An ice pack melting is not a failure; it is part of absorbing heat through a phase change. The material takes energy from its surroundings, and as va-Q-tec explains, its measured temperature stays nearly constant around the transition range (PCM overview). That steadiness belongs to the pack itself; not every point in the container, nor the product, stays equally flat.
How many hours: the calculation
Melting time is the heat the pack can absorb divided by the heat entering the container per hour. A water-based pack absorbs about 334 kJ per kilogram as it melts. As an assumed example, if a steady 10 W comes in through the walls and lid openings, 1 kg of pack melts in about 9 hours and 2 kg in 18; double the heat ingress and the time halves. The measurement below gives your container's real heat ingress.
Three zones of the melting curve
| Zone | What happens in the pack | Signal in the product reading |
|---|---|---|
| After preparation | Solid material approaches transition | Temperature depends on probe position |
| Phase change | Latent heat is absorbed; material can stay in a narrow range | A plateau may appear |
| After transition | No phase-change reserve remains | The warmest product point begins to rise |
The moment to watch: the curve's break
An upward break in a product reading shows that point is warming; lid openings and ambient changes can also bend the curve. So the warmest expected product point is tracked separately from a near-core point, and for freeze-sensitive goods the cold spots and lower limit are checked too. The route is planned with a margin inside the time the product stays within its acceptance range.
To compare the same pack in two containers, keep load, pack layout, ambient and lid pattern the same and record the warmest expected product point in each. Compare plateau end and first target crossing separately. A single centre probe may miss the temperature distribution across the load.
What changes the melting time
- the mass of the pack: more mass, longer phase change
- container insulation: slow heat ingress means slow melting
- contact with the product: direct contact speeds heat flow
- lid opening frequency: each opening accelerates the melt
- outside temperature: the bigger the difference, the faster
A pack that has started to melt can be refrozen; the time to become solid again depends on thickness, filling and freezer conditions. A half-frozen pack has already spent part of the heat it could absorb, so the duration shrinks. Do not dispatch until the filling is fully solid and the preparation time has been recorded.
The meltwater question
A cold pouch or plate can collect condensation when it meets warm, humid air; that does not mean the filling has leaked. Keep cartons and labels dry by checking the shelf, separation and leak-control arrangement in the pack-out.
