Cost per Frozen Pack: Adding Freezer Energy to the SAP Calculation

Ask three ice pack makers what a 500 g pack costs and you get three answers between $0.18 and $0.42, all of them correct, because none of them count the same things. Polymer, film, water and labour are the visible costs. Freezing, frozen storage and shrink are the ones that decide whether the line makes money, and they are usually left out of the quotation because they sit on a different utility bill.

Where the money actually goes

For a 500 g pack at 0.8% SAP: polymer 4 g at $2.60–3.40/kg is $0.011–0.014. Water, if deionised, adds $0.002. Film at 90 micron coex PE runs $0.055–0.085 depending on volume and whether you print. Labour and overhead on a semi-automatic line is $0.03–0.05. That totals $0.10–0.15 before anything is frozen. Notice that SAP is 8–10% of the material cost — the argument that a cheaper polymer transforms your economics is mostly wrong, and a polymer that lets you cut dose from 1.0% to 0.7% saves $0.004, not $0.04.

The freezing energy number most quotes miss

Cooling 500 g of gel from 20°C to -18°C takes about 228 kJ: 42 kJ of sensible cooling, 167 kJ of latent heat, 19 kJ to pull the ice down. That is 0.063 kWh of theoretical refrigeration. Real blast freezers run at a coefficient of performance near 1.2 when you include defrost cycles, door openings and fan heat, and loading a warm pallet drops it further. Budget 0.15–0.20 kWh per 500 g pack. At $0.12/kWh that is $0.018–0.024 — more than the polymer.

Then add frozen storage. A pallet of 1,000 packs occupies 1.2 m³ and a freezer holds it at -20°C for a week at 0.3–0.5 kW, which works out to another $0.01–0.02 per pack. Suddenly freezing and holding is $0.03–0.045, or 20–30% of the total, and it is the line item nobody puts in the quote.

Pre-frozen delivery versus freezing in house

If you buy pre-frozen packs at $0.30 delivered and your own cost is $0.14 plus $0.04 of freezing, in-house wins on paper. It stops winning when your volume is seasonal: a freezer sized for July idles in February, and the capital sits in a room at -20°C. The break-even is usually around 4,000–6,000 packs a week on a leased blast freezer. Below that, buy frozen or use a shared cold store and pay per pallet-week.

The other option is to ship gel concentrate and hydrate at the point of use, which cuts freight mass by 85% and moves the water and freezing cost to the customer. It works for industrial buyers with their own freezer; it fails with retail buyers who want a pack they can drop in a domestic freezer.

What to do with the answer. Build a one-page model with five lines: polymer, film, labour, freezing energy, frozen storage. Then test the two variables you control. Thickness: a 25 mm pack freezes in 8 hours and a 40 mm pack in 14, so the same mass in a thinner pack costs 40% less energy to freeze — but needs more film and more freezer cube per kilogram. Dose: dropping from 1.2% to 0.8% saves $0.005 per pack and may cost you a firmer gel, which is a bad trade if it raises leakage claims by 1%.

Run the model before you renegotiate a polymer price. In most plants the saving from a cheaper SAP grade is smaller than the saving from a shorter freeze cycle or a fuller freezer.