Printing paper smoothness testing measures the micro-scale surface evenness of paper and board, a property that directly governs ink transfer, dot reproduction, and overall print quality. Using internationally recognized air-leak methods such as Bekk (ISO 5627, GB/T 456), Bendtsen (ISO 8791-2), and Parker Print-Surf (ISO 8791-4), manufacturers and printers can objectively characterize printability, optimize coating and calendering processes, and resolve quality disputes. This article explains the principles, sample requirements, test procedures, and practical applications of smoothness testing for printing papers.
Technical Overview
Smoothness (or, expressed inversely, roughness) is one of the most influential surface characteristics of printing paper. It describes the micro-geometry of the sheet surface, that is, the height distribution of the microscopic hills and valleys formed by fibers, fillers, and coating layers. Because most printing processes transfer ink through direct contact between a plate, blanket, or toner/fuser nip and the paper surface, even minor surface irregularities can produce visible defects such as missing dots, broken halftone images, mottling, ink starvation in solid areas, and uneven ink gloss.
All mainstream smoothness tests are based on the air-leak principle. A precisely machined, optically flat measuring head or annular land is pressed against the paper surface under a defined pressure, and the rate at which air escapes through the micro-voids between the land and the sheet is measured. A smoother surface restricts airflow and therefore yields a longer passage time (Bekk, Oken) or a lower flow rate (Bendtsen, Sheffield); the Parker Print-Surf method converts the leakage into an equivalent gap height expressed in micrometers. Because results depend on instrument geometry, pressure, and backing, the specific method and standard must always be stated alongside the numerical result.
Typical smoothness levels vary widely by grade: supercalendered and coated art papers may exceed 600 to 2,000 Bekk seconds or fall below 1.5 µm PPS, while uncoated woodfree and newsprint grades commonly show 20 to 300 Bekk seconds. Testing is therefore essential for grade classification, process control, and printability prediction.
Testing Samples
Coated Printing Papers
Coated grades such as gloss, silk, and matte art paper, light-weight coated (LWC) paper, and cast-coated paper possess pigment coating layers that fill the base-sheet valleys and produce very high smoothness. Because differences between premium grades are small, the Parker Print-Surf method (ISO 8791-4) is often preferred, as it resolves fine surface differences under conditions that simulate an actual printing nip.
Uncoated Printing Papers
Woodfree offset paper, bond paper, newsprint, and business-form papers rely on refining, filler loading, and calendering rather than coating to develop smoothness. These grades are traditionally characterized with the Bekk method (ISO 5627, GB/T 456) or the Bendtsen method (ISO 8791-2), both of which cover the medium-to-rough surface range typical of uncoated sheets.
Paperboard and Specialty Grades
Folding boxboard, white-lined chipboard, label facestock, carbonless paper, and digital or inkjet-printable papers can also be evaluated. For boards intended for high-quality litho or gravure printing, Print-Surf roughness under both 1,000 kPa and 2,000 kPa clamping pressure is frequently specified, since board compressibility affects the apparent surface finish under load.
Sample Preparation and Conditioning
All specimens must be conditioned to equilibrium in a standard atmosphere of 23 °C ± 1 °C and 50 % ± 2 % relative humidity in accordance with ISO 187 before measurement. Test pieces must be free of wrinkles, folds, water marks, fingerprints, and visible defects, and must not be touched in the measurement zone. Because the two sides of a sheet almost always differ, both the felt (top) side and the wire (bottom) side are tested separately, normally with at least ten valid test pieces per side.
Testing Items
A typical smoothness test report for printing paper includes the following items:
- Bekk smoothness of each side, in seconds, including mean, maximum, and minimum values
- Bendtsen roughness, in mL/min, at the specified air pressure and clamping pressure
- Parker Print-Surf (PPS) roughness, in micrometers, at 1,000 kPa and/or 2,000 kPa with a defined soft backing
- Sheffield roughness, in mL/min, where specified by ISO 8791-3
- Two-sidedness, expressed as the difference or ratio between wire-side and felt-side values, a key indicator for duplex printing
- Standard deviation and coefficient of variation, reflecting sheet uniformity
- Identification of the test standard, instrument type, and conditioning atmosphere used
Testing Methods
Bekk Method (ISO 5627 / GB/T 456)
The Bekk instrument presses an optically flat glass anvil with an annular land onto the specimen and draws air through the interface under a regulated sub-atmospheric pressure. The time required for a defined volume of air, typically 10 mL, to pass is recorded; a shorter measurement range of 1 mL is used for very rough papers, with the result converted proportionally. Results are reported in seconds, and higher values indicate a smoother surface. This method is the reference procedure for uncoated printing papers and remains the most widely cited smoothness figure in Chinese and international trade specifications.
Bendtsen Method (ISO 8791-2 / GB/T 22363)
The Bendtsen head applies air at a low, constant pressure across a narrow annular land pressed against the sheet. The escaping airflow is measured by a float-type flowmeter and reported in mL/min. Lower flow rates correspond to smoother surfaces. The method is fast, well suited to routine mill control, and particularly useful for uncoated and lightly coated grades in the medium roughness range.
Parker Print-Surf Method (ISO 8791-4 / GB/T 22363)
The Print-Surf instrument clamps the specimen between a spring-steel measuring land and a soft, resilient backing that deforms much like a printing blanket. Air leakage under a high clamping pressure of 1,000 kPa or 2,000 kPa is converted into an equivalent hydraulic mean depth of the surface, reported in µm PPS. Because the geometry reproduces the compression conditions of offset and gravure nips, PPS values correlate closely with actual print density and missing-dot behavior, making this the preferred method for coated papers and paperboards.
Sheffield and Oken Methods (ISO 8791-3 / JIS P 8119)
The Sheffield method measures airflow in mL/min across a larger land area and covers a wide roughness range, while the Oken method, standardized in JIS P 8119 and common in East Asian practice, is a time-based air-leak technique conceptually similar to Bekk. These methods are used when regional specifications or legacy datasets require them.
General Test Procedure
- Condition the sampled sheets to the ISO 187 standard atmosphere and allow the instrument and reference roughness plates to stabilize in the same environment.
- Verify instrument calibration using certified smoothness or roughness reference plates and confirm zero settings before each measurement series.
- Cut at least ten test pieces per side, avoiding edges, watermarks, and damaged areas, and mark the felt and wire sides clearly.
- Place each test piece against the measuring land, apply the specified clamping pressure, and record the air passage time or flow rate once the reading has stabilized.
- Repeat for all test pieces on both sides, discard invalid readings caused by sheet defects, and calculate the mean, extremes, and two-sidedness.
- Report results together with the test standard, method, instrument parameters, and conditioning atmosphere.
Applications
Smoothness testing supports quality control and product development throughout the paper and printing value chain:
- Mills use continuous smoothness data to control calendering, coating weight, and supercalendering operations, keeping production within grade specification.
- Coating and surface-sizing formulation studies rely on smoothness trends to compare pigment blends, binders, and surface treatment strategies.
- Commercial and publication printers verify incoming paper quality and match paper smoothness to the printing process, for example selecting high-PPS-smooth grades for fine halftone and low-resolution reproduction.
- Grade specifications, purchase contracts, and export documentation cite smoothness limits, and accredited third-party laboratories provide impartial results for trade acceptance and dispute resolution.
- Failure analysis of print defects such as missing dots, mottle, snow-flaking in solid areas, and non-uniform ink gloss routinely begins with smoothness and two-sidedness measurements.
- Developers of inkjet and electrophotographic papers use smoothness data to balance surface evenness against ink absorption and toner adhesion requirements.
FAQ
What does smoothness actually measure?
It quantifies the micro-flatness of the paper surface by measuring how much air leaks between the sheet and a flat reference surface under pressure. A smoother sheet produces closer surface contact, slower air leakage, and better ink transfer during printing.
Which test method should be selected?
The choice depends on the paper grade and the specification involved. Bekk (ISO 5627, GB/T 456) suits uncoated and calendered grades, Bendtsen (ISO 8791-2) is convenient for routine control of medium-roughness papers, and Parker Print-Surf (ISO 8791-4) is preferred for coated papers and boards because it simulates real printing-nip pressure. Sheffield (ISO 8791-3) and Oken (JIS P 8119) are used where those conventions apply.
Is a higher number always better?
Not necessarily. For time-based Bekk and Oken results, higher values indicate smoother paper, whereas for flow-based Bendtsen and Sheffield results, lower values indicate smoother paper, and for PPS, lower micrometer values are smoother. Values from different methods are not directly interchangeable.
Why is two-sidedness important?
The wire side of a machine-made sheet is almost always rougher than the felt side because of wire-mark drainage patterns. Excessive two-sidedness causes visible print quality differences between the two sides of a sheet, which is critical for books, magazines, and duplex printing applications.
How many samples are required?
Standard practice requires at least ten valid test pieces per side, taken from several sheets distributed across the sample so that machine-direction and cross-direction variation is represented.
Can smoothness results be used for certification or dispute settlement?
Yes. When tests are performed by accredited third-party laboratories in strict accordance with ISO or GB methods, the resulting reports are widely accepted as objective evidence of conformity in supplier qualification, import-export inspection, and quality disputes.