A frozen gel pack sitting against the flat wall of a polystyrene shipper looks like it is doing its job. And in that specific geometry — flat pack, flat wall, tight contact — it is. The problem arises the moment the pack must interface with anything other than a flat surface: a tray of pharmaceutical vials, a rounded piece of fresh produce, a box of meal kit cold chain ingredients with irregular corners, or a curved glass bottle of premium wine.
In those scenarios, a rigid pack creates air gaps at every point where the pack surface diverges from the product surface. Those air gaps are the enemy of efficient cold transfer. Understanding why — and what pack design characteristics minimise them — is fundamental cold chain engineering that most buyers never learn from a product data sheet.
Why Air Gaps Are Thermally Catastrophic
The thermal conductivity of still air is approximately 0.026 W/m·K. The thermal conductivity of a frozen water-based gel pack is approximately 2.0–2.2 W/m·K — 85 times higher. Liquid water (a thawed pack) has a conductivity of approximately 0.6 W/m·K — still 23 times higher than still air.
When a rigid frozen pack sits against a curved product surface and leaves a 2mm air gap between them, heat transfer across that gap is governed by the air’s conductivity, not the pack’s. The effective thermal resistance of a 2mm still air gap is approximately 0.077 m²·K/W. The effective thermal resistance of a 2mm frozen gel layer at the same position would be 0.00095 m²·K/W — 80 times lower resistance, meaning 80 times faster heat transfer.
In practical terms: an air gap between pack and product essentially breaks the thermal circuit. The pack continues to absorb heat from the shipper walls and surrounding air, but its ability to absorb heat from — and therefore cool — the product is severely diminished. The product temperature rises while the pack appears to be working correctly from the outside.
This effect is measurable in pharmaceutical cold chain validation studies. Temperature logging at product surfaces versus ambient shipper interior temperatures consistently shows higher product temperatures when packs have poor contact geometry, even at identical refrigerant mass. The delta can be 1–3°C — enough to push a marginal cold chain from compliant to non-compliant.
What Determines Pack Rigidity When Frozen?
Not all frozen gel packs become equally rigid. Several design variables govern how stiff a pack is at its frozen state:
Freeze temperature: Packs frozen to -18°C are harder and more rigid than packs frozen to -4°C. At -18°C, the water phase is fully solid and the ice crystal structure is well-developed throughout the gel matrix. At -4°C, the gel may retain some unfrozen water fraction, remaining slightly more pliable.
Cell design: A finely divided cell grid (6×4 or 8×6) produces a pack that, even when fully frozen, retains some mechanical flexibility at the cell boundary seal lines. The film between cells can flex slightly at the seal zones, allowing the overall pack to partially conform to surfaces even while individual cells remain rigid. A single-compartment pack or coarse-cell design (2×2) freezes as a monolithic block.
Film properties: Thicker, stiffer film creates more rigid boundary structure. Nylon-containing laminates retain more flexibility at low temperatures than pure PE constructions. 2-ply constructions using nylon as one layer typically maintain better low-temperature flexibility.
Gel formulation: Propylene glycol-modified gels freeze to a softer, more pliable solid than pure water gels. A -12°C propylene glycol gel pack has a partially crystalline, partially amorphous solid structure that is less brittle and more conformable than a fully crystalline ice matrix.
Contact Efficiency in Practice
Contact efficiency — the proportion of the pack face in direct thermal contact with the product — is the metric that matters. Achieving high contact efficiency requires attention to several factors:
Pack selection matched to product geometry: For products with curved surfaces, a conformable fine-cell pack is the correct specification. For flat-sided products in rectangular trays, a coarser-cell pack achieves adequate contact and the rigidity penalty is less consequential.
Physical placement and orientation: Packs placed horizontally under a product achieve better contact under gravity than packs placed vertically. For irregular products, placing the pack below the product maximises gravity-assisted contact.
Void fill and constraint: In a well-engineered shipper, void fill material constrains the product and presses it firmly against pack faces. Without constraint, products shift during transit and create variable contact geometry. What was adequate contact at the packing bench may be zero contact after 200km of road freight.
Pre-conditioning packs before insertion: A brief equilibration period at 0°C to -5°C (10–15 minutes) allows the pack surface to soften slightly — improving conformability without meaningfully reducing the refrigerant reserve. This is standard practice in pharmaceutical cold chain packing.
The Australian Summer Context
In Australian summer conditions — 40°C ambient, 55°C vehicle interiors, high radiant load — marginal cold chain design is unforgiving. When the total heat ingress rate into a shipper is doubled by extreme ambient temperatures, every source of inefficiency becomes consequential. A 10% reduction in effective contact area can represent 10% less cold delivery to the payload — and in a system already working near its thermal limit, that 10% determines whether the payload survives transit within specification.
Australian cold chain operators who see their packaging perform adequately in winter but fail in summer often discover that the specification has not changed. What has changed is the ambient heat load. Under that greater load, contact efficiency deficiencies that were marginal in winter become decisive in summer.
Measuring Contact Efficiency
For pharmaceutical validation, thermal imaging (infrared camera) of the pack-product interface during simulated transit reveals areas of poor contact as warmer zones on the product surface. For food and e-commerce cold chain, temperature logging with sensors at pack contact and non-contact positions provides equivalent data.
This testing pays for itself in reduced product loss. A single batch of temperature-excursioned product in an Australian e-commerce food business costs more in spoilage, customer refunds and logistics rework than a day of packaging validation testing.
Specifying Conformability in Procurement
When procuring packs for applications where product contact geometry is irregular, request from suppliers: cell configuration matrix dimensions, film construction and material specification, film thickness at seal zones, flexibility test data at -18°C if available, and gel formulation type including propylene glycol content. If supplier data is insufficient, test empirically by freezing a pack to its specified temperature and assessing contact against a representative product surface under moderate hand pressure.
Conclusion
Pack rigidity when frozen is not merely a handling characteristic — it is a direct thermal performance variable. The air gap created by a rigid pack against an irregular product surface reduces heat transfer by a factor of 23–85 times relative to direct contact. In Australian summer conditions, where the system’s thermal reserve is already under pressure from extreme ambient heat loads, this contact efficiency loss can determine whether a cold chain succeeds or fails.
Conformable pack design — fine cell grids, flexible film constructions, propylene glycol formulations where appropriate — is a performance-critical engineering choice for any cold chain application involving products with irregular geometry. The buyers who understand this specify it intentionally. Those who don’t discover it expensively.