Reasons and solutions for the expansion of the hot

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Reasons and solutions for the enlargement of silk screen printing points (III)

for example,

printing pressure in order to control the enlargement of printing points, we must first control the amount of ink. Minimize the amount of ink, which can minimize the damage caused by the mid point expansion of halftone overprint. There is no doubt that the biggest reason for dot expansion comes from the pressure imbalance in the printing process itself. Part of the reason is inherent in the system itself, which is largely controllable. In addition to the non impact printing process (mostly inkjet), the transfer of ink from the template to the substrate requires the balance transfer of various forces. For silk printing, the role of all forces is reflected in the whole process of inking, filling the plate, applying pressure to make the ink pass through the universal tensile testing machine fixture structure design requirements: plate, and finally make the ink adhere to the substrate. In this process, the rubber scraper and the wire interact to balance these forces. These forces determine when and how the ink flows. In order to minimize the point expansion, you must know how to balance and control these forces, otherwise, the effect of printing will be unimaginable. The tension of the wire itself and the pressure on the wire are uneven at different positions on the whole plate. The tension of the fixed frame can be controlled within the range of 1 Newton/cm2, but the tension is different on the surface of the substrate. The uneven tension on the surface of the substrate is directly related to the wire opening area, wire tension, distance and rebound force. Due to the uneven tension on the surface of the substrate, the requirements for ink pressure at different positions are also different, and the work is stable and reliable. Therefore, the flow rate of the ink is also different. As a result, the ink has been pressed through the wire to form a point before contacting the substrate

as long as we continue to use chemical filaments, the debate about silk tension will be endless. I personally have an important point of view on tension, which is different from 95% of my peers in the world at present, that is, it can have a good printing effect under any silk tension. The difference lies in the productivity and net pass rate. From the perspective of printing quality, it is acceptable in a large range. This article only deals with cases where the tension is greater than 20 N/cm2. One difference needs to be considered. The tension of silk is static. Once we add the glue scraping pressure and distance, the static tension becomes dynamic tension. After setting various printing parameters, there is a particularly accurate dynamic tension value, or a very different dynamic tension value. The static tension and dynamic tension have the most influence on the ink flow and ink amount( Δ t) The difference between them and the non-uniformity of dynamic tension. We hope to keep the variation range of dynamic tension within 2 Newton/cm2

when the static tension of the wire is 22 n/cm2 and the distance is 2mm, the dynamic tension( Δ t) It is stable between 4~8 Newtons (i.e. the printing tension is 26~30 Newtons/cm2). The pressure on the outer edge of the plate is relatively high. If the distance is adjusted to 4.5mm, Δ The value of t increases to 15~80 Newton/cm2. A simple distance adjustment makes the tension change greatly. This is one of the reasons why many printers are used to increasing the squeegee pressure and distance in order to improve the printing quality. The higher the pitch is, the greater the wire tension will be, and the rubber scraping pressure will increase, which will also stretch the wire and increase the tension

in order to understand the influence of various factors on the point expansion, we set a continuously changing tension distribution. For any ink, the shear point of the ink is determined by the fluidity of the ink, the tension of the silk and the pressure of the squeegee. At the cutting point, the ink is extruded and hung on the back of the plate drop by drop. After each printing, more and more ink cannot be printed on the substrate, resulting in dot expansion. The more ink, the more serious the dot expansion. At this time, the printer will find that the quality of the first two or three times of printing is OK. By the fourth time of printing, there will be some problems of expansion, and by the sixth time, the plate needs to be wiped

read the sample when the number of halftone lines is 65~100. The surface tension of most full tone printed substrates is between 28~35 Newton/cm2. The thicker the ink is, the wider the reproducible range is, and the greater the tension is required. The ink with high viscosity has less expansion than the ink with low viscosity. The higher the wire tension, the easier it is to control the degree of point expansion. Δ The larger the T is, the worse the tension uniformity at the blade of the rubber scraper is. The ink is also diluted due to a new trend proposed by BASF Group's high performance materials division, resulting in point expansion

another problem is the stability of tension during printing. The higher the pitch, the greater the damage to the wire and the worse the tension stability. When the distance is 2.5mm, the fatigue cycle is 5mm; When the distance is 1mm, the fatigue cycle is 2mm. A fatigue cycle is a printing cycle. This is like a paper clip constantly bending back and forth. After bending, the material becomes poor or even breaks. Under the action of pressure, the polyester filament "cold flow", resulting in a decrease in tension. The higher the distance, the faster the tension drops

due to the fixed clip on the frame, the distance is always a problem. The minimum distance is limited by the thickness of the frame fixing clip. The thickness of the clip is rarely less than 3mm. Under this distance, it is difficult to control the expansion of the point. There is a direct relationship between the percentage of point enlargement and distance. Because the distance increases, the percentage of dot enlargement will also increase

another related to distance is the stripping force of silk, that is, the separation of silk and substrate under external force. When the squeegee moves to the end of the silk, the silk rebounds from the printed picture and the substrate. The problem is: when printing to the end of the printing process, the distance increases, resulting in different tension between the front and back parts of the printing process. In order to minimize this tension difference, we adopt the method of pressurizing the end frame. When you increase the stripping force in the process of silk printing, the silk is separated from the surface of the substrate, which helps to maintain the consistency of distance and plate tension

in addition, there is another relationship that is often easy to ignore - a good parallel relationship must be maintained between the plate and the squeegee device to ensure the uniformity of the ink return knife and the squeegee pressure. To check whether it is parallel, the operator can adjust the stroke of the ink return knife so that the ink return knife only scrapes the version. If the version is parallel to the ink return knife fixing device, the ink layer on the version must be very uniform from front to back, otherwise it is easy to judge whether the publication is parallel to the scraper from the thickness of the ink layer. The result of uneven ink return is that the pressure around some graphics is too high, and the ink is squeezed out too early, resulting in dot expansion, which seriously affects the consistency of printing. If you scrub the version after 6 times of printing, it is easy to see that the ink has been extruded from the hole too early

(to be continued)

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