Printing Paper Smoothness Testing: Principles, Standards, and Laboratory Methods

Printing paper smoothness testing quantifies the surface evenness of paper, a property that directly governs ink transfer, dot reproduction, and overall print quality. This article explains the air-leak measurement principles behind the Bekk, Bendtsen, and Parker Print-Surf methods, the relevant ISO, GB/T, TAPPI, and JIS standards, specimen preparation requirements, and how manufacturers and printers apply smoothness data in quality control. Whether you are specifying coated art paper or calibrating a calendering line, understanding these test procedures helps ensure consistent, predictable print performance.

Technical Overview

Smoothness is one of the most critical surface properties of printing paper. It describes how flat and even the paper surface is at a microscopic scale, which determines how uniformly the paper contacts a printing plate, blanket, or digital imaging roller. A smooth surface allows fine halftone dots to reproduce faithfully with sharp edges, while a rough surface causes dot gain, missing dots, mottled density, and increased ink consumption.

Because surface irregularities cannot be assessed reliably by visual or tactile inspection, the paper industry measures smoothness with instrumented air-leak methods. In these techniques, the specimen is clamped against a precision-ground, ring-shaped glass or metal land, and either the time required for a fixed volume of air to pass between the paper and the land, or the rate of air flow at constant pressure, is measured. A rough surface allows air to escape quickly, while a smooth surface restricts air flow.

The most widely referenced methods and standards include:

• ISO 5627 and GB/T 456 — Determination of smoothness (Bekk method), reported in seconds
• ISO 8791-2 — Roughness determination by the Bendtsen method, reported in mL/min
• ISO 8791-3 — Roughness determination by the Sheffield method
• ISO 8791-4 and TAPPI T 555 — Roughness determination by the Parker Print-Surf (PPS) method, reported in micrometers
• TAPPI T 479 — Smoothness of paper (Bekk-type instrument)
• JIS P 8119 — Oken-type smoothness testing used primarily in the Japanese market

Accredited third-party laboratories routinely perform these tests under conditioned atmospheres, providing objective data for product development, process control, incoming inspection, and commercial specification disputes.

Testing Samples

Coated Printing Papers

Coated grades such as art paper, matte coated paper, and lightweight coated (LWC) paper are the smoothest commercial printing papers. Their pigment coating layers, refined through calendering, typically achieve Bekk smoothness values ranging from several hundred to several thousand seconds. These papers are tested to verify coating uniformity and to confirm suitability for high-quality offset, gravure, and digital printing.

Uncoated Printing Papers

Uncoated woodfree offset paper, writing paper, newsprint, and business form paper rely solely on refining, pressing, and calendering to develop surface smoothness. Typical Bekk values for these grades are substantially lower than coated papers. Testing distinguishes between grades intended for fine text and image reproduction and those intended for utility printing applications.

Paperboard and Specialty Grades

Folding boxboard, white lined chipboard, label facestock, carbonless paper, and digital printing substrates are also evaluated for smoothness because surface topography affects coating adhesion, varnish leveling, toner fusing, and adhesive performance. For boards, the Parker Print-Surf method is often preferred because its measurement geometry and backing system better simulate printing nip conditions on stiffer materials.

Testing Items

A comprehensive smoothness test program typically includes the following measurement items:

• Bekk smoothness of the top (felt) side, in seconds
• Bekk smoothness of the wire (bottom) side, in seconds
• Two-sidedness — the calculated difference or ratio between the smoothness of the two sides, an important parameter for duplex printing and coated papers
• Bendtsen roughness in mL/min, commonly specified for uncoated grades and paperboard
• Parker Print-Surf roughness in micrometers, used for print-runnability specifications and coated board
• Statistical parameters including mean value, maximum, minimum, standard deviation, and coefficient of variation for each side

Some laboratory programs combine smoothness testing with related surface measurements such as air permeance, porosity, and gloss to build a complete surface characterization profile of the paper.

Testing Methods

Specimen Conditioning and Preparation

Specimens must be conditioned to moisture equilibrium in a standard atmosphere of 23 °C ± 1 °C and 50% ± 2% relative humidity in accordance with ISO 187 before testing. Because moisture content changes the flexibility of fibers and the conformity of the paper surface, unconditioned specimens can produce misleading results. Test pieces are cut to the size required by the instrument, commonly 50 mm × 50 mm or larger, free of wrinkles, folds, water marks, and visible defects. Operators handle specimens only by their edges, since fingerprints, dust, and lint in the measurement zone will artificially reduce the measured smoothness. A minimum of ten valid measurements per side is normally required.

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

In the Bekk method, the specimen is clamped against an optically flat, ring-shaped glass measuring surface under a defined pressure of 100 kPa. A vacuum is drawn beneath the specimen, and the instrument records the time, in seconds, required for a defined volume of air — normally 10 mL — to be drawn between the paper surface and the glass land as the vacuum decays from 50.66 kPa to 48.00 kPa. For very smooth papers where the measurement time becomes excessively long, a larger air volume is used and the result is converted using the appropriate factor. Higher second values indicate a smoother surface. Before testing, the instrument is verified for leaks and calibrated against certified reference plates.

Alternative Roughness Methods

The Bendtsen method measures the rate of air flow in mL/min passing between the specimen and a flat glass ring at a specified clamping pressure and constant air pressure, where lower flow rates indicate smoother surfaces. The Parker Print-Surf method measures air flow across an annular land while the specimen is backed by a resilient backing ring at pressures of 1 MPa or 2 MPa, chosen to simulate the printing nip; the result is expressed as an effective roughness depth in micrometers, which correlates closely with print density variation in offset printing. The Sheffield method operates on a similar air-leak principle with different land geometry and flow scale. The choice among methods depends on the paper grade, the applicable commercial specification, and the end-use printing process.

Data Evaluation and Reporting

The laboratory reports the arithmetic mean smoothness or roughness value for each side, along with the standard deviation and coefficient of variation to characterize measurement spread. Outliers caused by visible specimen defects are documented and excluded according to the criteria in the relevant standard. A complete test report identifies the standard applied, the instrument type, the conditioning atmosphere, the number of test pieces, the individual and mean results for each side, and any deviations from the standard procedure.

Applications

Smoothness test data support a wide range of technical and commercial decisions across the paper and printing value chain:

• Product development — comparing pigment coating formulations, binder levels, and calendering strategies to reach target surface quality
• Process control — monitoring the effect of supercalendering, soft-nip calendering, and coating station settings on surface finish in the mill
• Print quality prediction — estimating halftone dot reproduction, ink film uniformity, and print density before press trials
• Incoming material inspection — verifying that purchased paper or board lots meet contractual smoothness specifications
• Grade classification and certification — demonstrating compliance with product data sheets and national or international specifications
• Troubleshooting — diagnosing print defects such as mottling, missing dots, and uneven varnish or coating laydown

FAQ

Does a higher smoothness number always mean a smoother paper? No. Interpretation depends on the method. For the Bekk method, higher values in seconds mean a smoother surface. For Bendtsen roughness and Parker Print-Surf methods, the reported values express roughness, so lower numbers mean a smoother surface.

Why must the two sides of the paper be tested separately? The felt side and wire side of machine-made paper differ in fiber distribution and surface structure. Two-sidedness affects duplex printing, coating coverage, and laminating performance, so standards require independent measurement of each side.

How many specimens are needed for a valid test? Most standards, including ISO 5627, require a minimum of ten valid measurements per side. Additional measurements improve the confidence of the mean value when the material shows high variability.

Which method should be specified for coated paper versus uncoated paper? Bekk smoothness is widely specified for coated printing papers and fine papers in ISO and GB/T markets, while Bendtsen roughness is common for uncoated grades and boards. The Parker Print-Surf method is frequently specified where correlation with actual offset print quality is the primary concern.

Can smoothness alone predict print performance? Smoothness is a strong indicator, but printability also depends on porosity, absorbency, formation, compressibility, and moisture content. Laboratories therefore often recommend testing smoothness together with air permeance, gloss, and surface strength measurements for a complete printability assessment.

How long does a typical smoothness test take? After conditioning the specimens to the standard atmosphere, which usually requires several hours to overnight depending on the sample, instrument measurement itself is rapid. Accredited third-party laboratories typically issue a full report within three to five working days of sample receipt.