Also known as: Burst Factor, BF, Mullen Test, BS
Bursting strength is the pressure required to rupture corrugated board, measured with a Mullen tester and expressed in kg/cm². It indicates resistance to puncture and rough handling, and burst factor (BF) expresses that strength relative to the paper's GSM.
The test clamps a sample and forces a rubber diaphragm through it under rising hydraulic pressure until the board ruptures. The pressure at failure is the bursting strength, reported in kg/cm².
It measures resistance to a force pushing through the face of the board — which corresponds well to real-world puncture, rough handling and the pressure of a heavy product pressing outward from inside the carton.
Burst factor is bursting strength divided by the paper's GSM, giving a quality figure independent of weight. Indian corrugated trade normally specifies papers as "GSM + BF" for exactly this reason.
Bursting strength does not describe stacking performance. A box can have high bursting strength and still collapse under vertical load, because that behaviour is governed by edge crush resistance. Specifying only bursting strength when the real risk is warehouse stacking is a common and expensive mistake.
Bursting Strength at a glance
| Unit | kg/cm² |
|---|---|
| Test method | Mullen burst tester (IS 1060 / TAPPI T810) |
| Burst factor (BF) | Bursting strength ÷ GSM |
| Typical 3-ply | ≈ 6–11 kg/cm² (two liners) |
| Typical 5-ply | ≈ 9–17 kg/cm² (three liners) |
| Typical 7-ply | ≈ 14–22 kg/cm² (four liners) |
| Assumes | 120–200 GSM kraft liners at 18–28 BF; board BS ≈ the sum of its liners |
| Measures | Puncture and rough-handling resistance — not stacking |
Common questions
What is the bursting strength formula and unit?
The bursting strength unit is kg/cm² in India (kPa or psi elsewhere), read directly from a Mullen tester. The bursting strength formula most buyers need links it to burst factor: bursting strength (kg/cm²) = burst factor × grammage (g/m²) ÷ 1000. A 150 GSM liner of 18 BF therefore bursts at about 2.7 kg/cm².
Bursting strength or ECT — which should I specify?
Specify bursting strength when the risk is puncture and rough handling; specify ECT when boxes will be stacked in a warehouse or container. Many buyers ask only for bursting strength and then see stacking failures, because ECT is what governs vertical load.
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