Paper & Board Troubleshooting Guide

Unprinted areas appear at the bottom of the flutes and form bands larger in the centre of the board sheet

Causes:

• This effect is produced with rigid boards, such as microflutes or boards with a high grammage
• The rigidity of the press cylinder or anilox roll is at the origin of the problem
• Plate: Lead edge is very large

Solutions:

• Anticipate otherwise, study an alternative solution
• Contact the manufacturer
• To be adapted, if possible, according to the final design. If the printing edge is located in a cutting zone, cut slopes in the plate or put a gap on printing edge between different flaps.

Comment: Prepare a plate with mounting foam. Be careful: an increase of the pressure between the press cylinder and the board will emphasize the phenomenon. The only solution is to slow down the machine.

The color strength varies in comparison to previous prints or samples.

Causes if the color strength is too weak:

• The ink dries on the anilox roll
• The color of the printing ink being used is too weak
• The ink is too thin
• The anilox roll is worn
• The transferred ink volume is too low
• The ink is not suitable for this particular application

Solutions if the color strength is too weak:

• Clean the anilox roll
• Contact your ink supplier or, if possible, add concentrate
• Increase the viscosity with fresh ink
• Replace the anilox roll
• Select an anilox roll with a higher transfer volume
• Contact your ink supplier

Causes if the color strength is too high:

• The color strength of the printing ink being used is too high
• The ink is too thick
• The transferred ink volume is too high

Solutions if the color strength is too weak:

• Contact your ink supplier or, if possible, add extender
• Decrease the viscosity with water
• Select an anilox roll with a lower transfer volume

The bottom of the flutes are not sufficiently inked.

Causes:

• The board is not adapted to this printing job.
• The board sheet is too wet.
• Too much starch and heat at corrugator.
• The plate is too hard for this type of board.
• The combination plate/paper is not correct.

Solutions:

• Change the board.
• Wait until the board sheet is uniformly dry.
• Adjust corrugator settings.
• With A, B, C check shore hardness of the plate.
• · Try a plate made of another material.

Non printed areas appear randomly. Non printed areas are due to the crushing of the board sheet and will produce similar zones with an excess of ink on the following sheet.

Causes:

• The board sheet has been damaged or production settings in the corrugator are incorrect

Solutions:

• While handling the board sheet be careful in every process step; avoid possibilities of sheet damage

Spots like small or big shooting stars appear.

Causes:

• The printing unit is not cleaned sufficiently. Dry agglomerates are ground between the anilox and the doctor blade and transferred to the plate.
• Too much over inking on the anilox roll: Fine particles of dry ink are formed around the relief on the print. Particles can be removed within the next printing unit, then are scraped onto the anilox roll before being recaptured by the plate on the second revolution.

Solutions:

• Clean the printing unit and think about installing a filtration device on the inking system.
• Add Retarder.
• Decrease ink volume by changing to an anilox roll with lower transfer volume or add retarder.
• Check ratio binder to pigment and add binder if necessary.

The shape of the screen dot in the print deviates considerably from the original shape.

Causes:

• The hardness of the printing plate is too low.
• The relief of the printing plate is too high.
• Excessive thickness fluctuations of the printing plate or the double-sided adhesive tape.
• The pressure setting between the plate cylinder and the substrate is too high.

Solutions:

• Use a printing plate with a higher shore hardness.
• Use the right set up of the printing plate.
• Change the printing plate or use a different adhesive tape.
• Reduce the pressure setting.

The screen dot is surrounded by a line or a ring.

Causes:

• The pressure setting between the plate cylinder and substrate is too high.

Solutions:

• Reduce the pressure setting.

Spots are present outside the printed area.

Causes:

• There is too much ink on the anilox roll and at high printing speed micro drops are projected.

Solutions:

• Check the parameters at the origin of the excess.
• Viscosity is too high.
• Pressure between rubber roll and anilox roll is insufficient.
• Speed is too high.
• Reformulate ink, contact ink supplier.

Colored constituents of the printing ink become visible in parts of the motif which have not been printed.

Causes:

• The transferred ink volume is too high.
• Ink surface tension too high.
• The printing speed is too high.
• The absorption of the substrate is insufficient.
• The rub resistance of the ink is not good.
• Too much heat at the corrugator.

Solutions:

• Replace the anilox roll or reduce viscosity.
• Add wetting agent to reduce surface tension.
• Reduce the printing speed.
• Optimize the drying speed of the ink.
• Add wetting agent or reduce viscosity.
• Check the ink recipe.
• Add wax.
• Try to reformulate the recipe.
• Adjust the corrugator settings.

Dots are larger than expected – usual phenomenon observed in flexography which reduces the details and the contrast.

Causes:

• The thickness tolerance of the printing plate or the double-sided adhesive tape is too large.
• The printing plate is too soft or the double-sided adhesive tape too hard.
• The pressure setting between the plate cylinder and the substrate is too high.

Solutions:

• Change the printing plate or the double-sided adhesive tape.
• Change the printing plate or the double-sided adhesive tape.
• Reduce the pressure setting.

In areas with a high ink coverage, the colour overprint is not correct.

Causes:

• Ink coverage in overprinting is too high.
• Insufficient drying of the ink.
• The phenomenon appears particularly at the bottom of the flutes of corrugated board, especially with low grammage board.

Solutions:

• Try to avoid printing of two colors with 100%.
• Reduce ink amount on the plate or choose preferably a different plate for large flutes.
• Check the ink quality and the dryers efficiency.
• The flute is too large, the liner is too thin or badly tended.
• Add wetting agent.
• Reduce viscosity.

Comment: Choosing colours sequence: K, C, M, Y might reduce the effect.

The colour density is higher on the top of the flutes – this is due to the resistance of the flute which caused dot gain.

Causes:

• The composition of the board is not adapted.
• The thickness of the plate is not adapted to the flute.
• Too much starch and heat at corrugator.

Solutions:

• Prefer EB flute combination instead of single B flute.
• Check shore hardness and properties of the plate.
• Change the plate.
• Adjust corrugator settings.

A lack of ink appears at the limits of the back edge, in the running direction.

Causes:

• A distortion is created when the plate comes into contact with the anilox roll. The distortion disappears when it arrives on an edge, without taking ink.
• Indifferent speed of substrate and plate.

Solutions:

• Usually, the plate is new and the photopolymer is not completely stabilized. This defect will disappear with time.
• Prepare the plate in advance, to let the plate end its stabilization with time.
• Operator: Add pressure if it doesn’t affect too much the printing quality.
• Adjust the speed of substrate and plate.

Bridges are created between dots

Causes:

• The transferred ink volume is too high
• The anilox roll and width of the printing plate raster do not match
• The ratio between the plate halftone and the anilox roll is to low
• The ink viscosity is too high

Solutions:

• For the print job, select anilox rolls with a lower transfer volume
• Use a finer anilox roll
• Prepare a new plate with a good ratio or change anilox roll
• Check and adjust the ink viscosity

Changes of colour along the machine width.

Causes:

• These variations are observed mainly with a two roll system, when the rubber roll is not parallel to the anilox roll

Solutions:

• Adjust the parallelism between rubber roll and anilox roll

Overflow of ink on relief contours of the plate.

Causes:

• Excessive ink applied to plate.
• The printing plate is worn, the relief depth is too small.
• The pressure setting between the plate cylinder and substrate is too high.
• The printing speed is too low.
• The ink viscosity is too high.

Solutions:

• Adjust anilox roll impression and/or doctor blade pressure.
• Wash or replace the plate.
• Reduce the pressure setting.
• Clean the printing plate and increase the printing speed.
• Reduce the viscosity.

Dust particles on the substrate cause spots and defects. This is the case with solid printing, where white spots are surrounded by a lighter ‚halo‘.

Causes:

• The feed unit is dirty.
• The printing plate is dirty or the surroundings are too dusty.
• The substrate is dusty.

Solutions:

• Clean the feed.
• Clean the printing plate.
• Clean the substrate by an efficient dust removal device before entering the printing machine.

Comment: Use antistatic unit or antistatic brushes.

Foam visible in ink pail or pump, missing print areas.

Causes:

• Ink exposed to too much air e.g. by stirring
• Ink viscosity too high.
• Ink not correct/appropriate for job.

Solutions:

• Check pump speed & for leaks in ink lines.
• Optimize flow and return.
• Adjust pumping speed.
• Use stirrer and optimum filling level of ink.
• Reduce viscosity.
• Formulate an alternative recipe.
• Add defoamer and stir the ink continually.

Comment: Excessive and incorrect addition of defoamer leads to “Pinholing”, in this case wash and rinse the printing unit and fill with fresh ink.

The ghosting effect is a weak printed image that is visible in one part of the motif where it should not be. Usually this effect occurs in the case of a solid print with negative text or with windows.

Causes:

• Insufficient drying between the two colours.
• The anilox roll cells are inadequately filled.
• The printing speed is too high.
• The ink viscosity is too high.
• Inks are printed in the wrong sequence.

Solutions:

• Check the efficiency of the drying system or check the ink recipes and add retarder if necessary.
• Increase the pressure in the chamber doctor blade system.
• Decrease the printing speed or the viscosity of the ink.
• Add water.
• Change ink sequence.

Printing error owing to parallel lines (clear or dark) in the direction of printing.

Causes:

• Clear bands: dirty deposits on the doctor blade.
• Dark bands: deformation or erosion of the doctor blade.
• The anilox roll is damaged.
• The anilox roll is dirty because of dried ink.
• There are particles in the ink.

Solutions:

• Clean or replace the doctor blade.
• Replace the doctor blade.
• Replace the anilox roll.
• Clean the anilox roll.
• Clean the pump system, printing plates, doctor blade and anilox roll after replacing the ink.

Uneven printing on text edges or in lines.

Causes:

• The plate is dirty, damaged or worn.
• The ink is drying too fast.
• The pressure setting between the plate cylinder and the substrate is too low.
• The printing plate is dirty or the surroundings are too dusty.

Solutions:

• Wash or change the plate.
• Use extender which is drying slower.
• Increase pressure setting.
• Clean the printing plate.

In the overprinting of the process colors (CYMK) the individual grid structures can interact and create a pattern. The type of interaction can create an undesirable interference pattern, the so-called moiré effect.

Causes:

• The engraved photopolymer plate is the origin of the problem:
  • Screen angle is not correct.
  • Screen lines are different for the various colours.
• The ratio between the line number of the anilox roll and the screen frequency of the printing plate is too low.

Solutions:

• Prepare a new plate with a new photoengraving.
• The ratio should be greater than 4.
• Use at least a 200 anilox roll, for example, for a 48 plate raster.

Uneven ink layer in solid printing – this printing defect frequently appears on coated papers.

Causes:

• Ability of the ink to cover the paper uniformly.
• Too much ink on the anilox roll.

Solutions:

• Try to achieve an uniform ink layer by adding wetting agent, to reduce the surface tension.
• Decrease ink volume by changing to an anilox roll with lower transfer volume.
• Reduce viscosity.

When printing & die-cutting, the ink is torn off by the die-cutter press.

Causes:

• Coated substrate is used.

Solutions:

• Do not use coated substrates without dryer.
• Use fast drying inks.
• Use overprint varnish.

Clear or dark spots appear and shift in the print areas.

Causes:

• The anilox roll is dirty. Areas blocked by dry ink will produce clear spots.
• Clean areas due to the repetition of a previous printing will cause dark spots.
• A damaged plate (e.g. scratches, cracks, foldings) will reproduce undesirable effects.
• The ink is not homogeneous.

Solutions:

• Clean the anilox roll.
• Prepare a new plate.
• If water is left in the printing unit, this effect will leave after some sheets.
• Stir the ink.

Comment: This effect can appear with half-tone.

The solid printing is made of dark and irregular lines – this effect is observed only with a two roll system.

Causes:

• The anilox roll is worn.
• Viscosity is too low.
• Printing speed.

Solutions:

• Replace the anilox roll.
• Increase the viscosity with fresh ink.
• The increase of the printing speed will reduce the striation effect.