Common problems and methods in printing

The ink has poor adhesion to the substrate

1.1 Ink Adhesion Mechanism Wetting: Adhesion strength is the result of the interaction of intermolecular forces between the printing substrate film and the ink binder.

Compatibility: The polarity of the binder of the ink is similar to that of the substrate, ensuring good compatibility. Be sure to choose the ink that matches the printed matter.

1.2 Reasons and Countermeasures for Poor Adhesion: Base material factors.

A. The corona treatment on the surface of the base material is insufficient, failing to reach the required tension, or the surface of the treated base material, due to poor storage environment and excessive storage time, has experienced a decline in surface tension and thus failed.

Generally, the surface tension of PP and PE should be ≥ 38mN/m, that of PET ≥ 50mN/m, and that of NY ≥ 52mN/m. The surface tension of each batch of films should be checked before use.

B During the processing of the base material, certain lubricants, antistatic agents, moisture-proof agents, etc. are mostly added. Over time, these additives gradually penetrate from the film to the surface, forming a weak interface layer on the film surface, which affects the adhesion of the ink. Therefore, the film should be used within its validity period and the surface tension should be tested before use. It can be tested with a dyne pen. Substrate factors: The factors that the substrate absorbs moisture. For instance, materials such as PA, PVA, and PT are prone to absorbing moisture and reducing their adhesion. Such substrates should be stored under good conditions (temperature and humidity controlled) and fully preheated before use. The wrong type of ink was used and different types of ink were mixed.

There are many types of inks for gravure printing, including surface printing and interior printing inks. Different materials require different inks. All kinds of specialized inks must be used accordingly; otherwise, the adhesion will not be strong enough. Before production, the operation process instructions should be checked. Ink whitening. The deterioration and whitening of the ink cause a significant decline in adhesion. The ink is not dried sufficiently. Insufficient drying results in a large amount of residual solvents inside. Additionally, a certain drying temperature is extremely beneficial for enhancing the adhesion of some resin binders.

2.Registration error

Registration errors are mostly common defects in printing. The phenomenon is that during the multi-color registration process, each color cannot be completely overlapped and there is a certain deviation.

2.1 Personnel Quality Since color registration errors are objectively present, it is required that employees have extremely strong working abilities. Printing is a meticulous job that requires dedication, attention to detail and a sense of responsibility. When working, one should conduct self-inspections and self-examinations, observe from multiple perspectives, and promptly identify problems.

2.2 Equipment Factors The precision and performance of the entire equipment, the tension control during registration, the heating and air supply system, the coordination of systems such as the impression rubber roller, plate roller, plate shaft, and guide roller, all these factors determine the temperature, pressure, parallelism, elongation, and winding during registration. Moreover, the color registration system itself further limits its precision. The color-matching photoelectric device should be well maintained and operated correctly. For instance, if the light from the halogen light source is weak, the optical fiber or halogen bulb should be replaced. The distance between the reflector and the scanning head should be adjusted correctly, generally around 8mm. If the increase of the plate roller is inappropriate, it is easy to cause inaccurate longitudinal color registration, deformation of the rubber of the impression roller and insufficient elasticity. Keep the pressure roller in good condition.

2.3 The tension setting of the base material in the operation process is unreasonable. If the tension is too small, the base material will be loose, and the color registration in both the longitudinal and transverse directions will be inaccurate. If the tension is too high, the base material will contract laterally and the longitudinal color registration will be unstable. Adjust the tension accordingly. The temperature is not appropriate, especially when the oven temperature is too high and the cooling roller temperature is too high, causing the film to deform. For films that are not very flat, the temperature of the preheating box should be appropriately increased. The imbalance of the intake and exhaust air volumes in the oven causes the film to shake inside the oven. An anemometer should be used to measure the intake and exhaust air volumes and control them. If the plate roller is not installed properly, such as eccentricity, the ink on the plug must be wiped clean before installing the plate roller.

3.1 Causes of scraper line marks ① Line marks without ink. The main reason is that the ink is mixed with hard inorganic impurities, such as sand, etc. These hard particles get stuck at the edge of the scraper, causing line marks. Sometimes, such scraper line marks are also dotted lines. The appearance of such scraper line marks can easily damage the printing and the scraper.