Sizing, Coating and Finishing

1. Why the "surface" decides a sheet's fate

In EP06, EP07 and EP08 we have sent the stock onto the wire, removed water, pressed and dried it; the reeled paper is already qualified in physical strength. But in the printing plant, problems often arise in the surface few microns: once ink goes on, fibers are pulled up (picking); coating is uneven and dots cannot spread; gloss is insufficient and the image looks dull. The finishing section solves exactly this surface engineering of a few grams between the "base paper" and the "printed product."

This line usually has three steps. The first is sizing, hanging a film with a hydrophilic/hydrophobic balance on the paper surface to raise surface strength and water resistance; the second is coating, applying a coating color made of pigment and binder to the surface to cover fibers, fill pits and build a smooth bright-white printing layer; the third is finishing, using calendering, reeling and rewinding to bring width, roll diameter and surface topography into the order specification. The coating/sizing mass of the three steps together is usually only 5%–20% of paper weight, yet decides 80% of the paper's "appearance" and print performance.

On-site experience: defects in the finishing section are almost all amplified on the printing press. So this line has very low tolerance for variation — cross-machine basis-weight deviation, moisture spots, coating bars, calender spots, any of them directly becomes a customer complaint. This is why this episode treats "quality control" as a separate topic.

2. Surface sizing: the "thin glue coat" of the film-transfer era

The purpose of surface sizing is to form a continuous film on both sides of the paper, fixing the exposed fiber ends while adjusting the paper's hydrophobicity and ink absorption. Early machines used an on-machine size press to press low-consistency starch liquor into the sheet, but once machine speed exceeds 1200 m/min, the traditional "vertical dip roll + nip" structure becomes hard to run stably because of liquor-film fling-off and breaks. So film-transfer sizing (film transfer / SpeedSizer) became mainstream: two metering rolls scrape the starch liquor into a precisely thick liquid film, transfer it to the transfer roll, and finally onto the paper surface, avoiding high-speed liquor flinging.

The adhesive for film-transfer sizing is mainly oxidized starch or cationic modified starch, with solids kept at 10%–18% (too high → high viscosity, hard to film; too low → insufficient pickup). Single-side pickup is generally 1–2 g/m²/side; a thin layer is enough to markedly raise surface strength and printability. For printing papers, surface sizing also lowers Cobb water absorbency and reduces linting/dusting in printing.

After sizing, a short drying section (IR or cylinder) is usually needed to bring the film's moisture down to balance with the sheet. There is an often-overlooked balance: sizing raises surface strength, but if the adhesive film is too thick and too continuous, it clogs pores and slows ink absorption, instead causing "set-off" or poor drying. So sizing amount is not the bigger the better, but optimized jointly with the following coating and calendering.

3. Pigment coating: "burying" the fibers into a bright smooth layer

Coating is the technical core of the finishing section. The so-called coating color is essentially an aqueous dispersion of pigment + binder + additives. Pigment provides whiteness, opacity and the roughness skeleton; binder sticks pigment particles to the paper surface and to each other; additives adjust rheology, water retention and lubrication. For LWC (lightweight coated), art paper and SC grades, the coating layer is the real "printing surface."

Pigment is mainly ground calcium carbonate (GCC), because of its high whiteness, low cost and gloss improvement, usually prepared at high solids of 70%–78% to save drying energy; precipitated calcium carbonate (PCC) is also used to adjust morphology, and kaolin to adjust ink absorption. The binder is mainly styrene-butadiene latex (SB latex) and styrene-acrylate, at 8–15 phr per 100 parts pigment. The additive system includes dispersant (polyacrylate, 0.1%–0.3%), lubricant (calcium stearate, anti-stick-roll), water-retention agent (CMC, controls coating penetration and leveling on the paper surface).

Coating preparation is a rheology game: keep low viscosity at high concentration for pumping and coating, yet rapidly "stand up" without flowing or penetrating once on the paper. The role of water-retention and thickening agents is to open the time window between "flow before coating" and "set after coating." This is also the most time-consuming part when debugging coating formulations.

4. The three coating musketeers: blade, air-knife and film press

How the coating color is "placed" on the paper decides the final coating topography. Engineering-wise there are three mainstream methods, each with its fit:

Blade coating is the first choice for smoothness, but the blade is sensitive to the base paper's "peaks" — if the base paper has pits, the blade cannot pass and leaves uncovered points; air-knife is the most uniform yet limited by speed and solids; film transfer wins on high-speed stability and the economy of mid coat weights. Modern coaters often parallel multiple heads (pre-coat + top-coat) on one machine, optimizing in stages.

After coating comes coating drying. The coating holds about 30% water and must be evaporated in a dryer (flotation or cylinder type) tens to over a hundred meters long. Flotation drying uses air flow to suspend the web on both sides and advance it, avoiding contact damage to the coating; cylinder drying relies on contact heat transfer and is more compact. The drying curve must be gentle to prevent coating bubbling and cylinder sticking.

5. Calendering and surface topography: the gloss, smoothness and bulk triangle

Coated paper needs to be "bright," relying on calendering. The calender uses one or more roll pairs to press the sheet flat under heat and pressure; flattening the pits and compacting the coating raises gloss and smoothness. But the harder the press, the lower the bulk and stiffness, and the paper becomes "harder and thinner" — an eternal triangular trade-off.

Calendering has three tiers. Hard calender (machine calender) uses steel roll against steel roll, high line load, low temperature, giving uniform smoothness but limited gloss; soft-nip calender uses one steel roll paired with one elastic-covered roll (polyamide or epoxy), roll temperature 150–250 ℃, line load 200–350 kN/m; the elastic roll can "fill" rather than "collapse," achieving high gloss while keeping bulk — soft-calendered LWC gloss can reach 55–75 GU; supercalender uses multi-roll stacks (steel roll + paper/cotton roll alternating), relying on high line load and friction heat for ultimate smoothness on SC paper, gloss can exceed 80 GU but bulk loss is huge.

6. Quality control: the finishing section only cares about "variation"

The quality indicators of the finishing section almost all revolve around "uniformity." First is basis-weight CV (coefficient of variation), required below 2% (std-dev/mean); once the cross-machine basis weight drifts, the following coating and calendering amplify it into visible bars; second, cross-machine moisture controlled within ±0.5% — uneven moisture causes curling and print misregister; third, surface-topography indicators — PPS roughness (Parker Print Surf, μm scale), gloss (GU, 60° angle), IGT pick speed (printing surface strength, measured by IGT tester with wax/ink, result in m/s or wax bar number).

These indicators are not isolated. For example, raising coating weight lowers PPS roughness and raises gloss, but bulk and stiffness drop; insufficient sizing gives low IGT pick speed and printing linting. So on-site a scanner monitors basis weight, moisture, ash and thickness on-line, paired with lab off-line PPS, gloss and IGT measurement, forming a "on-line + off-line" double gate. Any one out of bounds must be traced upstream (coating formula, coating head, calender roll temperature), not just adjusted at one point.

7. Rewinding, sheeting and reeling: "delivering" the paper

The calendered web must be cut to the customer's spec and wound into a tight, uniform roll. The core difficulty of the rewinder is tension taper control: from core to outer wrap, if tension is constant, the outer wrap develops "ribbing" or burst from accumulated stress; so tension must decrease as roll diameter grows (taper curve), making the in-roll stress distribution uniform. The winding hardness (hardness profile) is scanned on-machine with a beam detector; hard/soft spots correspond to downstream print-tension nonuniformity.

Continuity relies on flying splice: when a parent roll is nearly used up, the new roll is automatically joined at high speed, the whole machine never stops. Splice quality (tape, overlap length) directly relates to non-stop roll change on the press. For cut-size sheets, the sheeter must guarantee cutting accuracy ±0.5 mm, squareness and paper-dust control, otherwise the copier jams on feed.

Here, a sheet of paper from forest fibers, through pulping, refining, forming, pressing and drying, is finally "set" in sizing, coating, calendering and rewinding into the bright, smooth finished roll in the printer's hand. Next episode we step back to see how this paper, after use, becomes fibers again — recovered fiber and printability, and the finale of the whole series.

8. Common finishing-line defects and on-site countermeasures

What paper mills fear most in the finishing section is "visible defects" — because these are direct customer complaints. Streaks are the most common type, appearing as periodic or random dark longitudinal bars on the sheet: if the streak period corresponds to a roll's circumference, it is mostly backup-roll runout or a nicked doctor blade; the countermeasure is replacing the doctor blade, polishing the metering roll, and checking the backup-roll dynamic balance. Orange peel is coating surface-tension nonuniformity and insufficient water-retention agent causing the coating to retract into scale-like patches on the paper; raising the CMC water-retention agent and appropriately lowering viscosity usually works immediately.

Calender spots come from foreign matter embedded in the roll surface or local aging stickiness of the elastic roll; cleaning the roll surface and checking transverse roll-temperature uniformity is the first step. Reel "ribs" (hardness bar) are from a poorly set tension-taper curve or parent-roll alignment deviation; re-setting the taper and alignment eliminates them. Linting/dusting points to insufficient surface sizing or weak coating bond; raising sizing amount or latex ratio fixes it at the root. There is an iron rule for on-site troubleshooting: first determine whether the defect is "periodic or random, MD or CD, surface or internal," then trace the corresponding equipment — never blindly adjust the coating formula; many so-called "coating problems" are actually roll or blade problems.

9. Energy and cost of the finishing line

The finishing section alone is not the machine's biggest energy user, but coating drying is its energy peak: the coating holds about 30% water, all evaporated in the dryer; flotation drying is coating-friendly but has large air volume and high heat loss. Engineering pushes coating solids all the way to 70%–78%, the primary purpose being to evaporate less — empirically every 1 percentage point rise in solids lowers drying energy and exhaust volume in sync, the most cost-effective energy lever on the coating line. Soft calendering trades heat for gloss; roll temperature 150–250 ℃ relies on steam or electric heating, and this heat should be recovered into the machine's thermal system rather than directly exhausted.

In the cost structure, pigment (GCC/PCC) and latex dominate coating cost; latex price fluctuates with petrochemical feedstock, directly moving the per-ton cost. So formulation engineers repeatedly trade off between "performance — cost": use high filler to lower basis weight rather than thick coating, use efficient film-press coating rather than slow air-knife. For the mill, the finishing line's KPI is never "made most beautifully" but "made cheapest within the customer's spec red line."

10. From base paper to printing surface: how finishing decides final pricing

Different grades' "value addition" depends heavily on the finishing process. LWC uses mid coating + soft calender to get adequate gloss while keeping bulk, the mainstay of mass-market magazines; art paper uses thick coating (12–20 g/m²/side) + blade coating for ultimate dot reproduction and high gloss, with the highest pricing; SC paper does the opposite, using high filler + supercalendering instead of thick coating to get smoothness, lowest cost but greatest bulk loss, aimed at low-price publications. For sales, the four numbers "coating weight, gloss, PPS, IGT" basically decide which market tier this paper can sell into.

This also explains why the finishing section is willing to pay a high price for "small items" like calender rolls and doctor blades — they directly lever the per-ton premium. Finishing is not a cost center but a pricing center: with the same base paper, the right finishing route can sell for several hundred yuan more per ton; the wrong one either fails spec and gets returned, or over-finishes and grinds away the profit. Truly understanding this line means truly reading a printing paper's value chain.

11. Automation and scanner of the finishing section

Modern finishing lines are almost entirely strung together by scanners and process-control systems: on-machine β-ray or X-ray for basis weight, near-infrared for moisture and ash, microwave or nuclear for thickness; transverse arrays of hundreds of measurement points assemble a "cross-machine profile." The operator watches not the single-point mean but the "profile shape" — one side high and the other low means the headbox slice or coating cross-profile is mis-set; periodic fluctuation means a roll is eccentric. Soft-calender roll temperature, coating metering-roll gap and rewind tension taper are all closed-loop adjusted by these signals.

For technicians, reading this profile is more useful than memorizing a string of formula numbers: it translates "bad paper" into the specific location of "which roll, which slice section." In addition, the finishing line's "digital twin" is gradually landing — connecting the coating-rheology model, drying heat-transfer model and calendering elasto-plastic model to real-time data to predict gloss and PPS in advance, rather than waiting for lab results to return before adjusting. This is especially critical for paper machines above 1200 m/min: from sampling to result return often several kilometers of paper have passed; closed-loop prediction can advance the adjustment before the defect occurs.

Finally a reminder: all parameters of the finishing line serve the ultimate criterion that "the printing press is willing to accept it." No matter how advanced the scanner or how expensive the calender roll, whether the paper can run at high speed in the print shop and reproduce dots well is the only standard to test finishing success. So the feedback loop between the finishing section and the print shop matters more than any single-machine indicator — treating the press's complaints as the finishing line's debugging signal is often more effective than staring at the dashboard.