Using Croda Ink to Detect Coated Paper Backtrap Printing Stain (I)

Print Mottling refers to the alternating light and dark patches that appear on some patches after printing. Printing stains are one of the paper diseases that can be caused by all coated paper and cardboard. It is not as obvious as some physical paper diseases, such as holes, scratches, etc., but it is an inherent quality defect. It has the characteristics of concealment and lethality. Once it occurs, it may cause the entire printed paper to be scrapped. If you can detect the paper disease before it leaves the factory and solve it in production, you can give the paper more excellent quality.

Printing spots are caused by the performance of the paper itself or the poor printing process conditions. Divided into the following three categories: The first one is due to the presence of wet repellent paper caused by the damp water spots (Water Refuse Mottle) [1]; the second is due to uneven transfer of ink on the paper a few colors The Backtrap Mottle [2,4,6] is caused by the blanket, and the third type is the Wet Ink Trap Mottle or Two Color Mottle due to the improper color sequence during printing. There have been many studies on the detection of paper wet repellent phenomena, and there have been few studies on the detection of paper Backtrap stains.

Now the detection of the paper Backtrap stains is mostly carried out using a printability tester [2, 4]. This test requires the use of expensive experimental instruments such as IGT and PRUFBAU, and the experimental methods are also complex, costly, and long test times. Combining the practice of printing in the factory, the author confirmed that the Croda ink coloring method is a simple, economical, and effective method for detecting the paper Backtrap stains. Currently, this Croda ink has been successfully used in the routine inspection of the Zixing Paper Co., Ltd. coated paper.

1 Causes of Backtrap Stain during Printing

An article abroad has described and defined the Backtrap stain: the phenomenon that the ink on the paper is printed on the next color blanket in multicolor printing is called Backtrap if the transfer (ink layer) is uniform , then the quality of printing will not be affected. However, Backtrap often occurs unevenly, and Backrap stains can occur. This stain is usually reflected in the color printed on the front, especially in blue. The premise of this stain is that the ink-printed paper comes into contact with the blanket of the back color group. Backrap stains are more often found on presses with 6 or more colors. It is mainly affected by the uniformity of the ink solvent absorption during the ink drying process. There are also other factors that affect the Backtrap stain, such as the time interval (from one color to another) determined by the print speed, the thickness of the ink layer, and the number of print color groups. When a Backtrap stain occurs, a patchy patch is produced, and the quality of the print is severely affected [4].

Offset printing inks use dry vegetable oils, resins and a small amount of mineral oil as the binder. The drying of the offset printing ink is due to the low viscosity of the mineral oil penetrating into the paper, the colloidal oxidation of the high-viscosity dry vegetable oil and the resin is oxidized, the viscosity is increased, and the final solidification occurs. Therefore, the speed at which paper absorbs mineral oil (solvent) will affect the speed of ink fixation and the ink thickness of the fixing layer.

If the speed of the paper absorbing the ink solvent is not uniform and the fixing speed of the ink layer is slow, the following situation will occur. When the paper reaches the next color printing, the thickness of the fixed ink on the paper surface is different (see Fig. 1) .

Fig.1 Effect of uneven paper absorption on ink drying

In the 4-color offset printing process, if there is a case where the ink is fixed unevenly in the above-described ink, after the printing is finished, the paper is subjected to sequential color printing. At this time, the unfixed ink on the paper is adhered by the back-colored blanket. The fixed inks left on the paper will show different thicknesses of the ink layer, resulting in differences in the density of printing inks, which will cause printing stains (see Figure 2).

Figure 2 Schematic representation of the backtrap stain

It can be seen here that the difference between the Backtrap stain and the above two other stains is as follows: From the above definition and illustration, it can be seen that the Backtrap patch may occur in the monochromatic region of the first color. Since dampening water is not present on the first color unprinted paper, wet-repellent stains do not occur. The overprint must have two colors, so overprinting will not happen. Conversely, as long as the first color of the monochrome area has a stain, it must be a Backtrap stain. In the last color, because there is no color group (blanket) at the back, Backtrap stains must not occur.