Packaging Machinery

The problem is evenness, not heat

Getting a sealing bar hot is easy. Getting every millimetre of it to the same temperature, cycle after cycle, is the actual job. A seal is only as good as its coldest point, and a bag that leaks at one corner fails the same as one that was never sealed.

Bars lose heat at their ends, into the mountings and the frame. So a bar with evenly spread wattage runs cool at the ends and hot in the middle. The fix is distributed wattage — a cartridge deliberately built with more power in some sections than others — or more, shorter cartridges arranged to compensate.

Cycling is the duty, and it decides the size

A packaging heater does not settle at a temperature and stay there. It loses a burst of heat into the film at every seal and has to recover before the next one. As the line speeds up, the recovery time shrinks.

That is why a bar can be perfectly stable in the morning and drift below sealing temperature when production is pushed. The heater is not failing; it was sized for a slower line. When choosing, size on the hardest duty the machine will run, not the average.

The fit in the bore does the work

A cartridge heats by conduction into the metal around it. In a new bar with a reamed hole that is efficient and the cartridge lives for years. In an old bar, the bore has worn oval, carbon and residue have built up inside it, and the same cartridge now sits in a loose hole with an air gap around it.

That is the usual reason a machine starts eating cartridges after years of trouble-free running. Clean and check the bores before blaming the elements.

Put the sensor where the seal is made

Control from the middle of the working length, in the bar itself, at the depth the seal happens. A sensor at the end of the bar, or in the frame, reports a temperature the film never sees. On narrow-window films — a few degrees between a weak seal and a burnt one — that offset is the difference between a good shift and a scrapped batch.

Leads fail before elements on moving machines

Sealing bars move thousands of times a day, and the lead flexes at the same point every time. Braided or spring-protected leads, a proper strain relief and a cable route that does not force a tight bend all matter more here than on a static machine. When a heater on a moving bar goes open circuit, check the lead where it enters before condemning the element.

Shrink tunnels are air heaters wearing a different name

Everything about air heating applies: fins for surface, moving air to carry the heat, an interlock so the elements cannot run with the fan stopped. A tunnel that starts scorching film usually has an airflow problem — a blocked inlet, a slipping fan belt — rather than an element problem.