Flexographic printing plate-blocking issues explained all at once—come and save this!

Flexographic printing plate-blocking issues explained all at once-come and save this!

Plate blocking is one of the most common issues in flexographic printing. Plate blocking refers to the inability of ink to be fully transferred onto the substrate during flexographic production, resulting in increasing accumulation of ink at the top and shoulders of the dots. Once this is reached, a small dot print can become large due to excessive transfer, or several dots may irregularly connect together, causing the printing plate to become dirty.

The most direct solution to this problem is to shut down the machine and wipe the plate. When the machine stops and the plate is wiped, poor ink transfer is observed. On the same plate, the ink on the substrate is thoroughly transferred, leaving the plate clean, while the network cable area is heavily stuck. When ink accumulation is severe, wiping with a cloth usually cannot clean it thoroughly; a soft brush dipped in solvent is needed for thorough cleaning.

There are two types of blocking: one is blockage with dots below 30%, especially below 5%, commonly referred to as high-gloss area blocking; the other is 45%~55% dot blocking. Since the dot expansion in the midtone of flexographic printing is generally about 20%, the 50% critical point for offset printing is generally around 70% for flexographic printing. Thus, 45%-55% of the dot area on the printing plate is precisely where the edges of two round dots are close together. If not careful, ink can block the depression between the two dots on the plate. Foreign flexographic industry peers call this phenomenon 'ink fill-in', also known as bridging, meaning dot bridging.

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Causes of flexographic clogging in the printing plate

Overall, the causes of flexographic plate clogging can be summarized as follows:

Improper matching between the number of printing plate lines and the number of lines on the anilox roller

The key difference between flexographic printing and other printing methods is the anilox roller, which is a metering roller for ink transfer. The type of anilox roller you use determines the amount of ink you get. The main parameters of anilox rollers are: number of lines, ink storage capacity, and cavity angle. The number of lines on an anilox roller refers to the number of mesh holes per unit length along the axial direction of the anilox roller. Printing plates also have lines, and the smallest unit in the image is dots. The number of dots per inch corresponds to the number of lines on the printing plate.

In flexographic printing processes, matching the number of printing plate lines with the number of anilox roller lines is very important. Our previous view was: the number of lines for anilox rollers should be 4~7 times the number of printing plate lines. However, since dot areas and cavity areas cannot be completely compatible, local ink overflow from cavity areas will occur at the edges of dots. The excess ink cannot be transferred completely from the mesh holes of the anilox roller to the dots on the printing plate like the corresponding ink parts; instead, it is transferred in arcs toward the dots based on viscosity and surface tension. The quality of this transfer poses a hidden risk of clogging the plate. Some subsequent issues prompted us to revise our previous view: when selecting the number of anilox roller lines to match the number of printing plate lines, qualitative analysis of 4~7 times should not be relying solely on 4~7 times. Instead, we should further propose quantitative analysis of the minimum dot diameter of the printing plate ≥ the opening diameter of the mesh hole on the anilox roller.

Abnormal printing pressure

The printing pressure referred to in flexo printing usually includes two types of pressure: first, the pressure from the anilox roller on the printing plate roller, which we call inking pressure; second, the pressure from the plate roller on the impression roller, which we call printing pressure. This pressure comes from the gap between the two rollers pressing the printing plate.

The main causes of pressure abnormalities are: mechanical accuracy errors and unreasonable adjustment amounts. Mechanical accuracy error mainly refers to the radial runout error of each roller, also known in the industry as concentricity error. Generally, whether it is a satellite flexograph or a unit-type flexograph, the radial runout error of each roller should be controlled within 10~20u, which is a very critical figure. Unreasonable adjustment mainly refers to the gap between rollers not being properly adjusted, especially between the anilox roller and the printing plate roller, which is the main cause of abnormal ink pressure application. If the ink pressure is too high, the dots on the printing plate will deform under pressure, and too much ink will adhere to the dots, inevitably spilling outward. At this point, if the printing pressure is not properly high, plate blockage is very likely to occur. In addition, uneven printing plates are also one of the many causes of abnormal printing pressure.

Improper coordination of ink and surface tension between the printing plate

The surface tension between paper and film can be measured, and the surface tension of ink can also be measured. Due to differences in resin and formulation, the surface tension of the ink also varies. The greater the difference in surface tension between the ink and the printing substrate, the better its printability.

In short-path printing in flexographic printing, the ink is first transferred onto the printing plate, and then onto the printing substrate. This first raises the issue of tension matching between the ink and the printing plate. For ink and printing plate, because the tension difference is small, ink easily transfers from the plate to the substrate, leaving little or no residue on the plate. If the surface tension difference between the ink and the printing plate is not small, the ink on the plate will not transfer thoroughly, causing ink to fill the depression between two dots, causing plate blockage.

02

Methods to solve flexographic plate clogging

Based on the above reasons, the following solutions can be tried to solve the issue of plate clogging in flexographic printing:

01

Select the number of anilox roller lines based on the principle of the minimum dot diameter of the printing plate ≥ the mesh opening diameter of the anilox roller; there is no need to limit the traditional 4~7 times.

02

Repair equipment with mechanical accuracy errors exceeding standards to ensure compliance.

Control the inking pressure well, and be sure not to use it too much. When adjusting the inking pressure, start from light to heavy, gradually increasing the pressure, rather than suddenly increasing too much and then retreating. After adjusting the ink pressure, gradually adjust the printing pressure. If the pressure is normal, ink will be applied everywhere on the plate, and the dots will not deform. At this point, reduce the pressure slightly to confirm the print is correct, then speed up again. If the ink pressure drops too much, hollow dots will appear on the print, causing some differences in ink saturation, but the blockage problem will greatly improve.

Adjusting the unevenness between the printing plate and double-sided tape is done by placing flat printing plates instead of increasing printing pressure, which is a very important concept in operation. Common leveling measures are: tape can be applied under the solid plate, usually with a single-sided tape thickness of about 50~60u. You can apply shellac paint underneath the cable panel; each layer should be about 4u thick; if one layer is not enough, you can apply several layers.

Use low-density double-sided tape to cushion abnormal deformation of the dots. Low-density double-sided tape has good elasticity. Although pinholes tend to appear when printing solid surfaces, it is the best choice for printing screen lines. The dots are resistant to deformation and have good recovery.

In terms of the order of plate compression, the pattern should be compressed gradually from the area with fewer mesh lines to the area with more lines. In other words, in the protruding parts of the plate, pressure should be applied gradually from the low-density to the high-density areas. At this point, pressure changes are gradual, not abrupt. If the opposite is true, the probability of blocking the board increases significantly.

Pay attention to continuous vertical layout during layout. Due to the intermittent nature of the typesetting, there is always a difference between the high and low positions of the protruding parts of the printing plate vertically, and the pressure impact will be regular. If pressure strips of equal width are added to both sides of the printing plate, the strips will wrap around the entire plate roller, greatly reducing the impact of pressure. Another method is staggered layout, so that the entire perimeter does not have a completely low position. This allows the plate roller to run more smoothly and reduces impact.

03

Choose the appropriate ink and look for inks with surface tension similar to the printing plate.

Select plate parameters to make the surface harder. You can choose the plate material or adjust UVC and UVA parameters during plate making. If the printing plate stops and is washed during machine printing, do not rush to turn it on. Use compressed air to blow dry the plate. Do not start the machine immediately if it is not completely dry. This is because whether a printing plate is completely dry is closely related to its surface tension.

Special solvents are used to adjust the surface parameters of the printing plate to make it close to ink.