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Why the bottle leaks — where the seal fails
Liquid leak and CO₂ loss are symptoms that always point to the same physical spot: the interface between the cap liner and the sealing surface of the finish. When that interface does not close hermetically, liquid escapes on still products and CO₂ permeates on carbonated products. A good share of the time, the cause is not at the capper — it is on the cap or on the bottle.
Chapter 8 of the packaging technical guide is the only one whose capper causes are all cross-references to other problems. In other words: CO₂ leak is not a defect on its own, it is the manifestation of cocked cap, high cap, under-applied cap or low torque. That is why diagnosis is cross-referenced.
Possible causes — Cap
| Item | Cause | Solution |
|---|---|---|
| 8.1.1 | Cap liner incomplete or missing. | Change the cap lot and notify technical support. Inspect samples from the same lot — injection defects are rarely isolated. |
| 8.1.2 | Wrong cap (PET vs glass). | Confirm by the box label that the cap is the one recommended for the package in use. |
Possible causes — Bottle
Here is the trickiest list. Small dimensional variations of the finish or microcracks in the sealing zone create leaks that look like a capping failure — but machine adjustment will not fix:
| Item | Cause | Solution |
|---|---|---|
| 8.2.1 | Bottle finish out of specifications. | Check with "go/no-go" gauge. Contact the supplier. |
| 8.2.2 | Chipped or deformed finish in the sealing zone. | Inspect for excess material, deformations or damaged threads. |
| 8.2.3 | Microcracks in the sealing zone (returnable); poor material distribution in the walls (single-use PET). | Eliminate problem bottles from stock. Measure material distribution on single-use PET with proper equipment. |
| 8.2.4 | High level of hot-end treatment on the finish — the cap jams before sealing. | Contact the bottle supplier. Recommendation: minimum 4 ctu, maximum 10 ctu of treatment. |
Possible causes — Capper (cross-reference)
Ruling out cap and bottle, the problem is always one of the 4 below. There are no capper-only causes for leakage — just symptoms pointing to other 4 guide problems:
teste-vazamento-co2.jpeg
| Item | Cause | Solution |
|---|---|---|
| 8.3.1 | Tilted cap. | See cocked cap diagnosis. Threads deformed toward the liner are the clear sign. |
| 8.3.2 | Low removal and/or incremental torque. | Symptom of an under-applied cap. Audit vertical load, turret, rear guide and static torque. |
| 8.3.3 | High cap. | Cannot be hand-threaded onto the finish — the liner does not reach the sealing surface. See the high cap diagnosis and audit vertical load. |
| 8.3.4 | Under-applied cap. | Hand-threadable — turret too high, vertical load low, static torque low. Check the under-application diagnosis. |
Practical diagnostic flowchart
1. Grab leaking samples.
2. Inspect the liner: complete? If missing, it is the cap (8.1.1).
3. Inspect the bottle finish: chipped, deformed, within gauge? If outside, it is the bottle (8.2.x).
4. Check cap application: hand-threadable? If yes, high cap or under-applied. Threads deformed toward the liner? Cocked cap. Comes off with normal tightening but leaks? Low torque.
That flowchart covers 95% of cases. The remaining 5% tend to be a combination of causes — bad cap lot and miscalibrated capper, for example. Hence the rule of auditing sampling from several heads before touching the equipment.
Next step
Leakage is the defect that shows up most often in consumer complaints. On carbonated products, the consumer discovers it on the first sip. On still products, they discover it in the bag. To close the loop, the complete technical guide brings all 13 correlated problems — leakage is just the tip of the iceberg.
Guide to Cap Application Problems
The 13 most common capper failures — diagnosis, root cause and correction. Free technical material.
Download the full guide →