Helpful coating tips: jet angle, gap, critical factors, etc.
Dec 21, 2021
Factors that increase coat weight: decrease of blade load, decrease tip angle, increased viscosity/solids, increase speeds, increase in sheet roughness.
Increase in color supply: must be high enough to prevent skip coating, but low enough to prevent backflow which contaminates top jet lip.
Longer dwell times: increase the forces under a blade and hence increase blade load the longer the dwell time is.
Gloss and smoothness: improved with longer dwell times (250-550mm / 9.8-21.6").
Smoothness and printability: slightly worse vs. roll applicator due to fiber swelling after blade metering. Need to have some dewatering in dwell zone to form a filter cake to prevent poor coating coverage under the blade.
Higher solids: get better printability, but lower gloss.
Jet angle: typically 20-50°.
Jet reflection: when jet is reflected off backing roll towards the blade beam. Jet backflow - jet angle is too big or velocity too high.
Gap: Applicator jet gap to backing roll ranges from 5-20mm. There is no film split pattern or overflow. Jet flow is laminar.
Critical factors: air induced coat skip, nozzle plugs, jet reflection, backflow, and barring.
Jet angle depends on operating conditions, web boundary air, paper roughness, and color viscosity.
Jet coated papers exhibit lower flock scale roughness than applicator roll coated board.
Benefits of jet coating: no spare rolls, cleaner and no splashing, minimal coating loss, easy to clean, easy to set dry edge deckles.
Jet impinges onto web at around a 0.1 bar pressure vs 1 bar for roll applicator. Less dewatering in dwell zone.
Increasing speed reduces gloss for both jet and applicator roll coaters, low viscosity colors have higher gloss than high viscosity colors.
Can do Burnout tests to measure coating coverage. Oxidized paper turns black, coatings do not, measure brightness of sheet.
For assistance improving your coating process, contact your Valmet representative.