Large color differences in screen printing and low equipment utilization? Solve it with just one trick
Screen printing is one of the commonly used printing processes at present, in the field of packaging printing is an auxiliary process, mainly used for the surface decoration of printed materials, such as on the packaging box such as cigarette packs, sprinkled bags to achieve frosting, refraction, ice flower, local varnishing, etc., so that the packaging presents three-dimensional, embossed, highlight, flashing, illusion, matte and other artistic effects.
Compared with offset printing, gravure printing, flexo printing and other process equipment, the structure of the screen printing machine is relatively simple, and its ink supply method is mostly manual inking, due to the intermittent nature of manual inking, resulting in large changes in the hue of the printed product, and when the color difference exceeds the standard range, it will produce scrap products; and manual inking often needs to be stopped, resulting in reduced equipment utilization; At the same time, the current screen printing ink is mostly UV ink, which has a certain degree of corrosiveness, and the ink is easy to be stained on the skin during the manual inking process, causing chemical burns, and there are many safety hazards and other problems.
In order to ensure product quality, improve equipment utilization, and reduce safety risks, we want to replace the original manual ink change operation method with an automatic ink supply system.
Cause analysis
The printing plate of screen printing belongs to the screen-like hole plate, when printing, the ink is first poured on the printing plate, the ink is transferred to the substrate through the mesh of the printing plate under the action of the squeegee pressure, and the viscosity of the screen printing matte ink is higher, the particle size of the frosted powder is larger (generally in 15~60μm), and the small particles of frosted powder will preferentially pass through the mesh when the new ink is put on the machine, and the frosting powder in the ink on the printing plate will be less and less, and the large particles of frosted powder will accumulate more and more on the printing plate, resulting in the corresponding ink layer on the substrate becoming lighter and lighter, and the color difference is getting bigger and bigger. When the color difference exceeds the standard range, the operator needs to stop the machine to replace the ink (take back the old ink on the printing plate and replace it with new ink), and the downtime is the main reason for the low utilization rate of the equipment.
Workaround
01/ Problem-solving ideas
To find effective solutions, we must first understand the operating principle of the equipment. When manually inking the automatic screen printing machine as shown in Figure 1, first lift the ink scraper and ink overprinter, and pour the ink into the blank space near the middle of the end of the printing plate; then put down the ink squeegee and ink overprinter, and the ink is in the parallel area formed by the squeegee and the ink overprinter.
Figure 1 Fully Automatic Screen Printing Machine
During printing, when the equipment is operating, the ink roller is lowered to a position 0.3–0.5 mm above the printing plate (the height can be adjusted according to the inking situation). The printing plate moves backward, with the rear end of the plate away from the squeegee and ink roller. The squeegee and ink roller remain stationary to complete the inking process. At this time, the impression cylinder has rotated 180°. When the impression cylinder grabs the paper with its grippers and continues to rotate, the movement direction of the plate starts to reverse, moving forward, with the rear end of the plate moving toward the squeegee and ink roller. The ink roller is lifted and the squeegee is pressed down. Under the pressure of the squeegee, the ink on the plate is transferred through the mesh holes of the image and text areas of the plate to the substrate, from the gripper side to the rear of the plate. At this point, the impression cylinder has rotated 360°. The equipment continues to operate, repeating the above actions in cycles, thereby allowing printing to continue. The printing unit and printing plate of the fully automatic screen printing machine are shown in Figure 2.
Figure 2 Printing Unit and Printing Plate of the Fully Automatic Screen Printing Machine
02/ Design Concept
The overall design concept is to make full use of the compressed air source near the fully automatic screen printing machine (generally, the operations of the printing plate clamping, squeegee, and flood bar on the screen printing machine are pneumatic, all equipped with an air compressor or using a central air supply system to provide compressed air, powered by two pneumatic diaphragm pumps or explosion-proof electric ink pumps. One diaphragm pump is connected to the ink supply pipeline installed between the squeegee and flood bar to provide new ink; the other diaphragm pump is connected to the ink return pipeline installed at both ends of the squeegee beam to collect used ink. Through the downward pressing action of the squeegee and the reciprocating movement of the printing plate, the ink collection device at the pipeline front end collects and pumps the ink back. The two diaphragm pumps work together to complete the cyclic process of supplying new ink and returning used ink.
Specific Measures
01/ Install Ink Supply Pipeline
The ink outlet of the supply pipe is installed in the middle position between the squeegee and the flood bar (both in the middle of the length and the middle of the parallel spacing between the two). The ink inlet of the supply pipe is placed in the ink reservoir. A diaphragm pump is installed on the pipeline between the outlet and the inlet to provide power to continuously draw ink from the ink reservoir and deliver it to the printing plate.
To accurately supply ink to the middle area between the squeegee and flood bar, and then allow the ink to level from the middle to both sides under gravity and the reciprocating movement of the squeegee and the flood bar within the channel formed between them, the ink outlet is designed in a flat-mouth shape (Figure 3).
Figure 3 Flat-mouth Ink Supply Port
02/ Installation of Ink Return Pipeline
Due to the ink on the printing plate flowing evenly from the center to both sides under the influence of gravity and the reciprocating motion of the doctor blade and ink fountain roller, most of the old ink gathers near the side frames of the plate close to the operating side and the transmission side (Figure 4). Therefore, we designed the ink return pipeline with two ink collection ports, installed respectively at both ends of the doctor blade crossbeam (Figure 5). To ensure effective ink collection, we additionally installed trapezoidal ink collection scrapers on the ink collection ports (Figure 6). The height of the ink collection scraper can be adjusted so that it lightly touches the printing plate when the doctor blade is pressed down, ensuring effective ink collection without damaging the printing plate. Through the downward motion of the doctor blade and the reciprocating movement of the printing plate, the ink collection scraper collects and pumps the ink back.
Figure 4 Location of Old Ink Accumulation
Figure 5 Ink Intake Port and Its Trapezoidal Ink Scraper
Figure 6 Installation position of the ink collection port and its ink scraper on the doctor blade beam
03/ Installing the 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 start and stop as well as the flow of the ink supply and return actions can be controlled independently.
Figure 7 Pneumatic Diaphragm Pump
04/ Configuring the Ink Storage Tank
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 with a lid, and a pneumatic agitator is installed on the lid. The pneumatic agitator has an independent switch and a speed control valve, allowing it to start, stop, and adjust speed independently. The end of the ink supply pipeline is immersed in the ink storage tank, and the diaphragm pump provides the power to continuously deliver ink from the storage tank to the printing plate. The pneumatic agitator installed on the ink storage tank is currently used to fully mix the old ink with the new ink. The old ink pumped back from the machine is not directly added to the ink storage tank; it must first be re-ground with a grinder, then a certain proportion of ink adjusting oil is added to restore its printability to the effect of new ink. After that, it is added to the storage tank in a certain ratio, and the pneumatic agitator stirs it to fully mix it with the new ink.
Figure 8 Ink Reservoir with Pneumatic Stirrer
05/ Installation of Centralized Control System
In order to facilitate operation, achieve automation of new ink supply and old ink recovery, as well as ensure the accuracy and continuity of ink feeding and recovery volumes, we installed a centralized control system, purchased a control touchscreen (Figure 9), and designed a control program (Figure 10) to achieve integrated control of various actions.
Figure 9 Physical diagram of the touchscreen
Figure 10 System Control Circuit Diagram
Effect Verification
After the installation work was completed and a period of operational testing was conducted, it was found that under normal operating speed, the issue of product color difference caused by intermittent ink supply was effectively controlled. Additionally, since the automatic cyclic ink supply system completely replaced manual ink replacement operations, downtime was reduced, equipment utilization was improved, and the safety hazards caused by ink chemical burns were eliminated.