Cold-Chain Packaging Thermal Storage Materials: 3 Practical Boundaries—Phase Change Materials / Dry Ice / Gel Packs—The First 3 Numbers to Ask for a “48-Hour Cold Retention” Order
💡 💡 At a Glance
Cold-chain packaging and the requirement to “maintain cold temperatures for 48 hours” must be evaluated as three distinct solutions. ① Phase change materials (PCM): heat is absorbed at the phase transition point to stabilize the temperature. This provides insulation rather than cooling; a product starting above the phase transition point will never be cooled. The phase transition point is fixed (a 0℃ material can only reach 0℃), and both effective duration and effective capacity are limited. The box’s total thermal storage capacity must be at least equal to the total heat generated over 48 hours plus the box’s own heat absorption; many failures result from errors in this inequality rather than the material. Paraffin-based materials offer precise phase transition points and high storage capacity but are not degradable; salt-based materials are inexpensive but prone to supercooling. ② Dry ice: sublimation absorbs heat continuously and can produce subzero temperatures, making it the only option for continuous cooling. Its boundaries include mandatory ventilation (CO₂ is denser than air and accumulates at floor level, so complete sealing is a safety red line), no direct product contact (-78℃ can freeze-damage products), and specific air transport rules under IATA/UN 1845, with freight potentially based on initial weight. Box material also determines the printing method. ③ Gel packs: the simplest option, but they depend on 12-24 hours of customer pre-freezing, produce uncontrolled temperatures if improperly frozen, and carry a leakage risk. The printing company’s primary role is securing the packs inside the box rather than printing them. First ask for three numbers: the target temperature after 48 hours, the product’s starting temperature, and the transport channel, distance, and season.
In June 2026, a fresh-food e-commerce customer contacted us to print a batch of paper boxes for transporting insulated containers.
The customer’s requirement was clear: “Keep it cold for 48 hours.”
We asked the first question: “Does the 48 hours refer to the temperature when the goods reach the customer, or must the temperature remain below a specific level throughout the entire journey?”
The customer replied: “Simply, it should still be cold after 48 hours.”
Our second question was: “What is the product, what is its starting temperature, and what transport channel will be used?”
The customer answered: “Frozen shrimp, taken out at -18℃, shipped by road to East China, during summer.”
The third question was: “Will you supply the thermal storage materials, or should we? Is the box to be made of paper or WPC?”
The customer said: “You can put the package together for us.”
At that point, we could already calculate that keeping frozen shrimp at -18℃ for 48 hours in summer road transport is almost impossible using phase change materials. The principle of phase change cooling is to “use the material to absorb heat and keep the product near a specified temperature.” It performs best in the 0-8℃ range. Maintaining a -18℃ product for 48 hours with phase change materials requires several times the thermal storage capacity needed for ordinary refrigerated scenarios.
The customer asked: “What should we do then?”
We replied: “First, let us clarify the boundaries of the three options. Then you can choose.”
The First Boundary: Phase Change Materials—Passive, Heat Absorption, Insulation Rather Than Cooling
The first option is phase change cooling (PCM, Phase Change Material). Its principle is straightforward: the phase change coolant changes state at a specific temperature, absorbs heat, and “pins” the temperature near that phase transition point.
Typical phase change coolants include:
- Salt-based types (such as sodium chloride): phase transition points of approximately -21℃ to 0℃, inexpensive and reusable, but prone to noticeable supercooling (the liquid may fail to freeze below the phase transition point, making it seem “not cold enough”).
- Paraffin-based types: phase transition points can be set precisely (commonly -40℃ to +60℃), with high thermal storage capacity and good stability. They are the most commonly used in cold chains, but they are not degradable and have higher unit prices.
- Organic acid types: phase transition temperatures are adjustable, but supercooling and crystallization problems are significant.
There are three practical boundaries to phase change cooling:
Boundary 1: It provides insulation, not cooling. This is the point customers most often misunderstand. If the product’s initial temperature is already above the phase transition point, phase change cooling will never bring it down. To use a phase change material to reduce a product from 25℃ to 4℃, the product must first be frozen below 4℃, or a material with a higher phase transition point must be used for primary cooling. When a customer says “I need it cold,” first confirm the product’s temperature when it leaves the factory.
Boundary 2: The phase transition point is fixed and cannot simply be made “a little colder.” A coolant with a phase transition point of 0℃ begins absorbing heat once the product reaches 0℃ and stops absorbing heat as the temperature falls further. Maintaining -18℃ requires a material with a phase transition point of -18℃ or -21℃, involving an entirely different product line.
Boundary 3: It has both an “effective duration” and an “effective capacity” limitation. Effective duration is the time before the phase change material becomes fully saturated with absorbed heat; effective capacity is the total amount of heat it can absorb. The 48-hour requirement must satisfy the following inequality: the box’s total thermal storage capacity ≥ total heat generation over 48 hours + heat absorbed by the box itself. In more than half of the “48-hour cold retention” failures we have encountered, the problem was a mistake in this inequality rather than unsuitable material.
The quotation logic for phase change cooling therefore works as follows: calculate the required kilograms of coolant × unit price × box volume ÷ coolant density, and then work backward to determine the box dimensions. This is entirely different from asking, “How much does one insulated box cost?”
The Second Boundary: Dry Ice—Active, Continuous Cooling Through Sublimation, but It Is a Gas
The second option is dry ice. Its mechanism is completely different: when dry ice sublimates (solid CO₂ changes directly into gas), it absorbs a large amount of heat and continues absorbing heat throughout the sublimation process, allowing it to reach subzero temperatures.
Dry ice also has three practical boundaries:
Boundary 1: Dry ice can “suffocate people.” CO₂ is denser than air and accumulates at the bottom of enclosed spaces. When someone crouches to retrieve goods or counts packages at a cold-storage entrance, CO₂ concentration may exceed safe limits first at floor level. This is why all dry ice packaging requires ventilation: dry ice must never be placed in a completely sealed container. This is a safety red line, not a recommendation.
Boundary 2: Temperatures may become low enough to freeze and damage the product. If dry ice comes into direct contact with the product, its surface temperature can reach -78℃. Dry ice solutions therefore require an insulation layer (such as corrugated material, foam, or cardboard) to prevent direct contact.
Boundary 3: Dry ice has additional transportation requirements. Air transport has specific rules for dry ice under the IATA Dangerous Goods Regulations and UN 1845, including packaging configuration, ventilation requirements, net weight limits per package, and CO₂ concentration limits in the aircraft cabin. Road transport is also subject to the carrier’s regulations. It is fortunate that the customer specified road transport to East China; if the shipment were changed to air transport, the packaging would need to be redesigned.
Because dry ice is a gas that sublimates on its own, the box’s requirements for ventilation and insulation conflict: either leave a vent (reducing insulation performance) or allow the box itself to release gas gradually. Our factory installs an openable vent valve on the box that opens automatically after use. However, the valve itself becomes a quality-control point: it may leak and compromise cooling if defective, or cause the box to burst if closed.
There is also an issue specific to printing companies: the choice of box material for dry ice packaging determines whether and how the box can be printed. Corrugated paperboard boxes and WPC boxes use entirely different printing methods. WPC boxes generally support only silk-screen printing or UV spray printing, while corrugated paperboard boxes can be offset-printed but require careful management of registration accuracy on corrugated flutes.
The Third Boundary: Gel Packs—Semi-Active, the Easiest Option, but Most Likely to Cause a Safety Incident
The third option is a gel pack. Its principle is simple: gel is filled into a bag and frozen solid. Before shipment, the frozen packs are placed in the box and absorb heat to maintain the required temperature.
Its advantages are direct: it is inexpensive, versatile, and readily available from suppliers. However, three boundaries must be clearly explained:
Boundary 1: It requires a pre-freezing step. Customers must freeze the packs in their own refrigerators for long enough before use (typically 12-24 hours). If the gel is not fully frozen or freezes unevenly, the temperature inside the box becomes unpredictable. For a gel-pack solution, “48 hours of cold retention” is really a question of whether the customer has frozen the packs properly.
Boundary 2: A punctured gel pack creates a liquid leakage incident. The gel itself is non-toxic, but if it leaks and contacts the product, the result is contamination. The gel pack’s edge-seal strength, pinholes, and method of securing it inside the box (preventing packs from pressing against one another during transport) must therefore be designed.
Boundary 3: Gel packs cannot be reused indefinitely. Repeated freezing and thawing causes gel separation and increases the probability of leakage.
From a printing perspective, gel packs are generally not manufactured by the printing company because they are purchased components. What the printing company must develop is a method to “secure them inside the box so they do not move around”—through dividers, positioning systems, and protective cushioning. This detail is the true bottleneck in implementing a gel-pack solution and is the most frequently overlooked element.
How to Compare the Economics of the Three Options
The following is the actual selection table we provide to customers:
| Option | Minimum Temperature | Continuous Cooling | Cost | Transport Restrictions | Printing Considerations |
|---|---|---|---|---|---|
| Phase change materials | Limited by the phase transition point | No; insulation only | Medium to high (paraffin-based types) | Generally none | Box can be printed normally |
| Dry ice | Can reach -78℃ (with insulation) | Yes; continuous heat absorption | Medium to high (dry ice plus increased box weight) | Specific rules apply to air transport; ventilation required | Box material determines the printing method |
| Gel packs | Approximately 0℃ to -18℃ (depending on the formulation) | No | Low (purchased component) | Generally none | Focus on the securing system |
The most important column in this table is “Continuous Cooling.” It clarifies the true meaning of the customer’s original “48 hours of cold retention”: if the product must remain at -18℃ or below throughout the journey, dry ice is required. If the product only needs to remain below 4℃, either phase change materials or gel packs will work.
One cost calculation is also often performed incorrectly: the cost of dry ice is not limited to the dry ice itself. Dry ice sublimates during transport. Is freight charged according to the initial weight or the current weight? This is not a major issue for road transport over a few days, but it can be significant for air transport—you may ship 10 kg of dry ice and arrive with only 3 kg remaining.
When a Printing Company Receives a “48-Hour Cold Retention” Order, What Are the First 3 Numbers to Ask?
The first number: the product’s target temperature after 48 hours. Not “still cold,” but a specific figure: 4℃? 0℃? -18℃? This directly determines the technical route. If the target is below 0℃ and the product starts colder, only dry ice will work. For a 0-8℃ range, both phase change materials and gel packs are suitable.
The second number: the product’s starting temperature and whether it is already in cold storage when packaged. If it starts at 25℃, “48 hours of cold retention” actually means both cooling and insulation, requiring far more thermal storage capacity than insulation alone. We ask this on every project because it directly determines the solution.
The third number: the transport channel, distance, and season. The channel determines whether there are transport restrictions, especially the dry ice rules for air transport. Distance and season determine the environmental heat load—the box design for summer road transport to East China is not the same as for winter transport.
Only after asking these three numbers can we give the customer an honest answer: “48 hours is achievable,” “48 hours cannot be achieved, but 24 hours can,” or “To achieve 48 hours, the solution must change to dry ice.”
One Sentence for Customers
The most expensive element in cold-chain packaging is not the box, but the undefined requirement.
The six words “keep it cold for 48 hours” may describe a completely achievable specification, or they may make a physically impossible promise. What is the difference? Whether “cold retention” means insulation or cooling, the product’s starting temperature, and the transport channel.
Our recommendation is: translate “48 hours” into three numbers on the order confirmation—target temperature, starting temperature, and transport channel. Both parties then sign a verifiable specification rather than an ambiguous statement that each may interpret differently. If it ultimately proves that 48 hours cannot be achieved, the customer will appreciate our warning in advance; discovering it after production means losing money.
Further Reading
- Prepared Food Packaging: 3 Practical Boundaries—Oil Resistance and Cold Resistance / Microwaveability / Cold-Chain Costs—prepared foods and cold chains involve the same cost calculation
- Instant Retail Packaging: 3 Practical Boundaries—30-Minute Fulfillment / Forward-Stock Stacking / Condensation and Insulation—30 minutes and 48 hours are different worlds, but both face condensation
- Packaging Void Ratio: 3 Practical Cost Calculations—Volumetric Weight / Loading Rate / Pallet Layers—cold-chain boxes are bulky, so this article covers the logistics costs
- Packaging Cushioning: 4 Plant-Fiber Materials and Their Practical Boundaries—Wheat Straw / Rice Straw / Corn Starch / Cotton Fiber—sustainable options for securing and cushioning products in cold-chain boxes
- How to Choose 3-Layer / 5-Layer / 7-Layer Corrugated Paperboard: 3 Practical Load-Bearing and Cost Comparisons—corrugated paperboard is the mainstay for cold-chain box materials
FAQ
How much thermal storage material is needed to keep goods cold for 48 hours?
There is no reliable rule-of-thumb figure. It depends on three factors: the difference between the product’s target and starting temperatures, the total heat transferred into the box over 48 hours (the road-transport vehicle temperature, stacking quantity, and exposure to sunlight can make a major difference), and the box’s own heat absorption. For phase change cooling, the calculation is: the box’s total thermal storage capacity (in kilograms) must be at least equal to the total heat generation over 48 hours plus the box’s own heat absorption. Divide by the coolant’s thermal storage density to obtain the required volume. In practice, the approach is reversed: define the box dimensions first, then calculate how many kilograms must be added. More than half of the “48-hour cold retention” failures we encounter result from errors in this inequality rather than poor material quality. One additional note: for a -18℃ frozen product that must remain at -18℃, the phase transition point of the material must match the required temperature. The thermal storage capacity needed is several times that of a 0-8℃ scenario, resulting in a significant increase in box volume.
What transport restrictions apply to dry ice packaging? What if our customer wants to ship by air?
Dry ice is a dangerous good subject to specific regulations. Air transport is governed by the IATA Dangerous Goods Regulations and UN 1845. Packaging must withstand pressure changes during transport, ventilation openings must not be sealed shut (to prevent CO₂ accumulation and excessive cabin pressure), each package has a net weight limit, and airlines and airports impose limits on the total CO₂ quantity permitted per shipment. Because dry ice continues to sublimate during transport, confirm with the carrier whether freight is charged on initial weight or current weight; the difference can be substantial for air shipments. Road transport is relatively less restrictive but remains subject to carrier requirements. Two hard safety requirements also apply: first, dry ice must never be placed in a completely sealed container because CO₂ is denser than air and accumulates at floor level, creating the greatest danger when someone crouches to retrieve goods; second, the product must not directly contact dry ice, whose surface temperature can reach -78℃ and freeze-damage the product. Corrugated or foam insulation is required.
When accepting an order, should a printing company recommend phase change materials or gel packs?
Choose based on the target temperature and the customer’s operating habits. Gel packs are inexpensive, versatile, and readily available, but they require the customer to pre-freeze them, typically for 12-24 hours. If they are not fully or uniformly frozen, the temperature inside the box becomes unpredictable, and gel packs may rupture and leak if they press against one another during transport. The printing company’s real work in a gel-pack solution is not to print the packs, but to design a system that secures them inside the box. Phase change materials provide more stable temperatures, are reusable, and do not depend on customer pre-freezing. Their disadvantages are higher unit prices, lack of degradability, and their inability to cool a product whose starting temperature is above the phase transition point. Gel packs are more economical for a 0-8℃ refrigerated range when the customer is willing to pre-freeze them. Phase change materials are more reliable when a stable temperature is required or pre-freezing is not performed consistently.
What special printing requirements apply to cold-chain boxes? Are paperboard boxes and WPC boxes completely different?
They are indeed different, and the difference depends on the box material. Corrugated paperboard boxes can be offset-printed, but ink coverage on the flutes must be controlled because corrugated material absorbs more ink, often causing poor registration and dull colors. Foam boxes (EPE/EPS) generally support only silk-screen printing or digital spray printing. WPC boxes (WPC/PP wood-plastic composites) also mainly use silk-screen and UV spray printing. Their dense surfaces do not readily accept ink, so surface energy must be treated and silk-screen adhesion must be secured through pretreatment. Another cold-chain-specific consideration is that dry ice solutions require an exhaust path, affecting both box design and the placement of printed artwork. A vent valve must not be covered by printed graphics or sealed over by a label. In addition, cold-chain box materials shrink at low temperatures, and offset ink adhesion can change in cold environments. Conduct low-temperature testing in advance rather than validating only at room temperature in the workshop.
The customer only says “keep it cold for 48 hours.” How should we clarify the requirements?
Ask for three numbers: ① the product’s target temperature after 48 hours (4℃? 0℃? -18℃? A specific figure rather than “still cold”); ② the product’s starting temperature when packaged and whether it is already in cold storage, which determines whether the requirement is insulation or cooling and can change the required thermal storage capacity severalfold; and ③ the transport channel, distance, and season. The channel determines transport restrictions, especially the dry ice rules for air transport, while distance and season determine the heat load. Once these three numbers are known, you can determine whether 48 hours is achievable, achievable with a modified solution (dry ice or a larger box), or impossible. If any of the three is unanswered, avoid quoting a “48-hour” specification, as that is the most likely cause of a delivery dispute.
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