A packaging line is a continuous stream of highly combustible material moving through, past, and sometimes into heat-producing equipment. Corrugated, film, labels, and void fill are all ordinary Class A combustibles, and a packaging operation concentrates enormous quantities of them into a small footprint with conveyors providing a continuous path from one end of the facility to the other.
The heat sources are localized and specific. Shrink tunnels operate at temperatures well above the ignition point of the film they process and depend on continuous product motion to avoid overheating any single item. Hot melt adhesive systems hold molten polymer in heated tanks and pump it through heated hoses. Both are reliable in normal operation and both fail in the same way: a line stop with the heat still applied.
The characteristic incident is a jam. Product stops in a shrink tunnel that keeps radiating heat, or accumulates against a hot melt applicator, and what was a controlled process becomes sustained heating of a stationary combustible. The conveyor then provides the propagation path, and the packaging material staged along the line and in the adjacent warehouse provides effectively unlimited fuel.