Label printing covers several major printing methods. In the plate-making stage, different printing methods are selected according to the properties of different products. For different printing methods, the plate-making process is also different.
1. The original design suitable for flexible printing should have the following characteristics: more colors, but less overprinting; no need to reproduce particularly small details; the screen line is not too high, but can achieve color printing effects; can be processed online for packaging.
2. Meet the needs of plate-making, clear graphics and accurate size specifications; use frosted film, require the four corners to have consistent density; use medicine film positive characters; measure with a transmission density meter, the white position density is below 0.06; the black position density is above 3.5.
3. Exposure includes back exposure and main exposure.
Ultraviolet light passes through the support film to solidify the photosensitive adhesive layer to establish a stable base, and can also control the depth of plate washing and strengthen the bonding between the support film and the photosensitive resin layer. The back exposure time is determined according to the required base thickness.
Ultraviolet light passes through the vacuum film and the transparent part of the film to polymerize and solidify the photosensitive part of the plate. The length of the main exposure time is determined by the plate type and the strength of the light source). If the exposure time is too short, the image slope will be too straight, the lines will be bent, and the small characters and dots will be washed away. On the contrary, if the exposure time is too long, the plate will be covered and the handwriting will be blurred. If there are large and small characters, thick and thin lines on the same printing plate, you can use a black film to cover them and expose them separately according to the situation. The small parts will not be lost due to washing to ensure the quality of the printing plate.
4. Washing.
Wash and dissolve the unexposed parts to retain the photopolymerized relief. The length of the plate washing time is determined by the thickness of the printing plate and the depth of the print. If the plate washing time is too short, the unexposed resin will be left on the plate and affect the depth of the plate making. If the plate washing time is too long, the plate will swell, causing the fine parts to deform or fall off.
5. Drying.
Remove the plate washing solvent to restore the printing plate to its original size and thickness. The baking temperature is between 50-60℃. The baking time is determined by the thickness of the plate and the length of the plate washing time. Generally, it takes two hours for thick plates and one hour for thin plates. If the baking time is too long or the baking temperature is too high, the printing plate will become brittle and affect the printing life. If the baking temperature is too low, the drying time will be prolonged. If the baking time is too short, the printing plate will be rotten during printing.
6. Post-processing.
That is, de-sticking and post-exposure. Make the photosensitive resin completely harden (polymerize) to reach the required hardness index, and eliminate the stickiness of the printing plate to facilitate the transfer of ink. The post-processing time is obtained by testing, and the purpose is to prevent cracking and adhesion.
