Colour control

In this article I will talk about the affect ink and water balance has on colour control. In lithographic offset printing we are mixing ink, which is an oil base, with water together as part of the process. This is where the skill of a press operator and years of experience comes into play. In theory ink/oil and water do not mix together but our press operators make this phenomenon happen every day.

If the press operator does not balance his ink and water level a number of quality issues will develop. Foremost is off colour, the ink can become water logged, emulsified, reducing the colour strength resulting in colour variation. The emulsified ink will not flow down the ink roller train evenly resulting in an uneven ink distribution across the rolls, resulting in colour variation.

So the first process control that must be put into place starts with water. Should tap water or treated water be used as the base of the fountain solution? Tap water changes with the seasons so in all cases treated water is your best and only bet. You can use a deioniser to remove the hardness from the water or a reverse osmosis system that filters the water until it is free of contaminates. RO water is corrosive so it is advisable to add some salts back into the RO water to achieve a conductivity reading of 50 micromhos.

Adding solution

Now that the water is controlled lets start adding fountain solution, FS, to the water. The first thing that must be done is to determine what effect, if any, the FS has on the ink. Some solutions can attack the ink causing the pigments to separate out; this is noticeable in the FS chiller tanks. If you see a lot of pigments floating around in the tank or if the filter you have in the tank has a lot of pigment in it then work with your FS or ink supplier to determine if the solution is compatible with the ink.

Once you are satisfied you have a compatible system run a conductivity test to determine the conductivity of your FS at different concentrations. The FS supplier will specify the amount of concentrate that should be used. It is important to note that FS is concentrated. A recommended mix could be a minimum of three ounces per five gallons of water with a maximum of six ounces per five gallons. Check with your supplier for the correct amount. It is important to start at the minimum level of concentrate to see how it performs while printing.

Now that we have our water and FS mixed up we need to add a wetting agent. Either alcohol or alcohol substitutes are both wetting agents that reduce the surface tension of the non-image area of a printing plate. Alcohol is a solvent, which should be used at a maximum concentration of five per cent, any more will cause the ink to breakdown especially if you run UV inks. If alcohol is used at a high enough level it will affect UV ink cure, it also has health and environmental concerns so it is not the first choice. Now run a conductivity test to determine the conductivity of your FS and alcohol substitute at different concentrations to determine the conductivity levels. Now you have an FS operating range to work with.

Fountain mix

  • Maintain conductivity readings for UV inks at 1,800 – 2,800 max.
  • Keep fountain and tank clean of scum build up.
  • Use only deionised water for fountain mix.

Water balance

  • Set water balance to the minimum to keep plate clean. UV inks are more receptive to take on water.
  • Setting water balance more than necessary can cause UV inks to have a reduction of cure. The result being (soft ink film) with possibility of offsetting in stack.
  • Dampers should be clean and set correctly.
  • Maintaining an even ink film (using densitometer) across the sheet will make water balance easier to maintain at a lower level.

Other colour variation contributing factors

  • Check the colour of the white base coating.
  • Monitor the mixed ink to make sure the recipe was mixed correctly.
  • Examine the printing press to make sure the ink rolls were cleaned properly, if they were not they will affect the ink and printed colour.
  • Make sure the ink is not emulsified on press resulting in colour variation.
  • Inspect to see if there is enough ink in the fountain, a low level will change the pressure on the ink fountain blade
  • affecting ink flow to the roller train.
  • Investigate to see what affect the varnish will have on the colour. Some varnishes have a tendency to yellow the
  • printed image.

If you have questions please submit them to me at [email protected] or to Alec at [email protected].

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