Plastic food storage containers: how to specify the seal, the stack and the markings
A storage container looks like the simplest product in a kitchen range. Most of the complaints about it come from one word on the box — sealed, airtight, leakproof — that nobody checked against the lid.
Start with what goes inside
The contents decide which features matter. Write them down first; the rest of the specification follows.
| Contents | What matters most |
|---|---|
| Dry goods and snacks | A lid that closes evenly and keeps moisture out; easy pouring |
| Seasonings | Separate compartments that do not mix, a scoop, a lid that opens with one hand |
| Leftovers, soups, sauces | A seal that holds liquid, and the right temperature rating for fridge, freezer or microwave |
| Baby formula and powders | Moisture protection, parts that can be fully cleaned, and food-contact documents for every part |
The lid decides the claim
| Lid type | What it does well | Where it falls short |
|---|---|---|
| Press-fit lid, no seal | Cheap, quick to open, easy to clean | Keeps dust out, not liquid; should not be sold as leakproof |
| Lid with a silicone seal | Holds liquid and keeps moisture out when the seal is seated | The seal must come out for cleaning, or food collects behind it |
| Clip-lock lid | Even pressure on the seal; the strongest closure | Clips are the weak point — see our lunch box inspection article |
| Screw lid | Good for powders and round jars | Thread and seal must match, or it leaks at the thread |
Words such as 'airtight', 'leakproof' and 'moisture-proof' are claims. If they go on the pack, agree with the supplier how each one will be tested, and write the test into the specification. For containers sold for liquids, our general leak method applies: fill, close, 30 seconds upside down, then a shake, with no drops.
Stacking and nesting
Two different things, and both affect cost.
- Stacking full: the lid should have a recess or ring that locates the base of the next container, so a stack in a fridge or on a shelf does not slide.
- Nesting empty: containers that sit inside each other take far less carton space, which lowers freight per piece. Check whether they separate easily, or stick together.
- Sets: if one lid size fits several depths, the set is cheaper to tool and easier for the customer. Decide this at the drawing stage, not after the moulds exist.
How the set is packed and loaded is covered in our article on cartons and container loading.
Material and use markings
PP is the usual material for containers that go in the microwave, the freezer and the dishwasher. Clear containers are often moulded in a different material, and their temperature limits are lower — check the material against our temperature chart before printing any use symbol.
- Mark the material on the base.
- Print microwave, freezer and dishwasher symbols only where that model has been tested for that use.
- If the lid and the base are different materials, say which part can go in the microwave.
- For baby formula and children's products, keep test reports for every part, including the scoop.
Our article on product markings and test reports explains how to make the pack, the product and the reports agree.
What to check on the samples
- Every lid fits every base in the set, including bases from different mould cavities.
- The seal sits flat in its groove all the way round, with no twist.
- The leak test passes on models sold for liquids.
- A full stack stands straight, and empty containers nest and separate cleanly.
- Lid corners and tabs survive repeated opening without cracking.
- No smell from the container or the seal after washing.
- Markings on the base match the specification and the pack.
Approve one sealed sample per size and keep it. It is the reference when production lids start to feel looser than the ones you approved.