Common Seal Defects

Partial Bonds Usually Start With Small Setup Errors

A partial bond means only part of the foil liner has attached to the container mouth, leaving an area that can peel away or leak. On induction lines this often points to the sealing head being out of position, a cap that is not sitting square, or a neck finish that has contamination on it. Even a thin film of product or dust can block heat transfer and stop the liner from bonding properly.

In Jacobs and across Durban, we often see this after changeovers when operators move from one bottle size to another and do not recheck head height. A small difference in cap profile can be enough to shift the field and leave one side under-sealed. Coastal humidity can also affect cardboard cartons and stored liners if packs are left open too long, so storage matters as much as machine setup.

• Check bottle neck cleanliness before filling starts.
• Confirm the induction head is centred over the cap path.
• Match liner type to the container material and closure size.
• Inspect caps for cracks, warp or thread damage.

What To Check First When Defects Start Appearing

When defects begin suddenly, work through the line in a set order. Start with the simplest physical checks, then move to machine settings. This prevents unnecessary downtime and helps the team isolate whether the problem is happening before, during, or after sealing.

A practical inspection routine should include the cap supply, liner condition, conveyor speed, head height, and bottle neck cleanliness. If seals are inconsistent across the same batch, look for changes in container moulding or supplier lots. Small differences in neck ovality can lead to weak areas that are easy to miss on visual inspection.

• Inspect liner placement inside the cap before capping begins.
• Check for bent guide rails or unstable bottle transfer points.
• Test one bottle at a time after adjusting settings.
• Record the exact time and shift when defects begin.

When To Bring In Support Before Waste Builds Up

Some defects clear up with a quick setting change, but recurring partial bonds, scorch marks and weak seals usually mean there is a deeper issue in the line. If the same fault returns after adjustment, the problem may be with liner matching, cap quality, worn components or the sealing head itself. At that point, outside support can save product and time.

For operations that run multiple shifts in Durban, Pinetown or nationwide distribution depots, response time matters. Preventive maintenance, breakdown repairs and commissioning support help keep the line stable, while remote checks can often narrow the fault before anyone travels. The aim is simple: less scrap, fewer stoppages and better control of every batch. If your seals are drifting, do not wait until returns start coming back. Sort the cause while the line is still producing usable stock.