Compressed pressboard thickness is a time-dependent dimension. Define unloading, conditioning and measurement pressure so recovery readings represent the same material state and support the required assembly fit. [1]
Calculation basis. Project-specific values and gates in the illustrative screens below are assumptions; cited catalogue values retain their source test conditions.
Pressboard compression experiments distinguish initial densification from subsequent loading behaviour. Moisture, temperature and precompression history also affect the response. A thickness recovery test must therefore identify the loading sequence and the measurement times before its results can be used for dimensional control.
Keep the full thickness record
Measure the initial blank at defined locations and with a specified measuring force. Record the loaded thickness where the test arrangement allows it, the time of unloading and the recovered readings. Use the same locations after unloading; a slight change of position can include local thickness variation rather than recovery.
| State | Illustrative thickness | Information retained |
|---|---|---|
| Before loading | 4.00 mm | Reference thickness under the chosen measurement condition |
| Under specified load | 3.70 mm | Load-associated reduction |
| Immediately after unloading | 3.85 mm | Early recovery, with actual elapsed time recorded |
| One hour after unloading | 3.92 mm | Change during the observation interval |
| Twenty-four hours after unloading | 3.94 mm | Later observed state, not an assumed equilibrium |
All values above are fictional. At the last reading, the observed recovery is 3.94 − 3.70 = 0.24 mm, compared with a loaded reduction of 4.00 − 3.70 = 0.30 mm. Their ratio is 80%. This arithmetic describes that observation only; it is not a generic pressboard recovery property.
Separate recovery from an altered reference
An uncompressed control from the same sheet, stored alongside the test blank, helps identify environmental drift. If that control changes thickness during the interval, the residual difference from the original 4.00 mm cannot all be labelled permanent compression set. Temperature, moisture pickup and the measurement procedure must be considered.
Check instrument and fixture compliance when interpreting loaded thickness. For a stack, total displacement can include interfaces and apparatus deformation. Dividing the stack displacement by the number of sheets does not automatically produce an accurate individual-sheet recovery curve.
Repeated loading is another separate variable. A first-cycle curve can include irreversible densification that is absent or reduced in later cycles. State which cycle supplies the production dimensional reference rather than combining first-cycle reduction with a later-cycle recovered thickness.
Choose inspection timing according to the next operation. A blank machined soon after compression may have a different recovered dimension when assembled later. A stable production requirement therefore specifies the conditioning and elapsed-time window associated with its tolerance.
Report time-dependent thickness with the blank’s material, process and measurement conditions. Keep laminate recovery data tied to their own construction and method when comparing routes.
Apply the control correction to the required assembly state
Pressboard compression evidence retains the material, conditioning and loading history [2]. Keep the assumed thickness record above. Now assume an uncompressed control gains 0.04 mm during the interval and, for this demonstration, that the same additive environmental shift affects the tested blank.
Corrected final thickness is 3.94−0.04 = 3.90 mm. Recovery from the loaded state is 0.20 mm, or 66.7% of the original 0.30 mm reduction. Against an assumed minimum corrected recovery of 0.22 mm at this inspection time, the blank fails by 0.02 mm; the uncorrected 0.24 mm would pass.
Validate the control correction and measurement force for the actual material. Environment and compression can interact, so the control does not automatically isolate irreversible set.
References
- IEC, IEC 60641-2:2004, Pressboard and presspaper for electrical purposes — Part 2: Methods of tests.
- C. Oria, I. Carrascal, D. Ferreño, C. Méndez and A. Ortiz, Experimental dataset on the compressive mechanical properties of high-density pressboard used in power transformers spacers, Data in Brief 50, 109471, 2023. DOI: 10.1016/j.dib.2023.109471.