Plastic film flexo process quality control

According to the ISO9002 quality assurance system, several process quality control points must be set throughout the flexographic printing process. We must formulate appropriate countermeasures and take effective measures in light of the weaknesses in the process.

This article intends to discuss some viewpoints and practices on how to set process quality control points in the plastic film flexographic printing process.

1. Printing Substrate For plastic films, there are two major categories of PE and PP in the domestic use of flexographic printing processes.

The PP film is divided into two types: CPP and OPP. The control of the CPP film is exactly the same as that of the gravure printing process. The OPP film is used on the flexo printing more often than the label film, and the OPP label film is controlled before printing, except for the same concave. The same film flatness, vertical and horizontal tensile strength, surface friction coefficient, and wet tension on the surface of the film are the same as those of the printing process, but the whiteness of the film is also emphasized. Different OPP label films with different brightness have different effects under the same printing process parameters. PE film is the leading product of flexographic printing materials. Two parameters are emphasized here. The first is the problem of the crystal point of the film. Crystal point, also called dead point, often encountered in the printing process due to the freezing point on the PE film crushed the flexographic plate, so the control of the film dead point is a very important part, we must develop a acceptance criteria, That is, the size of a single dead point diameter, the number of dead spots per unit area. The other is the printability of plastic films. For a long period of time, we have only specified wet tensions of 38 to 42 dyne. However, we have encountered production speeds of plastic films of only 150 to 160 m/min in our production practice. When we go upward, we will find that in the field printing, The phenomenon of pinholes is getting worse and worse. Only with different films, the production speed can be between 230 and 250 m/min. Therefore, the printing of the film should also be one of the parameters that we must control.

Second, ink ink quality control, in addition to the common fineness, hue, viscosity and other parameters, the author thinks that should emphasize the color concentration. In addition to the general products, many ink companies also provide base inks, or high-concentration inks. If the base ink is added too much in the production, the resin in the ink is less, and the adhesion of the ink to the plastic film is inevitable. It will be reduced. Therefore, the basic ink is not as good as possible. It is also necessary to control one degree. For the verification method, we feel that using a density meter to measure the density in the field is simple and effective. It should also be noted that the resins used in the selection of flexo inks, polyamides, nitrocellulose, and PVB have their own strengths, which is also not negligible.

Third, the production of manuscripts often encountered a puzzling problem. That is, the plate produced by the platemaking company according to the design in the MO was always unsatisfactory and sometimes had to be reworked. After repeated analysis found the reason. Is the use of heat-shrinkable PE film, the transparency is very bad, gray, the product is printed on the above, the whole feeling is not clear. White is the color with the highest lightness. When printing, two layers of white ink are used to make up for the lack of film. For the MOs transferred by various printing companies, the plate-making companies must make certain adjustments. The compensation is mainly based on the adjustment of the dot expansion compensation curve and the printing plate extension caused by the radius of curvature of the printing plate. For printing companies, they can propose to the plate-making company how to increase or decrease the number of printing plate lines, buckles, compensation for shrinkage of PE film, changes in gradient curves, and large-area field separation of small and medium-sized screens. The process quality control of the production process is to understand the design in the MO. We handed it to the plate-making company, not just a MO, but must be a complete set of instructions. The role of the technical department should not be just a setter, but should be a command.

Fourth, the general parameters of plate making process: back exposure time, main exposure time, cleaning time, post-processing, drying and so on. These parameters are based on specific devices. Different equipment, the gap between the parameters is very large. To economically check whether the equipment is in good condition, check the lamp radiation energy, check the lamp filament heat preservation circuit, and keep the equipment parameters in the normal range, which is the key to the quality control of the printing process. In addition, the cleaning section emphasizes the specific gravity of the cleaning solution. It is also worth emphasizing that the printing plates of plastics attach great importance to the hardness of the plates.

Fifth, the plate loading machine, the most important quality standard, is the loading plate to be accurate, the comparison point of the loading plate is the cross line, the front and rear cross lines are aligned, the printing plate is basically affixed, but the printing plate circumference In the direction, our strategy is to make two fine 0.1mm thick wires measuring 30mm long from the top and bottom of the printing plate on both sides of the printing plate. When the printing plate is wrapped up, it requires a smooth connection between the top and bottom of the longitudinal strips and the outside of the strips. Straight line connection, no skew, thus ensuring the verticality.

Six, printing in the ISO9002 quality assurance system, the control of the printing process is a top priority. One is the ink control. According to the process technology document configuration each color group position and the corresponding anilox roller, the ink must pay attention to the deployment to the normal production required viscosity. We emphasize that the viscosity of the ink is measured every 15 to 20 minutes and adjusted in time with a solvent. The emphasis here is on the use of mixed solvents. The printing ink confirmed that some companies had made three confirmation items such as the color density standard, the color light standard, and the acceptance standard, and required the ink color in the repeated production to be within three standard samples. The national standard makes ΔE ≤ 5.0 for the same batch of chromatic aberration of the same color, but there is no stipulation on the allowable value of the color difference for different batches. The author in the production practice in different batches of color control in the ΔE ≤ 3.0 range; the second is the control of the layout accuracy. Whether the tension control is normal or not has a great influence on the layout. It is also one of the keys that the adjustment of the amount of hot air around the central embossing roller is correct. The third is the control of printing pressure. The pressure of the flexographic printing press refers to the pressure of the anilox roll to the platen roller and the pressure of the platen roll to the embossing roll. A good length of the flexographic printing press, the most important criterion is how to grasp this pressure.

7. The basis of inspection and inspection is the standard. Should adopt the national standard, that is, GB/T17497-1998 "flexible version of decorative prints." If conditions permit, they should establish their own corporate standards as soon as possible. In addition to conventional appearance, color, pattern, and text, product inspections should emphasize the following points. One is the bar code, and the other is the combination of the ink layer. The third is the total amount of solvent residues.