Talking about the Causes of Cracking in Die-cutting Dark Lines and Its Troubleshooting

In the post-press processing of paper packaging, die-cutting and indentation is an important process. It is to cut the substrate into a certain shape on a die-cutting machine by means of a die-cutting plate lined with a steel knife and steel wire. Press marks or groove marks for bending, groove marks for bending are commonly called dark lines. The quality requirements for dark lines are wide and narrow, clear and full, do not burst, and are well formed. Burst is a relatively common quality problem. The causes are various, some are intuitive and some are hidden. This article tries to analyze from the following aspects.

Inappropriate pressure
1. There are foreign matter in the dark line of the bottom plate, which leads to the sharp increase in pressure on the dark line. This is the most common cause of bursting. It is also the most destructive cause. It often causes the entire dark line to break, causing the product to be scrapped, but it is also more intuitive, easy to judge and solve, as long as it is promptly cleaned up. Foreign objects can eliminate the malfunction. In this regard, in the process of die-cutting, it is necessary to ensure that the bottom plate is clean and foreign matter is prevented from entering the die-cutting machine, ensuring that the press-fitting part works smoothly.

2. Running version, ie die-cut version or bottom version. Causes the steel wire to often press on the edge of the dark trough, causing a large area of ​​dark lines to burst, and is mainly concentrated on dark lines in the same direction. When you find this situation, you should immediately re-print and lock the die-cut version and the bottom plate.

3. According to the requirements of the die-cutting process, different steel wires and different thicknesses of the bottom plate and different dark line groove widths should be selected for different papers. If they do not match, they can easily cause the dark line to burst. The following is a common calculation method for your reference.

(1) Die-cut cardboard
Let A: bottom paper thickness, B: dark groove width, C: cardboard thickness, D: steel wire thickness, there should be:

D≥C;

A≤C;

B=(C×1.5)+D

(2) Die-cut corrugated cardboard
Let A: bottom paper thickness, B: dark groove width, C: thickness of corrugated board (Kava), D: thickness of steel wire, E: thickness of corrugated board after flattening, there should be:

D≥E;

A ≤ E;

B=(E×2)+D

In general, in the case of ensuring that the dark line does not burst, the dark slot should be as narrow as possible, so that the dark line is full, easy to bend, and the product is well formed. Especially for PET type paper, the dark line slot is narrower than the general product, but the dark line The slot is too narrow and can easily cause the dark line to burst.

Improper surface treatment of printed products
1. The effect of embossing
The embossing process is an emerging surface finishing process in recent years. It presses the paper through a patterned steel roller and a rigid wool roller to press the desired lines on the surface of the paper. The embossing process was initially limited to greeting cards, desk calendars and books on the front page, and was gradually adopted by paper packaging products. The use of the embossing process enhances the decorative appearance of the product, but it also has a negative impact on the post-packaging process. First, the greater pressure during embossing (usually at 3 to 8 MPa) makes it easy to produce certain paper. Deformation, resulting in inaccurate pressure embossing or die-cutting position; Second, it is easy to cause the dark line burst, because the paper is subjected to the strong compression of the pattern steel roller and the wool roller, the surface will be subject to different degrees of damage, especially when the pattern roller has a sharp line At the same time, the surface of the paper will be punctured. If the dark line is pressed against the puncture site when it is die-cut, it will burst when it is bent. To avoid this kind of problem, first of all, when designing patterns, avoid using very small points and very thin lines, because they are very sharp on the patterned steel roller, which easily damages the surface of the paper and forms bursts; secondly, embossing When the pressure is moderate, if it is a sheet-fed product, you can grind more and more of the roller, so that the pattern is completely replicated on the roller, and the desired effect is achieved with less pressure. If it is a card, you can pick the paper first. Embossing, because after the crepe paper, the paper becomes thicker and softer, plasticity is improved, and clear lines are easily pressed out.

2. The effect of oil polishing
After the surface of the paper has been oiled and polished, the varnish solidifies into a mirror surface on the surface of the paper. The elongation is extremely small, and the creasing of the indentation can easily cause bursting. To do this, you can make the paper wet or soft, or use a gluing machine to apply water to the back of the paper.

Improper selection of process and materials
1. Effect of paper pattern
In the papermaking process, the paper fibers are aligned in a uniform direction due to the oscillation of the apparatus, which forms the directionality of the paper. This characteristic of the paper has a great influence on printing and post-processing. Practice has proved that, when die-cutting, if the indentation steel wire is perpendicular to the direction of the paper fiber, and the fiber of the paper is destroyed radially, the dark line is easy to bend, the molding is good, and the angle is small; if the indentation steel wire is parallel to the paper fiber direction, the paper is Without being damaged in the horizontal direction, the dark line is not easy to bend, and it is formed into a rounded corner with a large angle, and it is strong against the outer layer of the paper and is easily broken. This directionality of paper has little effect on the die-cut of sheetfed products, but it is not well formed, but it is not easy to burst the wire. However, it has a greater impact on the products of card cards. If the handling is not good, not only the molding is not Well, it is also easy to burst the line, mainly the dark lines of parallel paper lines burst at different positions, and the other direction does not burst. In order to avoid this kind of problem, it is necessary to specify that the paper lines and the lined paper lines are perpendicular to each other when the process is being developed. They cannot be parallel. If they are already present, they can only be obtained by humidifying the paper and widening the dark lines or adding steel. Knife curvature to make up.

2. Die Making Problems
Die-cutting is the basis for die-cutting work. If some of the creases are improperly handled, it can also lead to popping.

For example, in the post-processing and finishing process of packaging, the film is a relatively common process. The main material is biaxially oriented polypropylene film (BOPP). Its tensile strength is very good, and its tensile strength in both vertical and horizontal directions is 130Mpa. In the above case, it is not easy to burst, but if there is a slight gap, the burst will be easily extended from the gap. Therefore, if a die cutter is damaged or a burr is left when the indentation line is cut when making a die cut plate, That is, the BOPP film is damaged at the dark line or the blade line interface. When the dark line is bent, the BOPP film cannot be subjected to force, which may cause burst.

In short, the cutting knife and die cutting line at the dark line should avoid the knife edge and the interface. Try to make a round angle or a steel knife to shape once.

3. Material quality issues
Material quality problems, but also easily lead to dark lines burst, such as paper water content is too low, the paper becomes brittle, it is easy to cause the dark line burst.

In order to avoid bursting caused by material quality problems, paper, paper with low surface strength, recycled paper and sandwich paper should be selectively used. For BOPP film, it is necessary to prevent sun exposure and long-term storage to avoid the decrease of tensile strength.

The above only enumerated the common causes of the bursting of the paper dark lines. Actually, there are other reasons. This requires careful observation and careful consideration in the actual operation in order to properly eliminate the dark line burst failure.