Unwanted surface defects

Defects in surface structure can be generated anywhere during printing, varnishing, lamination, packaging or distribution.

In most cases the defects originate from the paperboard manufacturing procedure. It is thus a challenge to the paperboard manufacturer to obtain a surface as free from structure as possible. The basis of this ability is a profound understanding of the instances in the paperboard manufacturing process were surface problems are likely to originate and the knowledge of how to control the production.

Surface structure defects depend on the topography (three-dimensional irregularities) of the material as well as the in-plane (two-dimensional) irregularities that affect the absorption properties (for inks). Such irregularities can often have significant undesirable consequences.

The following table gives some examples of the most common defects caused by surface irregularities. These might not be visible until the paperboard is printed or varnished.

 

Defect on printed sample
  Conceivable origin in the paperboard
  Possible cause in paperboard manufacture

Missing dots in print

  Basic paperboard sheet
  Insufficient surface smoothness or low compressibility of the surface in the printing nip or both.
Fibrous structure (fibre wicking)
  Basic paperboard sheet
  Fibres not well bonded on the surface.
Ink or vernishing peeling   Improper combination of
paperboard, ink and varnish. Poor surface wettability.
  Choice of coating components. On plastic coated surfaces the cause can be insufficient corona treatment or incorrect handling of the paperboard.
Pinholes in plastic coated surface
  Paperboard surface or
plastic coating or both
  Excessively rough surface. The thickness of the plastic film is too low.
Ink smear on the printed sample
  Poor ink drying   Coating formula.
Mottling, print gloss
variation
  Absorption variation
  Uneven density in paperboard plies and/or uneven coating. Gloss variation.
Orange peel structure (porous, uneven pattern which makes the surface look rough)
  Coating
  Coating technique and settings.
Graininess (Z-direction)
  Basic paperboard sheet
  Choice of pulp and pulp treatment. Sheet forming technique.
Scratches – blade lines in print
  Straight lines in the machine direction (MD) on the coated surface from coating blade.
  Particles in the coating blade which disturb the smoothing of the coating colour. Rheology of the
coating formulation.
Indentations of different size experienced as unevenness on the surface   Machine equipment all the way from paperboard machine through to winders, sheeters, transport handling equipment and storing facilities for the paperboard.   Damage or impurities on rolls or other transport equipment for manufacturing, transportation handling and storage.

Contact

Iggesund Paperboard
825 80 Iggesund
Sweden

+46 650 - 280 00
info@iggesund.com

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