ISO 5628:2019

Overview

ISO 5628:2019 specifies general principles for determining the bending stiffness of paper and board using three laboratory bending tests: two-point, three-point and four-point methods. The standard defines measurement conditions, sampling and preparation, apparatus and evaluation so that results are comparable and do not cause significant permanent deformation of the test piece. Test pieces are treated as beams according to strength-of-materials theory.

Key topics and technical requirements

• Scope and applicability
  • Two-point and three-point methods: suitable for paper and low-thickness board.
  • Four-point method: recommended for corrugated fibreboard and thicker board.
• Theory and calculation
  • Bending stiffness is defined from beam theory (EI per unit width). The standard provides procedures and formulae to calculate bending stiffness from measured force vs. linear deflection or force vs. angular deflection.
• Measurement conditions
  • Tests are specified to avoid significant permanent deformation and to remain within the validity range of the calculation formulae.
  • The document suggests maximum allowable deflections for different methods and requires reporting of test rates and times for load or deflection application.
• Geometrical constraints
  • To minimize inter-laminar shear effects, the bending length l should not be less than approximately 40 times the specimen thickness for paper and board.
  • Width selection is left to the user but must be reported.
• Practical test elements
  • Apparatus, sampling, conditioning (reference to standard atmosphere), preparation of test pieces, test procedures, evaluation and reporting requirements are all covered.

Applications and users

ISO 5628:2019 is used by:

• Paper and board manufacturers for product specification and quality control
• Packaging engineers and converters evaluating stiffness for folding, stacking and shipment performance
• Independent testing laboratories and R&D teams developing or validating materials
• Standards writers and procurement teams requiring consistent, reproducible bending stiffness data

Practical uses include comparing material stiffness across grades, assessing suitability for boxes and cartons, and calibrating or validating commercial bending testers.

Related standards

Normative references in ISO 5628:2019 include:

• ISO 186 - Sampling to determine average quality
• ISO 187 - Standard atmosphere for conditioning and testing
• ISO 534 - Determination of thickness, density and specific volume
• ISO 3034 - Corrugated fibreboard - determination of single sheet thickness

Keywords: ISO 5628:2019, bending stiffness, paper and board testing, two-point three-point four-point methods, corrugated fibreboard, paper stiffness test, test methods, standards.