Gravure printing technology: to print good products, only need to control seven points

        Gravure printing process control is relatively complicated, even if some small process details are not handled properly, it may have a serious impact on printing quality. Therefore, in order to make good use of the gravure printing process in flexible packaging printing and obtain high-quality printing effects, some details in the gravure printing process cannot be ignored. Here, discuss with netizens how to deal with some details that deserve attention in gravure printing for flexible packaging.

1. Arrangement of printing color sequence

        In flexible packaging gravure printing, the arrangement of printing color sequence is very important, and generally follows the following rules.

(1) The printing color sequence of the inner printing ink is arranged according to the rule from dark to light, generally black, cyan, magenta, yellow, and white.

(2) The printing color sequence of surface printing ink 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 and after yellow inks, and can also be arranged after black or three primary color inks, but generally not between cyan, magenta, and yellow inks. Generally, there should be no more than 3 spot color inks in one printing job.

        It should be noted that for patterns with strict overprinting requirements, it is not appropriate to add any spot colors between the three mesh colors of cyan, magenta, and yellow. For series versions, sometimes to ensure that the ink tank is not replaced during the printing process, It is required that the printing color sequence must be consistent. In addition, because the surface printing ink is more vivid than the inner printing ink, it is customary to use surface printing ink when proofing. When using special inks such as printing inks, attention should also be paid to the reasonable arrangement and adjustment of the printing color sequence of surface printing inks.

2. Scaling of adjacent colors

        In gravure production, in order to make up for the gaps between adjacent different colors caused by inaccurate overprinting, it is usually necessary to shrink the printed part, also known as trapping or trapping.

        When the inks of two colors are connected, overlapped, or tangent, expansion and contraction are generally required, and sometimes mutual expansion is required. In special cases, reverse expansion and contraction are required, and even between two adjacent colors. Make up for the overprinting error by increasing the gap to make the printed matter more beautiful. The general principle to be followed in the expansion and contraction processing is: expand the bottom but not the top, expand the shallow but not deep, expand the flat network but not the solid field.

        The amount of expansion and contraction depends on the characteristics of the printing material, the overprinting accuracy of the printing machine, and the printing method. Generally, the expansion and shrinkage of offset printing is smaller, and the expansion and contraction of gravure printing and flexo printing are larger, generally 0.2 ~ 0.3mm (depending on the specific requirements such as printing accuracy), and the expansion and contraction of similar colors is usually 0.3 mm, the expansion and shrinkage of the opposite color is usually 0.2mm, and it is also required that the printed pattern after expansion should not be deformed.

3. Determination of Surface Tension

        In actual production, the surface of the printed film must be corona treated to ensure that it has a reasonable surface tension. The method of measuring the surface tension of the printed film is generally as follows: use a cotton swab to dip the dyne solution corresponding to the surface tension of the film to be measured, and apply a liquid film with a length of about 10mm on the surface of the film. If within 5 seconds, the surface of the liquid film film does not shrink or the shrinkage is small, the surface tension of the film can be considered qualified; if the liquid film is completely broken or shrinks into a strip with a length of less than 8mm, the film can be considered to be stable. Surface tension failed.

4. Determination of the unwinding direction

        During the compounding process, care should be taken to check whether the composite material rolls used are provided by the same designated manufacturer, whether there are special requirements for composite material rolls of special materials, and whether the winding direction of special films (such as yin and yang films) is correct.

        The principle of determining the direction of the roll is: the first stroke of the text or the first letter of the word is the first, the last stroke of the text or the last letter of the word is the last; the top of the pattern or the left is the first It is the head out, and the bottom of the pattern or the one on the right is the last out.

5. Determination of ink and adhesive viscosity

        In order to obtain a good printing effect, attention should also be paid to measuring the viscosity of ink and adhesive during the production process. The measuring method is as follows: after the Zahn cup is filled with the ink or adhesive to be tested, use a stopwatch to measure the time for the ink or adhesive to flow out from the small hole at the bottom of the cup, so as to measure the viscosity of the ink and adhesive.

        In addition, it is necessary to understand and be familiar with the manufacturer, scope of application and solvent ratio of the selected ink to ensure the accuracy of the measurement data.

6. Production of control signs

        In order to ensure accurate overprinting of the printing film and to facilitate the smooth progress of subsequent bag making and slitting work, it is usually necessary to make some marks for position control and judgment on the printing plate. The following issues should be paid attention to in the process of making control signs.

(1) Production of cross registration marks: cross registration marks are generally used together with bar marks, and the numbers indicate the printing color sequence, which are made on the printing plates of each color.

(2) Production of mark lines (spot marks): Mark lines can play a role in tracking and cutting in slitting bag making. Generally, the darkest color is used, the width is greater than 2mm and 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.

7. Key points of printing quality control

        The main points of printing quality control mainly include color difference, color registration deviation, ink fastness, defect detection, etc. The basic requirements are as follows.

(1) Color difference: ΔE≤5, ΔH≤1.5 for the spot color part; ΔE≤5, ΔH≤2.5 for the light-colored part of the hanging net.

(2) Color registration deviation: The color registration deviation of the main pattern of the biaxially stretched film is required to be ≤0.20mm, and the color registration deviation of the secondary pattern is ≤0.35mm; the color registration deviation of the main pattern of the non-biaxially stretched film is ≤0.30mm, and the color registration deviation of the secondary pattern ≤0.60mm.

(3) Ink fastness: Usually, a transparent tape with a width of 24mm and a length of 250px is pasted on the printing surface, the tape is torn off, and the ink imprint left on the printing surface is observed to judge.

        In addition, in printing production, strobe lights (synchronous lights) can also be used to observe whether abnormal conditions such as knife lines, stains, missing points, and color registration have occurred.

        The embankment of a thousand miles collapsed in an ant's nest. During the gravure printing process, these small details are often ignored and cause major quality problems. Therefore, special attention must be paid to these subtleties in gravure printing production to ensure high-quality printing. D.