The last meters of a paper machine are where surface, gloss, and density are set — and where cleanliness and machine vision now make or break yield. Mills are doubling down on calender/coater upkeep and online inspection to head off quality defects in real time.
Calender configurations and finish targets
Paper finishing hinges on the calender — a stack of rolls that compresses the sheet in the “nip” (the contact line between two rolls) to boost smoothness, gloss, and density at the expense of bulk. Valmet describes calendering as compaction that “improves the surface properties [–] smoothness and gloss” while balancing opposite properties (www.valmet.com).
Plants run both on-machine calenders at the dry end and off-line supercalenders (stacks of alternating hard and soft rolls). Hard‑nip calenders, often 2–6 rolls, target high gloss; soft‑nip or multi‑nip stacks — 10–20 rolls alternating steel and fiber covers — deliver very smooth surfaces with moderate density (www.ameteksurfacevision.com) (www.valmet.com). A supercalender — an off‑line stack of steel and cotton/felt rolls — can push gloss on coated papers, while on‑line calenders trim smoothness and caliper on newsprint or packaging grades (www.ameteksurfacevision.com) (www.valmet.com). Valmet’s “hard” and “soft” OptiCalender lines are deployed as final or pre‑coat calenders across board and fine‑paper grades (www.valmet.com).
Coating machines and integrated sequences
Alongside calenders, coating machines apply uniform layers for printability, barriers, or other functions. Common coater types include blade (knife), metering‑rod, air‑knife, and curtain coaters. One case in point: APP Sinar Mas installed a new off‑line coating machine in Indonesia in 2021, adding ~60,000 t/y of high‑end coated board (app.co.id). In practice, calenders and coaters often work in tandem: coated stocks may be pre‑calendered before coating and post‑calendered after coating to lock in finish (www.valmet.com).
Contamination sources and defect risks
Cleanliness on calender roll faces and within coating equipment is non‑negotiable. Airborne dust or oil films on a calender roll transfer every revolution onto the sheet, seeding periodic streaks or spots (www.mdpi.com). On the coater, residual grease or build‑up on a blade or roll leads to “repellant” spots — areas of missing coat where contamination prevents wetting (www.pffc-online.com).
Contaminants originate from powdered additives, machine oils, or shed fiber/pitch; one analysis highlights “oil, dirt, or grease from the substrate manufacturing process and from motors and bearings” as causes of coating spots (www.pffc-online.com). An Indonesian mill also logged “dirty paper” and foreign inclusions (e.g., fiber clumps) among top production defects (journal.ipb.ac.id). Left unchecked, defects degrade product grade and can escalate — a clogged coater slot or fiber lump on a calender can trip the process and cause downtime.
In‑situ cleaning routines on calenders and coaters
Best practice leans on scheduled purge cycles and doctor blades. Calender stacks often use in‑situ doctor blades and spray systems; before threading a new reel, a solvent or oil “mop” is applied to dissolve deposits (www.amirkabirpaper.com). Guidance includes flushing rolls with kerosene or other solvent for pitchy residues or surfactant solutions for starchy deposits, just before web threading (www.amirkabirpaper.com). Often the first meters of paper pick up the cleaning fluid and loosened dirt.
During operation, doctor blades or an oil application are kept in contact to continuously wipe the rolls; an oil mop at the calender exit “will clean most surface soils” and help eliminate roll bounce on start‑up (www.amirkabirpaper.com). Coating apparatuses receive similar care: blades and pan lips are cleaned or replaced regularly to avoid dried flake; air filters on coater dryers are maintained; pump and recirculation lines are kept free of polymer gel. Metered cleaning and additive application depend on precise delivery; equipment such as a dosing pump supports accurate chemical dosing without overfeed.
Inadequate cleaning is a known root cause of periodic defects. One consultant mapped defects repeating every 12.57 inches — exactly one roll circumference — back to a specific mandrel on that roll, removed the contamination, and stopped the defect (www.pffc-online.com). Inline filtration is often part of keeping recirculation loops clear of flakes and gel; industrial cartridge filters in robust steel filter housings help maintain clean flows without frequent shutdowns.
Automated web inspection and defect mapping
The other half of the strategy is real‑time, camera‑based inspection. Human vision cannot safely inspect a high‑speed calender or coater line; a non‑contact solution is required (www.ameteksurfacevision.com). Modern systems use arrays of high‑resolution line‑scan or area‑scan cameras (a line‑scan camera images the web line by line as it moves) with tailored lighting to continuously image the running paper. Software “scans the entire moving web and produces defect maps,” enabling immediate adjustments without interrupting the line; time‑stamped images aid root‑cause analysis (www.pffc-online.com).
Algorithms flag periodic defects quickly — any tiny roll blemish that repeats each revolution can “greatly” degrade quality, so early removal is critical (www.mdpi.com). A study showed an experimental sensor outperformed a leading commercial system in detecting such striping on hot steel strip (the principle applies to paper) (www.mdpi.com). Major suppliers (Valmet IQ, ABB WebView, Readwin, etc.) now deploy 8K+ line‑scan cameras and image processing that achieve sub‑millimeter resolution even at web speeds >1000 m/min (mobilityforesights.com). Valmet’s latest IQ Web Inspection integrates 8K line‑scan cameras with “multigeometry” lighting to capture defects at top speeds (mobilityforesights.com), and these systems claim near‑100% fault detection, flagging minute pinholes, foam marks, or fiber flakes that would escape human eyes.
Quality gains and adoption signals
Quantitatively, mills with online inspection report sharp defect reductions. For example, valves lines equipped with continuous vision have documented fewer off‑quality batches. Valmet reports over 1000 references globally for its IQ camera systems, and says its machine‑vision business “has grown significantly” in the last decade (www.valmet.com). Market context points in the same direction: a 2024 analysis projects ~4.5% CAGR for coated‑paper demand through 2030, spurring investment in quality control (www.databridgemarketresearch.com).
The maintenance payoff is tangible. A lean Six Sigma study at an Indonesian mill reported pre‑improvement sigma levels of ~3.4 for perforations and ~4.2 for “dirty paper” defects (journal.ipb.ac.id), implying sizable defect rates; thorough cleaning protocols are a key input to raising yields toward 5σ. Additional finishing equipment — e.g., size presses, laminators, or UV dryers — may be used for special grades, but calenders and coaters remain the core smoothing/glossing units (www.valmet.com) (www.ameteksurfacevision.com).
Bottom line on finishing quality control
Clean calender and coater hardware minimizes induced flaws (www.amirkabirpaper.com) (www.pffc-online.com), while real‑time inspection catches residual defects before they propagate (www.pffc-online.com) (mobilityforesights.com). Together, these measures reduce scrap and help finished paper meet the tight surface‑quality specs demanded by high‑value grades.
