A foam box has no hours of its own, because it makes no cold; it slows the escape of the cold placed inside it. Heat through the wall is inversely proportional to its thickness and proportional to the temperature difference between inside and outside: double the wall and heat gain halves.
An example shows the order of magnitude. A 46-litre EPP box measuring 600 × 400 × 320 mm outside and 538 × 338 × 257 mm inside has walls of about 31 mm. Taking EPP conductivity as 0.040 W/mK, between 25 °C outside and 4 °C inside, roughly 26 W flows in through about 0.95 m² of wall. Since 2 kg of ice absorbs 668 kJ as it melts, the calculation gives about 7 hours with the lid shut; 1 kg of ice halves that, at 35 °C it comes close to five hours, and a eutectic plate attachment on top extends it.
Measuring your own duration
- fill the box with a normal load and your usual amount of coolant
- put the logger probe at the core of the product
- keep the closed box somewhere close to summer conditions
- write down the hour the product crosses its acceptance limit
If the time falls short, a thicker box is the last step: first check pre-cooling, lid seating, filling of empty space and waiting in the sun. The measured duration goes into the shipping instruction as 'this product, this coolant, at this ambient, for this many hours'.
