What should I do if I encounter these problems during the flexographic printing process? See here for the key ideas!
Flexographic printing is a relatively environmentally friendly and efficient printing method that has developed rapidly in China, with various technologies and processes being updated quickly. However, companies that use flexographic printing often encounter problems during the printing process due to different technical capabilities, such as inaccurate overprinting, burrs, hollow dots, etc., which brings a lot of troubles to the company. This article lists several common flexographic printing faults and their solutions, hoping to be helpful to readers.
01
Rough edge
The uneven edges, jagged or irregular overflow of printed text and images are called burrs. The reason for the appearance of burrs is largely due to ink problems, such as excessive ink viscosity and rapid ink drying. Sometimes, ink may dry a part of the printing plate first, and then transfer it along with small scabs around it, forming burrs. Sometimes it's not necessarily because the ink viscosity is too high, it can also occur when the viscosity is low, and this should be treated differently. Normally, at a room temperature of 25 ° C, it is appropriate to control the pH value of the ink between 8.5 and 9.5.
In addition, excessive ink loading can increase the concentration of ink around the dot and also form burrs. At this time, a mesh roller with high thread count or low ink load should be selected.
Sometimes static electricity in printing can also affect the ink transfer to the dots, causing burrs. If necessary, anti-static devices can be used to reduce or eliminate static electricity.
The cleanliness of the substrate surface also has a certain impact on the leveling and diffusion of ink, especially when there are too many paper hairs or small dust on the surface of the substrate, which can lead to edge problems. Therefore, timely cleaning of the substrate surface can also help reduce edge problems.
The drying speed of the equipment can also affect the burrs. If the drying speed is too fast, it will reduce the leveling of the ink. At this time, some slow drying agents can be added to appropriately delay the drying speed of the ink on the surface of the substrate, making the ink more uniform.
Inappropriate surface tension of the substrate can sometimes affect the penetration of ink, causing ink to accumulate on the surface and not diffuse well. Generally speaking, the surface tension of the substrate should be slightly higher than that of the ink to achieve this
02
Hollow network points
Hollow dots are more common in flexographic printing, mostly caused by excessive printing pressure. With the emergence of flat top dot printing plates, this phenomenon has been effectively alleviated. As long as the pressure value is not too deviated, complete dots can basically be printed.
In addition, the low adhesion of ink can also make normal dots unable to hold the ink, resulting in a hollow feeling. You can use an appropriate amount of ink to adjust the oil. Low ink viscosity can also cause similar problems, so it is important to pay attention to the viscosity changes of the ink, especially when the printing workshop temperature is high. The viscosity changes of newly configured ink are still significant compared to ink that has been used for a period of time, so it needs to be checked regularly.
03
Overprint
Leakage is a typical example of poor ink transfer, where ink is very shallow or completely missing in certain areas of the printed material. The main reason for missed printing is due to insufficient printing pressure, such as inconsistent contact pressure between the anilox roller and the printing plate, and between the printing plate and the substrate, resulting in ineffective ink transmission. There are probably two possibilities for inconsistent contact pressure: one is insufficient pressure, and the other is poor horizontal balance between unit components, which can be checked regularly.
In addition, the surface condition of the printing plate (such as oil stains, dents, or insufficient thickness) can also cause ink leakage problems. If it is an oil stain, it can be wiped off with alcohol. If it is another problem, a new printing plate needs to be replaced.
Insufficient ink supply can also cause leakage problems. Currently, more advanced equipment uses closed ink chambers. If the amount of ink pumped by the air pump is too small, it cannot fully wet the mesh roller, and leakage problems will naturally occur. In addition, when the mesh roller is severely blocked, it cannot transfer ink normally, so the cleanliness of the entire ink pipeline should be taken seriously.
04
Inaccurate overprinting
Inaccurate overprinting is a common issue in flexographic printing, caused by inaccurate color group positioning, resulting in blurry or even distorted printed images.
There are many reasons for inaccurate overprinting, first look at the printing plate. At present, the film output is already very good, and the influence of the helix angle in the longitudinal and transverse directions of the film output should not be significant. However, the calibration standards of different manufacturers are not the same, so the printing plates of different manufacturers may not be able to fit together, including laser plates. Take another look at the sticker. Generally speaking, the initial alignment line is a set that looks at the center of the printing plate, so that when pasting, it will be pushed to both sides, which is less likely to cause significant errors, provided that the pasting machine needs to be calibrated in advance.
The beginning of overprinting is when there are no problems with the printing and pasting plates. Different models have different factory settings for overprinting accuracy. For example, the accuracy of a general unit printing machine is 0.2mm, while the accuracy of a satellite printing machine is 0.15mm. Therefore, when overprinting, it is not necessary to be 100% accurate. Of course, 100% accuracy is better, but it is difficult to achieve.
If inaccurate overprinting occurs even under normal conditions, it is necessary to consider whether it is a problem with the equipment or materials. The main equipment issues are the settings of the plate rod drive system, automatic correction settings, tension control, etc; The main material issues are the size of the roll, the degree of material stretching, and thermal deformation. Pay attention to corresponding adjustments in operation.
Above, we have shared some common problems encountered in the production process of flexographic printing and proposed some solutions. In fact, there are many other problems that can be encountered in the production process. It is important to remember to prescribe the right medicine and identify the root cause of the problem.