Pressboard cylinder shrinkage needs separate allowance for height, circumference and local profile. Track sheet directions, seam restraint and drying supports to deliver the specified finished dimensions. [1]
Calculation basis. Project-specific values and gates in the illustrative screens below are assumptions; cited catalogue values retain their source test conditions.
Define the initial and final states before calculating compensation. The measurements must refer to identified conditioning, support and datum conventions. A circumference measured along the mid-surface is not the same quantity as an outside diameter multiplied by π when wall thickness changes.
Calculate the axes separately
Let s be the fractional reduction from initial dimension D₀ to final dimension Df: s = (D₀ − Df)/D₀. The compensating initial dimension is therefore D₀ = Df/(1 − s). Apply this relationship separately to axial and circumferential dimensions using qualified factors for their actual sheet directions.
| Axis | Illustrative final target | Illustrative reduction | Calculated initial dimension |
|---|---|---|---|
| Axial height | 500 mm | 0.4% | 502.008 mm |
| Mid-surface circumference | 2000 mm | 0.6% | 2012.072 mm |
These fictional reductions are calculation inputs, not material properties or drying recommendations. Adding the percentage directly to the final target would give a slightly different result because that operation uses the wrong dimensional basis for the defined reduction.
Record which sheet direction becomes the cylinder axis and which becomes the circumference. Determine directional dimensional change for the actual grade and process [1]. Compression experiments show why material and conditioning state must remain identified [2]; they do not supply free-cylinder shrinkage factors.
The joint and support can change the result
A joined cylinder does not necessarily shrink like a free rectangular strip. A scarf overlap, bonded region or temporary restraint can restrict local movement. Compare seam and remote sections, and record whether a support was present during the process and during measurement.
Measure height at several circumferential locations and inspect the end planes. Check diameter or profile at multiple axial stations in a defined support state. A correct average circumference can coexist with an oval shape or a locally raised seam.
Track wall thickness as a separate quantity. If both thickness and circumference change, an outside measurement can move without the mid-surface having the same fractional change. Keep this distinction in the inspection record rather than fitting a single factor to whatever instrument reading is available.
Establish compensation from representative cylinders processed through the intended route. Include variation between sheets and process runs, not only repeated readings on one cylinder. A coupon helps measure free directional change but does not demonstrate seam behaviour or large-part roundness.
Keep finished requirements separate from manufacturing blank allowances. Update compensation for changed material or processing while verifying height and the complete local profile.
Quantify the error from adding the percentage to the target
Keep the assumed 0.4% axial and 0.6% circumferential reductions defined on initial dimensions. Exact compensation gives 502.008 mm for the 500 mm height and 2,012.072 mm for the 2,000 mm mid-surface circumference.
Simply adding 0.4% to 500 gives 502.000 mm, which contracts to 499.992 mm. Adding 0.6% to 2,000 gives 2,012.000 mm, which contracts to 1,999.928 mm. Against an assumed ±0.050 mm final-circumference tolerance, that second shortcut misses the lower boundary by 0.022 mm.
Use the correct initial-dimension basis, then establish the actual process variation, seam allowance, thickness and restraint. The numerical correction does not supply a production shrinkage factor or prove the achieved roundness.
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.