Why a seal can fail even when the cap looks tight
Power setting is often the first place operators look, but it should not be treated as a single fix. The right setting depends on cap material, bottle shape, liner size and product sensitivity. A thick chemical container may need a different setup from a light food bottle, even if both use similar cap diameters.
In Durban, humidity also matters. Moisture in the air can affect storage conditions around caps and liners, especially where cartons sit near open loading bays. If the liners pick up too much moisture before use, the sealing window can change. That is why storage and handling are part of seal quality, not just the sealing head itself.
- Check cap and liner storage away from damp floors and open doors.
- Keep containers consistent in height and finish.
- Confirm that the sealing head aligns centrally over the bottle mouth.
Dwell time is the part many lines get wrong
Dwell time is the period the induction field stays effective over each pack. If the dwell time is too short, the liner may not activate fully. If it is too long, heat can spread beyond the target area and create defects. The sweet spot is usually found through testing, not guesswork.
Operators sometimes chase speed and the seal suffers. Then the line is slowed down to hide the problem, which lowers output and does not solve the cause. A better approach is to test at the intended production rate and record the result. That way, the line runs at the pace it was meant to run, with fewer surprises later.
For a beverage or food plant in Pinetown, a line that runs well for a short trial can still fail after an hour if the caps warm up or the product room changes. Seal quality should be checked through the full shift, not only at start-up.
Line speed and power must be set together
Line speed is not separate from power and dwell time. As speed rises, the time available for each container falls. That means the machine may need more output, a different head position or a revised conveyor pace to keep the seal consistent. The main point is simple: change one setting and review the others.
Many plants make the mistake of setting the sealing unit once and leaving it alone. But a production line is not static. A change in bottle supplier, label thickness, fill temperature or cap insert can all shift the seal response. Even a minor change in cap skirt depth can alter the outcome. On a chemical line this can have serious cost and safety implications.
- Record the speed, head height and power setting for each product run.
- Recheck settings after maintenance or container changes.
- Do not use one setup for every bottle family.
A Durban example: when a small change causes a costly leak
At one Jacobs site, a client running a liquid product saw a rise in leaking packs after a supplier changed the bottle neck finish. The line was still operating at the same speed, and the operators assumed the problem was random. In fact the new neck finish changed how the liner compressed under heat. The line needed a fresh setup, not just more pressure.
After the sealing parameters were reset and the operator team was shown what to watch for, the leaks dropped sharply. The main lesson was not to treat the seal as a separate step. It must match the container, the product and the pace of the line. That is especially true where loads move between Durban, Pietermaritzburg and Richards Bay and packs need to hold up in hot trucks and long storage cycles.
Good validation also helps when load-shedding interrupts production. If a line restarts later in the day, the first packs should be checked again because temperatures and machine conditions may have shifted while the line was idle.
What to watch during setup and daily checks
Practical seal control is not complicated, but it does need discipline. Operators should know what a good seal looks like, how to spot a weak bond and when to call for support. The right training saves time and reduces waste, particularly on lines that handle more than one pack size.
Daily checks should be part of the shift routine. A quick look at bottle appearance is useful, but the real test is whether the seal survives handling, storage and transport. That is why validation and maintenance matter as much as first-time setup. A line that is left unattended for weeks often drifts out of spec without warning.
- Inspect the liner bond after the first start-up packs.
- Watch for scorched caps, misshapen necks or uneven foil pull.
- Check that spares, liners and heads are suited to the same container type.
With the right balance of heat, dwell time and speed, a pack stays protected and the line stays productive. That balance is the difference between a line that merely runs and a line that holds quality all the way to the customer.
