Discussion on Quality Control Experience of Water Offset Printing Gift Boxes

Discussion on Quality Control Experience of Water Offset Printing Gift Boxes

In the process of water offset printing, it is more important to control the balance of ink and water, which is closely related to whether the imprint is transferred normally, the depth of ink color, the accuracy of color registration, the dryness of the paper and whether it is sticky, etc. It is the key to ensure product quality stability. .

There are many factors that affect the quality of water offset printing Gift Boxes, and it is difficult to control them in actual production. Based on the accumulated work experience, the author summarizes some quality control points for sharing with readers.

inspection of paper

During the printing process, the results of various factors will be reflected on the paper, so the inspection of the paper is very important and necessary. The inspection of paper in printing operations can generally be divided into two stages: starting printing and normal printing.

1. Start the inspection at the printing stage

● Check the ink color of the paper

At the beginning of printing, the operator should take out the paper frequently, and conduct a comprehensive inspection from the mouth of the paper to both sides and toward the dragging direction. Because if the paper has quality problems, such as the depth of ink, the oil is dirty, etc., it always starts from the mouth and gradually expands to the drag. Therefore, when checking, the line of sight should scan the whole picture for quality problems and whether the ink color conforms to the proofs. When the ink color of the whole picture or part of the area does not conform to the proof, it should be adjusted in time. The means of adjustment should not be limited to the adjustment of the ink volume, and the control of the water volume should also be considered.

● Check the paper size

After checking the ink color of the paper, you should quickly check whether the T-line of the paper meets the requirements. Especially for paper with pictures printed on both sides, check whether the T-line and the size of the mouth on the front and back are consistent at the same time.

● Inspection of blotting dots

The dot is the most basic unit that constitutes the color of the printed product. If the dot is obviously deformed and enlarged, it will directly affect the color reproduction and reproduction effect. Therefore, you can use a magnifying glass to compare the difference between the dots and the lines between the prints and the proofs, and observe whether the dots and lines of the imprint are empty, whether there is obvious expansion, vertical and horizontal deformation, ghosting and other quality problems.

In addition to checking the ink color, specification and imprint of the paper, various quality defects that are easy to occur in the production of the enterprise are also the key points that need to be checked at the beginning of the printing stage.

2. Inspection at the normal printing stage

● Check paper overprint and ink color

Entering the printing stage, as the ink is gradually in a balanced state, the focus of the operator's inspection is still on paper overprinting and ink color. Generally, the two sides of the paper can be compared with the middle part, and compared with the paper extracted several times. In addition, you should also pay attention to whether there is a change in the moisture content of the layout. As for the inspection of the rules, 2 to 30 sheets of paper can be taken from time to time, collided with their mouths and spread out to observe whether the cross-rule lines are consistent, and whether the positioning of the front rules and side rules is accurate.

● Pay attention to the influence of powder, paper wool, etc.

After a period of continuous printing, the dust of the paper, paper wool and fine ink skin in the ink are easy to adhere to the printing plate or blanket, resulting in coarse lines or imprints. In this case, stop and wipe off the ink vellum. In order to prevent the change of ink color due to downtime, it is generally necessary to continue printing after placing 8 to 10 sheets of master paper. If too much powdery dirt adheres and affects the quality, the printing plate and blanket should be cleaned.

Control of water and ink

In the production process of water offset printing, it is very important to control the balance of ink and water, which is closely related to whether the imprint is transferred normally, the depth of ink color, the accuracy of color registration, the dryness of the paper and whether it is sticky, etc. It is to ensure the stability of product quality. one of the keys. During the printing process, the depth of printing ink is directly related to the amount of water supply and the moisture of the layout. However, in production practice, we cannot simply rely on reducing the water supply to improve the ink color, because the water is too small, the ink balance will be lost, and the blank parts will be easily stained. Therefore, in the printing process, when adjusting the amount of ink supply, it should be considered whether the amount of water supply is appropriate, and whether the ink and water are balanced in time and correctly. Experienced operators often control the water supply to the smallest possible range on the premise that the printing plate is not sticky and dirty, and keep the water supply and ink volume in a relatively stable state. This ensures that the ink color of the paper is consistent before and after, and the printing operation is stable.

1. Control of water supply

To properly control the water supply and achieve the ink balance in the printing process, it is a prerequisite to correctly identify the moisture content of the layout. For the identification of the moisture size of the layout, so far, there is no fixed value, so no mechanical regulations can be made. In production practice, the size of moisture is often identified by the operator's visual inspection of the intensity of the reflected light on the layout. This visual inspection method can play a certain role in identifying the water content of the plate, because a certain thickness of the water film can fill in the sand holes of the printing plate, reduce the diffuse reflection and increase the amount of reflected light. However, this method also has certain limitations, because it is limited by various reasons such as the difference between the area of the graphic and the blank area of the layout, the different printing plate materials, the different lighting intensity, and the different viewing angles. The water film cannot make the layout have the same amount of reflected light. Therefore, in daily printing operations, in addition to the above-mentioned visual inspection methods, the following points can also be observed. If the following conditions occur, it means that the layout is too watery. ① Use an ink knife to shovel ink on the ink roller, and there are small water droplets on the ink knife; ② There are small water droplets on the ink transfer roller, and there are also water droplets in the ink fountain groove; There is still no sign of drying; ④The imprint is empty, the mouth imprint is wavy and pale, and the ink is dull and dull; There is a big gap between the ink color of the paper during printing and the paper before the shutdown.

2. Mastering the relationship between ink and wash

In the process of printing preparation, when the plate loading, paper loading and adding ink to the ink fountain are completed, it is necessary to first apply the ink on the water and then correct the plate and color. When adjusting the moisture, the size and distribution of the graphic area on the layout, the thickness of the imprinted ink layer, the properties of the printing paper, the performance of the ink, the type of the printing plate, the running speed of the machine, and the ambient temperature of the workshop should be considered. During color calibration, if you feel that the ink color is too light, you can of course adjust the amount of ink used, but the influence of water cannot be ignored, and both ink and water should be considered at the same time. Generally speaking, for PS version and smooth paper, the moisture can be slightly smaller; for rough paper, the moisture can be slightly larger. When the machine is running at high speed, the moisture can be slightly smaller; for low-speed machines, the moisture can be slightly larger. In addition, environmental conditions and temperature and humidity cannot be ignored. Since the moisture of the layout is distributed directly and indirectly, in addition to meeting the needs of printing ink balance, most of the moisture will be distributed into the air. Therefore, the higher the ambient temperature, the faster the distribution. Sometimes it is encountered that the oily ink is added with gum in the fountain solution, the paper is rough, the fibrous structure is loose, and it is easy to lose hair and powder, and the amount of adjuvant added to the ink is improper, etc. The original ink-water relationship will also change.

The fluidity of ink is also affected by temperature. When the temperature is low, the fluidity of the ink is relatively poor. When the machine temperature rises, the fluidity of the ink becomes better. Therefore, with the same adjustment amount of the ink fountain screw, the amount of ink under the ink will also be different. In order to maintain the consistency of the ink color before and after the paper as much as possible, attention should be paid to the change in the amount of ink under the ink. During the printing process, new ink can be gradually added to keep the fluidity of the ink relatively stable as much as possible.

For the balance of ink and wash, we cannot simply look at the amount of the two. Because the water used in offset printing is not pure water, but an aqueous solution containing a certain amount of electrolyte and hydrophilic colloid, which constitutes a hydrophilic colloid system. The ink is not a completely non-polar oil. In addition to the binder, it also contains pigments, seasonings and other substances to form a water-repellent colloid system. Therefore, the relationship between ink and water is not only the relationship between water and oil, but also the relationship between hydrophilic colloids and water-repellent colloids. Water and ink are basically immiscible in a static state, but under the influence of emulsifier and outside air temperature conditions and pressure, the two will mix to a certain extent. This emulsification phenomenon more or less runs through the entire printing process, and the key is that the amount of emulsification should be controlled within a certain range.

3. Control the pH of the fountain solution

During the printing process, not only should pay attention to the amount of fountain solution, but also the pH value of the fountain solution. The pH value should be kept at 5-6, because the pH value is too low or too high will cause some bad results.

If the PH value is too low, it will cause problems such as corrosion of the printing plate, deinking of the ink roller, and slow drying of the imprint. This is because the base of the PS version is aluminum, and the hydrophilic blank part is the aluminum oxide layer. The aluminum oxide layer will not be corroded under weakly acidic conditions, but it will be corroded under strong acidic conditions. That is, the grit in the blank part is corroded. If the corrosion is further aggravated, the metal base of the graphic part will be damaged, causing the lines and dots on the layout to fall off. The strong acid dampening solution reacts with the surface of the metal ink roller to form a hydrophilic salt layer on the surface, thereby forming the deinking of the ink roller and affecting the normal transfer of the ink. If the acidity of the fountain solution is too large, the drying agent in the ink will change, and the drying effect will be lost, which will make the ink not easy to dry and the print will be easy to smudge.

If the PH value is too high, it will cause problems such as dirty printing, falling off of pictures and texts, and emulsification of ink. This is because the hydrophilic layer of the blank part of the printing plate may be damaged due to friction during the printing process, which requires the acidic solution to react with the plate-based metal in time to form a hydrophilic salt layer to supplement the damaged part. If the pH value of the fountain solution is too high, the process of repairing damage will not occur or will occur slowly, and the phenomenon of dirt will appear. The graphic part of the PS version is a hardened photosensitive resin, which will be dissolved in an alkaline solution, causing the graphic to fall off. The higher pH value of the fountain solution will also free fatty acid ions from the ink. It is an anionic surfactant, which can reduce the surface tension of the fountain solution and the ink, resulting in aggravation of the emulsification of the ink.

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