Simone Bonaria – FTA Europe shares his expert thoughts on main functions that inking systems must perform for a good ink layer deposit
The word “print” means presence of ink. Without ink we are not able to talk about printing. In the following article we are going to talk about the importance of inking and wash-up system in the flexo printing unit.
The main functions that inking systems must perform are mainly three: move the ink from the bucket to the doctor blade chamber – making it return to the bucket, recover the remaining ink and wash the circuit, prepare it for the next job.
The main phases of the process
Inking is the most common operation, because the one used during the printing phase. The ink from the bucket, thanks to the feed pump, is pushed into the pipes until it reaches the doctor blade chamber, usually through an ink loading hole located in a low position and in the middle of the cavity. In this way, the flow inside the chamber divides towards the two drain holes located at the end of the chamber, in a high position. The position of these holes guarantee the desired level inside the doctor blade chamber. Once the ink has reached the level, depending on the pump configuration, falling into the drain holes reaches the bucket by gravity or with the aid of a return pump.
The recovery of the ink, although it seems an obvious phase of the system, is present only on 40% of the flexographic machines on the market. This function has the task of emptying the circuit from the remaining ink. Due to the position of the loading hole, at the end of the inking cycle the chamber remains full of ink, while the return pipes, both by gravity or by the return pump, are empty. To empty the feed pipe and the doctor blade chamber, the only plausible solution is to invert the feed pump, thus allowing the pump to push the remaining ink in the circuit, bringing it to the bucket.
The final phase is the washing of the circuit, which in the most innovative systems is divided into pre-washing and washing. The prewash takes off semi-dirty solvent/water from the previous wash. Through the delivery pump the ink is push into circulation in the system for a variable time, increasing its speed to create a slight pressure in the chamber to help to wash it in depth. Some systems allow to add detergents in the prewash phase to increase the system cleanliness. Once this phase is complete, the system automatically drives this solvent/water to the drain via the appropriate valves, completely emptying the circuit, also thanks to the inversion of the delivery pump. After that, the washing phase begins. The appropriate valves, taking clean solvent/water from the line, putting it into circulation in the system with settable times and speeds. At the end of the cycle it will be necessary to empty the remaining liquid in to a special tank to use this liquid left over for the next prewash cycle In this way the ink system is able to reduce consumption and time.
Ways of configurations
The different inking systems on the market are distinguished from each other mainly by: presence of a return pump, type of pumps used and software.
Entering into the typologies of inking systems, we can say that there are fundamental differences between the single pump system and the double pump system.
As long as the machines print at relatively low speeds, everything is easy to manage, but as the speed increases, the problems are revealed.
Certainly, from a technological point of view the double pump system is more advanced due to the possibility of adjustments and optimisations, but both have strengths and weaknesses, and sometimes manufacturers prefer a basic but simple management system rather than a more complex one, which requires a greater technological investment but ensuring superior performance.
Single pump vs double pump
The strengths:
- price
- simplicity of management
The critical points:
- rough and not very stable adjustments
- limited machine speeds
- high consumption of water or solvent for washing.
- greater application flexibility
- better machine performance
- better optimisation of the washing cycles
- higher cost
- a management that, if not automated, becomes difficult and demanding
Another aspect to consider in the inking and washing system is the type of pumps it uses. The pumps most used are of two types:
- pneumatic membranes
- peristaltic
Currently the most commonly used are pneumatic pumps with membrane, both for the lower initial cost and for the greater possibilities of basic management, which often occurs through a pressure regulator that directly regulates the speed of the pump.
To be maintained, this type of pump requires time to dismantle and replace the internal balls and gaskets, inevitably causing a machine stop. Also for this reason, some manufacturers of printing press, have preferred the use of peristaltic pumps, which have as only consumable element the “milking” tube: this allows the replacement in a couple of minutes.
Other great advantages of the peristaltic pump are pumping accuracy, reduced noise, reduced consumption. However, to detriment there is the initial cost.
The importance of adjustments
The right pressure balance of fluid inside the doctor blade chamber is guaranteed only in presence of fine adjustments of the inking and washing system.
As previously seen, there is an inlet and two ink outlets in the doctor blade chamber, usually controlled by two pumps (feed and return). The balance of the flow of these pumps ensures correct operation that the chamber does not have any type of pressure/depression inside it. In the event that the feed pump works at 10lt/min and the return pump works at 8lt/min, there will be an overpressure in the chamber, which will quickly lead to ink leaks from the doctor blade chamber. Otherwise, there will be a depression, which pull the doctor blade chamber more in contact with the anilox, wearing out the blades and the end seals more and causing a deafening whistle.
For this reason, less and less manual pressure regulators are used, which force machine operators to continuously adjust, and often prevent standardising the process; but more and more advanced systems that use PLCs for control, ensuring fine and stable adjustments over time, with inking and washing cycles that can be set and interfaced with the machine, allowing the process to be completely standardised.
Most common problems and their causes
Foaming is a problem due to micro bubbles that are incorporated into the ink and other pumped liquids. It is a more obvious problem with water-based inks, while it is almost not influent for solvent-based inks and it becomes more evident, when the machine speed increases: normally reaches a critical point around 300m/min.
The most common solution for solving this problem is the use of peristaltic pumps, which thanks to their structure, do not have any internal agitation in the passage of the fluid, not going to incorporate air into the ink.
Another frequent issue is contamination from one job to another. This occurs when press operator notice color residues of previous job during the inking of the new colour in the machine. Among the main causes, in addition to that of a system not suitable for the press or poorly configured, there is the positioning of the pipes and the use of unsuitable filters.
The positioning of pipes from inking system to printing units plays a fundamental role in resolving the contamination: a low point will never be possible to empty. So it is important to check that the pipes maintain the right heights, or limit the presence of low points as much as possible. The choice of filters must also be evaluated thoroughly, in order to assess that the same filter creates a low point that cannot be emptied. For this we recommend the use of Y filters that do not create any stagnation of ink or washing water/solvent.
However, most of the problems are linked to unsuitable or badly used washing systems. Washing is a very important phase that should not be underestimated and to which you need to dedicate the right time and the right investments, in order not to run into the various problems mentioned above.
To conclude, the main objective of the inking and washing system is provide ink to the printing unit without problems, to recover the greatest amount of ink in circulation at every job change, to wash deeply every part of the circuit to allow changes between different colors without hanging and finally allow a wide range of simple, effective and repeatable adjustments over time.