The box arrived with 6.5 kg of gel inside, which was the number on the spec, and the top layer of tuna loins still read 8.9 °C against a 2 °C target. The pack mass was right. The packs were all lying flat on the floor of the shipper, under a 16 kg payload, doing nothing for the 30 cm of air above them.
A 20 kg insulated shipper is the workhorse of seafood and meat export: roughly 55-60 litres internal, a payload of 15-18 kg, and a gel load somewhere between 6 and 9 kg depending on lane length. Where those kilos sit matters as much as how many there are, and the difference is measurable in a single afternoon.
Why the floor is the worst place for all of them
Cold does not fill a box like water. Air in a sealed shipper moves in a slow loop: it cools against the pack surfaces, becomes denser, sinks, and displaces warmer air upward. Put every pack on the floor and you create a stable cold layer 10 cm deep with a stagnant warm cap above it. The product sitting in that cap is insulated from the cold by the product beneath it, which is itself a poor conductor at 0.5 W/m·K.
Stacking 16 kg of loins on top of the packs also compresses them. A 500 g pack under 30 kg of static load will thin at the centre, push gel toward the seams, and put the heat-seal under tension for the whole flight. That is one of the quieter causes of leaks that get blamed on film quality.
A three-way test with the same gel mass
We ran three layouts in identical 58-litre shippers, 16 kg of frozen loins, 6.5 kg of gel at -18 °C, 30 °C ambient, 36 hours. One logger went between the loins in the top layer and one under the bottom layer, both positioned against product rather than pack.
| Layout | Top logger | Bottom logger | Spread |
|---|---|---|---|
| A: all six packs on the floor | 8.9 °C | 0.4 °C | 8.5 K |
| B: all six packs under the lid | 0.6 °C | 8.6 °C | 8.0 K |
| C: two floor, two long walls, two lid | 2.1 °C | 3.4 °C | 1.3 K |
Layout C used the same 6.5 kg and cut the spread from more than 8 K to 1.3 K. Nothing about the gel changed. What changed was the surface area of product exposed to a cold face, and the fact that the loop had a cold wall on every side to run against.
The failure the winning layout almost caused
In the first run of layout C, one pack was taped directly to the top loin to stop it sliding. That loin arrived with a 3 mm frozen crust and a pale ring through the centre — one fillet per box downgraded, roughly $9 of product in a box that shipped for $46. Direct contact between a -18 °C pack and a 2 °C product is not a cold-chain solution; it is a freeze brand.
Leave 20-25 mm between any frozen pack and the product surface, and let a corrugated divider or a bubble sheet do the work. The gap costs almost nothing in performance — air at that thickness is a better conductor than the product itself — and it removes the contact-freeze argument from your claim file entirely.
Practical placement for the 20 kg shipper
- Two packs on the floor, laid flat, with a divider over them before the payload goes in.
- One pack against each long wall, standing upright, held by the box wall and not by tape on the product.
- Two packs under the lid, taped to the lid liner so they travel with the lid when it opens at destination.
- No pack in the four bottom corners; corners are where the panel is thinnest and where a dropped box lands.
- Total 6-9 kg for a 40-hour lane at 30 °C ambient, scaled by the heat-load sum rather than by habit.
Then check it. Two loggers, 36 hours, one under the lid and one under the payload, and you will know in a day whether your layout is A, B or C. If you are specifying packs for a new box size, start from the logistics ice pack guide and adjust from there.
Da maohua
Zhengzhou Flor New Materials Co., Ltd. specializes in SAP super‑absorbent polymer products. Serving personal hygiene, agriculture & horticulture, pet supplies and other industries, we leverage supply‑chain resources to provide particle‑size selection, performance matching and customized solutions based on clients’ practical requirements.
