Logistics

Packing Cartons for Sea Freight: Ply, Moisture and Palletising

Which ply should an export carton be for sea freight?

For standard sea freight, 5 ply double wall in BC flute is the workhorse carton. It holds shape at the bottom of a container stack for weeks. Step up to 7 ply triple wall, three flutes, for heavy or dense units, deep stacks and long rough-handling routes. Confirm the ply against your real weight and route.

A sea container is a harder test than any domestic van. The carton runs for weeks, not a day. It sits deep in a stack carrying the full column above it. It is loaded and unloaded by forklift at two ports, and it rides through a humid box that softens board the longer the crossing lasts. The ply that survives a same-city delivery is often the ply that slumps somewhere off the coast, and you find out only when the pallet arrives crushed at a port you cannot reach.

So work from the load, not from habit. Take three numbers: the unit weight, the stack height inside the container, and the length and roughness of the route. Heavier units and deeper stacks push the ply up. Longer, more humid routes add margin. Vikrama Industries builds these cartons factory-direct from NCR on recycled kraft that is GSM-verified and burst-tested by batch, so the strength on the spec sheet is the strength in your hands. The table below is a starting point, then a physical sample is approved before any run.

Getting the ply right is money in both directions. Over-specify and you carry grams of paper you did not need on every carton for the life of the SKU. Under-specify and you pay in slumped pallets and rejected consignments abroad. The board cost tracks the paper it uses, and recycled kraft trades roughly Rs 38 to Rs 46 per kg with the market, so a live estimate comes back as a band, not a single figure. Prices attract 5% GST (HSN 4819); a GST invoice is issued with every order.

A sea container is judged over weeks, not days, so the ply that survives a city delivery is rarely the ply that survives the crossing.

How do you protect cartons from container humidity?

Plain recycled kraft is not waterproof, and a sea container turns humid over a long crossing, which softens board and lowers stack strength. Protect the load rather than trusting the box: shrink-wrap each pallet, keep cartons off wet floors, load the goods dry, add desiccant for sensitive items, and specify a heavier ply for humid routes.

Moisture reaches an export carton three ways, and none of them care how strong the dry board tested. Container sweat forms when warm, damp air inside the box meets a cooler steel roof and rains back down on the top layer. Monsoon loading traps humid air in the container before the doors even close. And a carton packed damp carries that moisture with it for the whole voyage, weakening from the inside. The fix is to manage water at the load level, because the bare board will not shrug it off.

• Shrink-wrap or stretch-wrap the whole pallet so less board is exposed to damp air.
• Keep pallets off wet floors and dock surfaces at both ends of the trip.
• Load the goods dry; a carton packed damp holds that moisture the whole voyage.
• Add desiccant sachets inside the container or carton for moisture-sensitive goods.
• Step the ply up for long, humid routes instead of expecting bare board to shrug off water.
• Keep coatings minimal so the carton still recycles clean at the far end.

The comparison below shows the gap between packing an export load like a domestic one and packing it for the crossing. The difference costs a little more per carton and saves the whole pallet.

Container humidity, not the drop test, fails most sea-freight cartons. Protect the load and spec the ply for the damp, and the box lands standing.

How should cartons be palletised?

Unitise cartons on a pallet, do not ship them loose. Column-stack so each carton wall sits directly over the wall below, because a corner over a void crushes first. Keep every carton inside the pallet footprint with no overhang, then shrink-wrap and strap the load so it moves and lands as one block.

A pallet is not just a way to lift the goods. It decides how the stacking load travels down through the cartons. Corrugated board carries a vertical column best through its edges and corners, so alignment is everything. Column stacking, cartons squared straight up, keeps each box loaded through its strongest lines. Interlocking or brick-laying the layers ties the stack together and stops it sliding, but it puts carton walls over open gaps and gives up a large share of compression strength. For a heavy sea-freight stack, favour column stacking and add strapping for stability.

• Column-stack for strength; interlock only the top layers if you need extra stability.
• No overhang: a carton hanging past the pallet edge loses much of its strength.
• Square the load to the pallet so the corners line up floor to top.
• Shrink-wrap and strap the unit so it stays one block through forklift handling.
• Remember the lowest cartons carry the whole column, so they set the ply you need.

This is where the ply decision and the pallet decision meet. The taller the unit load and the deeper the container stack, the more weight the bottom cartons hold, and that is what pushes a 5 ply carton up to 7 ply. Plan the pallet pattern and the ply together on your real cartons and your real container, and a person walks that plan with you before anything is cut, so the stack is set on your goods and your route, not a guess.

What markings do export cartons need?

Every export carton needs marks a stranger can read: consignee, destination port, carton number, gross and net weight, and handling symbols such as this way up, fragile and keep dry. Print them on a planned panel rather than sticker them, which survives a wet, rough crossing far better. On a humid route the keep dry mark earns its place.

That is the short version, because marks are a whole job of their own and this post is about surviving the crossing. The panel-by-panel layout, the symbols and where each one goes are covered in full in the markings guide.

• Export carton markings guide
• Price your sea-freight carton

Frequently asked questions