Winding a thin sheet between layers is one of those interventions that works extremely well when it is needed and is pure cost when it is not. Deciding which case you are in is mostly about the product and the number of layers.
What interlayer does
Three useful things:
- It stops layers embedding in each other. Soft or tacky jackets will otherwise press into the layer below over time, and the pack effectively becomes a solid mass.
- It stops layers bonding. Some products, particularly with certain jacket compounds, will block or stick to themselves under pressure and warmth. A sheet between layers prevents it.
- It gives a clean separation point. Where a spool is part-filled and part-used, the sheet marks a boundary.
What it costs
- Material and handling. The sheet itself, and somebody loading it.
- A take-up at payoff. The sheet has to go somewhere as the spool empties, which means a second reel and the space for it. This is the capital item people underestimate.
- Capacity. The sheet takes up room in the pack, so you get less product on the same spool.
- A failure mode. If the sheet is narrower than the pack, or if it tears, it can leave debris in the wind — and a torn sheet caught in the product is worse than no sheet at all.
When it is clearly worth it
Products with soft or tacky jackets, products that block in storage, deep packs with many layers, and anything that has already given you dig-in or bonding problems that you could not fix with tension and pitch settings.
Also: long storage. A pack that will sit for a year before it is unwound has much more time to consolidate than one that is used next week, and interlayer is cheap insurance against that.
When it is not
Shallow packs, stiff product, short storage, and any situation where the payoff has no take-up for the sheet. Without a take-up, the sheet ends up on the floor, and an operator stepping on a sheet on the floor is a bigger problem than the one you were solving.
Choosing the sheet
The sheet should be wider than the pack so it covers the reversal ridges at the edges, and it should be strong enough not to tear at the point where the product crosses it. Thinner is better for capacity; stronger is better for reliability. For reusable spools, the sheet is consumable, so its cost is per trip rather than one-off — worth putting in the calculation properly.
Alternatives worth trying first
Before adding interlayer, try the two free adjustments: tighten the traverse pitch so there are no gaps for a layer to sink into, and hold or slightly increase the winding tension. Both are settings, both cost nothing, and together they solve most of the problems that interlayer is bought to fix.
If those do not work, interlayer is the answer — and it is a good one. Just price the take-up before you commit.
The cost nobody counts
Separation sheets are cheap as a line item and expensive as an operation. Somebody has to feed them, somebody has to remove them at payoff, and somebody has to dispose of them — or, in the reusable version, collect, flatten, store and reissue them. On a line running continuously, that handling time usually dwarfs the material cost.
There is also a waste question. Single-use sheets on a multi-trip spool programme undercut the environmental argument fairly directly, and if you are reporting on packaging waste it is a visible line. Worth counting before you commit rather than after.