Flexo printing starts from design and from printing(two)
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Of course, the level of plate making is also a very important part. Let us first understand the steps of plate making:
First, back exposure
- Do not vacuum, directly use UV light to illuminate the back of the plate for exposure
- Makes the plate more sensitive to UV light by consuming oxygen free radicals, reducing main exposure time
- Make the shoulders sharper
- Establish substrate thickness to limit plate depth
Second, positive exposure
Through the photopolymerization reaction, a relief is formed under the transparent part of the negative film.
Third, wash the plate
At the time of washing, the partially polymerized resin is melted by the washing solvent and is washed away by the brush scrubbing action.
Fourth, baking
During the washing process, the syrup in the bleed plate is volatilized
Five, to stick
Full exposure of the plate with UVC light to remove the surface tack of the plate
Six, post processing
Post-exposure refers to the full exposure of the dried plate, no need for film and vacuum
Plates of the same thickness have different hardnesses, some are suitable for on-site printing, some are suitable for dot printing, some are suitable for ink printing, and some are suitable for uv ink printing, so it is necessary to select the correct hardness of the printing plate according to customer needs. The thickness of the plate is 7mm, 3.94mm, 2.84mm, 2.54mm, 2.28mm, 1.7mm, 1.143mm, which are for carton printing, labeling, film printing. Carton-printed plates have a normal hardness of 33-42 (Shore), and labels and film-printed plates have a normal hardness of 56-70 (Shore).
In general, softer printing plates have better ink transfer performance and less pinhole appearance. Printed characters and dots are suitable for harder printing plates, so that the printed dots are less expanded and the writing strokes are clear.
The cut plate is placed on the exposure machine for back exposure to produce a substrate for the plate. If the back exposure time is too long, the relief becomes shallow, and such a printing plate is liable to block ink. If the back exposure time is too short, the relief is too high, and the graphic is easily washed out when the plate is washed, and the dot spread is large during printing, and the printing durability is not high.
Under normal circumstances, we advocate that the ratio of the base and the relief is controlled within the range of 6/4, 5/5 of the plate thickness.
The backing exposed flexo is peeled off the protective layer, and the film surface of the matte film is attached to the surface. In order to prevent residual air between the film and the plate during the plate making process, a special vacuum strip is pressed on the edge of the film and the plate (the vacuum bar has a horizontal and vertical groove for the air to be exported), the vacuum switch is activated, and the cleaning is performed. The vacuum film starts to perform positive exposure when the vacuum pressure is greater than 0.8 Bar.
Positive exposure is an important part of the plate making process. The exposure time required for different patterns is different. The exposure time is too long, which will make the relief slope become larger, resulting in shallower stroke space of the reverse white text, affecting the printing effect; exposure time is not enough , small dots, small text due to insufficient curing, was washed off during the washing.
After exposure, the plate is washed in the plate washer, the uncured resin is washed off, and the plate washing time also needs a certain time. If it is too long, the image will be washed off; if the time is too short, then The base cleaning is not smooth enough, the resin cleaning between the reliefs is not clean, the depth is not enough, and the printing effect is affected.
After the rinsed version, the syrup will penetrate into the plate during the cleaning process. At this time, the plate is expanded and the thickness is thickened. It needs to be placed in a baking machine for drying. Normally, it is dried at 60 degrees. -2 hours. The medicinal solution infiltrated into the plate is volatilized to restore the initial thickness of the plate. Too long baking time will affect the life of the printing plate. The baking time is too short. Because the syrup does not completely evaporate, the printing plate will appear uneven in thickness of the same plate.
The dried printing plate needs to be debonded. If the debonding time is too long, the printing performance of the printing plate will be affected, and the surface of the printing plate will be cracked; the debonding time is not enough, the surface of the printing plate is sticky, which affects the ink transfer and the service life.
After the final post-treatment (ie, post-exposure), the exposure ensures that the resin that has not completely undergone the cross-linking reaction is completed, so as to ensure the hardness of the printing plate and improve the printing durability of the printing plate. If the exposure time is too long, it will also crack the plate.
Of course, it is necessary to test according to different plate making equipment, exposure energy, plate type and other practical factors, in order to obtain the best plate making data.
After the version is completed, quality inspection is required. We need to check whether the surface of the printing plate is clean, whether the base is clean, whether the relief ratio is correct, whether the outlet is dropped, whether there are holes in the field, whether the characters and lines are complete.
At present, the plate making can be 175 lines 1%-97%, but we generally do not recommend customers to do too high number of lines. On the one hand, the number of plates to be plated needs to be determined according to the anilox roller configured by the printing factory. From the author's experience. It is said that the difference is within the twenty-line, usually there is not much visual difference, but there is a big difference in the printing durability of the printing plate. For example, the printing durability of the printing plate of 175# will be better than that of the printing plate of 150#. 40% less; it is higher in terms of cost. On the other hand, from the perspective of the printed yield, the 150# dot-blocking ink will have less image than the 175-line plugging ink, so the invisible yield will be higher.