ISO 8791-4:2021

Overview

ISO 8791-4:2021 - Paper and board: Determination of roughness/smoothness (air leak methods) - Part 4: Print‑surf method specifies the standardized Print‑surf method for measuring the surface roughness of printing papers and boards. The method quantifies the mean air gap between a flat circular measuring land and the sheet surface by measuring air flow or pressure drop. It applies where a substantially airtight seal can be formed against the sensing‑head guard lands.

Key topics and technical requirements

• Measurement principle: Air leak (Print‑surf) method - air flows across the measuring land; flow rate or pressure differential is converted to a roughness value expressed in micrometres.
• Instrument types:
  • Variable‑area flowmeter: standard pressure (6.2 kPa or 19.6 kPa) across the measuring land; flowmeters calibrated to show the “cube root mean cube gap” (see Annex A).
  • Impedance type: controlled pressure source and fluidic impedance with pressure transducers; typically displays 0.6 µm to 6.0 µm to 0.1 µm resolution.
• Sensing head geometry: three concentric annular lands - measuring land width 51.0 µm ± 1.5 µm, effective length 98.0 mm ± 0.5 mm; guard lands ≥ 1 000 µm; radial spacing between guard lands 152 µm ± 10 µm.
• Clamping pressures: standard clamping pressures of 980 kPa ± 30 kPa and 1 960 kPa ± 30 kPa (instrumentation and protective‑collar spring force must be accounted for).
• Backings: two resilient backings are specified:
  • Soft backing: offset blanket assembly, ≈2 000 µm thick, apparent hardness 83 IRHD ± 6.
  • Hard backing: polyester film assembly, apparent hardness 95 IRHD ± 2.
• Sampling and test pieces: sample selection per ISO 186; conditioning per ISO 187; at least 10 test pieces per side, 100 mm × 100 mm each.
• Calibration and maintenance: procedures and requirements for calibration (Annex C) and maintenance of testers (Annex B); precision data in Annex D.

Applications and users

• Primary use: objective measurement of surface roughness/smoothness to assess printability, ink transfer, and runnability for printing papers and boards.
• Who uses it:
  • Paper and board manufacturers - quality control and specification verification.
  • Printing houses and prepress labs - selecting substrates and optimizing press settings.
  • Test labs and standards organizations - accredited roughness testing and inter‑laboratory comparisons.
  • R&D teams - formulation, coating and surface treatment development.
• Practical value: provides repeatable, instrument‑based roughness metrics that correlate with tactile properties and printing performance; useful for acceptance criteria, process control and supplier communication.

Related standards

• ISO 186 - Sampling to determine average quality (referenced).
• ISO 187 - Standard atmosphere for conditioning and testing (referenced).

Keywords: ISO 8791-4:2021, Print‑surf method, paper roughness, air leak methods, roughness measurement, paper smoothness, printing paper testing, paper and board standards.