Smoothness Testing of Printed Products: Standards, Methods, and Quality Control

Smoothness testing of printed products is an essential quality-control procedure that quantifies the surface evenness of paper, paperboard, and printed matter, directly influencing ink transfer, dot reproduction, gloss, and overall print quality. This article explains the key test items, internationally recognized standards such as ISO 5627, the ISO 8791 series, and GB/T 456, the major measurement methods, and typical industrial applications. Whether you are a printer, publisher, packaging manufacturer, or sourcing specialist, understanding these protocols helps ensure consistent product performance and reliable results from accredited third-party laboratories.

Technical Overview

Smoothness (or its inverse, roughness) describes the micro-geometric evenness of the surface of a paper, board, or printed item. It is one of the most influential substrate properties in printing, because the printing nip reproduces fine images by transferring ink onto the peaks and valleys of the surface. A smoother surface provides more uniform ink film formation, sharper dot reproduction, higher printed gloss, and reduced defects such as mottling, missing dots, or uneven coverage.

The smoothness of a finished printed product is determined by a combination of factors, including fiber composition and formation, filler and coating formulation, calendering intensity, and the converting processes applied afterwards, such as offset printing pressure, varnishing, film lamination, or embossing. Consequently, smoothness must be evaluated at multiple stages: on the base substrate before printing, on semi-finished sheets between process steps, and on the final printed article.

• Poor smoothness causes incomplete ink transfer and increased dot gain in offset and letterpress printing.
• Insufficient smoothness leads to missing or broken dots in gravure and flexographic printing.
• Excessive roughness reduces achievable printed gloss and scuff resistance of coatings.
• Uneven two-sided smoothness can cause quality differences between the front and reverse printed faces.

Smoothness and roughness are quantified with standardized air-leak or pneumatic instruments, and results are widely used in product specifications, supplier qualification, incoming inspection, and failure analysis. Accredited third-party laboratories routinely perform these tests under international standards to support trade acceptance and dispute resolution.

Testing Samples

Base Papers and Paperboards (Unprinted)

Typical specimens include coated art paper, light-weight coated paper, supercalendered paper, uncoated woodfree offset paper, newsprint, and coated folding boxboard. Testing the base substrate establishes the inherent surface quality before any converting or printing operation is applied.

Printed Products

Finished or semi-finished printed items such as books, magazines, brochures, folding cartons, liquid packaging board, self-adhesive labels, and flexible packaging laminates can be submitted. Measurements are normally taken on unprinted margins or solid ink areas, depending on the purpose of the evaluation, because ink layers and varnishes alter the effective surface topography.

Coated and Specialty Substrates

Cast-coated papers, metallized papers, thermal papers, and digitally printed substrates are also tested, since their coating structures and toner or inkjet layers behave differently from conventional offset-printed surfaces.

Sample Preparation and Conditioning

Specimens must be free from folds, wrinkles, dirt, and watermarks, and must not be touched on the test area. Prior to measurement, samples are conditioned to equilibrium in a standard atmosphere of 23 °C ± 1 °C and 50 % ± 2 % relative humidity in accordance with ISO 187 (equivalent to GB/T 10739). Sufficient test pieces are cut so that a statistically valid number of measurements can be made on each side of the sheet.

Testing Items

Depending on the substrate type and the end-use requirement, the following items are commonly evaluated in a smoothness or roughness testing program:

• Bekk smoothness, expressed in seconds (s), applicable to fine coated and supercalendered papers.
• Bendtsen roughness, expressed in mL/min, suitable for uncoated papers and boards of medium roughness.
• Print-surf (PPS) roughness, expressed in micrometers (µm), designed to correlate with actual printing nip conditions.
• Sheffield roughness, expressed in Sheffield units, used for rougher paperboard grades.
• Two-sidedness evaluation, comparing the smoothness of the felt (top) side versus the wire (bottom) side.
• Comparative smoothness before and after printing, coating, varnishing, or lamination processes.

Testing Methods

Bekk Air-Leak Method

The Bekk method, standardized in ISO 5627, GB/T 456, and TAPPI T 479, measures the time in seconds required for a defined volume of air (10 mL) to pass between the specimen surface and an optically flat glass ring under a controlled vacuum, while the specimen is clamped at a standard pressure of 1 kgf/cm². A longer leak time indicates a smoother surface. Both sides of the sheet are tested, and results are reported as the mean value with statistical measures. This method is most appropriate for smooth coated and supercalendered papers where high sensitivity is required.

Bendtsen Roughness Method

The Bendtsen method, specified in ISO 8791-2 and the corresponding part of GB/T 22363, measures the volume of air per unit time (mL/min) that escapes between the specimen and an annular measuring head at a constant air pressure. Higher values indicate a rougher surface. The method covers a broad measuring range and is particularly suitable for uncoated papers; the same instrument can also determine air permeance, which is useful for a complete printability assessment.

Print-Surf (PPS) Method

The Print-surf method, defined in ISO 8791-4 and GB/T 22363, was developed to simulate real printing conditions. The specimen is clamped against a circular measuring land with a resilient backing at selectable clamping pressures of 1 MPa or 2 MPa, and the leakage airflow is converted into a mean roughness value in micrometers. Because the measurement compresses the surface much like a printing nip, PPS roughness correlates closely with actual print quality and is the preferred method for coated papers and paperboards intended for high-grade printing.

Sheffield Roughness Method

The Sheffield method, described in ISO 8791-3, is another pneumatic technique that measures airflow over the specimen surface at higher flow rates, extending the measurable range to relatively rough boards and heavyweight grades where Bendtsen or PPS instruments lose resolution.

Instrument Calibration and Data Treatment

All pneumatic smoothness and roughness instruments must be verified regularly using certified reference specimens, calibrated flow meters, and checked seals and lands. Test results are evaluated against product specifications or contractual limits, and both individual and mean values are reported together with the test atmosphere, instrument type, standard applied, number of measurements, and sides tested, ensuring traceability and comparability of results between accredited third-party laboratories.

Applications

Smoothness testing of printed products and printing substrates supports quality control and decision-making across the entire paper and printing value chain:

• Publishing and commercial printing: specification of art paper for magazines, catalogs, and fine books where high gloss and sharp halftone reproduction are required.
• Packaging and labeling: qualification of folding boxboard and label facestocks for offset, gravure, and flexographic printing of premium cartons and labels.
• Paper and board manufacturing: in-process quality control of coating, calendering, and supercalendering operations, and product development of new coated grades.
• Gravure and high-definition printing: verification that substrate smoothness is sufficient to prevent missing dots in fine highlight areas.
• Converting and finishing: evaluation of the surface effect of varnishing, film lamination, or UV coating on final product appearance.
• Supplier qualification and incoming inspection: comparison of competing substrates and verification of delivered goods against agreed technical datasheets.
• Trade acceptance and dispute resolution: submission of samples to accredited third-party laboratories for neutral, standards-based testing.

FAQ

What is the difference between smoothness and roughness testing?

Both describe the same surface property from opposite directions. Smoothness is usually expressed by air-leak time in seconds (Bekk method), where higher values mean a smoother surface, while roughness is expressed as airflow rate (Bendtsen, Sheffield) or mean roughness depth (PPS), where higher values mean a rougher surface. The appropriate expression depends on the applicable standard and the substrate grade.

Which method should be chosen for coated versus uncoated papers?

Coated and supercalendered papers are typically tested by the Bekk or Print-surf (PPS) method because of their superior sensitivity on smooth surfaces, whereas uncoated papers and rougher boards are usually evaluated by the Bendtsen or Sheffield method. PPS is often preferred when the results must correlate with actual printability under printing-nip pressure.

Can smoothness be measured directly on printed surfaces?

Yes, provided the test area is flat, dry, and free of loose particles. Measurements on printed samples are usually performed on unprinted margins and, when relevant, on solid printed areas to compare the surface contribution of the ink or varnish layer. Care must be taken to allow full drying and to avoid contaminating the instrument land.

What sample size and conditioning are required?

Sample dimensions sufficient for at least ten valid measurements per side are recommended. All specimens must be conditioned to equilibrium at 23 °C ± 1 °C and 50 % ± 2 % relative humidity in accordance with ISO 187 or GB/T 10739 before testing, because moisture content significantly affects surface compressibility and air-leak behavior.

How does smoothness affect print quality?

A smoother surface increases real contact area in the printing nip, producing more uniform ink transfer, lower dot gain, finer halftone detail, better solid ink density, higher printed gloss, and fewer defects such as mottling or missing dots. However, excessively smooth or sealed surfaces can affect ink setting and drying, so smoothness must be balanced with absorbency requirements.

Are these tests recognized internationally?

Yes. The Bekk, Bendtsen, Print-surf, and Sheffield procedures are covered by ISO standards and their national equivalents such as GB/T 456 and GB/T 22363, together with related TAPPI methods. Results from accredited third-party laboratories operating under these standards are widely accepted for supplier qualification, product certification, and commercial dispute resolution.