What to do if you encounter these problems during flexographic printing? Check out the key ideas here!
Flexographic printing is a relatively eco-friendly and efficient printing method that has developed quickly in China, with various technologies and processes updating rapidly. However, companies using flexographic printing often run into problems during the printing process, such as misregistration, rough edges, and hollow dots, which can cause a lot of headaches. This article lists some common flexo printing issues and solutions, hoping to be helpful to readers.
01
Rough Edges
When the edges of printed text and images are uneven, jagged, or irregularly smeared, it's called rough edges. The main cause of rough edges is often ink-related issues, such as ink being too viscous or drying too quickly. Sometimes, part of the ink may dry on the plate first and then transfer along with small crusts around it, causing rough edges. Even when the ink viscosity is low, rough edges can still occur, so it needs to be addressed accordingly. Generally, a suitable approach is to control the ink's pH value between 8.5 and 9.5 at a room temperature of 25°C.
In addition, too much ink load can increase the accumulation of ink around the dots and also cause rough edges. In this case, a screen roller with a higher line count or lower ink load should be chosen.
Sometimes static electricity in printing can also affect the transfer of ink to the dots, causing rough edges. If necessary, an anti-static device can be used to reduce or eliminate static electricity.
The cleanliness of the substrate surface also has some effect on ink leveling and spreading. Especially when the surface has too much paper fiber or small dust particles, it can lead to rough edges. So cleaning the substrate surface in time can also help reduce rough edges.
The drying speed of the equipment can also affect rough edges. Drying too quickly can reduce the ink's leveling properties. In this case, adding some slow-drying agent can appropriately delay the drying speed of ink on the substrate surface, making the ink more uniform.
If the surface tension of the substrate is not suitable, it can sometimes affect ink penetration, causing ink to accumulate on the surface and not spread well. Generally, the substrate surface tension should be slightly higher than the ink's surface tension.
02
Hollow Dots
Hollow dots are common in flexographic printing, mostly due to excessive printing pressure. With the emergence of flat-top dot printing plates, this phenomenon has been effectively alleviated. As long as the pressure isn't too far off, you can basically print complete dots.
In addition, if the ink's adhesion is too low, normal halftone dots won't hold ink properly, giving a hollow feeling. You can adjust this by adding an appropriate amount of ink modifier. Low ink viscosity can also cause similar problems, so it's important to keep an eye on the ink's viscosity, especially when the printing room temperature is high. Newly prepared ink will have a bigger change in viscosity compared to ink that has been used for a while, so regular checks are needed.
03
Missing Print
Missing print is a typical example of poor ink transfer, where some areas on the printed piece are very light or completely lacking ink. The main cause of missing print is usually insufficient printing pressure, such as uneven contact pressure between the anilox roller and the plate, or between the plate and the substrate, preventing the ink from transferring effectively. There are roughly two possibilities for uneven contact pressure: either not enough pressure was applied, or there's poor lateral balance between unit components. Just make sure to check these regularly.
In addition, the surface condition of the printing plate (such as oil stains on the plate, dents on the plate, or insufficient plate thickness) can also cause ink leakage. If it's oil stains, you can wipe them off with alcohol; if it's other issues, you'll need to replace the plate.
Insufficient ink supply can also lead to missing prints. Currently, more advanced equipment uses a closed ink chamber. If the amount of ink pumped in is too small, it can't adequately wet the anilox roller, which naturally causes the missing print problem. In addition, if the anilox roller is severely clogged, it won't transfer ink properly, so keeping the entire ink path clean is important.
04
Misregistration
Misregistration is a common flexo printing issue, caused by inaccurate color registration, leading to blurry or even distorted printed images.
There are many reasons for misregistration. First, look at the printing plate. Nowadays, film output is quite good, and the spiral angle during its horizontal and vertical output shouldn't have much effect. However, different manufacturers have different calibration standards, so plates from different suppliers might not match up, including laser plates. Then check the plate mounting. Usually, the starting registration line is based on a group near the center of the plate. This way, when mounting, you adjust the plate outwards, minimizing major errors, provided the plate mounting machine has been calibrated in advance.
The start of proper registration is when both the printing plate and the plate mounting are fine. Different types of machines have different factory-set registration accuracy. For example, a typical unit press has an accuracy of 0.2mm, while a satellite press has an accuracy of 0.15mm. So when it comes to registration, it's not about being 100% accurate. Of course, being 100% accurate is better, but it's very hard to achieve.
If everything above is normal and registration is still off, then you need to consider whether it's a problem with the equipment or the materials. Equipment issues mainly involve the plate cylinder drive system settings, automatic correction settings, tension control, etc. Material issues usually relate to roll diameter, material stretch, and deformation from heat. You just need to make the corresponding adjustments in operation.
That's it-I've shared some common problems in flexo printing production and some solutions. Actually, you'll encounter many other issues in the production process. The key thing to remember is to treat the specific problem correctly and find the root cause.