Void fill and gel packs: when the filler freezes the pack

We tested four void fills against the same payload and the same 800 g pack: air pillows, kraft paper, moulded pulp and a foam-in-place corner set. The paper came last on temperature by a clear margin, losing 3.1 °C at the twenty-four hour mark compared with air pillows. That was expected. What was not expected was the paper arriving at the destination as a damp, compressed mat that had stopped filling anything at all.

Void fill has one job in a cold box, and it is not cushioning. It is to stop air moving. But the material you choose to stop air moving will also decide whether the cold gets to the payload, and those two effects pull in opposite directions.

Air is the best insulator and the worst conductor

Still air has very low conductivity, which is why foam and pillows work. Moving air carries heat efficiently, which is why a box with a loose fill and a gap at the top performs worse than a box packed solid. The target is not “full”, it is “no path”. A carton that is full but has a continuous channel along one wall will convect heat from the lid to the base in a loop, and the fill will do nothing.

This is why the position of the pack and the position of the fill interact. A pack pressed against the lid blocks the top of the convection loop, but it also puts the coldest surface against the hottest wall. A pack in the middle with fill all around it is thermally isolated from the payload, which is worse than no fill at all in a small box.

Where the pack should sit relative to the fill

The workable arrangement in most shippers is: pack at the base or against one wall, payload beside it with a rigid divider, and fill only in the remaining gaps, never between the pack and the payload. Fill between them insulates the pack from the thing it is supposed to protect. We have measured a 2.4 °C penalty at twelve hours from a single layer of bubble wrap placed between a pack and a vial tray by an operator who was trying to be careful.

The correct mental model: the pack and the payload are one thermal system, and the fill’s job is to keep the outside world out of it. Anything that goes between them is a wall you built inside your own box.

The moisture problem with paper

Paper void fill absorbs water. In a cold box there is always some water, from condensation on the packs and from the product itself. Once paper takes up moisture it loses stiffness, compresses under vibration and stops filling the space it was put there to fill. A carton packed tight on Monday can have a loose load by Thursday, and a loose load convects.

If you must use paper, wrap it away from direct pack contact, or use a coated grade. If you are choosing fresh, air pillows and moulded pulp both hold their shape in a damp environment, and pulp has the advantage of blocking radiation as well as convection because it is opaque.

Testing fill properly

Run the test with the real payload mass, the real pack, and a vibration table if you have one, because the failure mode of most fills is not thermal, it is mechanical: the fill migrates and the box empties itself in transit. A static test in a chamber will tell you the fill is fine right up until the box is tipped forty-five degrees on a conveyor.

Then open the box at the end and look at the fill before you look at the logger. If the fill has moved, the temperature data describes a box that no longer exists at delivery, and the customer will see exactly what you see.

If you are re-specifying a shipper, the pack dimension tables in our logistics ice pack guide are laid out so the fill allowance is a separate line, which makes it much harder to accidentally insulate the pack from the product.

Take one carton, remove the fill from between the pack and the payload, and run it against your current build for twelve hours. That single change is usually worth more than switching to a more expensive fill.