7 details of gravure printing on packaging bags; one mistake can ruin it!

Gravure printing process control is relatively complex. Even minor process details, if handled improperly, can severely affect print quality. Therefore, to ensure that packaging bag printing can effectively utilize gravure printing and achieve high-quality printing results, some details in the gravure process cannot be overlooked. Here, Shunxingyuan Packaging, a professional flexible packaging manufacturer, has specially compiled its own gravure detail handling during production, showing you how high-quality and exquisite packaging bags are printed.

I. Arrangement of Printing Color Sequence

In gravure printing of packaging bags, the arrangement of the printing color sequence is crucial, generally following the rules below.

1. The printing color sequence of reverse printing inks is arranged from dark to light, generally black, cyan, magenta, yellow, and white.

2. The printing color sequence of surface printing inks is arranged from light to dark, generally white, yellow, magenta, cyan, and black.

3. Special color inks are generally arranged before white ink and after yellow ink. They can also be arranged after black or primary color inks, but generally not between cyan, magenta, and yellow inks. Usually, no more than 3 special color inks should be used in a single printing job.

It should be noted that for patterns with strict register requirements, no special colors should be added between the three halftone colors of cyan, magenta, and yellow. For series versions, sometimes to ensure that the ink tray is not changed during printing, the printing color sequence must remain consistent. In addition, since surface printing inks are more vibrant than reverse printing inks, proofing usually uses surface printing inks. However, in some special cases, such as when the customer provides a color sample, needs to use anti-counterfeiting printing, or needs to print special inks such as gold ink or anti-counterfeiting ink, the printing color sequence of surface printing inks should also be reasonably arranged and adjusted.

II. Expansion and Contraction of Adjacent Colors

In gravure plate making, to compensate for the white gaps caused by inaccurate register, the printed parts usually need expansion and contraction processing, also known as trapping or filling white gaps.

When two colors of ink meet, overlap, or touch, expansion and contraction processing is generally required, sometimes mutual expansion is also needed, and in special cases, reverse expansion and contraction is required, or even a gap is added between the two adjacent colors to compensate for the register error, making the printed matter more beautiful. The general principle for expansion and contraction processing is: expand down, not up; expand light, not dark; expand screen, not solid.

The amount of expansion and contraction depends on the characteristics of the printing material, the register accuracy of the printing machine, and the printing method. Generally, the expansion and contraction amount of offset printing is smaller, while that of gravure and flexographic printing is larger, generally 0.2～0.3mm (depending on the specific requirements of printing accuracy, etc.), the expansion and contraction amount of similar colors is usually 0.3mm, and the expansion and contraction amount of opposite colors is usually 0.2mm. At the same time, it is also required that the printed pattern does not deform after expansion and contraction.

III. Determination of Surface Tension

In actual production, the surface of the printing film must undergo corresponding corona treatment to ensure that it has reasonable surface tension.

The general method for determining the surface tension of the printing film is: use a cotton swab to dip in a dyne liquid 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 the liquid film does not shrink or shrinks slightly within 5 seconds, it can be considered that the surface tension of the film is qualified; if the liquid film completely breaks or shrinks into a strip less than 8mm in length, it can be considered that the surface tension of the film is unqualified.

IV. Determination of Unwinding Direction

During the lamination process, it is necessary to check whether the laminated rolls used are provided by the same designated manufacturer, whether there are special requirements for special material laminated rolls, and whether the unwinding direction of special films (such as yin and yang films) is correct.

The principle for determining the unwinding direction is: the one with the first stroke of the text or the first letter of the word first is the head out, and the one with the last stroke of the text or the last letter of the word first is the tail out; the one with the top or left side of the pattern first is the head out, and the one with the bottom or right side of the pattern first is the tail out.

V. Determination of Ink and Adhesive Viscosity

To obtain good printing results, it is also necessary to determine the viscosity of the ink and adhesive during production.

The measurement method is: after filling the Zahn cup with the ink or adhesive to be measured, use a stopwatch to measure the time it takes for the ink or adhesive to flow out from the small hole at the bottom of the cup, thereby determining the viscosity of the ink and adhesive.

In addition, it is necessary to understand and be familiar with the manufacturer, applicable range, and solvent ratio of the selected ink to ensure the accuracy of the measured data.

VI. Production of Control Marks

To ensure accurate register of the printing film and facilitate the smooth progress of subsequent bag making and slitting, some marks for position control and judgment are usually made on the printing plate. The following issues should be noted during the production of control marks.

1. Production of cross-registration marks: Cross-registration marks are generally used with bar codes, with numbers indicating the printing color sequence, and are made on each color printing plate.

2. Production of mark lines (light spot marks): Mark lines can play a role in tracking slitting in slitting and bag making, generally using the darkest color, with a width greater than 2mm and less than 10mm, and a length generally greater than 5mm.

3. Production of detection lines: Detection lines are mainly used to ensure that the film does not deviate laterally during slitting and bag making, and the color is generally dark.

VII. Key Points of Printing Quality Control

The key points of printing quality control mainly include color difference, register deviation, ink fastness, defect detection, etc., with the following basic requirements.

1. Color difference: ΔE≤5, ΔH≤1.5 for special colors; ΔE≤5, ΔH≤2.5 for light halftone areas.

2. Register deviation: The register deviation of the main pattern is ≤0.20mm and the register deviation of the secondary pattern is ≤0.35mm for biaxially oriented films; the register deviation of the main pattern is ≤0.30mm and the register deviation of the secondary pattern is ≤0.60mm for non-biaxially oriented films.

3. Ink Rub Resistance: A 24mm wide, 250px long transparent adhesive tape is typically applied to the printed surface. After removing the tape, the ink marks remaining on the printed surface are observed to determine the ink rub resistance.

In addition, during printing production, a stroboscopic lamp (synchronous lamp) can be used to observe whether abnormalities such as knife lines, stains, missing dots, and color registration occur.

A small leak will sink a great ship. In the gravure printing process, major quality problems are often caused by ignoring these small details. Therefore, these subtle aspects of gravure printing are the secrets to ensuring high-quality printing results at Shunxingyuan.

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