ISO 5628: The International Benchmark for Bending Stiffness Testing of Paper and Board
In the paper, packaging, and printing industries, the “bending resistance” of paper and board directly determines the core performance of products—the turning feel of book paper, the load-bearing stability of express cartons, and the forming effect of food packaging are all closely related to this property. ISO 5628 “Paper and board—Determination of bending stiffness by static methods—General principles” is precisely the internationally accepted standard formulated to quantify this key performance, providing a scientific and unified testing basis for the global industry.
I. Standard Origin and Core Positioning
ISO 5628 is developed under the leadership of the International Organization for Standardization (ISO). Its technical origin can be traced back to the German DIN 53121 standard, which was verified through the practice of regional standards such as the Nordic SCAN P65 and North American TAPPI T836, and finally integrated into an internationally unified method. Since its first release in 1990, the standard has undergone multiple revisions and improvements. The latest version currently is ISO 5628:2019, released in 2019, which replaces the 2012 version and further enhances the standardization and data comparability of testing methods.
As a “general guide” for static bending stiffness testing, the core positioning of ISO 5628 is not limited to a specific type of paper product, but to provide a unified testing framework for all related materials—from ordinary printing paper and coated paper to packaging board and corrugated board, as long as they meet the specific thickness and basis weight range, they can be tested in accordance with this standard. Its essence is to convert the “ability to resist external bending force” of paper and board into a quantifiable “bending stiffness” index through a standardized testing process, providing an objective basis for production control, product development, and quality certification.
II. Core Technical Content: Methods, Requirements, and Specifications
The technical system of ISO 5628:2019 is built around three cores: “classification of testing methods, control of key parameters, and standardization of operation procedures”. Among them, the differentiated application of the three testing methods is the essence of the standard.
1. Three Testing Methods: Precise Solutions for Different Materials
The standard clearly specifies three static bending testing methods: the two-point method, the three-point method, and the four-point method. Each method is designed for materials with different thicknesses and stiffnesses to ensure the accuracy of test results. The specific differences are shown in the following table:
| Method Type | Applicable Materials | Loading Method | Core Calculation Formula | Maximum Strain Limit |
| Two-point Method | Thin paper, low-thickness board | One end fixed, free end loaded | Sb = Fl³/3fb or 60Fl²/παb | 0.2% |
| Three-point Method | Thin paper, low-thickness board | Supported at both ends, single-point loaded at the center | Same as the two-point method formula | 0.2% |
| Four-point Method | Corrugated board, thick board | Dual support points + dual loading points, more uniform force distribution | Sb = Fl₁l₂²/8fb | 0.05% for corrugated board; 0.2% for thick board |
Note: In the table, Sb is bending stiffness, F is loading force, l is bending length, f is deflection, b is sample width, and α is bending angle.
2. Strict Testing Conditions and Parameter Control
To avoid the impact of environmental and operational factors on data accuracy, ISO 5628 puts forward strict requirements for the entire testing process:
•Sample Requirements: Samples shall be taken in accordance with ISO 186, and conditioned in the temperature and humidity environment specified in ISO 187 (usually 23℃±1℃, relative humidity 50%±2%) to avoid fingerprint contamination; the thickness of thin board shall be measured in accordance with ISO 534, and that of corrugated board in accordance with ISO 3034 to ensure the accuracy of basic parameters.
•Geometric Parameter Limitations: For the two-point method and three-point method, the bending length shall not be less than 40 times the sample thickness to prevent the interference of interlayer shear strain; the support spacing (l₂) of the four-point method shall be adjusted according to materials, 10-50mm for thin board and 150-300mm for corrugated board.
•Equipment Accuracy Standard: The force measurement error shall be ≤±2.5%, and the bending length adjustment accuracy shall be ±1% to ensure that the instrument itself meets the testing requirements.
3. Specifications for Key Terms and Symbols
The standard unifies the definitions of core industry terms to avoid misunderstandings caused by different expressions. The core terms include:
| Chinese Term | English Equivalent | Core Interpretation |
| 弯曲刚度 (Wānqū Gāngdù) | Bending Stiffness | The ability of a material to resist bending within the elastic deformation range, which is the core evaluation index |
| 弹性模量 (Tánxìng Móliàng) | Modulus of Elasticity | The ratio of stress to strain in the elastic deformation stage, reflecting the elastic performance of the material |
| 惯性矩 (Guànxìng Jǔ) | Second Moment of Inertia | A geometric parameter describing the ability of a material cross-section to resist bending |
| 挠度 (Náodù) | Linear Deflection | The displacement of the sample in the force direction, which is the key data for calculating stiffness |
III. Core Revisions of the 2019 Version: Detail Optimization to Improve Data Reliability
Compared with the 2012 version, the revision of ISO 5628:2019 focuses on the refinement of testing details. The most core change is reflected in Clause 6.4.2—a new note is added to clarify the “measurement method of loading force F” in the four-point method. Since some testing equipment will display double the loading force (2F), the standard specifically points out that the actual loading force shall be corrected through software to avoid calculation errors caused by differences in instrument display. This revision further improves the comparability of data between different laboratories.
IV. Practical Value of the Standard: Full-chain Guarantee from Production to Market
ISO 5628 is not just a “technical document”, but a practical tool throughout the entire life cycle of paper products, and its value is reflected in multiple dimensions:
1.”Quality Benchmark” for Production Control: Paper mills can monitor the stiffness changes of paper and board in real-time during the production process in accordance with the standard, and adjust parameters such as pulp ratio and calendering process in a timely manner to avoid batch product disqualification caused by unqualified stiffness.
2.”Design Basis” for Product Development: When developing products such as express cartons and food boxes, packaging enterprises can clarify the relationship between material stiffness and product load-bearing and formability through standard testing, realize “accurate material selection”, and reduce R&D costs.
3.”Universal Language” for International Trade: As an internationally accepted standard, the test results obtained in accordance with ISO 5628 are recognized by customers around the world, helping export enterprises break through “technical barriers” and ensure that products meet the quality requirements of the target market.
4.”Design Benchmark” for Testing Instruments: The current mainstream bending stiffness testing instruments (such as Jinan Sanquan Zhongshi TD-10A) are all designed with ISO 5628 as the core basis, and are also compatible with Chinese standards such as GB/T 2679.3, realizing “one test, meeting multiple standards” and improving testing efficiency.
V. Implementation Suggestions: Ensuring the Authority of Test Results
To give full play to the value of ISO 5628, enterprises should pay attention to the following key points during the implementation process:
•Conduct “Bidirectional Testing”: Each sample shall undergo at least 5 forward and 5 reverse bending tests in both longitudinal and transverse directions to eliminate the impact of the initial curvature of the material, and the average value shall be taken as the final result.
•Standardize Test Reports: The report shall clearly indicate the standard number (ISO 5628:2019), testing environment, sample information, method used, key parameters, and standard deviation to ensure the completeness and traceability of the report.
•Calibrate Equipment Regularly: The force sensor and bending length adjustment mechanism of the testing instrument shall be calibrated regularly in accordance with the standard requirements to avoid test errors caused by instrument aging.
Conclusion
With scientific testing methods and strict parameter specifications, ISO 5628 has established a globally unified “benchmark” for the bending stiffness testing of paper and board. In today’s era where the performance requirements for packaging materials are increasingly high, complying with this standard will become the key to improving product competitiveness, whether for quality control of small and medium-sized enterprises or international certification of large enterprises. With the continuous iteration of testing technology, ISO 5628 will also be continuously improved to provide solid support for the high-quality development of the paper product industry.
Our product GA-5006 Stiffness Tester meets this standard.
GA-5006 Stiffness Tester
Test Standard
GB/T 23144 “general principles for the determination of static bending stiffness of paper and paperboard”;
GB/T 22364 “determination of bending stiffness of paper and paperboard”;
ISO 5628 “paper and board – determination of static bending stiffness – general principles”;
ISO 2493 paper and board – determination of flexural stiffness
Use
Digital display (touch screen) crease stiffness tester, is suitable for testing all kinds of paper and paperboard and all kinds of new materials bending crease stiffness test instrument.The stiffness instrument is suitable for testing flexural crease stiffness of paper, paperboard and other sheet materials with thickness less than 3mm, and also for testing softness of wire, leather and film.
Specification
Easy operation via the integrated touch screen
Measurement range:0-30000mN
Load cell accuracy:±0.1%
Sample size:38*70mm Standard size
Measure thickness:0.01-5mm
Resolution:0.01mN
Bend angle:1-92° adjustable
Multi-angle measurement: the word test can measure 3 different angles at the same time,1° -90° can set
Angle speed control:0.5-5°/sec Three test angles and duration are independently adjustable
Angle accuracy:±0.1°
Bend length:Choose different force arms according to different materials
8 grade force arms choose:length span:1-60mm,5mm,10mm,15mm,20mm,25mm,30mm,40mm,50mm,8 grade adjustable.
Length span accuracy:±0.1mm
Bend time:5-35s adjustable
Self-calibrate:The equipment can realize self-calibration and standard calibration tools
Data ouput:7 inch touched screen,with test curve
Print output：Thermal paper printer
Test fixture: manual fixture operation, visual menu operation
Clamping mode: unlimited clamping, non – destructive continuous clamping
Signal output: the device can achieve self-calibration: standard calibration tools
Product link:
https://www.gonoava.com/product/ga-5006-stiffness-tester/
Contact:
Email: arlenliu@gonoava.com
WhatsApp: 0086 18670728798(We chat)
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