There are many hidden dangers in manual ink printing in screen printing, and this innovative system has become a savior!
Screen printing is one of the commonly used printing techniques, which is an auxiliary process in the field of packaging printing. It is mainly used for surface finishing of printed products, such as achieving frosted, refracted, ice flower, local gloss, etc. on packaging boxes such as cigarette packs and wine packs, making the packaging appear three-dimensional, embossed, high gloss, sparkling, iridescent, matte and other artistic effects.
Compared with offset printing, gravure printing, flexographic printing and other process equipment, screen printing machines have a simpler structure, and their ink supply methods are mostly manual ink. Due to the intermittent nature of manual ink supply, the color of the printed product changes greatly, and when the color difference exceeds the standard range, it will produce waste products; And manual ink application often requires shutdown, resulting in a decrease in equipment utilization; At present, screen printing ink is mostly UV ink, which has a certain degree of corrosiveness. During manual ink application, the ink is prone to contact the skin and cause chemical burns, posing safety hazards and many other problems. In order to ensure product quality, improve equipment utilization, and reduce safety risks, we want to replace the original manual ink changing operation with an automatic ink supply system.
cause analysis
The screen printing plate belongs to the mesh type perforated plate. During printing, ink is first poured onto the plate, and the ink is transferred to the substrate through the plate mesh under the pressure of the ink scraper. However, the viscosity of the screen printing frosted ink is high, and the particle size of the frosted powder is relatively large (usually 15-60 μ m). When new ink is applied, small particles of frosted powder will first pass through the mesh. As the printing time increases, the amount of frosted powder in the ink on the plate will decrease, while larger particles of frosted powder will accumulate more and more on the plate, causing the corresponding ink layer color on the substrate to become lighter and the color difference to increase. When the color difference exceeds the standard range, the operator needs to stop the machine to replace the ink (retract the old ink on the printing plate and replace it with new ink), and the downtime is the main reason for low equipment utilization.
solution
01/Solution ideas
To find effective solutions, the first step is to understand the operating principles of the equipment. When manually applying ink to the fully automatic screen printing machine as shown in Figure 1, first lift the ink scraper and ink coating knife, and pour the ink onto the blank space near the middle of the plate tail; Then put down the ink scraper and ink coating knife, and the ink is in the parallel area formed by the ink scraper and ink coating knife.
During printing, when the equipment is in operation, the ink coating knife is pressed down to a height of 0.3-0.5mm from the printing plate (the height can be adjusted according to the ink coating situation), and the printing plate moves backward. The tail of the printing plate moves away from the direction of the ink scraper and ink coating knife, and the ink scraper and ink coating knife are in a stationary state to complete the ink coating action. At this time, the impression cylinder has rotated 180 °; When the embossing cylinder continues to rotate while gripping the paper, the direction of movement of the printing plate begins to change and move forward. The tail of the printing plate moves towards the direction of the ink scraper and the ink coating blade. The ink coating blade is lifted and pressed down, and the ink on the printing plate is transferred to the substrate through the mesh holes in the graphic and text parts of the printing plate from the gripper to the tail under the pressure of the ink scraper blade. This time, the embossing cylinder has rotated 360 °. The equipment continues to operate and repeats the above actions, so that printing can continue. The printing unit and plate of the fully automatic screen printing machine are shown in Figure 2.
02/Design Ideas
The overall design concept is to make full use of the compressed air source near the fully automatic screen printing machine (generally, the plate locking, scraper blade, and overlay blade of the screen printing machine are all pneumatic and equipped with air compressors or use a central centralized air supply system to provide compressed air. Two pneumatic diaphragm pumps or explosion-proof electric ink pumps are used as power sources. One diaphragm pump is connected to the ink supply pipeline installed between the scraper blade and overlay blade, responsible for supplying new ink; the other diaphragm pump is connected to the ink return pipeline installed at both ends of the scraper blade crossbeam, responsible for old ink recovery. Through the downward pressure action of the scraper blade and the reciprocating motion of the plate, the ink collection device at the front end of the pipeline is driven to collect and pump back ink. Two diaphragm pumps work together to complete the reciprocating cycle of new ink supply and old ink recovery.
Specific measures
01/Install ink supply pipeline
The ink outlet of the ink supply tube is installed at the middle position between the scraper blade and the ink coating blade (the middle position of the length and the middle position of the parallel distance between the two), and the ink suction port of the ink supply tube is placed in the ink storage tank. A diaphragm pump is installed on the pipeline between the ink outlet and the ink suction port to provide power and continuously suck out ink from the ink storage tank and deliver it to the printing plate.
In order to accurately supply ink to the middle part between the ink scraper and the ink coating blade, and then make the ink flow from the middle to both sides in the channel formed between the ink scraper and the ink coating blade under the action of gravity and the reciprocating motion of the ink scraper and the ink coating blade, we designed the ink supply port in a flat mouth shape (Figure 3).
02/Install ink return pipeline
Due to the gravity and the reciprocating motion of the ink scraper and the ink coating blade, the ink on the printing plate flows flat from the middle to both sides in the channel formed between the scraper and the ink coating blade. Therefore, most of the old ink accumulates near the plate frame on both sides of the operating and transmission surfaces of the printing plate (Figure 4). Therefore, we designed two ink collection ports for the ink return pipeline, which are installed at both ends of the scraper crossbeam (Figure 5). In order to ensure the ink collection effect, we have installed a trapezoidal ink collection shovel on the ink collection port (Figure 6), and the height of the ink collection shovel can be adjusted so that it gently contacts the printing plate when the scraper is pressed down, which can ensure the ink collection effect without damaging the printing plate. Through the downward pressure action of the scraper and the reciprocating motion of the printing plate, the ink collection shovel is driven to collect and pump back the ink.
Figure 4: Old Ink Gathering Position
Figure 5 Installation position of ink collection port and ink collection shovel on the scraper beam
03/Install pneumatic diaphragm pump
Two pneumatic diaphragm pumps (Figure 7) are installed on the ink supply and return pipelines, respectively. The pneumatic diaphragm pumps are equipped with independent switches and gas flow control valves. Through the switches and gas flow control valves, the on/off and flow rates of the ink supply and return actions can be independently controlled.
04/Configure ink storage tank
In order to ensure that the old ink pumped back from the machine can be reused, we have configured an ink storage tank (Figure 8). The ink storage tank is cylindrical in shape with a lid, and a pneumatic mixer is installed on the lid. The pneumatic mixer is equipped with an independent switch and a fast/slow control valve, which can independently start/stop and adjust the speed. The end of the ink supply pipeline is immersed into the ink storage tank, and the ink in the tank is continuously transported to the printing plate through the power provided by the diaphragm pump. Installing a pneumatic mixer on the ink storage tank is currently aimed at thoroughly mixing old ink with new ink. The old ink pumped back from the machine is not directly added to the ink storage tank, but needs to be re ground by a grinder, and then a certain proportion of ink adjusting oil is added to restore its printing suitability to the new ink effect. Then, it is added to the ink storage tank in a certain proportion and mixed thoroughly with the new ink through the stirring of a pneumatic mixer.
05/Install centralized control system
For the convenience of operation, the automation of new ink supply and old ink recycling, as well as the accuracy and continuity of ink loading and recycling, we have also installed a centralized control system, purchased a control touch screen (Figure 9), designed a control program, and achieved integrated control of various actions.
Effect verification
After a period of running tests after the installation work was completed, it was found that the color difference problem caused by intermittent ink supply was effectively controlled at normal operating speed. Moreover, the automatic circulation ink supply system completely replaced manual ink replacement operations, saving downtime, improving equipment utilization, and eliminating safety hazards caused by ink chemical burns.