Flexographic continuous image solution

- How to achieve accurate flexo plate end-to-end connection

When printing productions using photosensitive sleeves or laser engraved printing cylinders, jobs that require the printing of seamlessly continuous images present new challenges to the production workflow.

As the technology in the printing industry continues to evolve, customers have begun to require that at least one continuous image or background color be included in the print job. These requirements cannot be achieved with simple color roller coating. Taking a typical gift wrapping paper as an example, there is usually at least one continuous background color, and there are some or many design elements dotted throughout. Wallpapers, banners, banners, slogans, etc. are all products that usually require continuous printing. Once upon a time, the ability to print seamless, continuous images has become an important factor in ensuring the production of high quality products.

The use of conventional flexographic printing plates to print such jobs usually leaves a gap or gap in the area where the plates meet, so that they will show up on the printed product. Therefore, we can use a laser-engraved rubber print sleeve or roller - or photosensitive resin sleeve - to print a seamless continuous image. Both of these flexo printing methods can be used to print high-quality continuous images or seamless background colors, making them ideal for demanding print jobs.

For many years, the rubber-engraved rubber printing cylinders have always been round, and the manufacturers of photosensitive polyester plates are also working hard to develop a continuous sleeve plate that can be circled and then exposed again to ensure that continuous images are printed. Ideally, we will use photosensitive polyester sleeves or laser-engraved printing cylinders to print jobs that require continuous printing. However, this method usually brings new challenges to our workflow.

Since the sleeves and rollers of most printing factories are usually purchased externally, the printing plate is damaged, worn out, and the printing plate errors, etc. will bring problems such as the plate transportation time is too long, the delivery time is too late. Retaining a spare sleeve is a very safe method, but it is expensive. It is likely that the printing plant will not have enough sleeves ready. And it is very likely that there will be no time to order a new sleeve or not have enough money to buy a new sleeve. At this time, it is practically feasible to make a printing plate with a gap that is barely visible or to use a substitute printing plate for printing.

Unseen gaps The gap created by the end-to-end contact of the plates can usually be eliminated by folding or cutting. If pressure-sensitive materials are used, gaps may inevitably occur, which must then be cut off during the post-press processing. Sometimes adding a color block of the same color to the other end of the plate will also help to mask the gap. At other times, customers also agree to add a shallow line to the reel to cover the gap. The key to such operations is to make the gap as invisible as possible.

Careful measurement calculations and careful precision cutting also make the gap almost invisible, thus maintaining the overall consistency of the print. The operation steps mainly include the following points:

1. Carefully study the product and the printed image and place the seams in the least conspicuous position possible. Normal creases and seals are the best place to hide seams, or some seams can be removed during the processing of the product. The placement of the seams may not be easy to find, but spending more time on this step can save a lot of time and expense for later plate revisions and post-printing processes. These times are very worthwhile.

2. Make the entire plate image area size exactly as the plate cylinder repeat length, and modify the file size according to the roller diameter.

3. Use professional techniques to cut the plates to ensure that the places where the plates meet are properly sized and connected.

Adjusting the Plate Length When printing continuous images using conventional flexo plates, it is a common practice to make the plate length longer than the repetitive length of the platen, and then adjust the plate end as required.

On the surface, it seems that if the plates are cut vertically and horizontally at the place where they overlap, a tight tight gap will be ensured. In fact, the result will only lead to the plate being too long. Therefore, in practice, the front and the back of the printing plate are overlapped during the mounting process, and then the length of the printing plate is accurately measured. One or both ends of the printing plate are cut off. Even with the help of a ruler, it is not easy to form a tight gap on the roller. Sometimes it gets worse. In addition, the pressure required to cut the plate on the roller often also forms scratches on the roller surface.

If you have good luck and certain skills, this approach is likely to bring good results. By the way described below, we can accurately predict the final length of the printing plate, and cut the printing plate according to the front and tail edges of the printing surface.

To cut a plate like this, I prefer to use a single-edged razor with a safety bracket, with the help of a ruler. This thin blade can only cut a very small amount of material each time, thus guaranteeing higher cutting accuracy. Even a small amount of incompleteness in the cutting process can affect the overall print quality. Therefore, it is very important to carefully verify the edges when cutting them. Using a sharp new blade helps to ensure the most smooth and clean cut.

The plate should be cut on a hard and smooth surface. A glass surface can be used, but care must be taken because the blade is likely to slide or misalign during cutting, causing plate damage or bodily injury. Prepare a 2-3 inch long strip underneath the precise plate length position after cutting, which helps to prevent the plate from turning and at the same time helps to ensure that the edge of the blade does not slip away. Use a thin, flat steel ruler as a cutting guide for the razor. Make sure that the ruler is placed horizontally on the surface of the plate, and that the edge of the ruler should be as level as possible with the edges of the printed surface.

In order to ensure the tightest connection, the blades should maintain a certain angle, slightly lower than the two ends of the plate. Try to cut along the edge where the printing surface meets the plate. If the cut is completely vertical and the plate remains horizontal, both ends of the plate can be perfectly matched on the surface. The backings of the plates can overlap and can be precisely adjusted to achieve precise cutting. The accurately vertically cut plates are weakened slightly inwards under the shoulders, thus avoiding the overlap of the lower plates of the plates, and ensuring that the plates are tightly connected.

It should be noted that the lower and upper layers of the printing plate cannot be weakened too much inward. Through continuous practice, you will quickly learn to grasp the blade method, while also using a light to medium cutting force. If you use too much force, the plate is likely to be damaged, crooked or bent. Even a very thin plate (0.067 inch) requires at least two cuts to cut completely with appropriate pressure. Each time the second cut must be closer to the inside than the first, care must be taken not to see jagged dents. The knife cannot be lifted too high during the cutting process. It is best to practice using the old plate material several times to find the right feel to ensure accurate cutting.

Remember to follow the following cutting principles to ensure safe operation:
The cutting blade must avoid facing any body part of you or the person next to you!
Be careful with all kinds of sharp instruments. Place them in a small container that cannot be penetrated (such as a tin can), and then label, block the container, and finally dispose of it as unused garbage.

Printing plate

After the cutting of both ends of the plate is completed, it is usually necessary to start printing plates on the rollers. The first two ends of the plate must be precisely aggregated. If it cannot be precisely aggregated, it means that one of the previous steps does not guarantee a sufficiently accurate operation, or that one or more spatial variables (such as plate thickness, deformation, etc.) are incorrect. Recheck all measurement parameters, distortion factors, and cutting techniques to ensure that each step is accurate.

During the printing process, you face a challenge: Ink, water, and various solvents spread on the printing plate to ensure that the plate remains free of cracks. The best defense is to ensure good adhesion between the plate and the liner tape at the seam. The operator should always pay attention to the printing plate, and reinstall the printing plate when necessary to ensure that unexpected accidents do not occur suddenly. Moving a plate using a smaller diameter drum can be more difficult. The drum is covered with a low-density polyester film, so for a certain period of time, the printing plate keeps its shape longer.

For some continuous printing jobs, there is no laser engraving rubber or photosensitive polyester sleeve that can be used as an alternative. The method described above is still the best way to ensure that the flexographic printing press gets a seamless continuous image. Although seams can also be hidden or removed during the processing of printed paper products, the methods described above can help you obtain a clean and accurate flexographic plate end-to-end, with increased accuracy.