Folding endurance testing of printed materials evaluates the ability of paper and printed products to withstand repeated double folding before fracture, a critical durability parameter for banknotes, maps, books, packaging cartons, and labels. This technical guide explains the test principle, applicable standards such as ISO 5626, ASTM D2176, TAPPI T 511, TAPPI T 423, and GB/T 457, sample preparation, instrumentation, and result interpretation. It is intended for quality engineers, printing and converting professionals, and accredited third-party laboratories performing folding resistance inspection.
Technical Overview
Folding endurance, also known as folding resistance or double-fold strength, is the number of double folds a strip of paper, board, or printed material can withstand under a specified tensile load before it breaks. Because many printed products—banknotes, maps, instruction manuals, catalogs, cartons, and labels—are folded repeatedly during manufacture, distribution, and end use, folding endurance is one of the most important mechanical durability indicators in the printing and paper converting industries.
The test is performed on a dedicated folding endurance instrument. A narrow specimen strip is clamped under a defined tension, and a folding head oscillates through a fixed arc until the specimen ruptures. The machine records the total number of double folds at failure, and the result is commonly expressed either as the fold count itself or as the logarithmic folding endurance value (log10 of the number of double folds), which reduces the strong scatter typical of fold data.
Internationally recognized test methods include ISO 5626 (paper and board—determination of folding endurance), ASTM D2176 (MIT tester method), TAPPI T 511 (folding endurance of paper by MIT tester), TAPPI T 423 (folding endurance of paper by Schopper-type tester), and GB/T 457, the Chinese national standard harmonized with ISO 5626. Specimen conditioning is carried out in accordance with ISO 187 or GB/T 10739 at 23 ± 1 °C and 50 ± 2 % relative humidity.
Testing Samples
Unprinted Base Papers and Paperboards
Incoming base stocks such as offset paper, coated art paper, uncoated woodfree paper, security paper, kraft paper, and light cardboard are tested to verify that the substrate meets the folding durability requirements before printing and converting.
Printed Products
Finished or semi-finished printed items—including banknotes and security documents, maps, textbooks, brochures, catalogs, tickets, tags, and self-adhesive labels—are tested to confirm that the printing, coating, and varnishing processes have not unacceptably degraded fold performance. Folded printed specimens are also visually inspected for cracking of ink layers and coatings along the fold line.
Packaging and Converting Materials
Folding cartons, micro-flute laminates, blister cards, and flexible packaging laminates are evaluated to ensure the material can survive creasing, gluing, filling, transport, and repeated opening without fiber fracture or delamination at the fold.
Sample Preparation and Conditioning
- Specimens are cut as strips of 15 mm width, with the length appropriate to the instrument used (typically 90–100 mm for Schopper-type testers and at least 100 mm for MIT testers).
- Specimens are cut in both machine direction (MD) and cross direction (CD), because folding endurance is strongly anisotropic.
- Edges must be clean, straight, and free of nicks, wrinkles, or water marks, as any defect causes premature failure.
- Specimens are conditioned in the standard atmosphere (23 ± 1 °C, 50 ± 2 % RH) for at least 24 hours before testing.
- A minimum of 10 valid specimens per direction is recommended for statistical reliability.
Testing Items
- Number of double folds to failure (MD and CD), reported as the primary result.
- Logarithmic folding endurance value: log10 of the double-fold count, used to normalize the log-normal distribution of fold data.
- Geometric mean folding endurance, calculated as the antilog of the arithmetic mean of the individual log10 values.
- Comparison of folding endurance before and after printing, coating, or laminating to quantify process-induced loss of flexibility.
- Assessment of surface cracking—ink film, coating, or varnish fracture along the fold line—on printed specimens after folding.
- Optional evaluation of folding endurance after accelerated aging or exposure to temperature and humidity cycles, relevant for archival and security documents.
Testing Methods
MIT Folding Endurance Method (ASTM D2176, TAPPI T 511)
The MIT tester clamps the specimen vertically and applies a adjustable tensile load, commonly 4.9 N or 9.8 N. The folding head oscillates through 135 degrees in each direction, producing approximately 175 double folds per minute. The counter stops automatically when the specimen breaks. The MIT method is suitable for a wide range of grammages, including relatively stiff boards, and is widely used for security papers and currency substrates, where very high fold counts are required.
Schopper Folding Endurance Method (ISO 5626, TAPPI T 423, GB/T 457)
In the Schopper-type instrument, the specimen is held between a folding head and a spring-loaded tension system, typically applying an initial tension of about 7.6 N. The folding knife oscillates through 180 degrees at approximately 120 double folds per minute until the strip ruptures. This method is well suited to printing and writing papers within the thickness range specified by the standard.
Test Procedure
- Verify and calibrate the instrument, including tension force, oscillation frequency, and counter accuracy.
- Mount the conditioned specimen without slack or pre-stress, aligned parallel to the clamps.
- Apply the specified tension and start the folding cycle.
- Record the number of double folds at rupture; discard and retest any specimen that fails inside the clamps or at an edge defect.
- Repeat for at least 10 valid specimens in each test direction.
Data Processing and Result Expression
Because double-fold data follow an approximately log-normal distribution, the arithmetic mean of the raw counts is not statistically appropriate. The individual fold counts are converted to log10 values, averaged, and the result is reported as the mean logarithmic folding endurance together with its standard deviation; the geometric mean fold count may also be given. Test reports must identify the test method and standard, instrument type, applied tension, test direction, conditioning atmosphere, and the number of specimens tested.
Applications
Folding endurance inspection is applied throughout the paper, printing, and packaging supply chains:
- Security printing: qualification and routine control of banknote and passport paper, where circulation durability is a core performance requirement and fold counts often reach into the thousands.
- Publishing and commercial printing: durability of book covers, spines, folded inserts, maps, and manuals subjected to repeated handling.
- Packaging: verification of carton board and coated folding boxboard before creasing and gluing operations, and evaluation of crack resistance of printed and varnished surfaces.
- Label and tag production: confirmation that label facestocks and printed laminates tolerate container bending and repeated flexing.
- Paper manufacturing: incoming fiber, furnish, filler loading, and calendering optimization, since high filler content and severe drying reduce folding endurance.
- Conservation and archival science: folding endurance loss is a recognized indicator of paper aging and degradation in collection management.
- Supplier qualification and dispute resolution: accredited third-party laboratories provide ISO 17025-based test reports for buyer–supplier acceptance.
FAQ
What is the difference between folding endurance and folding strength?
Folding endurance refers to the number of double folds a specimen withstands before breaking under a defined tension, expressed either as the fold count or as its log10 value. The terms folding endurance, folding resistance, and double-fold number are used interchangeably in most standards.
Why are MIT and Schopper results not directly comparable?
The two instruments differ in applied tension, folding angle, oscillation speed, and clamping geometry. A paper may register in the hundreds of folds on one tester and a different count on the other. Results must always be reported with the method identification, and comparisons should only be made within the same standard and instrument type.
Why is folding endurance tested in both machine and cross direction?
Paper is anisotropic due to fiber orientation during sheet forming. Fold performance in the machine direction can differ substantially from the cross direction, and printed products are folded along different axes depending on the design, so both directions must be characterized.
How does printing affect folding endurance?
Substrate folding endurance is governed mainly by fiber properties, moisture content, filler level, and calendering. However, heavy ink coverage, UV-cured coatings, and varnish layers can embrittle the surface and cause visible cracking along folds even when the base paper folds well. Printed specimens are therefore folded and inspected for surface cracking in addition to counting folds.
What factors most strongly influence test results?
Moisture content is critical—dry paper folds far fewer times than properly conditioned paper—so strict atmospheric conditioning per ISO 187 is mandatory. Specimen edge quality, applied tension accuracy, and the number of replicates also significantly affect reported values.
What acceptance criteria are typically specified?
There is no universal pass threshold; specifications are agreed between buyer and supplier according to the end use. For example, security and map papers are commonly specified with minimum double-fold counts in the thousands, while general printing papers may be accepted with counts in the tens to hundreds. Acceptance criteria, test standard, tension, and direction must all be stated in the purchase specification and test report.