ON-PRESS troubleshooting

ON-PRESS troubleshooting

HICKEYS – A hickey is any particle that sticks to the blanket or plate that transfers an imperfection to the printed page.  Usually they are a dot with a ring around it or an unprinted void surrounded by printing. If they are left uncontrolled, they cause us massive amounts of waste.

offset-printing-hickeys

offset-printing-hickeys

Causes:

  • Unground pigment or foreign matter in ink.
  • Poor grind or dried ink skin in the fountain.
  • Paper dust or surface of paper picking from poor paper surface or ink that is too tacky.
  •  Dust in press room environment.
  •  Disintegrating rollers.

      Remedies:

  •  Use adhesive tape to take hickey off blanket to examine.
  •  If pigment or ink skin than change ink.
  •  Dip out ink fountain.
  • If paper fibers,reduce tack of ink.
  • Inspect roller condition.
  • Use hickey picker roller.
  •  Sometimes you will have to change the paper.

PICKING/CONTAMINATION – If ink is too tacky, or if the coating is defective, bits of coating/fibre are pulled from the paper’s surface. This material adheres to the blanket and leaves a colour void,or surface crater, in the printed sheet where the pick-out first occurred. Subsequent sheets show partial filling, or may continue to show absence of one or more colours.

offset printing - Picking

Picking

Causes:

  • Poor paper coating
  • Ink too tacky too fast drying.

Remedies:

  •  Clean blankets.
  •  Soften ink with reducer or slow ink-drying with anti-oxidant.
  •  Reduce impression cylinder squeeze.
  • Try a different production run of paper.

PILING– Piling/Tail-edge pick occurs when ink builds up on the blanket,rollers, or/and plate until it eventually lifts off a portion of the image or pulls the fibres or coating from the sheet.

The defect is seen like patchy print, lack of details or an empty place on a print. Have the ability to show up both on screen and solid fields of a print, although it is really more prominent on solids. In many cases shows up at the back side of these fields while looking in print direction.
Shows up generally with inks printed at first of the color order. The inks printed in the initial units are separating on succeeding units (as is ordinary in multicolor offset printing).
Along at the separating stage rubber blanket is lifting small particles of ink as well as varnish onto the blanket – this could be viewed as a buildup on the suitable blanket, i.e. small particles of ink and varnish are piling onto the blanket. Very often ink piling comes about near the register stage of a printing process.

pilling

pilling

Causes:

  • defective paper coating.
  •  improper fountain solution mix.
  •  Ink is waterlogged.
  •  Ink drying on press.
  • Improperly packed cylinders.
  •  Improperly set or worn rollers.
  • Blankets too tacky.

     Remedies:

  •  Try a lower tack or lower set rate of ink.
  • Add anti-oxidant or retard ink-drying.
  •  Change ink.
  • Adjust dampener settings.
  •  Check specifications and adjust cylinders.
  • Treat blanket or change to less tacky blanket.
  • Change blanket wash.
  • Clean up the rubber blankets (wash-up). Run the work with much more repeated wash-up intervals
  • Run the press with ideal ink/water stability
  • Select any good ink which includes much better water-emulsifying capability
  • Ink could have weak water-emulsifying ability; transporting dampening water with itself (surface water).The water softens paperboard coating.
  • Test if the buildup shows up about the same color as it is observable – this may recommend that the ink features a weak water-emulsifying quality.
  • Determine should the buildup shows up on the following blanket – this might tell you an ink-setting trouble
  • Select much less tacky as well as slower-setting inks
  • Tacky ink is lifting small particles that came from the coating – resulting from excessive tack, UV inks are classified as the most critical.
  • Change the printing velocity
  • Reduce the interaction between a sheet and rubber blanket through the use of \”quick release\” type blankets.
  • Rubber blanket might not release a sheet easily.
  • Look at the compatibility of your ink as well as the dampening water together with the ink manufacturer
  • You might be feeding an excessive amount of dampening water, or the dampening water is extremely hard.
  • Test whether it is possible to adjust the printing order to ensure that unsettling color is a bit more toward the end the sequence
  • Allow time for the full printing procedure to warm up. The press completely, ink, dampening water or paperboard may be cold after weekend or other prolonged standstill.

DUST – Loose dust particles on the paper surface adhere to the blanket, take on ink and print as dark specks, or show up  as voids in print.

offset printing - Dust

Dust

Causes:

  • Dust deposits can occur during sheeting or trimming operations.

Remedies:

  • Predust on impression with a dry, blank unit.
  • Inspect all four sides of paper for cut quality.
  • wipe edges with a glycerine or tack cloth.
  • trim paper on all four sides or replace with a different production run of paper.

Ghosting– situation where printing form elements other than the

ghosting

ghosting

desired positive or negative ones duplicate themselves onto the printed surface. These “stencils” or “ghost images” emerge from repeated passes of the ink form roller over the plate cylinder, and from a reduction or accumulation of ink.

Causes:

  •  Poor job layout
  •  Ink film too thin
  •  Ink too transparent
  •  Used (hard) or poorly adjusted ink-rollers

Remedies:

  • Improve job layout
  • Consult ink manufacturer to weaken ink for heavier film or  to reformulate ink for greater opacity.
  • When maintaining rollers, only use the appropriate cleansing agents; a weekly application of wash paste removes lime deposits, and will regenerate the rollers.
  • Change used rollers: the rubber surface of older rollers will become glossy and over-smooth. At the same time, as their hardness increases, the edges bulge out in a trumpet shape. More pronounced abrasion becomes evident.
  • An optimal balance between ink and dampening solution helps prevent ghosting.

Mottling– Solid areas not of uniform density, resulting in uneven appearance (cloudy print output).

mottling example

mottling_example

The problem originates in paper that is partially uneven, and the resulting uneven absorption and back split characteristics. If the paper is spotted, or if a certain amount of cloudiness is already present in the coating, then the printing ink will be only partly absorbed. In 4-color sheetfed offset printing, the printing result will back split on the printing blankets of the subsequent printing units. When an uneven penetration of printing ink is especially pronounced, this will become visible after the back split process occurs, as an uneven print reproduction. The problem of a “cloudy printout” manifests itself in the greatest variety in offset printing.

Causes :

  • Non-uniform stock surface
  • Improper printing pressure
  • Improperly set or worn form rollers
  • Improper ink/water balance
  • Worn blanket

Remedies:

  • Consult paper manufacturer to change stock; consult ink manufacturer for ink  for less penetration, strength
  • Adjust printing pressure
  •  Adjust rollers to proper setting; replace if necessary
  • Adjust to proper ink/water balance
  • Replace blanket.

Smashed Blanket – Sometimes a foreign object or a paper defect can actually smash, or render useless, a blanket or a plate. The only remedy is to clear away whatever has caused the damage, spot-check the remaining paper and replace the blanket or plate.

Delamination

delaminated sheet

delaminated sheet

Causes:
If the internal bond of the paper cannot withstand the tack of the ink or other printing forces, the sheet will delaminate.

 Remedies:
Reduce ink tack on the rolls, reduce impression squeeze or try a different production run of paper.

More on-press troubleshooting’s and their solution you may find here:

 More at ON-PRESS troubleshooting Part 2