Newsprint folding endurance testing measures the ability of newsprint paper to withstand repeated folding under a defined tension, a critical durability indicator for high-speed web offset printing and newspaper production. Conducted in accordance with ISO 5626 and GB/T 457 using MIT-type folding endurance instruments, the test helps paper mills, publishers, and accredited third-party laboratories verify press runnability and product quality. This article explains the technical principles, sample requirements, test procedures, and practical applications of newsprint folding endurance determination.
Technical Overview
Folding endurance is the capacity of a paper strip to resist failure when it is repeatedly folded back and forth at a specified angle while under a constant tensile load. In the standard test, a strip 15 mm wide is clamped in a folding head, placed under tension, and folded continuously until the strip breaks. The result is reported as the number of double folds the specimen withstands, or as the base-10 logarithm of that number, which is formally defined as the folding endurance value.
For newsprint, folding endurance is a particularly important quality parameter. Newsprint is a lightweight grade, typically 40 g/m² to 49 g/m², manufactured largely from mechanical pulps such as thermomechanical pulp, stone groundwood, and deinked recovered fiber. These short, stiff fibers and the relatively high ash content give newsprint comparatively low folding strength. At the same time, newspaper presses run at very high speeds and subject the web to aggressive bending in the former and folder units, so insufficient folding endurance directly increases the risk of web breaks, press downtime, and waste copies.
Folding endurance correlates with, but is not identical to, tensile strength. It is highly sensitive to fiber length, interfiber bonding, filler content, calendering intensity, and moisture content. Because paper is anisotropic, the property must always be evaluated in both the machine direction (MD) and the cross direction (CD) of the sheet.
Testing Samples
Standard Newsprint Grades
Samples are typically taken from production rolls or sheets of standard newsprint grades, including common grammages of approximately 42.5 g/m², 45 g/m², and 48.8 g/m². Improved newsprint grades containing a small percentage of chemical pulp may also be evaluated, and results can be compared against mill specifications or purchase contracts.
Specimen Preparation
Test pieces are cut to a width of 15 mm with a length appropriate for the instrument, normally at least 100 mm for MIT-type testers. The following precautions apply during preparation:
- Specimens must be free of folds, creases, wrinkles, watermarks, and visible defects.
- Edges must be clean and parallel, cut with a sharp cutting device to avoid fiber damage.
- Each strip must be clearly marked to identify the machine direction or cross direction.
- Sampling of the lot shall follow a recognized sampling standard such as ISO 186 to ensure representativeness.
- At least 10 valid specimens per direction are normally tested, as required by ISO 5626.
Conditioning
Because folding endurance is strongly influenced by moisture content, specimens are conditioned in the standard atmosphere defined by ISO 187, namely 23 °C ± 1 °C and 50 % ± 2 % relative humidity, until equilibrium is reached before testing. Testing outside this atmosphere will bias the results, typically increasing folding endurance at higher humidity.
Testing Items
The core test items evaluated in a newsprint folding endurance experiment include:
- MIT folding endurance in the machine direction (MD), in double folds.
- MIT folding endurance in the cross direction (CD), in double folds.
- Folding endurance expressed as the logarithm (base 10) of the number of double folds, in accordance with ISO 5626.
- MD/CD folding endurance ratio, an indicator of fiber orientation anisotropy.
- Optional comparative testing after accelerated aging, used to characterize the embrittlement of newsprint during storage or archival studies.
Testing Methods
MIT Folding Endurance Method
The MIT method described in ISO 5626, GB/T 457, and TAPPI T 511 is the most widely used procedure for newsprint. The instrument applies the following fixed conditions:
- Folding angle of 135° ± 1° to each side of the vertical.
- Specimen tension of 9.81 N ± 0.10 N as the standard load, with 4.91 N and 14.72 N available for special purposes.
- Folding frequency of 175 ± 10 double folds per minute.
- Specimen width of 15.0 mm, with a free folding length of 10 mm.
The test procedure consists of the following steps:
- Verify and calibrate the instrument, including the tension system, folding head, and counter, before use.
- Clamp the conditioned specimen vertically in the folding head without touching the folding zone.
- Apply the specified tension and start the folding action.
- Allow the instrument to fold the specimen until it breaks, and record the number of double folds automatically counted.
- Repeat for all specimens in each direction and discard results from strips that break outside the folding zone or slip in the clamps.
Schopper-Type Method
A Schopper-type tester, covered by ISO 5626 and TAPPI T 423, may also be used for comparison or historical data alignment. This instrument uses a different folding head geometry with the strip folded through a different arc under spring-applied tension at approximately 120 double folds per minute. Results from MIT and Schopper instruments are not interchangeable, and the instrument type must always be stated in the report.
Calculation and Expression of Results
Because the number of double folds is scattered log-normally, ISO 5626 specifies expressing folding endurance as the logarithm base 10 of the number of double folds. The report should include:
- Individual and mean double-fold values for MD and CD.
- The mean folding endurance calculated as log10 of the mean double folds, or the geometric mean.
- The instrument type, applied tension, folding angle, and test atmosphere.
- The number of specimens tested and any discarded results with reasons.
Factors Affecting Accuracy
Key influences on the reproducibility of results include the moisture content of the specimen, wear or contamination of the folding rollers and head, calibration drift of the tension spring, specimen damage during cutting, and improper clamping. Regular maintenance and verification against reference materials are essential for reliable data.
Applications
Newsprint folding endurance data support a wide range of industrial and technical activities:
- Quality control in newsprint mills, ensuring each production batch meets internal and customer folding strength specifications.
- Runnability assessment for high-speed web offset presses, where low folding endurance correlates with breaks at the former and folder units.
- Incoming inspection by publishers and printing houses before committing large press runs to a paper supply.
- Research and development for furnish optimization, including mechanical-to-chemical pulp ratios, filler loading, refining levels, and calendering strategies.
- Trade acceptance and dispute resolution, where testing by accredited third-party laboratories provides impartial conformity evidence against ISO 5626 or GB/T 457.
- Conservation and permanence studies, since the progressive loss of folding endurance is a classic indicator of the embrittlement and aging of lignin-rich newsprint in archives and libraries.
FAQ
What is folding endurance and why is it important for newsprint?
Folding endurance is the number of double folds a paper strip can withstand under a specified tension before breaking. For newsprint, it predicts how well the lightweight web survives the repeated bending imposed by high-speed presses and folder units, directly affecting web break frequency, press efficiency, and waste rates.
Which standards govern newsprint folding endurance testing?
The principal standards are ISO 5626 and GB/T 457 for general folding endurance determination, along with TAPPI T 511 for the MIT instrument and TAPPI T 423 for Schopper-type instruments. Specimen conditioning follows ISO 187, and sampling follows ISO 186.
Why must both machine direction and cross direction be tested?
Paper fibers align predominantly with the machine direction during sheet formation, so folding endurance is significantly higher in MD than in CD. Testing both directions provides a complete picture of sheet durability and reveals the degree of fiber orientation anisotropy, which influences both printing behavior and folding performance.
How are the results expressed?
Results are expressed as the number of double folds to failure and, in accordance with ISO 5626, as the folding endurance value defined as the base-10 logarithm of the double-fold count. Both the instrument type and the applied tension must accompany the reported values.
How many specimens should be tested?
At least 10 valid specimens in each principal direction are recommended by ISO 5626, because the double-fold count shows considerable scatter. Testing more specimens improves the confidence interval of the mean, especially for weak newsprint with low fold counts.
What can be done if newsprint folding endurance is too low?
Common corrective measures include increasing the proportion of long chemical pulp fibers, reducing filler content, adjusting refining to improve interfiber bonding, moderating calendering pressure, and controlling moisture levels. A root-cause investigation supported by accredited third-party laboratory testing can confirm whether the deficiency originates in furnish, pressing, or finishing operations.