A January Pack and a July Pack: Setting Two Fill Weights for One Gel Line

Most ice pack specifications are written once, in a meeting room, and then run unchanged for years. The fill weight that gets approved is usually the one that survived a July trial, because that is the trial that fails. Come January, the same pack goes out at 550 g into a 2 °C ambient and does two things wrong: it over-cools product that needed to stay above 0 °C, and it wastes gel that could have been sold into a heavier summer pack.

What ambient temperature does to the melt curve

A 600 g gel pack frozen to −18 °C and placed in a 35 °C box gives up its cold in about 26 hours. The same pack in a 2 °C box gives it up in 60 to 70 hours. The cold is the same; the rate at which it leaves is not. As a rule of thumb, hold time scales roughly with the temperature difference between pack and ambient, so moving from a 35 °C summer lane to a 5 °C winter lane extends hold time by a factor of about 2.2.

That means a summer fill of 800 g is over-specified by 300 to 400 g in winter for the same 36-hour route. Trimming it is not just a cost exercise. Over-cooling is a real failure mode: produce with a 2 °C minimum, vaccines, and chilled ready meals all suffer when an oversized pack drives product below its lower limit for the first 10 hours of transit.

Winter also breaks the assumption about freezing

In an unheated warehouse at −2 °C, packs left on a loading dock will not freeze, but packs that arrive from a −18 °C freezer will not thaw either. The problem moves from insufficient cold to uncontrolled cold. Three practical adjustments matter more in winter than in summer.

• Condition packs before loading. In July, a −18 °C pack in a 20-litre box is acceptable. In January, it will freeze the product against the wall, so packs should be pulled to −8 or −5 °C.
• Shorten freezer dwell. A pack that sat 60 hours at −25 °C is harder than one at 24 hours, and hardness changes the contact area against the product.
• Watch the transit vehicle, not the outside air. A heated cab with an unheated cargo box can run at −5 °C on a two-hour leg, then at 18 °C during a midday stop.

Two fill weights, one filling line

Running two fills on the same line is a changeover question, not a formulation question. The cleanest approach is to keep the same SAP grade and change only the water-to-gel ratio and the fill volume, so the pump calibration stays valid and the only variable is the set point.

A workable pairing for a 20-litre box with a 36-hour target:

• Summer fill: 850 g, frozen to −18 °C, packs on two walls, product band 2 to 8 °C.
• Winter fill: 550 g, conditioned to −8 °C, packs on one wall plus base, same product band.
• Shoulder months, ambient 10 to 20 °C: 700 g at −14 °C, which covers both without a third SKU.

Label both SKUs identically except for a fill weight line printed on the film. Warehouse staff pick by weight far more reliably than by a colour code, and a scale at the packing bench catches the wrong SKU before it reaches a box.

Deciding when to switch. Do not switch on a calendar date; switch on the lane’s mean ambient. Pull 90 days of temperature logger data from the routes you actually run, and find the month where the 90th percentile ambient crosses 22 °C. That is the summer-to-shoulder switch. The shoulder-to-winter switch sits where the 10th percentile falls below 8 °C. In northern China and northern Europe those dates can be six weeks apart from the calendar solstice, and in export lanes to tropical ports summer fill is used year-round because the destination, not the origin, sets the load.

For destinations that mix climates — a February shipment from Hamburg to Lagos — size for the warmest leg and control over-cooling with placement rather than weight. Putting the pack against the carton wall rather than the product, or adding a 5 mm foam sheet between pack and product, removes the freeze risk without removing cold capacity.

The cost side of getting it wrong. Gel is cheap per kilo and expensive per shipment. A 250 g reduction on 500,000 packs a year saves 125 tonnes of gel, and more importantly 125 tonnes of frozen water that no longer has to be cooled to −18 °C — at roughly 90 kWh per tonne of gel frozen from 20 °C, that is a meaningful freezer load. Oversized winter packs also add volumetric weight, and couriers bill the outer box, not the gel.

Grades that freeze and thaw predictably across both fills are described on the SAP powder products page.