A paper core’s crush strength depends mainly on its wall thickness relative to its diameter: a thicker wall raises resistance steeply, while a larger inside diameter at the same wall lowers it. Paper grade, number of plies, adhesive, moisture content and the pressure built up by winding tension then decide whether the core survives the reel.
Key points
- Flat crush and radial crush measure different things; state which one your requirement uses.
- Doubling the wall adds far more than double the crush resistance; doubling the diameter at the same wall cuts it sharply.
- Inside diameter tolerance decides chuck fit, and crushing under a tight reel can close the bore.
- Moisture is the most common hidden cause of weak cores.
- Winding tension builds radial pressure on the core layer by layer, so light film reels can still crush cores.
This guide goes deeper into strength than how to specify an industrial paper core or tube, which covers the basic dimensions and what to send with an enquiry.
What is the difference between flat crush and radial crush strength?
Flat crush strength is the load a short length of core withstands when squeezed between two flat plates across its diameter. Radial crush describes resistance to pressure applied evenly all round the outside, as a wound reel does.
Flat crush is the most common test because it is simple: cut a sample of fixed length, place it on its side in a compression tester and record the peak load. Results are usually reported per unit length, so a 100 mm sample and a 50 mm sample can be compared. Radial pressure is closer to what a reel does to a core, but it is harder to test directly, so it is often inferred or tested with special fixtures. Other properties matter for some applications: axial crush for cores stacked on end or clamped at the ends, beam strength for long cores supported only at the chucks, and torque strength at the chuck for high-speed winding. Agree the test, sample length, conditioning and how the result is reported, or two suppliers’ figures cannot be compared.
How do wall thickness and inside diameter affect crush strength?
Crush resistance depends on the ratio of wall thickness to diameter. Thicker walls raise it steeply; larger diameters at the same wall lower it.
A core under flat crush behaves like a ring being bent. In simple ring theory, its stiffness rises roughly with the cube of the wall-to-diameter ratio and its load at failure rises faster than in proportion to wall thickness. In practice that means a small increase in wall thickness often gives a large gain in crush strength, while moving from a 76 mm to a 152 mm inside diameter at the same wall gives a much weaker core. Paper and board reels for converters commonly use 76.2 mm (3 inch) and 152.4 mm (6 inch) inside diameters, and a 152 mm core usually needs a noticeably thicker wall to carry a comparable reel.
| Change | Effect on flat crush | Other effects |
|---|---|---|
| Thicker wall, same ID | Rises steeply | Heavier, costlier, larger OD; better beam strength |
| Larger ID, same wall | Falls sharply | Better beam strength, fits larger shafts |
| Longer core, same section | Unchanged per unit length | More sag between chucks; straightness harder to hold |
| Stronger, denser board | Rises | Higher cost; may need adjusted adhesive |
| Higher moisture content | Falls | Swelling, ID change, softer ends |
| Weak or starved glue lines | Falls; plies can separate | Delamination at chucks and cut ends |
Does core length change crush strength?
Not per unit length, but length changes how the core is loaded. A long core supported only at its ends must resist bending as well as crushing.
Long cores, such as those for wide film or paper reels, sag under the reel weight between the chucks. That bending stress adds to the crush load near the supports, and it causes vibration and uneven winding at speed. Beam strength rises with diameter and wall thickness, so a long, heavy reel may need a larger diameter or thicker wall even if the flat crush figure looks adequate.
Why does inside diameter tolerance matter so much?
Because the core must grip the chuck or shaft without slipping and still slide on and off. If the bore is too large, the core spins or chatters; too small, it will not load or it splits.
Typical ID tolerances for converting cores are a few tenths of a millimetre, tighter for high-speed lines and expanding chucks. Crushing also changes the bore: as a tight reel squeezes the core, the inside diameter closes, and a core that loaded easily on the winder may not come off the unwind shaft. Specify ID with tolerance, and if your process is sensitive, add a limit on ID reduction under the finished reel.
How do plies, paper grade and adhesive build a core’s strength?
A paper core is built from many plies of paperboard wound and glued together. The paper’s strength and density, the number of plies and the quality of every glue line together set the wall’s strength.
Spiral-wound cores have plies wound at an angle and are cut continuously to length; parallel-wound (convolute) cores wind plies straight around a mandrel and are often chosen for very heavy duty. Strong, dense core board gives more strength for the same wall than lighter board, and a smooth, stronger inner and outer ply can protect the surface from chuck damage or the wound material from marking. The kraft or board grade, its moisture and its caliper must be consistent from reel to reel, or the wall build-up varies. Each glue line must be continuous and fully cured: starved or uneven adhesive leaves weak planes where plies separate under load. Bollant winds its cores from recycled paper made at its own mills, with adhesive from its own adhesives unit, then cures them, cuts them to length and finishes the ends (company-stated; see paper tubes and winding cores).
How does moisture affect paper core strength?
Paper loses strength as it absorbs moisture, so a core stored in humid air or wound with damp material can have well below its tested crush strength.
Core strength is normally tested on samples conditioned in a standard atmosphere, commonly 23 °C and 50% relative humidity, so results are comparable. Real cores sit in monsoon warehouses, cold stores and containers. Agree a moisture content range at dispatch, store cores off the floor and wrapped, and allow cores moved from cold stores to warm, humid rooms to equalise before use.
How does winding tension affect the load on a core?
Every wrap wound under tension squeezes the layers beneath it. The pressures add up, so the core carries a radial load that depends on winding tension and the number of wraps, not only on reel weight.
This is why a light reel of thin, stretched film can crush a core that easily carries a heavier reel of paper. Film can also shrink after winding as it cools or relaxes, adding pressure in storage. Heavy reels add their own weight at the chucks and when reels are stood on end or rolled. When you ask for a core, give the material, its thickness, winding tension, finished reel diameter and weight, and whether reels are stored for long periods. The supplier can then choose wall thickness and board grade with sensible margin.
What should go on a paper core drawing or specification?
A drawing should give dimensions with tolerances, the strength requirement with its test method, and the conditions the core will see.
- Inside diameter with tolerance, and any limit on ID reduction under load.
- Wall thickness or outside diameter, with tolerance.
- Length with tolerance; end finish (plain, notched, chamfered, capped).
- Flat crush (or radial, axial or beam) requirement, sample length, conditioning and units.
- Ovality and straightness limits for high-speed lines.
- Moisture range at dispatch.
- Wound material, winding tension, reel weight and diameter, and line speed.
- Storage conditions and whether cores are returned for reuse.
An example specification line: Paper core, ID 76.2 mm +0.3/−0 mm, wall 10 mm ±0.3 mm, length 1,000 mm ±1 mm, plain ends; minimum flat crush to be agreed from a 1,000 kg paper reel at the stated winding tension, tested on conditioned 100 mm samples; moisture at dispatch within an agreed range; for 600 m/min unwinding. The figures are illustrative. Bollant makes paper tubes, winding cores, textile cones, fibre drums and composite cans to the customer’s drawing; the range is under industrial paper products, and a strength test result for the agreed construction is available on request.
Frequently asked questions
What is flat crush strength of a paper core?
It is the peak load a short sample of core withstands when squeezed between two flat plates across its diameter, usually reported per unit length. It is the most common way to compare core strength.
How do I increase paper core crush strength?
Increase wall thickness, use a stronger and denser core board, make sure glue lines are complete and cured, and keep the core dry. Increasing wall thickness usually gives the largest gain.
Why do paper cores crush on film reels?
Film is wound under tension and can shrink after winding, so pressure from many tight wraps builds on the core. The load depends on tension and number of wraps as well as reel weight.
What are standard paper core inside diameters?
Common inside diameters for paper, film and label reels are 76.2 mm (3 inch) and 152.4 mm (6 inch), with many other sizes made to suit specific machines and products.
Does humidity weaken paper cores?
Yes. Paper absorbs moisture and loses strength, and a damp core also swells, changing its inside diameter. Agree a moisture range and store cores dry and off the floor.
What is the difference between spiral and parallel wound cores?
Spiral-wound cores have plies wound at an angle and are produced continuously; parallel-wound cores have plies wound straight around a mandrel. Parallel winding is often chosen for very heavy duty applications.
What wall thickness does a paper core need?
It depends on the inside diameter, reel weight, winding tension, core length and humidity. Light textile and label uses can run on thin walls, while heavy paper and film reels need much thicker ones. A supplier should size the wall from your load data and confirm it with a crush test.
Can paper cores be reused?
Industrial cores and tubes are built for repeated handling and can be reused when they are returned in good condition, then recycled through paper streams. A core meant for several trips needs a stronger construction than a single-use one.
Bollant Industries · Published 2026-11-15
