Last updated: September 8, 2026. A tray sealer looks like a closed cabinet with a conveyor, but inside it runs a precise, repeating cycle: load the tray, control the atmosphere above the product, weld the lidding film to the flange, cut and release. Every step has an engineering failure mode, and understanding the sequence is what separates a specification that works from one that leaks.
The core idea of this page: the tray sealer is a five-step cycle, and seal failure is almost never a machine mystery — it is a flange contamination, temperature-drift, film-mismatch or parameter problem. Learn the cycle and you can diagnose the failure before the supplier arrives.
The cycle, step by step
- Tray infeed: a de-nester or infeed conveyor places a preformed tray into the sealing tool. On automatic machines the tray is transported by servo-driven clamps or walking beams into the sealing station.
- Film feed: lidding film unwinds from a roll above the sealing station, held under controlled tension. Film registration sensors keep printed film aligned to the tray position; mis-registration is a classic cause of printed-film waste.
- Gas flush (MAP): before sealing, the headspace is flushed with the protective gas mixture. The mixture is product-specific — gas suppliers publish guidance per product category, and research documents mixtures such as 70% O2 / 30% CO2 for red meat. The flush displaces air so the sealed tray keeps the intended atmosphere.
- Sealing: the heated sealing tool presses the film onto the tray flange under controlled temperature, pressure and dwell time, welding the film’s seal layer to the tray material. The parameters are developed per film recipe on the real tray and film — a drifting zone makes a weak weld that still looks fine.
- Cutting and ejection: the film is cut around the tray, the finished pack is ejected, and the cycle repeats. Vacuum-skin versions evacuate the headspace and draw the film down onto the product before sealing instead of gas flushing.
Why the seal fails — the four usual causes
- Contaminated flange: product, moisture or film dust on the tray flange prevents the weld; flange wiping and clean infeed matter as much as the sealer itself.
- Tool temperature drift: multi-head tools can run unevenly; a cold zone makes a weak weld that passes visual inspection and fails leak testing.
- Film-tray mismatch: the film seal layer must match the tray polymer; a “universal” film usually means a compromise on at least one tray material.
- Pressure and dwell: short dwell at high temperature can thin the film at the flange edge; the recipe must be developed on the real film, tray and product, not the demo pack.
How the machine is validated
Acceptance runs measure seal strength with a peel test to ASTM F88, leak-test packs by water bath or pressure decay, and — for MAP — verify the headspace gas composition with a gas analyser, the documented verification step for MAP flushing. The sampling plan and reject threshold belong in the purchase order. Machines: Proseal GT2s, Sealpac A7, G.Mondini Trave LC. Format choice: MAP vs Vacuum Skin Packaging; machine-class choice: Tray Sealing vs Vacuum Chamber Sealing.
Last verified: September 8, 2026.
