Technical guide with sources · ROLL Lab
How to choose the right paper core
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Start from the application, not the catalogue
A paper core always does one of three jobs: it carries a roll that squeezes it (film, tape), it carries a weight that crushes and bends it (paper, metal, heavy textiles), or it holds and protects (postal tubes, formwork). The first question is which of the three.
| If the core… | what matters most | test to ask for |
|---|---|---|
| carries a roll of elastic film | radial strength and ID stiffness | radial crush in bar, CCTI T-158 or T-159 |
| carries heavy, wide reels | flat crush, bending, critical speed | flat crush ISO 11093-9, bending ISO 11093-6 |
| is cut with the roll (tape, labels) | cut quality, dust | flat crush and a cutting trial |
| comes back and is reused | cycles, moisture, cleanliness | comparison with plastic, cost per use |
How a paper core is made
A paper core starts as strips of board wound around a steel mandrel and glued ply on ply. In spiral winding the strips go in at an angle to the axis: the seams run in a helix and are staggered from one ply to the next. The tube comes off the line continuously and is cut to length. In convolute (parallel) winding, a sheet is rolled up in concentric layers and the length is limited by the width of the sheet.
The manufacturer adjusts four levers: number of plies, board grade, winding angle and adhesive type, usually water-based. They are the same levers that shift strength, stiffness and finish. That is why two cores of the same size can behave differently under the same roll.
The 7 things to give your supplier
- Inside diameter with tolerance: under ISO 13542 the tolerance is plus only (+1.2 mm on 76 mm). See the table of diameters and tolerances.
- Wall thickness or outside diameter (OD = ID + 2 × wall).
- Length with tolerance and type of cut (saw, knife, lathe).
- Minimum strength with the method: flat crush ISO 11093-9 in kN/m or N/100 mm; for film, also radial crush in bar with method T-158 or T-159 (how to read flat crush and radial crush).
- Maximum moisture on delivery, usually 6–8%, measured in an oven (why moisture matters so much).
- Finishes: ends (standard, bevelled, notched), inner surface (smooth, grooved), outer ply.
- Packing: pallet, frame, pieces per pack. The figure that takes up space is the outside diameter: how cores are packed and loaded.
The Prof’s rule
The mistakes that cost you a roll
- Specifying only flat crush for a film core. Flat crush barely reacts to the variables that drive radial strength: two cores with the same flat crush can have very different radial strengths.
- Forgetting warehouse humidity. One extra point of moisture costs about 12% of ID stiffness. A core follows the air it lives in, not the air it was made in.
- Approving a core straight off the line. Under cast film, failure typically comes within 72 hours; under blown film, within 7 to 14 days. Recheck at 72 hours and at 14 days.
- Comparing suppliers with samples of different ages. A freshly made core is weaker than a seasoned one: the comparison only makes sense at the same age and moisture.
- Drying too much. Below 4–5% moisture, paper turns brittle.
Straightness and chuck grip: the two things people forget
On fast machines the core must be straight. A bent core runs eccentric, vibrates and can mark the material. ISO 11093-5 measures exactly that: the eccentricity of the spinning core. If the core is long or the machine runs fast, ask for a straightness or concentric rotation limit.
The second is the grip at the ends. During winding and unwinding, toothed chucks transmit the starting and braking torque. If the wall is weak, the teeth tear the paper and the core spins freely: this is called chew-out, and it is measured with a torque test on the ends. If you run on chucks, tell your supplier the chuck type and the maximum torque.
What drives the price of a core
Cores are sold per piece or per metre, but the cost follows the kilos of board: diameter and wall decide the weight, the board grade decides the price per kilo. Then come length and cutting tolerances, finishes, quantities, packing and transport. That is why a stronger board grade can cost less than an extra millimetre of wall: compare them in the Lab.
Frequently asked questions
What is the difference between a paper tube, a core and a winding core?
They are the same object under three names. "Tube" is the general term; "core" is used when the tube sits at the centre of a roll or reel; "winding core" stresses that material is wound onto it. Italian says anima or mandrino, French mandrin, German Hülse.
Spiral or convolute core: when should you choose each?
A spiral core winds strips of board in a helix and is by far the most common: it is made continuously, in any length. A convolute (parallel) core winds the board in concentric layers: it is stiffer in bending and has smoother surfaces, but it costs more and its length is limited. For film and paper reels, spiral cores are the standard.
How do I choose the right wall thickness?
Start from the strength you need, not from the wall. For film, radial strength matters: how well the core withstands the squeeze of the roll. For heavy reels, flat crush and bending matter. The ROLL Lab calculator "How much wall do you really need?" gives the minimum wall for a strength target.
What do I need to give a supplier to order a paper core?
Seven things: inside diameter with tolerance, wall thickness (or outside diameter), length with tolerance, minimum strength with the test method (flat crush ISO 11093-9, radial crush CCTI T-158 or T-159), maximum moisture on delivery, edge and surface finish, and packing. Without the test method, a strength figure cannot be compared.
3-inch or 6-inch core: what changes?
The 3" (76 mm inside diameter) is the standard for film, tape and many paper grades; the 6" (152 mm) is used for large, heavy reels, high-speed machines and paper. With the same wall, a 6" core has lower radial strength than a 3", because the diameter is twice as large: it needs a thicker wall or stronger board.
Can paper cores be reused?
Yes, if they come back intact, dry and with no crushing of the inside diameter. In practice, paper cores are used once and then recycled; plastic cores, according to customer experience reported by manufacturers, reach 3–35 cycles depending on the material.
Why does an "identical" core from another supplier fail?
Because "identical" often means only the same diameter and the same flat crush. Two cores with the same flat crush can have very different radial strengths, and moisture on delivery and the age of the core matter too. A comparison only makes sense with the same test method, moisture and age.
What drives the price of a paper core?
The weight of board (diameter and wall), the board grade, the length and cutting tolerances, finishes, quantities, packing and transport. Cores are usually sold per piece or per metre, but their cost follows the kilograms of board.
Sources
- ISO 13542:2006 — internal diameters of cores for reels
- ISO 11093-9 — flat crush resistance
- CCTI T-158, T-159 — radial crush
- ISO 11093-3 — moisture content by oven drying
- Osservatorio Packaging (University of Bologna), «Spiral winding» sheet; W. Soroka, Fundamentals of Packaging Technology, 5th ed., IoPP 2014
- ISO 11093-5 — concentric rotation
- US Patent 5,379,648 (1995), chew-out test for paper cores
- ROLL Lab calculation
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