Engineering for Sublimation: Designing Vented Shippers to Prevent Pressure Buildup

A package designed and labeled for shipments using dry ice as a refrigerant, commonly used for frozen goods and biological materials.

What is happening: sublimation and why airtight is unsafe

Dry ice is solid carbon dioxide (CO2). At normal ambient pressures and temperatures, dry ice does not melt into liquid but sublimates directly into CO2 gas. When dry ice is confined inside an airtight container, the generated gas has nowhere to escape and the internal pressure will rise as sublimation continues. Because gases expand to fill available volume, even a modest amount of dry ice can produce a large volume of CO2 gas. If the container has strong seals and lacks a controlled venting path, that pressure can deform or rupture the package, dislodge closures, or force lids open abruptly—creating safety hazards and compromising the package contents.

Primary safety hazards

• Pressure rupture risk: airtight packaging can fail catastrophically, scattering contents and creating sharp fragments.
• Asphyxiation and elevated CO2 concentrations: escaped CO2 can accumulate in confined spaces such as vehicle trunks, loading docks, or small rooms, displacing oxygen and posing an inhalation hazard to personnel.
• Thermal shock to product: a sudden loss of dry ice containment or an unplanned rapid pressure release can alter the internal thermal behavior and expose product to warmer temperatures.

Design objective for a dry ice shipper

The engineering goal is to create an insulated thermal barrier that keeps the product within its required temperature window for the intended transit time while simultaneously providing one or more controlled paths for CO2 gas to escape safely. In practice this means accepting that the package is intentionally not airtight and designing venting so gas release is steady, predictable, and directed away from handlers and the product.

Key design elements and considerations

• Insulation strategy: Use insulating materials that minimize conductive and convective heat transfer without sealing gas. Common materials include expanded polystyrene (EPS), polyurethane (PU) foams, and vacuum insulated panels (VIP). Insulation should be continuous around the payload but need not form an airtight barrier—thermal performance and venting must be balanced.
• Venting path and location: Vents are typically placed near the top of the shipper because CO2 is denser than air when cold but may warm and mix; placing vents up assists passive escape of gas and reduces the chance of gas pooling at the lid. Vents should create a low-resistance path for gas to exit so internal pressure remains close to ambient.
• Venting type: Simple passive openings or covered slots are commonly sufficient. In higher-risk applications, controlled pressure-relief devices that open at a modest overpressure can be used. Gas-permeable membranes may be employed where insect ingress and moisture are concerns but these need to be selected to permit CO2 permeability.
• Diffusion and thermal management inside the box: Ensure vents do not create direct drafts across the product that would produce local overcooling or freeze damage. Internal diffusers, baffles, or an internal secondary barrier (an inner liner with limited openings) can allow gas to flow out while preventing direct cold jets on the payload.
• Seals and closures: Avoid gasketed, clamped, or welded closures that produce a gas-tight enclosure. Use simple lid overlaps, taped edges designed to peel away at low pressure, or closures with intentional small gaps. Any adhesive tapes used should not completely block vent paths.
• Compatibility with handling and transport modes: Design choices must reflect the intended transport mode. For example, air transport is subject to IATA/ICAO regulations for dangerous goods (dry ice is UN 1845 and regulated as a Class 9 miscellaneous dangerous good). Most carriers require vents and specific labeling; verify carrier-specific instructions.

Selecting or modifying containers: practical guidance for facility managers

• Start with a purpose-built dry ice shipper when possible: Commercial shippers designed for dry ice shipping combine effective insulation with engineered venting and documented performance. These products reduce the risk of improper modification and often come with compliance documentation for certain transport modes.
• When modifying existing containers:

1. Ensure the base container provides adequate insulation for the required hold time; add insulated liners if needed.
2. Introduce one or more vent openings near the top or in the lid; ensure vents are sized to avoid pressure build-up but protected against ingress of water or debris.
3. Use internal baffles or a secondary liner to prevent direct cold airflow onto temperature-sensitive items.
4. Do not retrofit airtight gaskets or clamps—these defeat venting and create hazards.

• Labeling and documentation: Mark packages clearly as containing dry ice (UN 1845), indicate net weight of dry ice, and include appropriate hazard and handling labels required by carriers and regulators. Maintain documentation showing that the container and venting approach are compatible with the mode of transport.

Operational controls and facility-level measures

• Storage and handling procedures: Store packed dry ice shippers in well-ventilated areas away from confined spaces. Personnel should move shippers in a way that prevents blocking vents.
• Monitoring and detection: In loading areas and enclosed workspaces, CO2 sensors and alarms help detect elevated concentrations. Establish evacuation and ventilation procedures if sensors trigger.
• Training and PPE: Train staff on the hazards of CO2, the rationale for non-airtight packaging, correct handling techniques, and emergency response. Provide appropriate gloves and eye protection for direct dry ice handling.
• Carrier coordination: Communicate with carriers about the shipment contents and container design; some carriers have strict requirements for net dry ice weight per package and venting practices.

Regulatory context and compliance

Dry ice used as a refrigerant is regulated as a hazardous material in many transport regulations (e.g., IATA/ICAO for air, DOT for ground in the United States). These regulations address classification, labeling, documentation, and limits on quantities. Engineering choices for venting should be evaluated against applicable rules—particularly for air transport, where pressure changes during ascent and descent are additional considerations.

Common mistakes and failure modes

• Using airtight containers or sealing vents completely, which allows pressure to build and can cause rupture.
• Placing vents at the wrong location (e.g., bottom) where gas cannot escape efficiently or where vents allow ingress of contaminants or moisture.
• Neglecting internal diffusion design, leading to cold spots or product damage from direct exposure to sublimating dry ice.
• Failing to label or document shipments correctly, resulting in carrier rejection or unsafe handling.

Practical example

An e-commerce fulfillment center shipping biologic samples chooses a purpose-built dry ice shipper with EPS walls and a non-sealed lid that includes two top-mounted passive vents. Inside, a thin perforated liner separates the payload from the dry ice pockets so that CO2 can exit via the upper vents while preventing direct cold jets onto vials. The center also requires CO2 monitors in the packing area and trains staff on packing limits and labeling. The result is predictable thermal performance with low risk of pressure buildup and a documented chain of compliance for air and ground carriers.

Summary

Designing dry ice shippers requires accepting that the package must not be airtight. The engineering challenge is to balance thermal performance with controlled venting: provide an insulated environment to preserve product temperature while offering safe, predictable paths for CO2 gas to escape. Facility managers should prefer purpose-built solutions, or when modifying containers, incorporate top-mounted vents, internal diffusers, and non-gasketed closures, together with clear labeling, training, monitoring, and carrier compliance checks to manage the associated safety risks.