Dimensional Measurement
Evaluating the dimensional stability of paper when exposed to changing humidity requires a specific testing protocol to establish a baseline for comparing different cellulose substrates. Compliance with ISO 8226-1 allows manufacturers to evaluate how much a sheet will expand or contract as moisture levels fluctuate between specified limits. This evaluation is critical for multi-pass printing where precise registration is required.
The protocol is designed primarily for flat sheets of paper rather than finished, folded cartons.
Standardized Testing
To execute the test, the laboratory exposes conditioned strips of paper to a relative humidity of thirty-three percent and then to sixty-six percent, measuring the change in length along both the machine and cross directions. Under the rules of ISO 8226-1, the measurement must be conducted using a high-precision extensometer or an optical comparator capable of resolving minor dimensional shifts. These shifts are expressed as a percentage of the original sample length, providing a clear coefficient of hygroinstability.
Because cross-direction fibers expand more than machine-direction fibers, the standard requires reporting both values separately. This detailed reporting prevents misunderstandings when converters evaluate substrates for high-humidity environments.
Converter Application
Register errors during multi-color offset printing are often caused by the hygroinstability measured by this protocol. By using the values derived from ISO 8226-1, converters can select paperboard grades that resist warping and misregistration during humid production runs.
