Six reasons the seal fails
- The layers do not match. The film and tray do not seal, no matter how high you raise the temperature.
- The settings are wrong. Too little heat, time or pressure — or, on the contrary, overheating.
- The machine needs attention. Build-up on the plates, a worn frame, miscalibrated sensors.
- The tray does not hold its shape. A narrow or uneven flange, soft walls.
- Storage was not considered. Freezing, modified atmosphere, stacked transport.
- No seal requirements in the specification. There are defects but nothing to check against.
How a seal works
It is not “the tray and the film” that seal together, but two layers in contact: the sealing layer of the film and the contact layer of the tray flange. The outer layer of the film provides stiffness and printing, the barrier layer — shelf life, the tray base — the shape. They do not take part in the seal.
That is why the specification should state not the general material name but which layer faces the product and the flange.
Which pairs seal
| Tray contact layer | Film layer | Result | What matters |
|---|---|---|---|
| PE | PE | Seals | The most common and reliable pair |
| PP | PP | Conditionally | The film layer must be heat-sealable PP |
| PP | PE | Conditionally | The PE layer needs a PP-compatible additive |
| PET | PET | Conditionally | A heat-seal lacquer on the film is needed |
| PVC with a PE layer | PE | Seals | The seal forms on the tray’s PE layer |
| Adhesive rim | Any | Seals | The seal forms on the adhesive, not on the tray plastic |
Pay attention to the codes: a “PP/PE tray” means a PP base with a PE layer on top, so the seal forms on PE.
Adhesive rim: when you do not need to match a pair
- Almost any film works: it bonds to the adhesive on the flange, not to the plastic. You can even use a laminate with printing between layers.
- Tolerates a contaminated flange: if product got onto the flange during loading, the seal suffers less.
- Lower sealing temperature: heating elements last longer.
Peel or lock seal
Seal strength is a balance between tightness and ease of opening.
- Peel — the film is removed by hand, without a knife. Ready meals, salads, fruit and vegetables, snacks.
- Lock — the film cannot be peeled off, it is cut with a knife along the flange. Meat, poultry, fish.
If there is one tray but customers need different opening, the film solves it: some films give a peel at one sealing temperature and a lock seal at a higher one.
For a chilled display, add antifog: the coating keeps the film from fogging up, and shoppers see the product, not condensation.
A seal is welding, not gluing
- Heating. Both contact layers must melt. Overheating causes odour, discolouration and loss of strength.
- Time. While the layers are molten, the polymers interpenetrate and become one.
- Pressure. The frame presses the film tightly and evenly against the flange along the whole perimeter.
- Cooling. The seal gains strength and becomes tight.
Hence the order: first find out what the contact layers are made of, and only then set the temperature.
Tray and machine
A wide, flat flange distributes pressure and compensates for small irregularities. On a narrow or soft flange, any shift under the frame goes straight into the seal and the tray springs back — seal failures appear.
Before changing the film, check the tray sealer:
- the heating plate is clean, without build-up or film residue;
- the pressure frame is flat, without wear or distortion;
- the sensors show the real temperature;
- the pressure is the same along the whole perimeter;
- the tooling matches the tray shape;
- the tray flange is clean after the product is loaded.
Storage changes the requirements
A chilled product needs a tray that holds its shape when stacked. In a modified atmosphere, tightness and barrier matter: a leak changes the gas composition and the product spoils early. When freezing, the product expands and presses on the seal, while plastic becomes brittle in the cold — freezer-grade film and trays are needed.
How to choose a pair and test it
The order of selection: the product and its condition → the opening method → storage conditions and shelf life → line and settings → tray. Only then are the film structure and thickness chosen. Often a thinner film than the one bought “just in case” is enough.
Test the whole system, not an empty tray: seal strength and peel force, tightness, the seal after freezing and thawing, the seal’s appearance, a box drop. The best test is a small batch with the real product along the whole route: production, warehouse, transport, store shelf.
