No. 21
How do container shipments of empty nested bread trays get configured to maximize unit density per container?
- 6 sections
- 2 tables
Density in a container comes from two numbers multiplied together: how many nested columns fit on the floor, and how many trays fit in each column. The floor count depends on the tray footprint and the container’s internal length and width; the column count depends on the usable height, the tray’s overall height and, above all, its nest increment, the height each extra tray adds. A small change in nest increment can move the total more than a change in loading pattern. Weight stays far under the limit in the worked example below, but it has to be checked, and the load has to be braced, kept free of untreated wood, and classified for customs before it ships.
Step 1: count columns on the floor
Start from the container’s internal dimensions as measured or stated by the carrier, not the nominal size. ISO 668 lists minimum internal dimensions for a 40-foot high-cube container of 11.998 m (39 ft 4.375 in) long, 2.330 m (7 ft 7.73 in) wide and 2.655 m (8 ft 8.5 in) high (ISO 668 summary); actual boxes can be larger, so use the figures for the equipment you book.
For example, take a tray with a 26.9 x 22.0 inch footprint (683 x 559 mm, the published size of SPF Plastic Group’s SPF 650) on that minimum floor:
| Orientation | Along the length | Across the width | Columns per floor |
|---|---|---|---|
| 26.9 in along the length | 17 | 4 | 68 |
| 22.0 in along the length | 21 | 3 | 63 |
The first orientation wins by five columns because four 22.0-inch sides fit across the width with room to spare, while only three 26.9-inch sides do. Check both orientations for your tray, and check whether a mixed pattern (most rows one way, a last row turned) adds a column in the leftover strip; with some footprints it does, with others the strip is too narrow.
Step 2: count trays per column
A nested column of N trays stands H + (N − 1) x n tall, where H is the tray’s overall height and n its nest increment. Solve for N with the usable height, which is the internal height minus a margin for loading and any base under the column.
For example, keep the 6.1-inch tray height, leave about 2 inches (50 mm) of margin under the 2.655 m roof, and try two nest increments:
| Nest increment | Trays per column | Trays per container (68 columns) |
|---|---|---|
| 1.5 in | 65 | 4,420 |
| 2.0 in | 49 | 3,332 |
Half an inch of nest increment changes the load by 1,088 trays, about a third more than the 2.0-inch case carries. That is why the nest increment on the supplier’s sheet deserves close attention when containers are part of the plan. The increments in this table are illustrative; use your tray’s figure.
A column this tall is a single nested stack, not loaded product, and the bottom tray carries only empty trays above it. Still confirm with the supplier that the tray tolerates the column height you plan, and that nested trays will release after a long, warm voyage.
The door can set a lower limit than the roof. One leasing company’s specification for a 40-foot high-cube unit lists an internal height of 2,698 mm but a door opening height of 2,585 mm (101.77 inches). The 65-tray column in the example stands 6.1 + 64 x 1.5 = 102.1 inches, so it cannot pass upright through that door: build such columns inside the container, or cap columns that are built outside and moved in at the door height, here 64 trays (6.1 + 63 x 1.5 = 100.6 inches). That figure is for the bare column; it leaves about 1.2 inches for anything the column stands on and for handling, so if columns travel on a base, a slip sheet or a loading aid, measure the full height against the door and drop trays until it clears with room to spare. Use the door opening listed for the box you book.
Step 3: check weight and floor load
Empty trays are light for their volume, so the example container fills by volume long before weight; check your own load rather than assume the same.
- Payload. In the example, 4,420 trays at 4.10 lb is 18,122 lb (8,220 kg). ISO 668 lists a maximum gross mass of 36,000 kg for this container type; the rating that applies to your shipment is the one on the box’s own data plate, which also gives its tare and maximum payload.
- Floor line load. Load spreads along the floor; a German transport-insurance reference cites maximum line loads of 4.5 metric tons per meter for a 20-foot container and 3.0 metric tons per meter for a 40-foot container (TIS container floor loading). Ask the carrier for the figure for the box you book. The example load of 8,220 kg spread over about 12 m of floor is roughly 0.7 metric tons per meter, far below the 40-foot figure; pallets or racks concentrate load and need their own check.
Step 4: hold the columns in place
Ships roll and pitch, and the load moves with them for days. Tall nested columns standing free can lean, slide or topple.
- Pack columns tightly against each other and against the walls, so no column has room to start moving.
- Fill remaining gaps with dunnage or airbags sized for the gap.
- Band or wrap groups of columns into blocks where the column height is high relative to its footprint.
- Close off the door end with a barrier, such as a bulkhead, bars or straps across the last row, so the load cannot fall against the doors when they are opened.
- Write a loading plan with the pattern, column count, trays per column, bracing and void fill, and have the loading crew sign it.
Step 5: wood packaging, documents and customs
- ISPM 15. Wood packaging material in international trade, including pallets and wood dunnage, must be debarked and then heat treated or fumigated, and marked (ISPM 15 summary). Products made from alternative materials, such as paper, plastic or wood panel products like OSB, hardboard and plywood, are exempt. The rule follows the wood, not the pallet: floor-loaded columns still need ISPM 15 treatment and marking for any solid-wood blocking, bracing or dunnage, and only a load with no solid wood at all is outside it.
- Food-contact documents. Ship the supplier’s food-contact compliance statement with the commercial documents so the receiving quality team can release the trays without delay; what that statement should contain is covered in our guide to food-contact regulations and certifications.
- Customs. Confirm the tariff classification, duty and any origin documentation with a licensed customs broker before the first shipment, and use the same classification on every shipment after that.
When a container is not the right comparison
Bread trays are also made domestically; a U.S. buyer comparing an overseas supplier with a domestic one should compare landed cost per tray, including ocean freight, duty, port and drayage charges and the inventory carried while trays are at sea, with the domestic price delivered by truck. Return-trip density in domestic trucking is covered in our guide to the economics of reusable trays, and the geometry behind nest increments in our guide to stacking and nesting.
Get the supplier’s nest increment and footprint, the carrier’s internal dimensions and payload, and run steps 1 to 3 before you negotiate a price per container.