Corrugated cardboard printing is a commonly used printing process in the production of paper boxes. This process is known for its simplicity, speed, multifunctionality, short production cycle, and high efficiency. However, corrugated cardboard is made by mechanically bonding multiple layers of paper, which can lead to quality defects such as warping and poor flatness. Additionally, the accuracy of paper feeding and positioning is relatively low, and the registration in printing may not be as good as that in pre-printed cardboard boxes, especially in the case of multi-color overprinting. Therefore, it is essential to scientifically and reasonably arrange the printing color sequence and operate the printing machine correctly to improve the registration accuracy of the two-color flexographic printing machine and enhance the printing quality of corrugated cardboard products.

Analysis of Quality Incidents in Corrugated Cardboard Overprinting

I have witnessed a quality incident in cardboard printing where a factory’s cardboard workshop used a two-color flexographic water-based printing machine that combined printing, creasing, and cutting in one pass to print a 3-color overprint on 5-layer corrugated cardboard (specifications: 1150mm × 920mm). This resulted in a serious registration issue and a significant economic loss of over 3000 yuan due to the rejection of a large batch of products that did not meet quality requirements.

After investigation, observation, and analysis, I found that the improper arrangement of the printing color sequence was the main reason for this overprinting quality incident. In addition, inadequate machine operation skills also contributed to the registration errors. The product’s design structure required partial overprinting of red, blue, and black 3-color layouts. Two circular symbols in the blue layout needed to be overprinted with both black and red layouts, with only a 2mm blank margin around the edge. However, there was no precise registration relationship between the black and red layouts. Clearly, if the registration was off by just 1mm during printing, the misalignment in the symbol layout would become very noticeable.

For such 3-color overprinting on corrugated cardboard, the ideal result is achieved when using a multi-color flexographic printing machine to complete all 3 colors in one pass. However, with a two-color printing machine, the printing process needs to be divided into two passes, which requires careful attention to the printing process to ensure effective registration. Due to a lack of knowledge in printing technology and production experience, the machine operator decided to print the red and blue colors first and then overprint the black color. This approach, combined with the inherent warping of corrugated cardboard, led to instability and inaccuracies in paper feeding and positioning. When attempting to overprint the black color after the red and blue colors had already been applied, obvious misregistration issues arose. Unfortunately, the operator mistakenly believed that a millimeter-level misregistration was normal for cardboard box production. As a result, the finished products were rejected by the customer, leading to significant economic losses. In my opinion, this printing quality incident was primarily due to errors in the printing process. When printing 3-color corrugated cardboard with a two-color printing machine, the scientific and rational arrangement of the printing color sequence is crucial.

How to Properly Arrange the Color Sequence for Printing 3-Color Corrugated Cardboard with a Two-Color Flexographic Printing Machine

For products like the aforementioned 3-color overprinted corrugated cardboard, a proper arrangement of the color sequence can ensure accurate registration for both the blue layout with the black and red layouts. The product’s layouts can be categorized based on area size, with the black layout being the largest, followed by the red layout, and the blue layout being the smallest. In terms of registration accuracy, the blue and red layouts have higher accuracy than the black and blue layouts. There is no registration relationship between the red and black layouts, and an error of 2mm to 3mm will not be noticeably visible. Therefore, the color sequence can be arranged as follows:

1. In the first pass, print the blue layout and overprint the red layout on top of it. By printing the smaller blue layout first and the larger red layout later, you can improve registration and prevent dirty marks caused by friction during printing.
2. In the second pass, place the blue layout that needs to overprint the black layout in the front color group and the larger black layout in the rear color group for overprinting. This arrangement can achieve better registration accuracy and prevent dirty marks on the printed products. When switching to the second pass, you only need to change the ink from red to black in the red color group, making it easy to avoid common color-shifting issues during initial printing. This color sequence arrangement utilizes the extra color group in the second pass when using a two-color printing machine for 3-color products.

With this arrangement, although the blue layout is printed in two passes, the machine’s operating cycles remain the same. This allows you to split the blue layout, which needs to be overprinted with both the red and black layouts, into two separate prints, ensuring precise registration for the blue and red layouts and the blue and black layouts. This approach has been adopted by the factory for printing corrugated cardboard, and there have been no more registration issues, resulting in improved customer satisfaction with the printing quality.

Improper Equipment Operation Can Also Lead to Inaccurate Corrugated Cardboard Overprinting

The KYO-1600 two-color flexographic printing machine is commonly used for printing corrugated cardboard, and improper machine operation can also result in registration inaccuracies. In this case, the technical competence of the machine operators played a significant role in the quality incident. The machine has four sets of roller devices at the front and rear of the two printing cylinders, which play a role in supporting and frictionally conveying the corrugated cardboard. The lower rollers in each set of roller devices can be adjusted up and down to accommodate the frictional conveyance of corrugated cardboard with different thicknesses. The upper rollers in each set consist of two movable rollers, each with a screw that can be tightened or loosened to adjust for different widths of cardboard. During on-site observation and analysis, I noticed that the machine operators had not properly understood the use and operation of the equipment. They had a very limited understanding of the machine’s operation and believed that the corrugated cardboard was mainly conveyed by the pressure of the printing cylinders. Moreover, they had never encountered precise overprinting issues with two-color products in the past, so they did not adjust the roller clearances or positions when printing corrugated cardboard of different thicknesses or specifications.

It became apparent that the lack of familiarity with the machine’s performance and operational requirements was another contributing factor to the registration issues. In reality, the clearance of the rollers has a significant impact on the conveyance of cardboard. If the roller clearance is too small, it can leave roller marks on the paper surface, and excessive pressure can lead to misalignment of the cardboard. Conversely, if the roller clearance is too large, it will affect the speed of frictional conveyance and can result in misregistration between the front and rear color groups. Therefore, the clearances of all roller groups should be adjusted to match the thickness of the cardboard, and they should be kept approximately equal to ensure the normal conveyance and registration speed of the cardboard. Adjusting the clearances of the roller groups can be done by inserting the corrugated cardboard to be printed between the roller groups and conducting a pull test to adjust the clearances. The cardboard should provide a slight resistance during the pull test. Currently, when printing corrugated cardboard, the machine adjusts the roller clearances and positions before printing, and registration issues similar to those in the past have not occurred.

The occurrence of the quality incident mentioned above provides valuable insights: just like other pre-printing processes, corrugated cardboard printing can be improved by scientifically and reasonably arranging the printing color sequence. High-precision layouts should be printed in one pass, while layouts without precise registration relationships can be printed in subsequent passes. Printing smaller layouts before larger ones, and effectively utilizing extra color groups on the machine, can lead to improved registration accuracy and prevent printing quality issues. Furthermore, providing adequate technical training to printing operators and ensuring the correct use and operation of the printing machine are crucial for preventing printing quality problems or incidents and enhancing both the printing quality of products and the efficiency of cardboard box production, ultimately benefiting the company’s economic performance.