The blast freezer was set to −25 °C and the packs had been in it since the previous evening. At 06:00 the loader squeezed one from the top of the stack, felt it hard, and started loading. At 09:20 a driver in the yard cut a strap and found the bottom layer of the second pallet flexible, like a half-frozen juice pouch. Those packs had been in the freezer for fourteen hours.
Freezing a gel pack is a heat transfer problem, and a stack of packs is one of the worst geometries you can choose for it. The pack on the outside freezes in three hours. The pack in the middle of a 400 mm stack can take a day and a half, because it is insulated by the frozen packs around it.
Why insulation gets worse as freezing proceeds
Frozen gel conducts heat better than liquid gel, roughly 2.2 W/m·K against 0.6. That sounds like it should help. The problem is that the frozen layer is between the cold air and the liquid core, and the latent heat that has to be removed to freeze the core is large: about 300 kJ per kilogram of gel, on top of the sensible heat.
So the outer shell freezes quickly, then the process slows dramatically. The remaining liquid is surrounded by ice at −18 °C, and the only path for heat is conduction through 40–60 mm of frozen gel. Meanwhile the packs around it are also frozen, so there is no cold air movement anywhere near the surface.
Thickness dominates. A 25 mm pack freezes through in about 4–6 hours at −25 °C with air movement. A 40 mm pack takes 8–11 hours. A 50 mm pack, in the middle of a stack, can take 18 hours or more. Doubling the thickness roughly quadruples the time, because the heat has to travel twice as far through a path that is also getting less effective.
The stack makes it worse than the maths suggests
In a single layer on a rack with air moving over both faces, the numbers above hold. Stack packs in a carton, put cartons on a pallet, and wrap the pallet, and the centre pack sees almost no air at all. The dominant resistance becomes the carton wall and the air gaps between packs, both of which are excellent insulators.
That is why the surface of the pallet reads −18 °C on an infrared gun while the core is at −2 °C. The gun is measuring the part that froze first.
What to change
Freeze single layer, or at most two layers, on racks with gaps. This is the change that fixes most of the problem, and it usually costs nothing but floor space in the freezer.
- Single layer on wire racks, 20 mm gaps between packs
- Air velocity over the packs of at least 2 m/s
- Freezer at −25 to −28 °C, not −18
- Minimum 8 hours for 40 mm packs, 12 for 50 mm
- Probe the centre pack, not the top one, before releasing a pallet
If floor space is the constraint, freeze in a tunnel with forced air and load directly from the tunnel exit. A tunnel that freezes 40 mm packs in five hours takes a fraction of the room a static freezer needs for the same throughput, because the air is actually moving over every surface.
The check that catches it
An infrared thermometer is the wrong tool. It reads surface temperature and will tell you a pack is ready when its core is still liquid. Use a probe, and probe the pack that is hardest to freeze: the one in the middle of the bottom layer.
The practical test is a squeeze and a shake. A properly frozen pack is rigid and silent. A pack with a liquid core feels slightly yielding and makes a soft slosh when shaken next to your ear. Teach the loader that sound. It is faster than a probe and nearly as reliable.
Then record it. A pallet that goes out with a core temperature of −2 °C instead of −18 °C loses roughly 60% of its latent cooling capacity, which is two thirds of the reason it was in the box.
The counter-intuitive finding
A plant in Iceland lowered its freezer to −35 °C to speed things up and saw almost no improvement in core freeze time. The reason: at that temperature the outer layer froze so fast that it formed a thick insulating shell within 40 minutes, and the core then froze at the same slow rate as before. The real gain came from spacing the packs and adding fans, not from more cold.
Before the next load, walk into the freezer with a probe and test the middle of the bottom layer. If it is not below −15 °C, the pallet is not ready, whatever the outside feels like. The SAP for gel ice packs range includes freeze-curve data per pack thickness so you can plan freezer time against your actual format.
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.
