The control of the gravure printing process is relatively complicated. Even if only some small process details are not handled properly, it may have a serious impact on the printing quality. Therefore, in order to make good use of the gravure printing process in flexible packaging printing to obtain high-quality printing effects, some details in the gravure printing process cannot be ignored. Here, we will discuss with the reader the handling of some details that are worth paying attention to in the gravure of flexible packaging.
1. Arrangement of printing color sequence
In flexible packaging gravure printing, the arrangement of printing color sequence is critical, and generally follows the following rules.
(1) The printing color sequence of the Indian ink is arranged according to the rules from dark to light, generally black, cyan, magenta, yellow, and white.
(2) The printing color sequence of surface printing inks is arranged according to the rules from light to dark, generally white, yellow, magenta, cyan and black.
(3) Spot color inks are generally arranged before white inks, after yellow inks, or after black or tri-primary inks, but they are generally not arranged between cyan, magenta, and yellow inks. Generally, the spot color inks in one printing operation should not exceed 3 kinds.
It should be noted that for patterns with more strict requirements for overprinting, it is not advisable to add any spot colors between the cyan, magenta, and yellow screen adjustment colors. For series versions, sometimes to ensure that the ink tank is not replaced during the printing process The printing color sequence must be consistent. In addition, because the surface printing ink is more vivid than the inside printing ink, it is customary to use surface printing ink when proofing, but in some special cases, such as customers providing color samples, the need to use anti-counterfeiting plates, the need to print gold ink or anti-counterfeiting In the case of special inks such as ink, attention should also be paid to the reasonable arrangement and adjustment of the printing color sequence of surface printing inks.
Second, the expansion and contraction of adjacent colors
In gravure production, in order to compensate for the leakage between adjacent different colors caused by inaccurate overprinting, it is usually necessary to expand and shrink the printed part, also known as trapping or trapping.
When the two colors of ink are connected, overlapped, and tangent, generally need to do expansion and contraction, sometimes mutual expansion, in special cases, reverse expansion and contraction, or even in the two adjacent colors By increasing the gap to compensate for the error of overprinting, so as to make the printed matter more beautiful. The principles generally followed for expansion and contraction are: expansion without expansion, expansion with shallow expansion without expansion, and expansion with flat network expansion without expansion.
The amount of expansion and contraction depends on the characteristics of the printing material, the overprint accuracy of the printing machine and the printing method. In general, the shrinkage of offset printing is smaller, the shrinkage of gravure printing and flexo printing is larger, generally 0.2 ~ 0.3mm (according to specific requirements such as printing accuracy), the expansion and shrinkage of similar colors is usually 0.3 mm, the amount of expansion and contraction of the opposite color is usually 0.2mm, and also requires that the printed pattern after expansion and contraction is not deformed.
3. Determination of surface tension
In actual production, the surface of the printed film should be corona treated to ensure that it has a reasonable surface tension. The method for measuring the surface tension of the printed film is generally: use a cotton swab dipped in dyne solution corresponding to the measured surface tension of the film, and apply a liquid film with a length of about 10 mm on the surface of the film. If the surface of the liquid film does not shrink or the shrinkage is small within 5 seconds, the surface tension of the film can be considered qualified; if the liquid film is completely broken or shrinks into a strip shape with a length of less than 8 mm, the film can be considered The surface tension is unacceptable.
4. Determination of the unwinding direction
During the compounding process, you should pay attention to check whether the compound material roll used is provided by the same designated manufacturer, whether the special material compound material roll has special requirements, and in addition, it is necessary to judge whether the direction of the special film (such as Yin and Yang film) is correct.
The principle of determining the unwinding direction is: take the first stroke of the text or the first letter of the word as the first out, take the last stroke of the text or the last letter of the word as the last out; take the top of the pattern or the first out of the left For the first out, the first out under the pattern or on the right is the last out.
V. Determination of the viscosity of ink and adhesive
To get a good printing effect, pay attention to the viscosity of ink and adhesive during the production process. The measuring method is: after the Zahn cup is filled with the ink or adhesive to be tested, a stopwatch is used to measure the time for the ink or adhesive to flow out of the small hole at the bottom of the cup, so as to determine the viscosity of the ink and the adhesive.
In addition, it is necessary to understand and be familiar with the manufacturer of the selected ink, the scope of application and the solvent ratio to ensure the accuracy of the measurement data.
Sixth, the production of control signs
In order to ensure the accuracy of printing film overprinting and facilitate the smooth progress of subsequent bag making and slitting, it is usually necessary to make some marks for position control and judgment on the printing plate. The following issues should be noted during the production of the control logo.
(1) Production of cross registration marks: Cross registration marks are generally used in conjunction with bar marks. The numbers indicate the printing color sequence and are produced on each color printing plate.
(2) Production of mark line (light spot mark): Mark line can play the role of tracking and slitting in slitting and bag making. Generally, the darkest color is adopted, the width is greater than 2mm, less than 10mm, and the length is generally greater than 5mm.
(3) Production of detection line: The detection line is mainly used to ensure that the film does not deviate laterally during slitting and bag making, and the color is generally dark.
Seven, the main points of printing quality control
The main points of printing quality control mainly include color difference, color deviation, ink fastness, defect detection, etc. The basic requirements are as follows.
(1) Color difference: the spot color part ΔE≤5, ΔH≤1.5; the light-colored part of the net is ΔE≤5, ΔH≤2.5.
(2) Chromatic deviation: the main pattern chromatic deviation of the bidirectional stretch film is required to be ≤0.20mm, the secondary pattern chromatic deviation ≤0.35mm; the non-bidirectional stretch film is to be chromatic deviation ≤0.30mm, the secondary pattern chromatic deviation ≤0.60mm.
(3) Ink fastness: usually use 24mm wide and 250px long transparent tape to stick on the printing surface, tear the tape, and observe the ink marks left on the printing surface to judge.
In addition, stroboscopic lights (synchronized lights) can also be used in printing production to observe whether abnormal conditions such as knife lines, stains, missing points, and color registration occur.
Thousands of miles of embankment, collapsed in the ant cave, the gravure process is often caused by ignoring these small details and caused large quality problems. Therefore, special attention must be paid to these subtleties in gravure production to ensure high quality. D.
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