printing is the result of the interaction between the plastic film and the ink carrier. Therefore, the closer the polarity of the ink to the film, the better the compatibility, and the choice of ink must be related to the type of film. Match. The factors that lead to poor adhesion are divided into plastic film and ink factors, as follows:

Plastic film factor:

1. The corona treatment degree of the surface of the film is not enough, the tension is not reached, or the surface of the substrate after the treatment is too long, the storage time is too long, and the surface tension is attenuated and fails. Generally, the surface tension of PP and PE is m38mN/m, PET≧50mN/m, and NY≧52mN/m. The surface tension of each batch of film should be checked before use.

2. Most of the film is added with certain lubricants, antistatic agents, moisture-proofing agents, etc. These additives slowly penetrate into the surface from the film over time, forming a weak interfacial layer on the surface of the film, affecting the ink attachment. Focus on. Therefore, the film should be used within the validity period and the surface tension should be measured before use. A dyne pen test can be used.

3. If PA, PVA, PT and other materials are easy to absorb moisture and reduce adhesion, such substrates should be stored under good conditions (temperature control and humidity control) and fully preheated before use.

Ink factor:

1. The wrong type of ink is used, and different types of inks are mixed. There are many types of gravure inks, such as surface printing and printing inks. Different materials correspond to different inks. All kinds of special inks must be used correspondingly. Otherwise, the adhesion fastness is not enough. The operation process instructions should be checked before production.

2. The ink is degraded and whitened to make the adhesion extremely lower.

3. Ink drying is insufficient, and there are many residual solvents in the interior. In addition, a certain drying temperature is extremely advantageous for improving the adhesion of some resin binders.

2. Overprinting error in the printing process

Overprinting error is a common defect in printing. In the multi-color overprinting process, each color cannot be completely overlapped, and there are certain deviations. The main reasons are as follows:

Personnel quality: Because the color registration error is objective, it requires employees to have a strong ability to work. Printing is a meticulous work, and must be done with care, care, and responsibility. When doing work, do a good job of self-inspection, self-examination, multi-party observation, and timely discovery of problems.

Equipment factors: 1. Accuracy and performance of the whole equipment, tension control of overprinting, heating air supply system, embossing rubber roller, plate roller, plate shaft, guide roller and other systems. These conditions determine the temperature and pressure during overprinting. Parallelism, elongation, take-up, etc., and the color registration system itself limits its accuracy. 2. The color photoelectric device should be well maintained and operated correctly. For example, if the halogen light source is weak, the fiber or halogen bulb should be replaced, and the distance between the reflector and the scanning head should be adjusted correctly, generally about 8 mm. 3. The increment of the plate roller is not suitable, and it is easy to produce longitudinal color misregistration. The embossing roller rubber is deformed and the elasticity is insufficient. Keep the pressure roller in good condition

Operating process factors:

1. The substrate tension setting is unreasonable, the tension is too small, the substrate is slack, and the vertical and horizontal color registration are not accurate; the tension is too large, the substrate is lateral, the shrinkage and the longitudinal color registration are unstable, and accordingly the tension is adjusted accordingly.

2. The temperature is not suitable, especially the oven temperature is too high, the temperature of the chill roll is too high to deform the film, and the temperature of the preheating box should be appropriately increased for the film which is not flat.

3. The amount of air in and out of the oven is unbalanced, causing the film to shake in the oven, and the anemometer is used to measure the amount of air in and out and to control it.

4. The installation of the plate roller is not suitable. If the eccentricity is generated, the ink on the plug must be cleaned before the plate roller is installed.

3. The cause of the scraper line marks, the line marks without ink and the line marks with ink

Ink-free line marks

Mainly the ink is mixed with hard inorganic impurities, such as sand, which are embedded in the edge of the blade to cause line marks. Sometimes such scraper line marks are also dashed, and the appearance of such scraper line marks can easily damage the print and the doctor blade.

Ink line mark

1. A thick continuous line of ink with a doctor blade. This is mainly due to the fact that some of the thicker soft particles or impurities are caused by the squeegee or the blade edge is damaged. This is the simplest and most common scraper line mark.

2. Meteor-like scraper line marks. Such scraper line marks are generally present in the screen dots of the printing plate which are very shallow or unengraved, and are present at a time when the product quality is extremely harmful. The causes of such scraper line marks are more complicated. Over the years, people have been explained from ink, plate making, printing environment, etc., but so far, no one is perfect. Better is the "active particle hypothesis."

3. Filament-like continuous scraper line marks. Such scraper lines are small, like hair strands, often appear in the printing process, and are difficult to observe on the printing machine. When a large number of products have been printed at the time of discovery, the quality is extremely harmful. Such scraper line marks can be divided into two types, one is soft and can be wiped off with a bamboo knife, mainly caused by impurities; the other is that the hair is initially filamentous, and then slowly distributed in a strip shape. Can not be wiped off with a bamboo knife, the reason for its production is very complicated, involving ink, plate making, ambient temperature, humidity, scraper hardness and other aspects, need specific analysis of specific issues.