In short: specifying custom cardboard dividers comes down to five inputs: the internal case dimensions, the true product envelope, the functional clearance (usually 1–3 mm per cell), the board caliper, and the blade height. A 2 mm error on clearance is enough to turn a perfect fit into a product that rattles. This guide gives the full calculation method, typical values by material, and the mistakes that cost tooling money.
The five variables that define a divider
A divider — also called a partition, cell pack or insert — is a grid of slotted blades that interlock to form one cell per product. Its geometry is entirely determined by five inputs.
- Internal case dimensions (length × width). Internal, not external: on a double-wall RSC the difference is 8–10 mm.
- Cell count along each axis, which follows from your pallet pattern.
- Functional clearance, the gap between product and blade.
- Board caliper, which consumes space and must be deducted.
- Blade height, which sets the contact area and therefore the quality of lateral protection.
The calculation, step by step
Take a case with 380 × 285 mm internal dimensions, holding 12 cylindrical bottles of 60 mm diameter in 4 columns × 3 rows, using 0.7 mm solid board.
Step 1: product envelope
4 × 60 mm = 240 mm along the length; 3 × 60 mm = 180 mm across the width. Critical point: use the product’s maximum dimension, not the nominal one. Blown glass routinely runs ±1.5 mm.
Step 2: count the blades
Four columns need three internal blades along the length, plus optional edge blades. Three rows need two internal blades. Each blade consumes its own caliper: 3 × 0.7 = 2.1 mm and 2 × 0.7 = 1.4 mm respectively.
Step 3: distribute the remaining space
Remaining length: 380 − 240 − 2.1 = 137.9 mm across four cells, or 34.5 mm per cell. Far too much — the product will move.
Two corrections are possible: add cells (move to five columns) or shrink the case. This is precisely why the divider study should come before the case is frozen, not after.
Step 4: validate the clearance
Ideal clearance sits between 1 and 3 mm per cell for a rigid product, and 2 to 4 mm for a product with wide dimensional tolerance or a fast manual packing line.
Recommended clearance by product type
| Product type | Clearance per cell | Note |
|---|---|---|
| Standard glass bottle | 2 to 3 mm | Account for the neck ring and label |
| Perfume / cosmetic flacon | 1 to 2 mm | Minimum clearance to avoid decoration rub |
| Glass food jar | 2 to 4 mm | Wide moulding tolerances |
| Machined metal part | 1 to 2 mm | High mutual scratching risk |
| Electronic board | 3 to 5 mm | Easier manual insertion; ESD is the priority |
| Injection-moulded housing | 2 to 3 mm | Watch post-moulding shrinkage |
Choosing the right board
Solid board (chipboard / greyboard)
0.4 to 1.2 mm caliper, 300 to 900 gsm. The reference material for fine cells, light products and sensitive decoration. Its smooth surface does not mark labels.
E and B flute corrugated
Around 1.5 mm and 3 mm. Flute adds cushioning capacity, useful once unit weight passes 500 g. It consumes far more space, which must feed back into step 2.
Honeycomb board
10 to 60 mm thick. Used for heavy-duty separators, honeycomb wall partitions and heavy part packing. It combines low weight with very high compression strength.
The five most common specification mistakes
- Working from external case dimensions. The most frequent and most expensive error: the divider either will not go in, or goes in under force and buckles.
- Ignoring product tolerance. A CAD drawing gives a nominal; production gives a distribution. Always ask for measured maximums.
- Undersizing blade height. Low blades let products touch near the top. Rule of thumb: at least 60% of product height, 80% for fragile items.
- Forgetting grain direction. Slots must be cut across machine direction, or blades tear during assembly.
- No grip provision. On manual packing, a divider with no finger notch slows the line by 15–20%.
Matching the spec to the assembly method
Flat-supplied, operator assembled
The cheapest option for freight and storage. Slot depth should be roughly half the blade height, with slot width 0.1–0.2 mm above board caliper so blades interlock without forcing.
Pre-erected
Saves 4–8 seconds per case on a manual line, at the price of three to five times the transport volume. It pays as soon as packing labour cost outweighs the freight premium — which happens quickly in Western Europe and North America.
Glued or stapled
For very fine cells or heavy products, permanent assembly guarantees squareness. Allow slightly more overall clearance: a rigid divider will not flex into a slightly undersized case.
Validate before you commit to tooling
Three simple checks eliminate 90% of nasty surprises.
- Cold insertion test — assemble ten dividers into ten cases from different batches. Case variation is often larger than divider variation.
- Shake test — close the case, invert it and shake by hand for thirty seconds. No product should change cell or touch another.
- Stack test — reproduce the real load of the pallet above for 24 hours and check that blades have not buckled.
Run in-house, these three tests cost half a day and prevent die corrections worth thousands.
From drawing to prototype
Digital cutting has transformed this stage. A prototype made-to-measure partition can now be produced in 48 hours with no cutting die, then iterated two or three times before tooling is committed. Prototyping cost is trivial next to the cost of reworking a die.
Specifying for automated lines
Automation raises the bar on tolerance. Where a manual operator compensates for a slightly out-of-square divider, a pick-and-place head does not. Four points belong in the specification whenever a divider feeds an automated cell.
Dimensional tolerance
State it explicitly — typically ±0.5 mm on external dimensions and ±0.3 mm on slot position. A supplier who cannot commit to a written tolerance is telling you something useful.
Flatness and set
Board carries a memory of how it was stored. Pallets stored on edge or under uneven load develop a curl that jams magazine feeders. Specify flat storage, banded stacks and a maximum bow.
Denesting behaviour
Flat blanks that stick together stop a line as effectively as a mechanical fault. Surface finish and any anti-stick treatment should be agreed at sample stage, not discovered at commissioning.
Pallet presentation
Layer count, orientation, interleaving and shrink method all affect how quickly the line can be replenished. Write the pallet pattern into the specification and check it on the first delivery.
Cost drivers you can actually influence
Three levers move divider cost far more than negotiating the price per unit.
Blank nesting. How efficiently blades nest on the sheet drives material yield. A 5 mm change in blade height can move yield by several percent, which is often worth more than a price negotiation.
Number of distinct parts. Every additional blade profile means another die station and another line item to stock. Designing a divider from two blade types rather than four reduces both piece price and inventory complexity.
Order consolidation. Dividers are produced in continuous runs. An annual commitment across several references typically improves unit price more than a spot negotiation on a single order.
Frequently asked questions
What clearance suits a 75 cl wine bottle?
Allow 2 to 3 mm per cell on a standard Bordeaux, measured at the maximum body diameter rather than the neck. For textured or spot-varnished labels, go to 3 mm to avoid any rubbing.
Can cells be different sizes in one divider?
Yes. Asymmetric cell layouts are common for mixed gift packs — one bottle and two glasses, for instance. This requires custom cutting and a clear orientation marker for the operator.
What is the minimum order for a custom divider?
With digital cutting, runs of a few hundred are workable. With a conventional cutting die, the break-even usually sits between 3,000 and 5,000 pieces.
Do I need a bottom and top pad as well?
Usually not — the case handles both. A bottom pad becomes useful when the product rests on a fragile base, and a top pad is essential whenever two product layers are stacked in the same case.
Key takeaways
A good divider is not the one that fills the case. It is the one that immobilises the product with the minimum clearance compatible with your packing rate. The maths is simple, but it demands reliable inputs: real internal dimensions, measured product tolerances and documented line constraints.
Estic-Maillot’s design offices have run divider studies since 1924 and operate digital prototyping across their European and US sites. Send us your product drawing for a dimensioned proposal.