1. Mottling
= Unwanted variations in optical properties of print/paper
Optical mottling (halftone mottling, 1-colour mottling)
• usually seen with high brightness glossy papers
• paper looks "dirty"
• usually not very obvious
• similar as calander blackening
• connected to calandering conditions
Water induced mottling (water repellence, wet
repellence mottling)
• not common in heatset printing due to blistering resistance
Backtrap mottling

2. Wet repellence mottling
Mechanism 1: Water from the previous unit has no time to absorb
into the coating before ink transfer
ink
water
coating
layer
Mechanism 2: Ink cannot emulsify all
water and water stays on top of the ink
surface layer (surface water)
water

3. Back-trap mottling
• Mechanism
• uneven pore structure in coating layer
• thickness of set and unset ink layer varies locally
• splitting line follows the profile of set ink layer
variation in the amount of ink after back-trap
mottling in print
• Always present at some degree - usually well controlled
• Has to be taken into consideration in paper development
• Higher speed with heatset presses helps
• problems can arise when printing heatset papers in sheet-fed
unset ink layer
set ink layer