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3 factors that affect print repeat length

FLEXOGRAPHIC PRINTING | Danilo Gatti, 6 October 2020

One of the greatest challenge in the printing process, especially in the hygienic industry, is ensuring a constant repeat length throughout the print job. This because the materials used for hygiene products are characterized by very low grammages and therefore they are extremely sensitive to factors such as tension and temperature.

The materials, such as LDPE or spunbond, are subjected to a determined tension during the printing process and, if this tension is not correctly set up and maintained, it can cause variations in the print repeat length outside the defined tolerances, with the result that the non-compliant reel must be discarded or, even worse, that the entire batch of reels in which the defective reel was present becomes the subject of dispute by the converter.

Specifically, there are three main factors to consider that affect the print repeat length: the tensioning of the material during the entire process, the structural stability of the machine and the temperature of the air used in drying.

1. Tension control

Proper and constant tensioning of the specific material before, during and after the printing process is of the utmost importance when it comes to ensuring a constant print repeat length. 

Overall, the printing machine can be divided into three tension zones, i.e. lengths in which the web is under the same tension: the unwind zone, intermediate tension zones (typically multiple) and the rewind zone. In order to properly control the tension in any of the zones, it must be measured, or approximated, by some means.

If we consider, for example, the case of a slitter-rewinder integrated with the flexo printing machine, the tension will be checked in 5 different points along the process:

  1. Unwinder
  2. In-feed nip roll
  3. Printing unit
  4. Out-feed nip roll 
  5. Winder pulling unit
  6. Winder

Instead, the print repeat length will be checked immediately after the winder pulling unit, as close to the winder as possible.

Each of the nip rolls are responsible for maintaining the proper, uniform tension of the material throughout the entire process. If at any of these points a different tension is detected, in excess or in defect, the flexographic printer will automatically make the needed corrections in order to achieve the desired repeat length. The preferred technologies for precise tension measurement are the load cell (tension transducer) or the dancing rolls.

2. Structural stability of the printing machine

The structural stability of the machine is another key factor of the machine that will affect the tensioning of the material, and consequently the repeat length, during the printing process. Since this stability is the result of a balance between all the electronic and mechanical parts  of which the printer is composed, there are many different aspects to take into consideration. Some of these are:

  • Each independent color deck must operate at the same speed as the others, in relation to the specific needs of the processed material. To achieve this, it is necessary to ensure that the master encoder positioned on the Central Impression drum communicates to the encoder of each color deck the speed at which its motor must operate.
  • The angular position of each color deck in relation to the angular position of Central Impression drum must be correct. In order to achieve this, the master encoder will detect and communicate the position of the drum to the encoders of each color deck every 3 milliseconds, then the individual groups will correct the misalignment and in turn send feedback to the master encoder. Since this happens for all color decks at the same time, the machine must be able to make the necessary corrections in the fastest and most efficient way possibile.
  • The motor of each color deck must have an appropriate weight in relation to the weight of the plate holder cylinder and its support system (inertia ratio). If this doesn’t happen due to an incorrect motor sizing, the color deck will not be able to make the necessary corrections communicated by the master encoder.
  • The mechanical rigidity of the coupling between the color deck motor and the plate holder cylinder support system must be such that the motor can properly transfer the command sent by the encoder of the color deck to the cylinder. In order to achieve this, A.Celli uses a rigid and direct coupling between the motor shaft and the plate holder cylinder shaft, without using gearboxes or other devices.

3. Air temperature

Finally, the print repeat length can be affected by the temperature of the air used to dry the ink.

During this part of the process the printed material pass through a section of the machine called drying hood, where it will be transported by motorized rollers at the same speed of the printing machine and the inks will be dried thanks to the use of air emitted by nozzles positioned at each roll. Since the more a material is heated, the more easily it will stretch under tension, the temperature inside the drying hoods must be set to ensure the material does not incur in these problems.

The variables of the drying process will obviously be different depending on the inks used. When it comes to water-based inks, the problem is that their drying time is significantly slower than solvent-based ones. It is precisely to solve this problem that A.Celli designed the drying hoods of the IRIDIUM® flexo printing machine specifically to dry this type of inks in the most efficient and fast way possible, without using temperatures that could affect the structure of the material and without slowing down the printing process. In addition, the flow rate and speed of the air emitted by the nozzles, designed for sensitive materials, contribute to improving the performance of the hood system.

Conclusions

As you can see, it is possible to obtain, provided the material is of high quality, the tension is correct and maintained throughout the printing process, the machine is structurally stable and the drying process is suitable for the type of ink and material used. All of this is critical to ensuring the production of a high quality, defect-free printed reel.

To find out more about controlling print repeat length and printing automation, check out our free eBook "How to Optimize the Flexo Printing Process for the Hygiene Products"!

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