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Commonly used base papers for corrugated carton surfaces include boxboard paper, coated surface paper, kraft card paper, tea board paper, white board paper, and single-sided coated white board paper. Due to variations in raw materials and manufacturing processes, these base papers differ significantly in physical and chemical properties, surface characteristics, and printability. This discussion outlines the challenges associated with these paper types in water-based flexographic printing on corrugated cardboard and provides corresponding process optimization strategies.
I. Challenges Associated with Low Grammage Base Paper
When low grammage base paper is used as the outer layer of corrugated board, corrugation patterns may become visible on the surface, resulting in exposed flute areas where graphic and text elements cannot be properly printed. To mitigate print defects such as blurred images and exposed substrate caused by surface unevenness, it is recommended to use flexible resin printing plates with excellent resilience. In particular, for Type A corrugated board manufactured from low grammage paper, printing can significantly reduce flat crush strength. Additionally, edge crush strength may be substantially compromised after passing through the printing and impression rollers.
Significant variations in surface roughness can also lead to warping of corrugated board during production. Warped boards may cause misregistration and inaccuracies in die-cutting or grooving operations. Therefore, warped boards should be flattened prior to printing. Forcibly printing on uneven substrates may result in irregular registration deviations and a reduction in overall board thickness.
II. Challenges Related to Variations in Surface Roughness
When printing on rough and loosely structured base papers, ink exhibits high permeability and dries rapidly. Conversely, on smooth, densely structured papers with tough fibers, ink drying is slower. Consequently, higher ink film thicknesses are required for rough papers, whereas lower ink volumes are sufficient for smoother surfaces. Ink dries quickly on uncoated paper but more slowly on coated paper, although the latter offers superior image reproducibility. For example, coated white board paper has a lower ink absorption rate compared to boxboard and tea board paper, leading to slower drying. Moreover, its higher surface smoothness enhances dot resolution and improves pattern fidelity, outperforming boxboard, coated paper, and tea board paper in print quality.
III. Challenges Arising from Differences in Base Paper Absorbency
Differences in raw materials, as well as variations in sizing, calendering, and coating processes, lead to differing levels of ink absorbency among base papers. For instance, when performing overprinting on single-sided coated white board paper or kraft paper, ink drying is slower due to limited absorbency. To address this, the concentration of ink in the initial print pass should be reduced, while the viscosity of subsequent inks should be increased. Lines, text, and fine details should be printed in the first color station, while solid fills should be reserved for the final station to enhance overprint accuracy. Furthermore, darker colors should precede lighter ones in the printing sequence, leveraging their superior opacity to mask potential registration errors. Darker inks provide better coverage, contributing to improved overprint precision, whereas lighter inks, having lower opacity, are less likely to reveal minor misalignments when printed later.
The surface sizing level of the base paper also influences ink absorption. Poorly sized paper absorbs more ink, while heavily sized paper absorbs less. Therefore, the gap between ink rollers should be adjusted according to the sizing condition-reducing the gap helps control ink transfer and prevents excessive ink deposition. It is essential that incoming base paper undergoes absorbency testing upon receipt. The resulting data should be shared with printing, slitting, and ink preparation personnel to support appropriate ink formulation and machine setup. Additionally, ink viscosity and pH should be adjusted based on the specific absorbency characteristics of each base paper type.
