Dry containers: how to stack cartons

Loading cartons into a dry container comes down to four questions: pallets or floor loading, can the cartons carry the weight of the stack for several weeks, do any cartons hang over the pallet edge, and what happens to the humidity inside. Color of Vietnam is preparing for export and has not announced a market. This piece is what we have learned during that preparation.

What a dry container is

A dry container is the standard steel box, unrefrigerated, for goods that do not need temperature control. A few basics, based on the general figures shipping lines publish:

• Three common sizes: 20 foot, 40 foot, and 40 foot high cube, which is about 30 cm taller than standard. Internal volume is roughly 33, 67 and 76 cubic metres respectively.
• Snacks are light and bulky, so a container fills by volume long before it reaches its weight limit. For goods like snack jars, a 40 foot high cube is almost always the better deal per carton.
• Inspect before loading: dry floor, no strange smell, intact door seals, and stand inside with the doors shut to look for pinpricks of daylight.

Pallets or floor loading

The right answer comes from the receiver, not from us.

• Floor loading: cartons stacked directly on the container floor to full height. It fits more and costs nothing in pallets. But unloading is by hand, carton by carton, takes hours, dents cartons, and many warehouses in developed markets refuse it.
• Palletised: cartons on pallets, stretch-wrapped, with a forklift unloading each pallet in minutes. Cartons are better protected and large warehouses usually require it. In return, pallets take space and must match the size the receiver uses. The two common sizes are the Euro pallet at 1200 x 800 mm and the industrial pallet at 1200 x 1000 mm, and the number per container is best calculated with your forwarder.
• Wooden pallets crossing borders must be heat-treated and stamped under ISPM 15, the international standard for wood packaging.

Before planning, ask the receiver: pallets or floor loads, which pallet size, and what maximum height.

Cartons must be strong, and must not overhang

The bottom layer of cartons carries the entire weight above it for weeks, through vibration and high humidity. Corrugated board weakens as it absorbs moisture. So when we order cartons we ask the supplier for two figures: ECT (edge crush test, the strength of the board's edge) and BCT (box compression test, the load a whole box can take), and then work out the allowable stack height with a safety factor for the sea voyage.

Three common mistakes:

• Cartons hanging over the pallet edge. The unsupported part costs the bottom layer a large share of its load capacity.
• Interlocking layers for stability, which leaves carton corners sitting on nothing. Straight column stacking carries far more load. Stability comes from stretch wrap and corner boards instead.

Humidity, the quiet enemy

Containers at sea suffer what the trade calls container rain. During the day the air inside is hot and humid. At night the steel roof cools, water condenses on it and drips onto the goods. For crisp snacks, the laminated film pack protects the product itself, but the carton soaks up water, softens, and a whole column can topple.

What we put on our loading checklist:

• Dry wooden pallets only. A wet pallet is a moisture source sitting directly under the goods.
• Kraft paper or film on the floor and beneath the roof over the cargo area.
• Container desiccant bags hung along both walls, in the quantity the bag supplier recommends.
• A small gap between cartons and the walls, so board never touches steel.
• No loading on rainy days.

A paper exercise for the Maize Ball jar

With no real container yet, we ran a paper exercise for the Let's PLAY Maize Ball jar together with the warehouse lead at our partner facility. The jar is light and its carton is large, so the problem is volume rather than weight. We calculated cartons per layer on an industrial pallet, layers per pallet under a maximum height that a hypothetical receiver might allow, and checked the total against the door height of a 40 foot high cube.

The first version chose the pattern that fitted the most cartons per layer, and the cartons overhung the pallet by a few centimetres on two sides. The warehouse lead pointed at the overhang and said the bottom layer would collapse before the ship docked. The second version dropped one carton per layer, kept every carton inside the pallet edge, stacked in straight columns, and added stretch wrap and corner boards. That calculation now sits in the packing specification for the jar, with lines for jars per carton, cartons per layer, layers per pallet and pallets per container.

If you are a logistics partner or a buyer who would like our packing specification, reach us through the Contact page. If you would like to run calculations like this in a small team preparing its first shipment, keep an eye on our Careers page.