Every delivery kitchen we talk to has had this problem and most of them started by blaming the film.

What the flange is

The flat lip running around the top edge of the tray, which the film is heat-sealed onto. Everything about sealing performance happens there.

It has to be genuinely flat, wide enough for the sealing tool on your machine, and free of the small forming ridges that a worn or cheap die leaves behind.

Why a ridge is a leak

The sealing head presses film against the flange under heat and pressure. Where there is a ridge, the film bridges over it and leaves a channel underneath.

That channel runs from the inside of the tray to the outside. It does not matter how good the film is; there is a hole.

Match the flange to the machine

A hand sealer and an inline tray sealer want different flange widths and tolerate different amounts of variation. Buying a tray without saying what machine it is going on is how you get a tray that seals beautifully in the supplier’s factory and not in your kitchen.

Tell your supplier the machine at the quote stage. It is a one-line question.

And do not blur oven-safe and chilled

They are different materials with different temperature limits. A chilled-only tray put into an oven is a safety problem, not a quality one.

Any supplier who is relaxed about that distinction is telling you something about the rest of their specifications too.