The core, also called the arbor, is the cylindrical centre of a galvanized coil. It carries the entire coil weight, keeps the inner diameter stable for the uncoiler mandrel, protects the inner wraps from crushing, and provides the anchor point for lifting and lashing. Choosing the wrong core material for a thin or heavy coil is a common cause of dents, telescoping, loose inner wraps and handling damage. This article explains steel, paper and composite cores, their standard dimensions, and how to select them for a given coil.
What the core does
A galvanized coil weighs from a few hundred kilograms for narrow slit strip up to several tens of tonnes for wide heavy-gauge material. The core must support that weight during lifting, storage and transport, maintain a round inner diameter so the uncoiler mandrel can grip and expand, and protect the first inner wraps, which are the most easily damaged zone of the coil. A failed core causes coil collapse, edge crushing, telescoping and production stops at the uncoiler, so the core specification is part of the packaging and handling contract, not an afterthought.
Steel cores
Steel cores are made in two ways. Welded spiral cores are formed from steel strip into a cylinder and joined by high-frequency welding; this is the strongest and most reliable construction and can carry heavy coil weights safely. Seamless cores are rolled and locked without a weld seam, which removes the weld zone as a potential weak point. Standard inner diameters in the trade are 508 mm (20 inches), 610 mm (24 inches) and 762 mm (30 inches), with a positive tolerance so the core fits the uncoiler mandrel without force. Steel cores are reusable and give stable geometry for high-speed uncoiling, but they are expensive, and a hard steel core can dent very thin strip at the inner wrap when the coil is wound tightly.
Paper and fibre cores
Paper cores are spiral-wound from kraft paper or fibreboard. They are soft, light and inexpensive, and they provide cushioning that prevents dents and scratches on very thin strip, typically material below 0.3 mm, where a steel core would mark the inner wraps during unwinding. They are single-use, have limited load capacity, and are sensitive to moisture, so they suit light coils, short-term storage and specialized export packaging. A paper core that gets wet loses its stiffness and can collapse under the coil weight, which is why dry storage is a condition of using them.
Composite cores
Composite cores combine a paper or fibre core with a thin steel sleeve fitted over it. The sleeve provides crush resistance and a precise inner diameter while the paper layer keeps the cushioning against the strip. This design is a practical middle ground for medium coil weights and thin strip, and it is increasingly common in slit-coil packaging where both surface quality and handling rigidity matter.
Selection criteria
| Factor | Steel core | Paper core | Composite core |
|---|---|---|---|
| Coil weight | Heavy (several tonnes and above) | Light only | Medium |
| Strip thickness | ≥ 0.3-0.5 mm typical | Thin, < 0.3 mm preferred | Thin to medium |
| Storage | Long, including outdoor | Short, dry indoor | Medium |
| Reusability | Yes, multiple cycles | No, single use | Limited |
| Cost | High | Low | Medium |
| Moisture resistance | High | Low | Medium |
The decision rule is simple: heavy coils need steel cores; very thin strip needs a soft core surface; everything in between can be served by steel or composite, with the choice driven by storage conditions and cost.
Core damage prevention
Most core damage happens at four moments. During lifting, use a coil lifter or C-hook sized to the inner diameter, and expand the mandrel before lifting so the core does not bear point loads. During transport, secure the coil on saddles or chocks so it cannot roll, and protect the core ends with plastic or wooden plugs. During storage, keep coils on saddles rather than flat on the floor, and keep paper cores dry. During unwinding, match the mandrel expansion range to the core inner diameter tolerance; over-expansion cracks the core, under-expansion lets the coil spin loose. Inspect the core bore and the inner wraps after unwinding, because repeat damage at the same position signals a mandrel or handling problem that will affect the next coil.
Why do steel cores dent thin coils?
A hard steel surface concentrates pressure on the soft inner wraps of thin strip, marking or denting them during winding and unwinding. A paper or composite core cushions the strip and prevents these marks.
What inner diameters are standard?
508 mm (20 inches), 610 mm (24 inches) and 762 mm (30 inches) are the common coil core inner diameters in the trade, specified with a positive tolerance to fit standard uncoiler mandrels.
Can a paper core support a heavy coil?
No. Paper cores are limited to light coil weights, typically a few hundred kilograms, and are not suitable for heavy-gauge coils; steel cores are required where the weight is several tonnes.
How should coils with paper cores be stored?
Store them indoors, dry, on saddles, and do not stack. Paper cores lose stiffness when wet, so moisture control is essential for the integrity of the coil.
Steel or paper core for 0.2 mm strip?
Choose a paper or composite core. At 0.2 mm the strip is thin enough to be dented by a hard steel core, so the cushioning of a soft core surface is the priority.
What is a composite core?
A composite core is a paper or fibre core with a thin steel sleeve fitted over it, combining the cushioning of paper with the crush resistance and dimensional precision of steel for medium coil weights.
How is a coil lifted without damaging the core?
Use a coil lifter or C-hook sized to the inner diameter, expand the mandrel so the load is spread over the bore, lift vertically, and lower onto saddles rather than onto the floor edge.
Frequently Asked Questions
Which core materials are available for galvanized coils? Three families are used in the trade: steel cores, made as welded spiral or seamless constructions; paper and fibre cores, spiral-wound from kraft paper or fibreboard; and composite cores, which combine a paper or fibre core with a thin steel sleeve.
What are the two constructions of steel cores? Welded spiral cores are formed from steel strip into a cylinder and joined by high-frequency welding, which gives the strongest and most reliable construction. Seamless cores are rolled and locked without a weld seam, removing the weld zone as a potential weak point.
Why is a positive tolerance specified on the core inner diameter? The positive tolerance lets the core slide onto the uncoiler mandrel without force while the mandrel expands to grip it. Standard inner diameters in the trade are 508 mm, 610 mm and 762 mm.
When is a composite core the better choice? For medium coil weights combined with thin strip, where a steel core risks marking the inner wraps and a paper core alone lacks rigidity. The steel sleeve gives crush resistance and a precise bore while the paper layer cushions the strip.
At which moments does most core damage occur? During lifting, transport, storage and unwinding. Damage is prevented by lifting with a properly sized lifter and an expanded mandrel, securing the coil on saddles or chocks, keeping paper cores dry, and matching the mandrel expansion range to the core bore tolerance.
What happens when a coil core fails? The coil can collapse, suffer edge crushing and telescoping, and develop loose inner wraps, which causes production stops at the uncoiler. Repeated damage at the same bore position signals a mandrel or handling problem that should be corrected before the next coil.