Paper Core Crush Strength Explained: Flat, Radial and Units

Published 26 October 2026 · By Ranjeet Kumar, Proprietor, Solution Enterprises

Short answer: crush strength is the load a piece of core can take before its wall gives way. There are two common tests: flat crush, where a short ring is pressed between two plates, and radial crush, where pressure is applied evenly all round the outside. A crush figure is only comparable with another if you know which test, how long the sample was, and the unit used.

Flat crush: the standard handling test

A short ring cut from the core is laid on its side between a fixed plate and a moving plate. The moving plate comes down at a steady rate and the instrument records the peak force before the wall buckles. The international method is ISO 11093-9:2019, Paper and board — Testing of cores — Determination of flat crush resistance, which applies to wound paper and board cores. One detail from its scope is worth knowing: if the wall is thinner than 5 mm, the breaking point cannot be detected by this method.

Flat crush tells you how well a core survives being handled, stacked and shipped. Core makers have been saying for decades that it is a poor guide on its own for cores carrying film or elastic yarn, because it loads the wall at two lines, not all the way round.

Radial crush: closer to what film does

In a radial crush test the core sits inside an inflatable bladder that presses evenly around its circumference until the core collapses, creases or compresses. That is much closer to how wound film or stretch yarn squeezes a core. So a high flat crush figure on its own does not prove a core will survive a tightly wound film roll; ask which test the number came from. Why film loads a core this way is covered in paper core for stretch film and tape.

Strength is more than crush

Core-selection articles from the converting trade list a wider set of properties: beam stiffness (how little a long core bends under a roll), dynamic load strength (how it holds up while spinning on an unwind stand), and resistance to vibration at speed. A wide, heavy roll on a long core can pass a crush test and still fail because the core bends. If your rolls are long, mention it, because the answer is a different wall, not just a stronger one.

What changes crush strength

• Wall thickness and ply count. More plies of board means more wall to carry load.
• Board grade and density. A denser core board carries more for the same wall.
• Adhesive between plies. Weak bonding lets plies separate under load; adhesive suppliers market their products on crush results for this reason.
• Moisture. Paper weakens as it takes up water, and its size moves with it.
• Inner diameter. For the same wall, a larger diameter ring behaves differently from a small one, so compare like with like.

Worked example: two quotes, two units

Bench testers sold in India take ring samples from about 25 to 100 mm long, and results are reported in kgf, newtons, or force per unit length. Quotes therefore arrive in forms that cannot be compared at a glance. Suppose, for the same 76 mm inner diameter core:

• Quote A says “850 kgf flat crush”, tested on a 100 mm ring.
• Quote B says “6 kN flat crush”, tested on a 70 mm ring.

Convert A to newtons: 850 × 9.81 = about 8,340 N. Divide by sample length: 8,340 ÷ 100 = about 83 N per mm. For B: 6,000 ÷ 70 = about 86 N per mm. The core that looked weaker on paper is, to a first approximation, the same or slightly better. Dividing by length is a rough normalisation, not a conversion the standard promises, so treat small differences as no difference. Large ones are worth a question.

Ask	Why it matters
Which test?	Flat and radial results do not compare
Sample length?	Needed to compare per mm
Unit?	kgf, N and kN are easy to mix up
Conditioning?	A damp sample reads lower
Average or minimum?	Your roll fails on the weakest core

Specifying without a lab

Most buyers do not own a crush tester and do not need one. What works is to describe the load — the heaviest roll, what is wound on it, how long it is stored and how it is handled — and let the core maker propose a wall. Then keep a few cut rings from an approved delivery as a reference. If a later batch feels softer or rolls start failing, you have something to compare against. We do not publish a crush figure for our cores; we specify the wall against your roll, as set out on the paper core tube page and in how to specify core sizes. For crush and stacking tests on whole packages rather than cores, see drop and compression testing basics.

Common questions

Is a higher crush figure always better?

Only up to the load you need. Beyond that you pay for board you do not use, and a heavier core adds weight to every roll.

Why is crush given in kgf in India?

Many bench testers in use here display kgf. It is fine as long as you know the sample length; multiply by 9.81 to get newtons.

Can thin cores be flat-crush tested?

ISO 11093-9 notes that below a 5 mm wall the breaking point is not detectable with its method, so thin-wall results need care.

If you have two quotes you cannot compare, send them to us with your roll details and we will tell you what we would specify and why.

Sources

• BSI Knowledge, BS ISO 11093-9:2019 Paper and board. Testing of cores. Determination of flat crush resistance — scope and the 5 mm note.
• Ox Industries, What is Crush Strength for Paperboard Cores? — flat and radial test descriptions, influencing factors.
• Sonoco, Film Cores (EMEA) — radial crush method.
• Paper, Film & Foil Converter, Core selection — the difference between success and failure — beam stiffness, dynamic load, critical speed.
• Fibertec Instruments, Paper Core Compression Strength Tester — sample lengths 25–100 mm, kgf readout.
• Unit conversion: 1 kgf = 9.80665 N (standard gravity).

The worked example uses invented quote figures to show the arithmetic; it is not data for any real core. Dividing by sample length is an approximation. Agree a test method with your supplier before using crush figures for acceptance.