Newsprint Smoothness Testing: Principles, Standards, Methods, and Applications

Newsprint smoothness testing is an essential quality control procedure that quantifies the surface evenness of newsprint paper, a property that directly governs print sharpness, halftone dot reproduction, ink transfer, and ink consumption in high-speed newspaper printing. This technical article explains the principles of air-leak smoothness measurement, the governing international and national standards such as ISO 5627, ISO 8791, and GB/T 456, correct sample preparation and conditioning, step-by-step test procedures, and the practical applications of smoothness data for paper mills, printers, and publishers.

Technical Overview

Newsprint is a lightweight, low-cost printing paper with a grammage typically ranging from 40 g/m² to 49 g/m², manufactured predominantly from mechanical pulps (stone groundwood, thermomechanical pulp, or chemi-thermomechanical pulp) blended with a smaller proportion of chemical pulp for strength. Because newsprint is generally unsized or only lightly sized and uncoated, its surface topography is dominated by fiber contours, fines distribution, and filler particles, making surface smoothness one of its most important printability attributes.

Smoothness is defined operationally as the degree of surface evenness of paper, quantified by the resistance the surface offers to the flow of air between itself and a reference flat surface under specified pressure conditions. Almost all routine smoothness measurements are air-leak methods: the specimen is clamped against a polished, optically flat measuring land, and either the time required for a fixed volume of air to pass through the residual gap (smoothness, expressed in seconds) or the rate of air flow through the gap (roughness, expressed in mL/min or µm) is determined. A smoother surface forms a tighter seal against the land, producing a longer passage time or a lower flow rate.

For newsprint, smoothness determines how completely and uniformly the ink film contacts the paper surface in the printing nip. Insufficient smoothness leads to missing dots, mottled solids, reduced print contrast, increased ink demand, and a higher risk of print-through on the low-grammage web. Because newsprint is two-sided, with a smoother felt (top) side and a rougher wire side, both sides must be characterized to control two-sidedness and to assign the more printable side to the image-bearing units of the press.

Testing Samples

Sample Selection and Conditioning

• Sampling should follow a recognized sampling standard such as ISO 186 so that the specimens are statistically representative of the lot.
• Specimens must be free of wrinkles, creases, folds, water spots, visible dirt, and any area that has been touched, folded, or printed.
• Each test piece must be large enough to fully cover the clamping head of the instrument, typically at least 50 mm × 50 mm, cut with clean edges without altering the surface structure.
• Before testing, samples shall be conditioned to equilibrium in the standard atmosphere defined by ISO 187, i.e., 23 °C ± 1 °C and 50 % ± 2 % relative humidity, because moisture content strongly influences the conformity of the paper surface against the measuring land.
• A minimum of ten valid test pieces per side is recommended so that the arithmetic mean, range, and 95 % confidence interval can be calculated with acceptable precision.

Side Identification and Two-Sidedness

The top (felt) side and the wire (bottom) side of the newsprint web must be clearly marked during specimen preparation and tested separately. The difference between the two sides, reported as two-sidedness, is a key acceptance parameter because excessive two-sidedness causes visible print quality differences across the newspaper spread.

Testing Items

Depending on the standard invoked and the instrument available, the following quantities are reported for newsprint:

• Bekk smoothness in seconds (s), the mean time for a fixed air volume to pass between the specimen and the glass land; a larger value indicates a smoother surface.
• Bendtsen roughness in mL/min, the mean air flow rate escaping between the specimen and the annular measuring land at a defined pressure; a larger value indicates a rougher surface.
• Parker Print-Surf (PPS) roughness in micrometers (µm), representing the mean gap between the paper surface and the land under printing-nip-like pressure; a smaller value indicates better printability.
• Two-sidedness, calculated as the difference or ratio between the values obtained on the top side and the wire side.
• Statistical descriptors of the data set: arithmetic mean, maximum, minimum, standard deviation, and the 95 % confidence interval, as required by ISO 8791-1.

Testing Methods

Bekk Smoothness Method (ISO 5627 / GB/T 456)

The Bekk method is the classical and most widely referenced smoothness procedure for newsprint in markets following ISO and GB standards. A test piece is clamped under a pressure of approximately 100 kPa against an optically flat, ring-shaped glass surface. A vacuum is applied, and atmospheric air is drawn through the microscopic gap between the paper surface and the glass land. The instrument measures the time, in seconds, required for a defined volume of air (nominally 10 cm³) to pass, detected by the movement of a liquid column between two calibrated marks. For smooth papers that would require excessively long measurement times, a smaller measuring land is used and the instrument converts the reading to the equivalent common scale. GB/T 456, the Chinese national standard, and corresponding national adoptions such as JIS P 8119 are technically aligned with ISO 5627, so results are broadly comparable when instrument and conditioning requirements are respected.

Bendtsen Roughness Method (ISO 8791-2)

In the Bendtsen method, the specimen is held against a flat measuring head containing a central air inlet and an annular land. Air at a defined low pressure (nominally 1.47 kPa) escapes through the interface between the paper surface and the land, and the flow rate is read in mL/min. Because rougher surfaces allow more air to escape, the Bendtsen value increases with roughness, which makes the method particularly suitable for the relatively open, uncoated surface of newsprint. The measurement is quick, and different flow ranges cover everything from supercalendered to highly rough grades.

Parker Print-Surf Method (ISO 8791-4)

The Parker Print-Surf method was specifically developed to correlate with actual printing performance. The specimen is backed by a resilient, deformable backing into which surface fibers can be pressed, closely simulating the deformation that occurs against a rubber blanket in an offset printing nip. A selectable clamping pressure of 500 kPa, 1000 kPa, or 2000 kPa permits testing under conditions representative of different press configurations. The result, expressed in µm, estimates the effective void volume between the paper surface and the land, and it is widely used to predict missing-dot risk and required ink film thickness on newsprint and other publication papers.

General Test Procedure

• Verify and calibrate the instrument using certified reference surfaces and calibrated flow or time standards, and confirm the zero reading before the test series.
• Condition the specimens to equilibrium in the ISO 187 standard atmosphere and perform the test in the same atmosphere.
• Place the first specimen against the measuring head with the side to be tested facing the land, without touching the measurement area.
• Apply the specified clamping pressure and observe the pre-loading or waiting time defined by the method so that the surface fully conforms.
• Record the reading, then repeat on subsequent specimens, distributing test positions across the sample width to capture cross-direction variability.
• Repeat the entire sequence for the reverse side, then calculate the mean, range, standard deviation, and 95 % confidence interval.
• Report the standard used, instrument type, side or sides tested, clamping pressure, land or orifice used, conditioning details, and all statistical results.

Applications

• Mill process and quality control: routine monitoring of machine calendering and soft-calender settings, furnish changes, filler addition, and wet-press performance to keep smoothness within the grade specification.
• Product specification and acceptance testing: verification of delivery conformity against product standards such as GB/T 1910 for newsprint, or against contractual limits agreed between producers, printers, and publishers.
• Printability assessment: prediction of ink film requirement, missing-dot behavior, print-through tendency, and mottling before full-scale web offset or flexographic production.
• Research and development: evaluation of new pulps, refining strategies, filler systems, and calendering technologies for improved surface quality at minimal fiber and energy cost.
• Troubleshooting and claims analysis: comparison of smoothness and two-sidedness data from retained samples to diagnose the origin of print defects or pressroom complaints in accredited third-party laboratories.
• Audit and certification support: generation of traceable, standards-compliant test reports required for supplier qualification and periodic requalification programs.

FAQ

Why is smoothness so important for newsprint?

Newsprint is printed at very high speeds with thin ink films and without a coating layer to level the surface. Smoothness therefore directly controls halftone dot contact, dot sharpness, print density, ink consumption, and the risk of print-through on a low-grammage web, making it one of the most closely monitored printability properties of the grade.

What smoothness values are typical for newsprint?

Indicative ranges for standard newsprint are approximately 15 s to 60 s Bekk smoothness, roughly 100 mL/min to 250 mL/min Bendtsen roughness on the top side, and roughly 2.0 µm to 3.5 µm PPS roughness at 1000 kPa, while supercalendered and improved newsprint grades achieve noticeably better values. Exact limits depend on the grade specification and the agreements between producer and printer, so the applicable product standard or contract values should always be consulted.

Which side of the sheet should be tested?

Both sides should be tested. The felt side is normally smoother than the wire side, and the difference is reported as two-sidedness. Printers aim to present the smoother side to the image-bearing units, so separate characterization of each side is essential.

How do Bekk smoothness and Bendtsen or PPS roughness results relate?

They are inverse expressions of the same surface property measured with different geometries and pressures. Bekk reports the time for a fixed air volume to pass, so the number increases as the surface becomes smoother, whereas Bendtsen and PPS report air flow or gap height, so their numbers increase as the surface becomes rougher. The methods correlate only approximately, and each result must be interpreted against the scale of the method used.

What factors influence newsprint smoothness?

The dominant factors are the mechanical pulp content and fines fraction, filler loading and particle size, refining intensity, basis weight and formation, wet pressing, drying strategy, and above all the calendering conditions, since machine and soft calendering flatten fiber ridges and consolidate the surface. Moisture content at calendering and at testing also has a measurable effect, which is why conditioning is strictly controlled.

How frequently should smoothness be tested?

For production control, mills typically test each reel or set at defined intervals during each production run, while incoming inspection at printing houses and acceptance testing by accredited third-party laboratories is usually performed per delivery lot according to the sampling plan agreed in the purchase specification.