How to specify sheets for digital post print.

When it comes to the digital post print presses some differences are noticeable in the sheet quality they can accept. (Flatness, printing surface, sheet damage, etc.)

Let’s have a look at top sheet surface properties. To have consistency in print it is important to specify exactly what grade you want for the top sheet. It is not enough to specify “Coated” or “Mëtsa” or “Rieger”.

Mëtsa and the other suppliers have several coated grades available. Each grade has its own surface properties related to the printing technique (aniloxes, dryers, no dryers, etc.).

My advice is to talk to your supplier and let him explain which grade will do the trick.

For UV inkjet, the surface properties are less critical. For water-based systems the surface properties are extremely important. Make sure your supplier can also guarantee consistent quality.

Once the top liner’s grade and grammage have been identified, one can start selecting the other components like flutings and inner liners, a lot of surprises may pop up if one is not careful.

The working window of most of the digital presses is rather small when it comes to flatness. Bear in mind that print heads are close (1.5 to 2.7 mm) to the board surface. If edges of corrugated sheets are facing too much, they will not pass or may damage printheads.

Considering using single wall E flute, the vacuum systems can compensate the warpage, but only with down curl. When there is up curl – in the perspective of the digital press that is a top printer – then the vacuum system will not be able to compensate for the curl. When double wall (BC,EB) is used, it is almost impossible to have the shape corrected by the vacuum.

Flatness should be over the full width of the corrugator

One should have strict rules for the sheetfeeder or the internal production related to sheet flatness. I suggest expressing warpage in mm and not in percentage, as using percentages can be misleading if the sheets are very narrow. Make sure the corrugated sheets keep the specified flatness by using adequate packaging, beware of climatical variations. Use flat wooden boards, corner protection and protection so metal or plastic straps cannot damage the sheets. Avoid foreign objects like nails sticking out pallets that can penetrate in the sheets and be carried along in the press. If the truck driver accidentally entered a stack with the forks, then it is better to remove the damaged immediately.

Even perfect flat sheets, perfectly packed, can give us issues. One of the most common issues is the surface rubbing from the hotplates on the flute tips. This causes post glazing or burnishing of the coated surface especially on the flute tips. This results in shiny areas on the flute tips. These shiny areas will have another way of accepting the water-based ink. The effect is noticeable after printing as irregular white/pale lines on the flute tips. Sometimes with bigger flute types even the valleys can be damaged.

These are caused by the corrugator’s conditions and settings. It is too complex to explain this in detail here, but I can offer a master class about this in case interested. Feel free to reach out.

In my experience a lot of improvement can be gained by working correctly on the corrugator parameters and the state of the hotplates, belts etc. Bear in mind that the choice of liner is crucial (talk to your supplier!).

So what you should take away from this is that specification and protection of the specification is the key, whether you produce internally or work with a supplier. In case you determine that this would require a mentality shift from your team, then training is a great tool to drive the change.

It is a bit silly that one invests millions in a sophisticated digital postprint press and does not invest in the training of the corrugator team, because the final quality is made by the TEAM!

Published by CORRADDICT

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