Specifying Slit Width, Width Tolerance and Core Diameter

Most converting problems that get blamed on the mill trace back to an incomplete purchase specification. "300 mm wide" leaves out the tolerance, the core, the roll diameter and the winding, and every one of those can stop a line just as effectively as the wrong alloy. This is what to pin down.

How slit foil is actually produced

Foil is rolled as a wide master coil. Slitting cuts that master into narrower strips with circular knives, then each strip is wound onto a core. The width you receive is therefore a cut edge, and any cut edge carries variation: knife wear, tension across the web, and the position of the knives all contribute. The tolerance band is not an admission of poor quality, it is a property of the process, and the sensible approach is to specify a band your line can genuinely tolerate rather than demanding a figure no mill can hold.

What width tolerance is realistic

Standard commercial tolerances are workable for most converting, printing and packaging operations. Tighter bands are achievable but expensive: they mean slower slitting, more frequent knife changes, more measurement and more scrap, and the cost lands in the tonne price. Before you insist on a tight tolerance, measure what your own process actually needs. If a two millimetre variation causes no jam and no visible defect, you are paying for control you will never see.

Core diameter is a machine-fit question

Cores are specified by inner diameter, and the common sizes are 76 mm and 152 mm, with 38 mm used on smaller rolls. Getting this wrong is expensive in the least forgiving way: a roll that will not seat on your mandrel cannot be run at all. Confirm the core inner diameter your unwinder is built for, and specify it explicitly rather than assuming a default. If you operate more than one machine, record which core size each one takes.

Roll diameter and weight

Outer diameter determines how long the line can run between roll changes, and it is limited by winding stability. Larger rolls reduce changeover frequency but must still arrive undistorted in the container, which brings packing back into the picture. Tell the mill the maximum roll weight and diameter your handling equipment and shaft can accept, and tell it the minimum that makes a roll change acceptable to your operators.

Tension, edge quality and telescoping

Winding tension determines whether the roll arrives as a usable cylinder or as a telescoped, edge-damaged coil. Consistent tension is what keeps the edges flush. If you have been receiving rolls with stepped or protruding edges, the conversation to have is about winding tension and packing, not just about the width figure.

Traverse winding for automated lines

Automated and high-speed lines often need spooled or traverse-wound material rather than a plain wound roll, so the foil can be fed continuously. The requirements are more specific: spool dimensions, bore, winding pattern, and often a limit on splices or a maximum length per spool. Specify these before the order, because converting a wound roll into a spooled package afterwards is not always possible.

What a complete width and core specification looks like

• Slit width, with the tolerance stated as a plus and a minus figure
• Core inner diameter, and the core material or specification if it matters to your machine
• Roll outer diameter or maximum roll weight
• Direction of winding as you will unwind it, and whether the surface you need faces in or out
• Whether splicing is permitted, and the maximum length per roll or spool
• Edge condition required, and whether interleaving is needed to protect it

Width, tolerance and core are everyday requirements for us. Tell us the slit width and its tolerance, the core inner diameter and the roll weight your equipment accepts, and we will confirm the achievable specification before quoting. See our aluminium foil roll range, or aluminium foil sheet where flat cut material suits you better than a roll.