How to Solve the Knife Phenomenon in Plastic Gravure Printing

In the plastic gravure printing process, the knife wire phenomenon often occurs. The reason for this is very complicated. It involves platemaking, ink, squeegee and environment, etc. Now it is analyzed and discussed by Anmei Mall for your reference.

1. Plates

The surface finish and hardness of the chrome plating layer on the plate will have an important influence on the production of the knife wire. The pigments used in plastic gravure inks are generally dispersed in the binder in the form of particles. In the printing process, after the ink is filled with the mesh, the excess ink is scraped off by a doctor blade while constantly rubbing the surface of the printing plate. If the hardness of the chrome plating layer on the plate surface is not enough, it can easily be scratched by the coarse-grained ink to produce a knife wire.

On the other hand, if the plate cylinder is badly polished at the time of plate making, there are fine patterns or too deep imprint on the plate. In the printing process, the ink will remain on it and cannot be scraped off by the doctor blade, and it will be transferred to the printing material. It will produce irregular, thin knives. Therefore, special polishing equipment and special polishing sandpaper should be used for polishing.

In addition, the accuracy and installation accuracy of the drum should be strictly controlled to prevent the eccentricity from exceeding the standard, so as to avoid the impact of the squeegee on the high speed printing.

2. Ink aspects

If the fineness of the ink used in the plastic gravure is not enough, the pigment particles are too large or mixed with the particulate impurities, it will lead to the emergence of knife wire. Therefore, a better quality ink should be selected, and its pigment particles should be less than 10 μm. Filtering devices should be added to the inking system, and they should often be inspected to avoid coarse particles of the ink damaging the plate and squeegee. If the viscosity of the ink is too large and the drying is too fast, the relative friction between the plate cylinder and the squeegee may be increased, so that it is difficult for the squeegee to scrape off excess ink on the plate to form a knife wire. Therefore, the viscosity of the ink should be controlled. For a fast drying ink, a slow-drying solvent can be used or the ink can be diluted.

In addition, if the ink is not diluted properly, the use of immiscible solvents will result in the precipitation of pigments in the ink, forming large particles, resulting in the production of knife threads. Therefore, the solvent that matches the gravure ink should be selected. If the antistatic performance of the ink is poor, or the lubrication performance is poor, it will also lead to the occurrence of the knife wire phenomenon, so it should be strictly controlled whether the quality indicators of the test ink are analyzed.

3. Squeegee

The squeegee is an important component in the plastic gravure. The self-grinding squeegee should be made of a material with tight, soft and hard quality. In general, a hard squeegee produces more cuts than a soft squeegee. When the squeegee blade is damaged, a regular left and right ink trace is created on the substrate. Subtle squeegee gaps can be polished with fine sandpaper until it is clean and smooth, and if there is a large gap, it needs to be replaced. Therefore, when installing the squeegee, it must be flat. Carefully check whether the squeegee has a notch before loading the blade. Carefully grind it after mounting. If there is a knife-spinning phenomenon during the printing process, it must be stopped for grinding, and it is strictly prohibited to mix the squeegee oil. In ink.

In addition, during the printing process, the angle and pressure between the squeegee and the printing plate must be mastered to reduce the relative friction between the squeegee and the printing plate. In general, the angle between the doctor blade and the plate cylinder is 70° to 75°, the inclination is upward, and the blade pressure is controlled between 200 and 400 kPa.

4. Environmental aspects

The temperature, humidity, and air cleanliness of the printing shop can all affect the appearance of the knife wire. If the temperature is too high, the solvent will volatilize too quickly and the ink will dry too quickly. If the humidity is too high, the ink will dissolve poorly and the viscosity will increase, making it difficult to wipe the ink. Both of these conditions can easily lead to the appearance of knife threads. Therefore, during the printing process, the temperature should be controlled at 18 to 23°C and the humidity controlled at 50% to 60%. If there is more dust in the air, the plastic film will generate electrostatic adsorption during printing. The dust will be transferred from the film to the printing plate cylinder during printing. When the dust gathers in the ink, it will adhere to the squeegee and produce a knife. wire. For this purpose, static eliminators should be installed and antistatic wire mesh or antistatic plant strips should be suspended above the plastic substrate film. The gravure production environment should be kept as close as possible, install an air filter and determine the dust content, so as to maintain the cleanliness of the air and avoid the occurrence of knife wire.

In short, in the process of plastic gravure printing, there are many reasons for the occurrence of knife wire phenomenon, and often affect each other, should be checked from the plate, ink, scraper and environmental aspects of inspection and analysis, and ruled out one by one.