Problems with ink use in screen printing

As a manager responsible for production, I heard from a long time ago that my duty was to troubleshoot problems. However, over the years, I finally realized that my responsibility is actually to answer the questions and explain clearly the production problems that employees have reflected to me. In order to help employees identify the cause of the problem, I purposely hung out a description of the ink on a billboard: Problems and Causes 1. Poor coloring performance = too low resin content 2. Subtle difference in image quality after drying = Viscosity ( Low plastic viscosity) 3) Low printing speed of the printer = High viscosity (plastic viscosity) 4) Inability to print on the surface of the ink film = Low surface energy of the ink film 5) Lightening after washing over a time = High plasticizer content Billboards? In fact, at first my employees couldn't read the billboard, but now it is beginning to work, and employees are beginning to look at the underlying causes. One day, the boss, who did not normally talk about screen printing, came to the workshop. After seeing the billboard, he asked for a detailed explanation. The content is as follows: Some employees once came to my office and told me that more print heads should be used to print the multi-color jobs that have become more and more common. The production task they received was to use the ten-color printing presses of our factory to print eight-colored jobs. As a result, they encountered problems in the production process. I said to them that the problem may be in ink. I first checked the film and took a look to see if I needed to use all eight colors. I really needed to use eight colors. Then I went to see the press. At that time, the eight-color job was being accepted twice at the third and sixth print heads. Fast drying process. I checked the rapid drying setup parameters - temperature, distance, and speed of the press and found that everything was normal. Next I saw two cooling steps immediately after the rapid drying process. The eight-color printing adds two more drying treatments and two cold treatments, a total of twelve processes. More than two processes. If there is no problem with the fast drying process, the ink layer is thin, and the press is in place, then the problem can only be resolved with the last remaining factor: ink. After rapid drying, ink stickiness becomes a big problem. Manufacturers mix vinyl and plasticizers together when producing hot melt inks. In the mixing ratio, the amount of resin should be higher than the amount of plasticizer. Unfortunately, manufacturers do not always produce the ideal ink, which will give users trouble in print production. If the ratio is not appropriate, the simple solution (from the manufacturer's perspective) is to dilute the ink with a plasticizer. However, for printers, if the plasticizer content is high and the resin content is low, the resin will prevent the ink from becoming sticky during printing (Note: if the plasticizer content in the ink is too high, the ink is not only more difficult Curing, but become thin, fluidity, feel softer). You should understand the relevant standards so that you can cross-examine your own judgments. The rapid drying process does not affect the resin until the plasticizer is affected by the temperature. The problem at this time is that if the liquidity of the ink is too strong, the reaction of the ink to the temperature will be too strong, and the viscosity will not be reduced for a long time, which is unfavorable for production. In this regard we have two options: switch to other brands of inks, or use dry processing additives. I told the boss that adding more resins (drying additives) can only temporarily solve the problem, so I have to switch to other brands of ink. "After quick drying, the ink surface is smooth, but it is slightly different." Last week, Jimmy rushed into my office with a print and told me that the film should be remade, or at least it should be replaced with a wire mesh. He sat in a chair all of a sudden and put the prints on my desk and said, "The subtle differences between the pictures." Then he told me about the details of the operation: First, use 125 mesh screens. Printed in white; after rapid heat treatment, PMS 286 was used to match the values ​​of the 230 mesh screen. Pure dark blue was printed on some white solids, and uniform white was printed on the black part of the shirt. In the heavy-duty cotton T-shirts, the white printing effect is good, but the blue is getting worse after printing more than a dozen pieces. In less than four dozen prints, they had to stop and scrub the screen. The job can be reprinted. However, only 48 pieces can be printed at a time. At that time, at the request of Jimmy, Sam and Dave of the Department of Silkscreen Printing appeared at the door of my office. Actually they do not really need them. I let them stay and listen to my explanation. The problems encountered at the time were: the extremely low plastic viscosity of blue, plus the temperature increase caused by rapid drying. The blue ink is further reduced in viscosity. The plastic viscosity is the flow characteristic of the ink at the time of printing under the pressure of the squeegee and the squeegee. In the beginning, the consistency of the ink is not fixed. As the ink is scratched on the screen (to be transferred to the shirt through the screen), the ink is also diluted. The situation we encountered at the time was that the ink became thin and it was impossible to reproduce the subtle levels. Newcomers often think that the problem lies with the screen, the film, or other links that may be thought of at that time. However, most problems are caused by ink, and the higher the production temperature, the greater the problem. The problem also involves printing. The weight of the blue pigment is not very light, the oil absorption is strong, and the shape is like a needle (the pigment particle is long and thin). When the PMS color matching parameter is 286, the amount of pigment input is quite large. Due to the pigment's own characteristics, the ink has a large fluidity. Furthermore, overprinting pure blue on a white background will reveal all the flaws. Scrubbing the wire mesh is only a symptom, not a permanent cure. Although there is indeed dirt underneath the screen, what helps (at least temporarily) to solve the problem is the time for scrubbing the screen. It allows the ink to return to a more appropriate printing temperature and viscosity. At the time we recorded the time and temperature used for the white and blue colors. Our suppliers also have the ability to improve the ink so that it does not become too thin, to ensure that we get a stable and qualified print quality. After listening to the explanation, Sam and Dave went back to work and happily made another set of screens. "At this speed, we can't deliver on time, and we can't speed up production." Carlos's face sweated. It's not because of the sweat that fills the press padding, but Carlos's sweat. He was concerned that he could not deliver on time at a rate of 35 dozen per hour. In order to ensure on-time delivery, he intends to arrange for the second shift to be overtime. Together with his colleagues, he tried all the dimensions of the squeegee he could think of, the top angle, the height, the shape, and all the angles that could be placed. They adjusted the distance between the screen and the substrate several times, even at a very small distance. Finally they used thinner to reduce the viscosity of the ink. However, the ink loses opacity after passing through the mesh. I said to Carlos that the problem was with ink. He was very surprised. Next, I first checked the calibration technical requirements of the printing press and the actual adjustment of the printing press. There is no problem in the distance between the screens. The front of the squeegee is pointed and the embossing angle is 80 degrees, which is in line with the standard. The fine mesh holes we use (large screen tension and small screen distance) are best suited for fast printing. The screens used meet the technical requirements of tension, coating, exposure and development. So ink is the only factor that should be considered. The ink in the container has a relative viscosity. When we heat the ink on the press, its plastic viscosity is still high. The viscosity of the ink was not low enough at the time and it was not suitable for high-speed printing. We know that the independent characteristic of tack is determined by the mixing state of the plasticizers of the resin. The viscosity of the ink is too high in connection with the following ink manufacturing processes: overheating, poor dispersion of resin and aged plasticizers (all plasticizers are absorbed by the resin), improper use of fillers. The above link is usually related to the ink production speed or raw material cost. So we used the ink distribution method to solve the speed problem. First, we purchased a low-viscosity, high-viscosity ink from the manufacturer. Then we added an appropriate amount (about 10%) of the above ink to the ink we used. In this way we can keep the viscosity of the ink constant (which cannot be achieved only with thinner) and the opacity is no longer a problem. The quality of the job is good. Later we switched to inks with suitable properties: high-viscosity, low-viscosity inks. "Seven screens, dried for seven hours, the ink is still not printed." Dan intends to return the color ink. It is a specially formulated color ink, but it is particularly difficult to print on the substrate. Dan would like to seize the time to deploy a very similar ink as a substitute ink. But I remind him not to forget that the red ink is used to print the customer's company logo. In addition the formulated ink does not give very close results. Dan tested seven screens, three of which were coated with a thick emulsion layer with 150-230 screens. Dan tried all the squeegee and adjustment parameters he could get on each screen (he said that changing only one parameter at a time would not work at all). The scraping of the squeegee twice successively makes the ink pass through the screen better, but it seriously affects the printing quality and production speed of the logo color. The color matching ink (close to PMS 185) is printed on the fast drying light yellow ink. Light yellow is superimposed on the rapidly drying white ink. The last two inks were screen printed with 150 screen lines to enhance opacity. When I began to look up the reasons, I first identified what was the correct link, as usual, and then looked forward to the problem with the hope. The rapid drying process meets our internal standards and this is the first inspection step. The screen tension also meets the requirements, and the problem is not related to the template, because the red graphic is a large piece of bold ink. Then I inspected the chemical verification certificate provided by the ink manufacturer (Class A, this certificate is generally only available upon request and should be requested) and learned that the red ink complies with the technical standards we used in the previous months. Obviously, the color red ink can not be used to solve the problem, nor is it the problem. The only remaining link is the printing of the bottom surface. According to the time, it is the top surface of the yellow pressed white correction layer that is overprinted with other kinds of ink. The signs that can indicate the problem are the high gloss yellow ink, and the light is smooth. Because of its low surface energy, it is impossible to print any other ink. The surface energy makes the printed yellow with the characteristics of PTFE fiber, like the bright egg on the front of the pan, the red ink is not easy to print. The low surface energy of the fast-drying ink is mainly due to the following two reasons: excessively high plasticizer content and excessive rapid drying (too long or too high temperature). The former is out of our control (and the third class logo on the yellow ink also convinces us that the yellow ink is suitable for rapid drying), so we focused on the latter, I let Dan in the first The white ink is overprinted with red ink, and then cross-checks the overprint effect of the yellow ink. Sure enough, the red effect printed with the original screen was very good. It turns out that the rapid drying temperature of the yellow ink was too high and the ink was overheated and became too smooth, making it difficult to print other inks. So we reduced the temperature and the result was red printing. "The Wiched Witch of the West returned the printing job after the first trial wash." The job faded and Jamie from the Customer Service asked about the temperature of the drying unit and wanted to know the drying Processing is in place. The production team member reduced the belt speed of the dryer to a slow crawl speed. When the drying unit reached the hot temperature (using broken shirts and returned defectives for testing), they increased the speed of the dryer belt and allowed the shirts to pass again. As a result, it has been found that the fading problem has not been solved. Since all other aspects met the technical requirements, we once again looked at the ink to see if it faded. Under the high magnification magnifying glass we observe that the problem is apparently caused by fibers protruding from the ink film. As long as the fusion is sufficient, the hot melt ink is not easy to fade. Jamie's test results show that the root of the problem is not the degree of ink fusion. In order to test whether the ink is discolored, we printed the ink on a piece of cotton cloth. After it has dried out, we put the ink film in two. After rinsing, the printing effect is still good. Although there are some interpretable changes in the test method, the problem is not that of ink fading. When washing T-shirts, the fibers in the spinning break, and these tiny fibers protrude through the ink film. Since the hot melt ink printed on is only a