Excerpt from WeChat public account: Tag Technology

Plate blocking is one of the most common problems in the production process of flexographic printing.The term 'stuck plate' refers to a situation in flexographic printing where the ink fails to transfer fully onto the substrate. This results in an accumulation of ink on the top and shoulders of the halftone dots. When this accumulation reaches a certain level, a small halftone dot can become significantly larger due to excessive ink transfer, or several intermediate dots may irregularly connect, leading to plate contamination.

In response to this problem,The most direct solution is to shut down the plate.When the plate is stopped for rubbing, it can be found that the transfer of ink on the printing plate is not good. On the same printing plate, the ink transfer in the solid part is very thorough, and the printing plate is clean, while the mesh part is sticking a lot of excess ink,When the ink is seriously stained, it can not be cleaned by wiping cloth, and it needs to be carefully cleaned with a soft brush dipped in solvent.

There are two types of blocking: one is the blocking of less than 30% network, especially less than 5% network, that is, the blocking of high light part, and the other is the blocking of 45%~55% network,Because the dot expansion of flexographic intermediate tone is generally about 20%, so the critical point of offset printing at 50% is generally about 70% on flexographic printing. In this way,In the 45%-55% dot area on the printing plate, it is the part where the edges of two circular dots are close to each other. If you do not pay attention to the ink, the depression area between two adjacent dots on the printing plate will be blocked,Foreign counterparts in the flexographic industry refer to this phenomenon as Ink fill in, also known as Bridging, that is, dot bridge.

01 Causes of flexographic plate blocking

In summary, the causes of flexographic plate blocking can be summarized as follows:

The number of lines on the printing plate does not match the number of lines on the anilox roller properly

The key difference between flexo printing and other printing methods is the anilox roller. The anilox roller is a metering roller for ink transfer, and the amount of ink transferred will be the same as the anilox roller used.The main parameters of the screen roller are: line number, ink storage and screen hole Angle.Among them, the number of lines of the screen roller refers to the number of holes per unit length along the axial direction of the screen roller. The printing plate also has the number of lines, and the smallest unit that constitutes the image is the dot, and the number of dots per inch is the number of lines of the printing plate.

In flexographic printing process, the matching of plate line number and screen roller line number is very important.Our previous view was that the number of lines on the anilox roll should be 4 to 7 times that of the printing plate. However, because the dot area and the cell area cannot perfectly align in position, some ink from the cells will overflow at the edges of the dots. This excess ink, unable to transfer as smoothly as the corresponding ink from the dots to the anilox roll, instead moves in a curved path toward the dots based on its viscosity and surface tension. The quality of this transfer can lead to potential blockage issues. Later problems prompted us to revise our previous stance,That is, when the number of lines of the screen cylinder matches the number of lines of the printing plate, it should not only rely on the qualitative analysis of 4~7 times, but also put forward the quantitative analysis that the minimum dot diameter of the printing plate is greater than or equal to the opening diameter of the screen hole of the screen cylinder.

Abnormal printing pressure

The printing pressure mentioned in flexographic printing usually includes two pressures:One is the pressure of the screen roller on the printing plate roller,We call it the pressure of ink,The second is the pressure of the printing plate roller on the embossing roller,We call this printing pressure. This pressure comes from the gap between the two rollers pressing against the plate.

The main causes of abnormal pressure are: mechanical accuracy error and unreasonable adjustment quantity.The so-called mechanical accuracy error mainly refers to the radial runout error of each roller, also known as concentricity error in the industry. Under normal circumstances, whether it is a satellite flexographic printing machine or a unit flexographic printing machine,The radial runout error of each roller should be controlled within 10~20u, which is a very critical number.Unreasonable adjustment amount, mainly refers to the gap between each roller is not adjusted well,In particular, the gap adjustment between the screen roller and the printing plate roller is the main cause of abnormal ink pressure.If the ink pressure is too large, the printing plate is deformed under the pressure, and the dot is stained with too much ink, which will inevitably overflow to the surrounding areas. At this time, as long as the printing pressure is not properly large, the blockage is likely to occur.besides ，Uneven printing plates are also one of the many reasons for abnormal printing pressure.

The surface tension of the oil does not match the printing plate properly

The surface tension of paper and film can be measured, and the surface tension of ink can also be measured. Due to different resins and different formulations, the surface tension of ink is also different.The greater the difference between the surface tension of ink and printing substrate, the better its printing adaptability.

In the short-ink-path printing process of flexographic printing, the ink is first transferred to the printing plate and then to the substrate. This process involves a tension match between the ink and the plate. When the tension difference between the ink and the plate is small, the ink easily transfers from the plate to the substrate, leaving minimal or no residue on the plate. However, if the tension difference is significant, the ink does not transfer thoroughly, leading to ink filling the recessed areas between dots, which can cause plate clogging.

02 Solutions to the problem of flexographic plate blocking

According to the above reasons, to solve the flexographic plate blocking problem, you can try to adopt the following solutions:

01

According to the principle that the minimum dot diameter of the printing plate is greater than or equal to the opening diameter of the mesh hole of the anilox rollerSelect the number of wire mesh rollers,Don't be bound by the traditional 4 to 7 times.

02

Repair the equipment whose mechanical accuracy error exceeds the standard,Make it up to standard.

Control the ink pressure, do not be too large.When the ink pressure is adjusted,From light to heavy, the pressure gradually increases,The ink pressure should not be increased too much at once and then reduced. After adjusting the ink pressure, gradually adjust the printing pressure. Once the printing pressure is normal, all areas of the printing plate are properly inked and the halftone dots remain undistorted. At this point, slightly reduce the ink pressure and observe that the printed product is error-free before increasing the speed. If the ink pressure is reduced too much, the printed product may show hollow halftone dots, which will result in a difference in ink saturation but significantly improve the plate blocking issue.

Adjusting the unevenness of the printing plate and double-sided adhesive is to use the printing plate instead of increasing the printing pressure，This is a very important concept in operation. Common leveling measures are as follows: the bottom of the field plate can be covered with tape, generally the thickness of the single-sided tape is about 50~60u. The bottom of the wire mesh plate can be coated with shellac paint, the thickness of one layer is about 4u, if one layer is not enough, you can apply several layers.

Use low density double-sided adhesive to buffer the abnormal deformation of the network.Low density double-sided adhesive has good elasticity. Although it is easy to appear pinholes when printing the ground, it is the best choice when printing the net line. The network is not easy to deform and has good recovery.

According to the sequence of pressure on the printing plate, it is necessary to select the pattern from the parts with less wire number to the parts with more wire number gradually under pressure,That is to say, in the protruding part of the printing plate, the pressure should be gradually compressed from the low density part to the high density part. At this time, the pressure change is gradual, not sudden. If the opposite is true, the probability of blockage will be greatly increased.

Pay attention to the vertical continuity of the layout.Due to the intermittent layout, the protruding part of the printing plate will always have a difference in height between high and low positions, resulting in regular pressure impacts. If equal-width pressure strips are added to both sides of the printing plate, these strips will encase the entire plate roller, significantly reducing pressure impacts. Another method is to stagger the layout, ensuring there are no completely low areas along the entire circumference. This makes the plate roller run more smoothly and reduces pressure impacts.

03

Choose the right ink,Find an ink that has a similar surface tension to the plate.

Select the plate parameters to make the surface of the plate harder. You can select the plate material or change the UVC and UVA parameters during the plate making process.When the printing plate is stopped for cleaning after being printed, do not rush to start the machine. Use compressed air to blow the printing plate dry. Do not start the machine immediately when it is not dry. Because the drying of the printing plate has a great relationship with its surface tension.

Adjust the surface parameters of the printing plate with special solvent,Make it close to ink.