When a parcel arrives with broken glass in it, the natural assumption is that it was dropped. Sometimes it was. More often, nothing dramatic happened at all — the package simply spent several hours vibrating on a belt or in a vehicle, and a vial that was free to move by two millimetres moved by two millimetres several hundred thousand times.
Two different failure modes
Impact is a single high-energy event. A drop onto a corner, a heavy parcel landing on top. Cushioning is what addresses it: a material that compresses and dissipates energy over a longer time.
Vibration is low-energy and continuous. It does not break glass directly. What it does is work things loose, abrade labels, scuff coatings, fret against closures, and gradually enlarge whatever gap already existed until the movement becomes large enough to matter.
Insert design addresses both, but through different properties, and it is worth knowing which problem you are solving.
What restraint actually means
A well-designed insert holds the vial before the box is closed. That is the test: open the loaded box, turn it over, and the vial should stay where it was put.
If it needs the lid to hold it, the vial has room to move, and it will use that room.
How the restraint is achieved depends on the material:
- Foam grips through interference. The cavity is cut slightly smaller than the vial, so the wall compresses on insertion and pushes back continuously.
- Paperboard grips through geometry. A punched hole with small relief tabs flexes against the glass while the vial’s base rests on the box floor.
- Formed trays grip through pocket geometry, with tapered walls that seat the vial at a consistent depth.
The four gaps that matter
Movement happens wherever there is a gap. There are four worth checking.
1. Radial — around the vial body. The most obvious, and the one insert design usually addresses well.
2. Vertical — between the top of the closure and the underside of the lid. Frequently overlooked. A vial can be perfectly held radially and still bounce vertically. A thin foam lid pad removes it.
3. Between vials — where two vials share a cavity or a shallow depression. Glass against glass is the most damaging contact in a pack. A physical wall is needed; distance alone does not help once the box is dropped.
4. Between the box and the shipper — outside the insert entirely. A perfectly restrained vial in a box that slides around inside an oversized outer is still going to have a bad journey.
That fourth one is the one most often missed, and it is usually the cheapest to fix.
Why void reduction beats adding cushioning
The instinct after a breakage is to add padding. It is usually the wrong response.
Empty space lets contents accelerate before they hit something. Adding soft material to an oversized box gives the contents something gentler to arrive at, but does not stop them arriving. Sizing the box to its contents so nothing can accelerate in the first place is more effective, and it costs less rather than more, because you use less material and ship less air.
If you have had transit damage, measure the empty space in the pack before changing the insert.
Depth: the number that matters at weight
For heavier vials, cavity depth does more than cavity diameter.
A cavity that grips only at the shoulder concentrates all the load on a narrow band of the insert. That is where paperboard tears and where foam takes a permanent set. Supporting the vial over more of its height spreads the same force across more material.
As a working rule, foam and tray cavities for 10ml vials are specified to support at least half the vial height. Below that, the insert is doing locating work rather than restraining work.
Density, briefly
For foam, density is as important as dimension and is the property most often left unspecified.
Too soft, and sustained load and vibration cause the cavity to take a set. The grip that was correct on day one is gone by the time the pack reaches a customer. Too firm, and insertion becomes difficult, the cavity mouth tears, and users struggle to remove the vial.
It is selected against the vial weight and how often the pack is opened, then confirmed on a physical sample rather than from a specification.
How we test it
Nothing exotic, but it is done on every insert design before a run is released:
- Load the box with the customer’s actual vials
- Close it, invert it, shake it, listen
- Open it and check for movement marks inside the cavity
- Cycle the pack open and closed if it will be opened repeatedly
- Low drops onto corners with a loaded sample
If a sample makes a noise, the specification changes. We do not ship a design that rattles.
For documented transit performance, that needs an accredited laboratory testing your final packed configuration — we supply samples in whatever configuration you want tested, and adjust based on what you find.
Further reading
EVA Foam vs Cardboard Inserts for Vial Packaging
Foam is not automatically the better insert. The question that settles it is whether the box travels on its own or inside an outer case.
Packaging Considerations for Temperature-Sensitive Shipments
No supplier can tell you how long a shipper will hold temperature without testing your configuration. Here is what can be specified, and what has to be established.