How to handle base paper of different materials in carton printing

Common base paper varieties for corrugated box surface paper include: boxboard, linerboard, kraft paper, tea board paper, white paper, and single-sided coated white paper. Due to differences in papermaking materials and processes for each type of base paper, the physical and chemical indicators, surface properties, and printability of the aforementioned base papers vary greatly. The following discusses problems encountered with the above paper products during the ink printing process of corrugated cardboard.

1. Problems caused by low-gram base paper

When low-gram base paper is used to make corrugated cardboard surface paper, problems with corrugated marks appear on the surface of the corrugated cardboard. This can easily lead to obvious ripples, and the required graphic content cannot be printed in the low grooves of the ripples. For surface unevenness caused by wrinkles in corrugated cardboard, a flexible resin plate with good elasticity should be used as the printing plate to overcome printing failures. Transparency and bottoming defects. Especially for A-type corrugated cardboard produced with low-gram paper, the flat crush strength of the corrugated cardboard will be significantly damaged by the printing press. Furthermore, the corrugated cardboard will have reduced edge crush strength after passing through the printing and embossing rollers. The damage will be even greater.

If the grammage difference of the corrugated cardboard is too large, it can easily lead to warping of the corrugated cardboard produced by the corrugated board production line. Warped board can cause inaccurate overprinting and printing slotting issues. Therefore, warped board should be flattened before printing, or reverse flattened on the flip line before printing. Forcibly printing uneven corrugated cardboard can easily cause irregularities. This also leads to a reduction in the thickness of the corrugated cardboard.

2. Problems caused by different surface roughness of base paper

When printing on paper with a rough surface and loose structure, ink penetration is high and ink dries quickly. When printing on paper with high smoothness and dense, tough fibers, the drying speed is slow. Therefore, the amount of ink should be increased on rough paper, and reduced on smooth paper. Ink dries quickly on unsized paper, and slowly on sized paper, but the reproducibility of printed patterns is good. For example, coated white paper absorbs less ink than boxboard and tea board paper, dries slower, and has higher smoothness than boxboard, linerboard, and tea board paper. Therefore, printing fine dots on it results in higher resolution, and its pattern reproducibility is superior to that of linerboard, boxboard, and tea board paper.

3. Problems caused by differences in base paper absorption

Due to differences in papermaking raw materials and in the sizing, calendering, and coating of the base paper, the absorption performance also differs. For example, when overprinting on single-sided coated white paper and kraft paper, the absorption performance is low, and the ink drying speed is reduced. Since the speed is slower, the viscosity of the first ink should be reduced, and the viscosity of subsequent overprinting inks should be increased. Lines, text, and small patterns should be printed with the first color, and full-plate designs with the last color to improve overprinting effects. Furthermore, dark colors should be printed on the front, and light colors on the back. This can cover overprint errors, as dark colors have strong covering power, which is beneficial for accurate overprinting, while light colors have weaker covering power.

Different sizing conditions on the base paper surface also affect ink absorption. Paper with less sizing absorbs more ink, while paper with more sizing absorbs less ink. Therefore, the gap between ink rollers should be adjusted according to the sizing state of the paper, meaning the interval between ink rollers should be reduced to control the ink amount on the printing plate. It can be seen that when base paper enters the factory, its ink absorption performance should be tested, and the printing slotter and ink dispenser should be provided with base paper ink absorption performance parameters for ink distribution and equipment adjustment. Also, the viscosity and pH value of the ink should be adjusted according to the absorption state of different base papers.

4. Problems caused by differences in base paper surface color

There are significant color differences among boxboard, linerboard, tea board paper, and white board paper. Even for the same batch of white board paper or single-sided coated white board paper, after processing on a corrugated cardboard production line, the production machine speed and heating conditions for the white board paper differ, and its whiteness can change, becoming lower or turning gray and dark; this leads to variations in whiteness for the same batch of products. White board paper printed on an offset press will still maintain good whiteness. Therefore, when printing corrugated cardboard processed from a corrugated board line with white board paper on an ink cleaning machine, the chosen ink should be darker than that used for an offset press, which can improve the contrast of the printed pattern and make the colors brighter. Furthermore, even when using the same variety from the same company, different batches of base paper products may have different tonal variations. For example, the whiteness of ordinary white board paper is above 75%, and that of coated white board paper is above 75%. However, different batches of white board paper may appear blue, while some are yellow. To ensure color consistency for the same batch of products, the same batch of base paper should be used as much as possible for printing the same batch of products. However, when the quantity of the same batch of base paper is insufficient, even if different batches of base paper are used, the color difference between different batches of base paper should be considered, and products printed on different batches of base paper should be marked for distinction to ensure that the molded products have the same color.