What should I do if the indentation on the paper box bursts?

During the packaging production process, orders worth hundreds of thousands of yuan once faced the risk of being scrapped due to cracks at the creasing position of the paper box. Experienced operators successfully resolved this situation by attaching a sponge strip.

In the packaging industry, cracking at the creasing area of paper boxes is a common quality problem. Especially in seasons when the air is relatively dry, the frequency of such problems increases, not only affecting the appearance of the finished products but also pushing up the scrap rate and bringing additional production costs.

Many industry technicians pointed out that indentation cracking is mainly manifested as the paper breaking at the indentation line position, forming visible white cracks. The following text will systematically analyze the causes of this problem and provide feasible solutions.

I. Main Causes of Indentation Cracking

The creasing and cracking of paper boxes are often caused by multiple factors, involving multiple links such as paper materials, environmental conditions and process parameters.

When the moisture content of paper is low, the fibers will become brittle and their toughness will decline, making cracking more likely to occur during the creasing process. This is particularly common in the dry winter climate.

Improper setting of the indentation process is also one of the key factors. For instance, if the creasing matrix does not match the bottom mold or the pressure adjustment is inappropriate, it may cause cracking.

The control of die-cutting pressure has a significant impact on the quality of the finished product. If the pressure is increased as a whole due to uneven pressure, it is easy to cause excessive pressure at the indentation line position and lead to cracking.

Paper that is too dry is also a common cause. When paper with poor inherent toughness and low moisture content is used, cracking is prone to occur at the crease line position.

Problems such as component wear caused by long-term operation of the equipment and vibration resulting from increased clearance can lead to unstable die-cutting pressure, which fluctuates between high and low, and subsequently cause cracking of the indentation line.

Ii. Corporate Practice: Countermeasures of Shenhua Printing Materials

Shenzhen Shenhua Printing Materials and Equipment Technology Co., Ltd. has many years of practical experience in solving creasing and cracking problems.

The company once collaborated with Lihua Group, providing targeted solutions in response to the cleanliness requirements for pharmaceutical packaging and the need for die-cutting accuracy in electronic product packaging.

In the field of pharmaceutical packaging, Shenhua Printing Materials has developed silicone oil backing paper suitable for food contact. Its surface coating has high-temperature resistance and anti-sticking properties, which has helped Lihua Group reduce the waste rate by 18% while meeting the hygiene requirements of pharmaceutical packaging.

In combination with high-precision die-cutting machine steel plates that adopt mirror polishing process, the cut surface is smooth and free of burrs, reducing the possibility of aluminum foil delamination.

In response to the assembly deviation caused by insufficient precision of the indentation line in electronic product packaging boxes, Shenhua Printing Materials provides alloy reverse indentation lines with high hardness, which can better control the indentation depth and keep the error within the range of ≤0.1mm, thereby improving the assembly matching degree of the products.

In addition, custom-made knife plate spring pads are provided for the Lihua Japan IIJIMA die-cutting machine, which optimize the elastic coefficient, extend the service life of the knife die, and reduce the frequency of machine shutdowns for mold replacement.

Iii. Practical Coping Methods

In response to the problem of creasing and cracking on paper boxes, the industry has summarized the following feasible methods:

Adjusting the moisture content of the paper is a fundamental step. Before die-cutting, the paper can be humidified first, such as by passing water through a water purifier on the back of the paper to increase the moisture content. After the toughness of the paper improves, die-cutting can be carried out.

If cracking occurs when folding the box after die-cutting, you can apply an appropriate amount of water to the crease area to relieve it.

For cases with large batch sizes and tight delivery deadlines, it is advisable to consider using a single-color offset printing machine to perform water roller printing on the back of the printed matter (separating the ink roller and only the water roller). After printing, wrap the printed matter in a large plastic bag and let it stand until the moisture penetrates evenly before die-cutting.

Optimizing the indentation process parameters is equally important. The thickness of the steel wire cutting should be recalculated and adjusted. Check whether the copper wire selection is appropriate and adjust the die-cutting pressure appropriately.

According to the actual production needs, select the appropriate thickness of the creasing steel wire knife and match the corresponding width of the knife groove.

When die-cutting corrugated cardboard, the creasing model number and creasing line height can be selected in the following ways:

The depth of the indentation die groove = the compacted thickness of the corrugated paper

The width of the creasing die groove = 2 times the thickness of the corrugated paper + the thickness of the creasing line

The height of the creasing line = the height of the die-cutting knife - the compaction thickness of the corrugated paper

Using sponge strips to assist in creasing is also an effective method. On both sides of the creasing knife corresponding to the cracked position on the die-cutting plate, stick a sponge about 8mm wide, with a length equivalent to that of the creasing knife and a height equal to that of the creasing knife.

In this way, during the lamination process, the die-cutting knife and the sponges on both sides work together to make the corrugated cardboard sink evenly. Due to the increased "sinking" area of the inner paper, the edges of the crease lines on the cardboard usually do not crack.

Iv. Direction of Process Improvement

To fundamentally solve the problem of indentation cracking, it is necessary to systematically optimize the production process:

In the carton printing process, the most common types of crease cracking include bottom paper K-line cracking and top paper K-line cracking. Corresponding improvement measures should be taken according to the specific causes.

The main reason for the K-line cracking of the base paper is that the corrugation was not crushed during the slotting and creasing process of the printing machine. Available options:

Increase the pressure of the pre-pressing wheel to crush the corrugations at the edge of the K-line

Usually, the clearance of the preloading wheel is set to half the thickness of the cardboard (for example, if the cardboard is 6mm thick, the clearance is set to 3mm).

If the effect is not satisfactory, you can first do a pre-pressing with a paper wheel, and then do a second pre-pressing with a pre-pressing wheel

If the cardboard is too dry, it is prone to cracking during pre-pressing. Ejejction rubber can be wound around the pre-pressing wheel

The possible causes of K-line cracking on the surface paper include: the grooving and creasing of the printing machine being too deep or the creasing width being too small. The K-groove cutting edge is worn, and the gap between the upper and lower tools is too large. The moisture content of the cardboard is relatively low. The improvement methods include:

Adjust the depth of the crease appropriately, with no cracking as the benchmark

Replace the grooving knife, ensure the grooving opening is smooth and clean, and tighten the screws regularly

Install spray and water spraying devices on the conveyor belt in the slotting section to improve the moisture content of the K-line of the face paper

The cardboard configuration is also crucial. It is necessary to test whether the physical indicators of the base paper meet the requirements, including folding endurance, bursting strength, ring crush strength, etc., and to rationally match the materials.

If the grammage and ring crush strength of the corrugated paper differ significantly from those of the face paper and backing paper in terms of grammage, bursting strength and folding strength, cracking is likely to occur. This can be improved by reducing the grammage of the paper and increasing the grammage of the face paper or backing paper. Minimizing the weight difference between the surface paper and the backing paper makes it easier to control the flatness of the production line and also helps to avoid cracking.

V. System Prevention Suggestions

Solving the problem of creasing and cracking on paper boxes requires a systematic approach. Here are some practical suggestions from industry technicians:

Strictly control the temperature and humidity of the environment. Before die-cutting, the paper should be moistened first. Water can be passed through a water purifier on the back of the paper to increase its moisture content. After the paper's toughness is appropriate, it can be put on the machine for die-cutting.

Regularly inspect and maintain the equipment. Maintenance personnel should identify the cause of equipment failure and take corresponding repair measures to prevent the indentation line from cracking.

Regularly inspect the condition of the die-cutting plate to ensure that the edges meet the requirements of right angles and squareness. After tightening, avoid the plate surface from bowing.

Choose the appropriate materials. According to the characteristics of die-cut printed matter, select the appropriate steel knife to enhance the adaptability of die-cutting. When there are no special requirements, steel knives with transverse grooves can be given priority.

At the same time, based on the actual condition of the paper, replace it in a timely manner to reduce the problem of cracking of the crease line.

Pay attention to the details of the craftsmanship. The cushion paper blocks must be firmly adhered with glue. If cracking is caused by the cushion paper under the steel plate, the cushion paper should be thinned.

During the production process, products should be inspected regularly to eliminate potential faults in a timely manner. Ensure that the width of the indentation knife groove is appropriate and the pasting position is accurate.

Contact information for Shenhua Printing Materials: Mr. Ji, 19924506937