When a container's holding time drops, insulation is the first suspect. The loss, though, often comes from one point around the lid where the seal has lost pressure: the gasket is not torn, only crushed or hardened, and the eye does not see it. The paper test finds that point without taking anything apart.
How to run the test
- Empty the container; keep the lid and gasket clean and dry. Fold an A4 sheet lengthwise into a strip.
- Lay the strip across the gasket line, close the lid normally and lock the latch.
- Pull the strip straight out and feel the resistance.
- Repeat every 15-20 cm around the lid; do it at the corners and level with the hinges without fail.
- Mark the points where the strip comes out easily on the container edge with a pen.
How to read the result
A sound gasket pulls the paper out against clear resistance, and sometimes tears it. A point where the strip slides out with no resistance has a gap between gasket and lid; heat gets in there and water gets in during washing. Because pressure is already high next to a latch, the weak point usually shows at the corners and in the middle stretch furthest from the latches.
| Reading | What it means | What to do |
|---|---|---|
| Paper comes out against clear resistance | The gasket holds pressure | Note it; no mark needed |
| Paper tears | Gasket sound, paper weak | Pull once more at the same point; a tear alone is not a fault |
| Paper slides out with no resistance | Gap between gasket and lid | Mark the edge and look for a second point on the same container |
Example: test points on a 600 × 400 mm lid. Take a 600 × 400 mm lid as an example; this is the Euro crate size. The lid edge is a closed ring, so the perimeter is 2 × (600 + 400) = 2000 mm. The number of points on the ring is the perimeter divided by the spacing, rounded up (an example calculation, not a measurement).
| Spacing | Points |
|---|---|
| At most 15 cm | 14 |
| 20 cm | 10 |
Assuming 20 seconds at each point (example assumption, not a measurement), 14 points take 14 × 20 s = 280 s, about 5 min. The same test on 20 containers comes to 20 × 280 s = 5600 s, about 93 min. That is the arithmetic behind putting the test into a planned maintenance slot rather than squeezing it in at the end of a shift.
How often, and when to replace. How often depends on the operation's own records. Start by testing when a new container is commissioned, after every repair and when the washing programme changes; repeat it on any container that gets a closing problem in the dispatch record. This is an operating suggestion, not a legal or brand interval.
The leak happens at the joint between gasket and body. In a 2024 study, Zhao and colleagues describe four separate heat leakage paths at a refrigerator door: gasket against cold air, against the outer shell, against the inner door lining and against warm air. Those figures are for refrigerators and do not transfer to a container, but they support the same finding: the leak gathers between the gasket and the surrounding body.
If the paper test is not enough, there are two more checks. In a dark area, put a torch inside the container and close the lid; if light escapes between lid and gasket, the gap is there. Press the gasket with a finger and release it: a sound gasket springs back at once, while a hardened or crushed one leaves a mark.
When to replace the gasket
Replace a gasket that has hardened, taken a permanent set or lost pressure at more than one point; a gasket weakened at a single point is not patched. After replacement, repeat the test at the same points and write down the result.
The record holds the container number, the marked point, the date and the result. That turns 'it does not hold' into a finding about which container and which point.