Liquid products shipped long distance or by sea frequently arrive with one recurring complaint: the outer carton has deformed — the box bulging, corners crushed, glue lines open. With heavy, dense products such as liquid fertilizer, the problem usually starts on the bottom layer of the truck.

The first reaction is almost always "the carton grammage is too low". But after following liquid fertilizer, juice, sauce and dairy shipments for years, the cases we have handled were rarely one thing. They were four accounts stacked on top of each other: grammage, structure, moisture, and what is inside the box and how it is stacked. This article breaks down all four — not only why cartons deform, but what to look at and how to inspect. If you are building a bag-in-box solution, start with our liquid fertilizer packaging solution and come back for the details.

Account One: Grammage — the same grammage is not the same paper

"Grammage" (g/㎡) describes the weight of the paper per square meter. It does not describe its strength.

A paper marked 160g may be high-strength corrugating medium, or ordinary corrugating medium. The liner may be coated white-board, or ordinary white-board, or recycled liner. The inside liner may be kraft, or recycled pulp. Pulp blend, fiber length, moisture content, sizing and surface treatment all differ — and the resulting compression performance can differ a great deal.

So the two figures that actually matter are two you can measure and two you can write into a purchase order: Edge Crush Test, ECT (N/m) — the force the board withstands per unit length along the flute direction, which maps directly onto stacking strength; and bursting strength (kPa) — the board's resistance to puncture, which maps onto impacts and sharp edges in transit.

The same sentence — "five-ply, same grammage" — can mean two completely different things from two different paper mills. The cheaper quote usually differs right here: not because the paper is expensive, but because the paper is weak.

Our advice: write edge crush strength, bursting strength and dimensional tolerance into the carton technical agreement, and ask for a test report with every batch. When you change paper mill or batch, run a comparison first — never price from the grammage line alone.

Account Two: Structure — flute, ply and box shape decide where the force goes

The flute profile is the carton's skeleton. At the same ply count, the difference between flute types is not small: B flute (2.5–3mm high) gives good compression, flat printing and medium cushioning; C flute (3.5–4mm) gives good cushioning and medium compression; A flute (4.5–5mm) gives the best cushioning but the weakest compression and the roughest print.

In short: the taller the flute, the better the cushioning — but the weaker the compression strength and the rougher the print. Cartons for liquid bags are generally heavy, so a single tall flute is rarely the right answer; the usual approach is a combination that trades compression against cushioning. That combination is one of the variables we work out when we set the carton standard for a liquid fertilizer customer.

Ply count and box shape matter just as much: at the same outer dimensions, five-ply has higher compression strength than three-ply and costs more; and the closer the box base is to square, the better the compression strength. The length-to-width ratio of the base is not chosen because it "looks nice" — it also determines how the box stacks and how the load is carried.

One more easily overlooked point: printing and die-cutting cut the paper fibers, and creasing that is too deep or too shallow also affects the strength of the finished box. Two cartons of identical specification can measure differently once formed, simply because the print area or slotting position differs — so test the formed, assembled box, not just the board.

Account Three: Moisture — what cartons fear is not pressure, it is water

Paper is hygroscopic. Once moisture content rises, compression strength drops noticeably — this, rather than insufficient grammage, is the real culprit behind many "the cartons suddenly failed" cases.

Three scenarios dominate: rainy-season transport; sea freight containers (large day-night temperature swings, condensation inside); and cold-store loading and unloading (condensation on the surface). The corresponding measures: film lamination or moisture-barrier treatment on the surface, stretch film around the pallets, goods stored off the floor and away from walls, desiccants where needed, and letting the goods stand in a dry environment after arrival before going to the racking.

There is also a time account: long-term stacking deforms cartons gradually under sustained load (creep). So storage duration belongs in your stacking height calculation — a stack that sits for three months and a stack that sits for three weeks should not be the same height.

Account Four: what is inside the box, and how it is stacked

Liquid packaging concentrates weight: liquid fertilizer typically runs 1.1–1.5 in density, heavier than water at the same volume; a filled bag bulges and holds internal pressure, so the weight is unevenly distributed; and inside the carton the bag rubs and shifts against the walls. All of this erodes the compression strength you calculated from the carton grammage alone.

Stacking is not just piling up — it is a set of rules you can actually calculate. Start from the standard pallet (1200×1000mm) and calculate truck loading and maximum stack height together. When the box base is not square (length ≠ width), you must stack with a 90° rotation, alternating odd and even layers; straight-up stacking is not an option, because the load is uneven between layers and the stack leans. When rotating, keep the number of boxes per layer as equal as possible between odd and even layers, so the whole stack carries load evenly and nobody has to distinguish layers during loading. A full case of converting from drums to bags for a liquid fertilizer plant is written up here.

Our actual sequence is: fix the box base dimensions (they must match the inner bag) → calculate boxes per layer and stack height → verify the bottom layer's load → then decide the paper grade, flute combination, and whether internal reinforcement is needed.

What our carton recommendation actually looks like

What we give a customer is never the sentence "use five-ply". It is a carton standard developed specifically for liquid fertilizer: the paper grade and strength thresholds, the flute combination, the moisture barrier, whether internal reinforcement is required, and the packing and stacking method — all set according to bag dimensions, weight per carton and transport conditions, and handed over with the proposal as verifiable parameters.

That standard came out of years of following liquid fertilizer shipments — different densities, different specifications, different seasons and transport modes — and it keeps being updated. It is also why we are reluctant to lay the numbers out in an article: carton parameters without their application context mislead easily, and copying someone else's figures can be more dangerous than having none at all. So this article sticks to what to look at and why; the specific figures belong in your project discussion.

The boundary, stated plainly: we do not manufacture cartons. What we do is work out the bag dimensions, the packing method, the stacking and pallet plan, and the transport and storage conditions — then hand over the parameters and inspection criteria the carton should meet. The specific paper mill and process are handled by the customer's carton supplier, and we are happy to align the specification with them. Where conditions allow, a full-box stacking or drop test before mass production is the safest route.

For customers just starting out, or running small batches, we go through exactly the same process — trial small, measure the parameters, then scale. That is much cheaper than ordering tens of thousands of cartons and then having to redo them.

FAQ

Q: Why can cartons of the same grammage differ by half in price?

A: Grammage describes the weight of the paper, not its strength. Grade, pulp blend and moisture content can change edge crush and bursting strength significantly. Comparing quotes on the grammage line alone is how you end up buying a carton that is "full weight, short strength".

Q: What should our purchasing team specify?

A: Edge crush strength + bursting strength + ply count and flute type + dimensional tolerance + whether the board is laminated or internally reinforced — and require a test report per batch.

Q: Cartons arrive soft with open glue lines after sea freight. What now?

A: The root cause is almost always moisture pick-up reducing compression strength. What helps: lamination or moisture-barrier treatment, stretch film and desiccants, storage off the floor, a lower stack height, and letting the goods stand before racking.

Q: When is internal reinforcement necessary?

A: It depends on weight per carton and stack height — where the load concentrates on the base and the stack is high, we state the requirement directly in the proposal. Ultimately, the full-box stacking test decides.

About Qichen Packaging: Qichen Packaging has focused on liquid flexible packaging for more than twenty years (founded in 2001), with our own factory in Wuxi, ISO 22000 certification and a Class 100,000 cleanroom. Our products meet GB 4806 series, EU 10/2011 and FDA 21 CFR food contact material requirements. From aseptic bags and bag-in-box to fitments and filling support, we deliver a complete path from bag to running production line.

Want to talk about your carton? Tell us your bag dimensions, weight per carton, stacking plan and transport mode — we will work the carton parameters and stacking out with you, then arrange samples. From sample to mass production, we run the line with you.