Principle
The Short Range Compression Tester is a key device for evaluating the compressive strength (ECT) and short range compressive strength (SCT) of cardboard edges, which simulates the pressure on the edges of the cardboard during stacking or processing by applying compressive force to the specimen over a very short distance (typically 0.7mm) through two platens. This test result is crucial for packaging design, quality control. In the testing process, the preparation and sampling direction of the sample are the basic factors affecting the accuracy of the data. As an anisotropic material, the fiber arrangement and laminated structure of cardboard lead to inherent differences in mechanical properties in machine orientation (MD, i.e., paper manufacturing operation direction) and vertical machine orientation (CD, i.e., transverse). Therefore, clarifying and unifying the sampling direction is a prerequisite for obtaining comparable and reliable data.
The effect of sampling direction on test results
The compressive strength of cardboard is highly dependent on the orientation and bonding strength of the fibers. In the machine direction, the fibers are arranged more neatly, which can transmit and bear loads more efficiently; In the vertical machine direction, the fiber arrangement is relatively messy and the strength is usually low. When testing with a short-range compression tester, failure to distinguish or incorrectly confuse the sampling direction can directly cause the measurement to deviate from the true properties of the material.
Its impact can be understood through the fundamental principles of material mechanics. Compressive strength (σ) is related to the maximum load (F) and the cross-sectional area of the specimen (A) and is expressed as: σ = F/A. While the formula itself does not directly contain directional variables, the elastic modulus (E) and failure strain (ε) of a material differ on MD and CD, stemming from the anisotropy of their internal structural tensors. In the actual test, the maximum load F is strongly dependent on the specimen orientation. Therefore, the measured short-range compressive strength value may produce systematic deviations in the same batch of cardboard only due to different sampling directions, and this deviation often exceeds the repeatability limit allowed by the laboratory.
Bias analysis
In order to quantify the influence of sampling direction, refer to the comparative experimental data. The table below shows typical results for testing short-range compressive strength of a common corrugated board under standard temperature and humidity conditions along the machine direction and vertical machine direction, respectively.
| Test direction | Average short-range compressive strength (kN/m) |
| Machine Orientation (MD) | 7.2 |
| Vertical Machine Orientation (CD) | 5.1 |
From the data, it can be seen that there is a significant difference between the results in the two directions. In this example, the intensity value in the MD direction is approximately 1.41 times that of the CD direction. This variation varies depending on the type of cardboard (e.g., single corrugated or double corrugated), the slurry ratio, and the manufacturing process, and can range from about 1.2 times to more than 1.5 times. If the direction is not specified in the quality control, the CD direction specimen is mistakenly accepted as the standard value of the MD direction, which will lead to overestimation of material properties and may cause the risk of packaging failure. Otherwise, it may lead to over-design and increase costs. Therefore, "inaccuracy" is not a problem with the accuracy of the instrument itself, but rather from the unclear or inconsistent direction of the sample preparation stage.
Standard specifications
Major domestic and foreign standards, such as ISO 9895, TAPPI T826, and GB/T 2679.10, clearly specify the sampling direction for short-range compression testing. Standards often require results to be reported separately for MD and CD orientations, or to clearly specify test directions based on product use. To ensure measurement accuracy, the laboratory should establish the following operating specifications: first, clearly identify the machine orientation of the cardboard before sampling (it can be judged by fiber orientation, tension marking or production information); secondly, use a special mold to accurately cut the specimen in the desired direction to ensure that the edges are flat and free of burrs; Finally, the corresponding direction of each specimen must be clearly marked in the test record. For quality control applications, it is recommended to test with a cardboard orientation that corresponds to the vertical pressure direction of the box to make the data more useful.
Conclusion
In summary, the sampling direction is the key factor affecting the accuracy and comparability of the test results of the short-range compression tester. The degree of influence depends on the degree of anisotropy of the cardboard and usually results in a difference of 20% to more than 50% in strength values between different directions. Ignoring the directional factor will make the test data meaningless and even mislead decision-making. Therefore, strict implementation of standardized sampling procedures, clearly distinguishing and recording machine orientation and vertical machine orientation are fundamental requirements for obtaining reliable data on cardboard compressive performance, ensuring packaging safety and optimizing material use.
References
ISO 9895: Paper and cardboard - Compressive strength - Short range test method
TAPPI T826 om-21: Test method for short-range compressive strength of corrugated cardboard
GB/T 2679.10: Determination of short-range compressive strength of paper and cardboard
J. A. Bristow, P. Kolseth. Compression properties of paper and cardboard. Paper Science and Technology Series, 1986.