Snowflake process screen printing is changed to single concave, which process parameters are determined? How to resolve quality defects?

"Snowflake" process screen printing is changed to single concave, which process parameters are determined? How to resolve quality defects?

Original form Minong Zhang Yanhui April 14

        "Snowflake" printing is a popular surface finishing process in packaging printing nowadays. "Snowflake" ink is a semi-transparent UV curable ink. Its process characteristics are: the surface of the printed product presents uniform and fine "small wrinkles" and dot patterns, accompanied by a matte effect, smooth ink layer, soft gloss, and good hand feeling , The surface of the printed product seems to be covered with a layer of fine snow, hence the name "snowflake" printing, also known as "freezing point" printing. Figure 1 and Figure 2 both use the "snowflake" printing process.

"Picture 1 "Red Flag Canal (Mango)" Cigarette Pack "Snow" Effect"

"Figure 2 "Dragon and Phoenix Chengxiang (Meet)" Cigarette Pack "Snow" Effect"

Process improvement background

        "Snowflake" ink contains two types of resins-"flowering" resin and curing resin. Although both resins are cured by cross-linking reaction initiated by ultraviolet light, the curing reaction rate is different. "Snowflake" printing The process is achieved by using this difference in curing rate.

        The specific process is as follows: after printing, first pass a 12kW UV "detonation" lamp, the power of the "detonation" lamp can be adjusted, generally controlled between 30% and 60%. The power of the "detonator" lamp is small, not enough to completely cure the ink. Its function is only to initiate the curing of the ink surface. Due to the difference in the curing rate of the two resins in the "snowflake" ink, the speed of the surface filming on the ink layer is fast or slow during the curing process Different, stress and strain are generated, which promotes the appearance of fine wrinkles on the surface of the ink layer, thereby forming a "snowflake" printing effect. After the "detonation", the substrate is then put into 3 UV light boxes with a power of 18kW to be completely cured to complete the "snow" printing.

        Due to the particularity of the printing effect of "Snowflake" ink, the thickness of the ink layer needs to meet certain requirements, so that the ink can reach the state of "cured on the surface and uncured on the inner layer" when the ink is irradiated by the first UV lamp, so as to become the surface of the ink. Stress and strain provide conditions for forming wrinkle paths. In order to meet the requirements of the thickness of the ink layer, the early "snowflake" printing process was completed by screen printing (the screen printing ink layer is the thickest in the traditional printing process) and is widely used in cigarette pack printing.

        With the development of market competition and the large-scale production of cigarette packs, the problems of low production rate and high cost of screen printers have been unable to meet the development requirements of cigarette pack printing enterprises. In order to improve efficiency, cigarette package printing companies have gradually developed a single-concave "snowflake" process to achieve the effect of screen printing "snowflake" process, but the following four problems will be encountered in the process of process replacement. The author will address these problems below Expand the analysis and share how to find the corresponding solution.

Common problems and solutions

1

Determination of the parameters of the plate roller

        The plate roller parameters must comprehensively satisfy the two factors of ink loading and leveling. Take the "Hongqiqu (Mango)" cigarette pack as an example. To achieve the "snow" printing effect, a large amount of ink is required, the thickness of the ink layer needs to be about 16μm, and the printing is required to be even and smooth. The factors that affect the amount of ink and leveling mainly include the engraving depth of the printing plate and the number of screen lines. The correlation is shown in Table 1.

 "Table 1 Correlation of Engraving Depth, Number of Screening Lines, Ink Volume and Leveling of Printing Plates"

            Determination method: Taking the "Hongqiqu (Mango)" cigarette pack as an example, in order to determine the parameters of the plate roller, a proofing test was carried out. The "snowflake" effects of 12 schemes were engraved on the same plate roller, and then jointly evaluated after printing on the machine , It is finally determined that the engraving depth is 65μm, and the number of screen lines is 68lpc. When printing under this plate roller parameter, the thickness of the "snow" ink layer meets the requirements, and the leveling effect is the best.

2

Determination of the thickness of "snowflakes"

        The thickness of the "snowflake" will affect the hand feel, three-dimensional effect and the appearance of the "snowflake" form of the printed product surface. The surface of the fine "snowflake" is smooth and delicate, while the surface of the thick "snowflake" is rough and three-dimensional. Therefore, the thickness of "snowflake" is the key control point of the "snowflake" printing process. It is affected by two factors: ①The wax powder content in the ink; ②The actual power of the "detonation" lamp.

        The wax powder content in the ink is non-linearly related to the thickness of the “snowflakes”. The wax powder content is generally controlled at 10% to 20%. There is a threshold between them. When the value is less than the threshold, the higher the wax powder content, the finer the “snowflakes” ; When greater than the threshold, the higher the wax powder content, the thicker the "snow".

        The power of the “detonation” lamp is generally set between 30% and 60%, which is also nonlinearly related to the thickness of the “snowflake”. As the power of the “detonation” lamp increases, the “snowflakes” first become thicker and then thinner.

        Determination method: According to the printing effect of the standard sample provided by the customer, follow the sample and reprint. Take the "Hongqiqu (Mango)" cigarette pack as an example, increase the wax powder content in the "Snowflake" ink from 10% to 20%, observe the change in the thickness of the printed sample, and finally determine when the wax powder content is 15% , The thickness of the “snowflakes” of the printed sample is consistent with that of the sample. The actual power of the “detonation” lamp is determined by a similar method. During the 30% to 60% change of the power, observe the change in the thickness of the “snowflake” of the printed sample. Also take the "Hongqiqu (Mango)" cigarette pack as an example. It is finally determined that when the actual power of the "detonation" lamp is 40%, the thickness of the "snow" of the printed sample is better. In the actual production, due to the precipitation and rheology of the ink components, the thickness of the "snow" of the printed sample will change. At this time, the power of the "detonation" lamp can be fine-tuned to match the thickness of the "snow" of the sample.

3

"Pattern" defects in the printing process

        Problem phenomenon: After printing with new ink for 2 hours, "pattern" defects appear, and the frequency is getting higher and higher.

        Reason analysis: After the "snowflake" ink is used in the ink tank for a period of time, the ink pigments will precipitate, causing the local viscosity and fluidity of the ink to be inconsistent. When two inks with inconsistent viscosity and fluidity are transferred to the printing plate, there will be a separation line, that is, a "pattern" defect, as shown in Figure 3.

"Figure 3 "Flower Version" Defects"

        Solution: As shown in Figure 4, machine the mounting holes on the top cover of the ink tank of the single-concave machine, install the mixing gun (as shown in Figure 5) on the ink tank of the single-concave machine, and connect compressed air as the stirring power. Stir the ink to ensure the uniform and consistent ink mixing of the ink tank, improve the printing suitability of the ink, and eliminate the "pattern" defects.

                                                                                                                                            "Figure 4 Ink tank with mixing gun"

                                                                                                                                            "Figure 5 Pneumatic mixing gun"

4

"Snowflake" rubbish dot defects in the printing process

        Problem phenomenon: There are missing dots on the printing surface of "snowflakes" that are not inked, and the location and frequency of occurrence are not fixed.

        Cause analysis: After observation, it was found that the defect was caused by paper powder and paper wool on the surface of the raw material. As shown in Figure 6, there are many paper wool and paper powder on the end face of the raw material paper stack to be processed. These paper wool and paper powder will fall to the paper surface during the printing process. After the "snowflakes" are printed on the paper wool and paper powder and fall off, the missing dots as shown in Figure 7 are formed.

                                                                                                        "Figure 6 Paper wool and paper powder on the end surface of the raw material paper stack"

                                                                                                                        "Figure 7 "Snowflake" Missing Spot"

            Solution: The paper wool and paper powder on the end surface of the raw material paper stack are caused by the cutting knife wear and dullness during slitting. As long as the raw paper is being slit, new knives and cleaning equipment should be replaced in time according to the production of paper wool and paper powder Dust can be solved, and the defects of "snowflakes" garbage spots are also greatly reduced.

            The solution of the above technical problems paved the way for the single-concave "snowflake" process to replace the screen printing "snowflake" process. The characteristics of high production efficiency and stable quality of the single-concave process are more in line with the large-scale production of cigarette packs, and also in line with the internal requirements of enterprises to reduce costs and increase efficiency. It is believed that the application of this process will become more and more extensive in future production.