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Will your core hold the roll? The radial duel

ROLL Lab · app 6 of 11 · Prof: verdict now, full report by email · updated 24 October 2026

In shortA 400 mm cast film roll can squeeze a 3" core at about 20 bar, more than twice the pressure in a truck tyre. The core must hold it with a margin.

Will your core hold?The roll squeezes, the core resists. Who wins? Let’s find out.

safety margin

What you wind

Roll diameter Wall of the 3" core

Roll pressure

Radial crush

Wall for 1.5×

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How the calculation works

Wound film is elastic: each wrap squeezes the ones below and the core receives a uniform radial pressure, in practice from about 5 to over 100 bar. The calculator starts from the simplest elastic model, P = σ × ln(R/r): film tension times the logarithm of the ratio between the roll radius and the outer radius of the core. Since the wraps are not rigid (trapped air, radial compressibility), the ROLL Lab applies a reduction factor of 0.6. The 0.6 factor is a ROLL Lab calibration, not a published value: the result is an order of magnitude, not a measurement.

Radial strength is estimated from flat crush with the thick-walled tube formula, fitted to three ROLL Lab reference estimates (spread up to 20%), and corrected for moisture. Flat crush alone is not enough: two cores with the same flat crush can have very different radial crush values.

Frequently asked questions

How much pressure does a film roll put on its core?

In practice, from about 5 to over 100 bar. It depends on winding tension, film type and gauge, number of wraps, lay-on roller and time. Film keeps shrinking after winding and keeps squeezing: under cast film, cores typically fail within 72 hours; under blown film, within 7 to 14 days.

What is the difference between flat crush and radial crush?

Flat crush squeezes the core between two plates, along a line. Radial crush squeezes it with uniform pressure over the whole surface, as a wound film does, and is measured in bar (CCTI T-158 with steel balls, T-159 with an inflatable bladder). The two values correlate poorly: for film rolls, radial strength is what counts.

What is ID stiffness?

It is the pressure in bar needed to reduce the inside diameter by 0.1 mm. It is not a failure but a stiffness, and it decides whether the roll comes off the shaft. It is also the property moisture degrades most: about −12% for each extra point of moisture, according to published studies.

Sources

Gerhardt, ASME J. Eng. Mater. Technol. 112 (1990) 144–150
CCTI T-158 and T-159 — Radial crush test
ROLL Lab calculation

Want the real number? ROLL Lab estimates help you choose. A specification needs a flat crush and radial crush test on the real core, at the moisture it will work at. Ask for a test or a quote →