Mục lục
Reviewed and updated by ICD - Week 24/2026
Quick summary:
- A dunnage bag (load-securing air bag) is an essential cargo-protection tool that fills the voids between loads in a container, truck or rail car.
- Its core purpose is to create strong bracing pressure that locks the load in place, stopping shifting and tipping and absorbing shock in transit.
- It works through two forces: static compression (the inflated pressure that holds the load) and dynamic compression (the push-back when the load tries to move).
- The outer-shell material - multi-ply kraft paper or woven PP - sets its AAR load level (1-5).
What is a dunnage bag? Definition and core role
A dunnage bag is a specialized void-fill material, made as a hollow bag and inflated with compressed air after the cargo is loaded into the vehicle. In other words, it is a tool that creates strong bracing pressure, turning loose loads into one unified block held firmly in place against the large forces of a long journey.
For exporters, the biggest cost is not freight - it is damage. A container worth a fortune only needs to shift 5 cm to cause disaster. A dunnage bag is, in effect, physical insurance for the load.
Core role in logistics and multimodal transport
- Securing the load: the most important function, especially in sea and rail transport. Once inflated, the bag fills the void and presses the loads against the container wall or each other, preventing lateral or longitudinal movement (such as rail shunting forces).
- Absorbing shock: when the vehicle brakes hard, corners or meets heavy seas, the bag acts as a giant shock absorber. The compressed air inside absorbs the impulse and converts it to heat, reducing the force reaching sensitive goods.
- Preventing damage: it protects goods from striking the container wall or each other - the leading cause of breakage, denting and loss in transit, which directly affects insurance cost and reputation.
How it works: bracing force and the science
The effectiveness of a dunnage bag is not about size but about the pressure it creates and how it interacts with the load.
Static vs dynamic compression
Static compression is the pressure inflated to the technical recommendation (typically 0.2 bar for a Level 1 kraft bag). It creates the friction and stiffness needed to fix the load initially, so it cannot shift when the container is lifted or set down.
Dynamic compression is the key factor. It arises when the load is thrown during transit (a ship braking hard or meeting waves). As the load tries to move into the filled void, it presses the bag harder; the bag responds by increasing its counter-pressure, pushing the load back to its original position. This ability to absorb and respond to dynamic force is exactly what the AAR level rates. When seas are heavy, lateral force on the load can be 2-3 times the normal weight - if the initial static pressure is too low, the bag cannot respond and the whole stack can collapse.
Optimizing the void and inflation
The ideal void for a dunnage bag usually falls in a standard range (for example, 10 cm to 40 cm). Calculating the void is essential:
- Void too small (under 5 cm): the bag cannot inflate enough to create the needed pressure - wasted material.
- Void too large (over 50 cm): the bag becomes over-rounded, reducing the contact area and the bracing force. It turns from a rectangular block into an oval and cannot distribute force evenly.
The bag must be inflated to the ideal fullness - strong bracing while keeping the elasticity to absorb shock - avoiding an over-hard bag that damages the load’s edges.
Construction and materials
A standard dunnage bag combines materials science and packaging engineering in three specialized layers. For the full breakdown, see dunnage bag structure and types.
Outer shell (strength and durability)
It resists compression, abrasion, tearing and puncture. Multi-ply kraft paper (2-6 plies, PP-laminated for mild moisture resistance) is the economical choice for medium loads and normal transport. Woven PP is the premium material with the highest mechanical and tensile strength - its cross-woven matrix is ideal for heavy, sharp-edged goods (such as steel coils) and allows repeated reuse. See what kraft paper is for the shell material.
Inner bladder (air retention)
It holds the compressed air and prevents leaks under dynamic force and temperature change. It is a PE film or a premium PA/PE composite; the PA/PE composite keeps pressure stable when temperatures swing sharply (such as a container heated by the sun, expanding the air inside).
Valve and accessories
A large-diameter fast valve speeds installation at the yard and auto-locks after the inflation gun is withdrawn. A dedicated inflation gun with a built-in pressure gauge ensures accurate pressure - avoiding under-inflation (weak force) or over-inflation (burst risk).
Classification by material and use
| Type | Outer-shell material | Key use | Load level |
|---|---|---|---|
| Kraft paper bag | Multi-ply kraft | Standard goods, medium load, normal road and sea transport (cartons, palletized goods) | Level 1-3 |
| Woven PP bag | Woven polypropylene | Very heavy, sharp-edged goods (steel coils, machinery, liquid chemicals) | Level 3-5 |
| In-box air bag | Thin PE/LLDPE | Filling small voids inside cartons, cushioning single items (e-commerce, cosmetics) | Shock-fill |
Benefits and safety standards
Using dunnage bags cuts the rate of damaged and crushed goods, lowers insurance exposure and speeds loading at the yard. Quality bags should meet international standards such as AAR; see our guide to the SGS standard for how material durability is verified.
Frequently Asked Questions (FAQ)
What is a dunnage bag used for?
To fill the voids between loads in a container, truck or rail car and brace them in place, preventing shifting, tipping and impact damage in transit.
Why inflate to 0.2 bar - can I inflate less?
No. The static pressure (around 0.2 bar for Level 1) is what lets the bag respond to dynamic force. Under-inflate, and the bag cannot push back when the load surges, and the stack can collapse.
What void size suits a dunnage bag?
Typically 10-40 cm. Under 5 cm the bag cannot build pressure; over 50 cm it over-rounds and loses contact and force.
Kraft or woven PP?
Kraft for medium loads and normal transport; woven PP for heavy, sharp-edged or reusable applications such as steel coils.
Contact ICD Viet Nam
For the right dunnage bag for your load and void size, contact our team for specifications and a quote.
- Northern Vietnam: 0983 797 186 / 090 345 9186 / 090 5859 186
- Southern Vietnam: 098 6784 186
- Email: sales@icdvietnam.com.vn
- Zalo: Chat on Zalo
