Choosing a lunch box to sell: microwave use, sealing, and how to check for leaks
A lunch box is several parts made from different materials. 'Microwave safe' and 'leakproof' are claims about the whole assembly, not about the plastic the base is made of.
A lunch box is four parts, not one
When a buyer asks whether a lunch box is microwave safe, the honest answer depends on which part you mean.
- The base. Usually PP on our range. Whether it can go in the microwave depends on the grade and wall design of that specific product, so it should be judged by the product's own marking and instructions.
- The lid. Often a different material, or the same material with clips, hinges or vents that behave differently when heated.
- The seal. A silicone ring or gasket in the lid. It has its own temperature limits and is the part most likely to be damaged by careless heating.
- Stainless steel inserts. Some of our bento boxes have a stainless inner tray. Metal does not go in a microwave, so those products are not microwave products, whatever the outer box is made of.
Why we do not say 'PP is microwave safe'
It is common to see one material rule applied to every product: this plastic is safe, that one is not. It is not reliable. Two PP boxes can behave very differently in a microwave depending on the resin grade, the wall thickness, the lid design and how long they are heated.
So the claim belongs to the finished product, not the resin. For each model, the marking on the box, the user instructions and the test conditions behind them are what decide it. If you are putting a microwave claim on your packaging, ask the factory which test it is based on and under what conditions, and keep that in writing.
For the general temperature behaviour of each plastic, see our temperature and dishwasher chart — but treat it as background, not as a label claim.
Two kinds of leak, two different causes
Buyers usually say 'it leaks'. There are two different failures, and they are fixed in different places:
- Leaking out of the box. Liquid escapes past the lid. This is a sealing problem: gasket seating, clip pressure, or the fit between lid and base.
- Leaking between compartments. Soup or sauce runs from one section into the next while the outside stays dry. This is a divider problem: whether the inner walls reach and press against the lid at all.
A box can pass the first and fail the second. If the product is sold as 'divided' or 'no mixing', both need checking.
The leak check we suggest
This is the method we recommend to buyers for checking samples and production. It is our suggested practice, not an industry standard, and you should agree your own pass criteria with the supplier before production.
- Flat: fill to the normal working level with water, close the lid as a user would, and leave it standing. Check the rim and clips for seepage.
- Upside down: turn the closed box over and hold it inverted — our general leak method uses 30 seconds. Pass is no drops.
- Shake: shake it the way it will be carried in a bag. Pass is no drops.
- Between compartments: fill one compartment with coloured water, leave the others dry, close, turn and shake. Pass is dry neighbouring compartments.
Write the fill level, the hold time and the pass rule into your inspection standard, so a third-party inspector runs the same test you ran on the sample.
What to ask before you choose a model
- Which parts of this model are marked for microwave use, and which are not?
- Is the lid seal removable for cleaning, and is a spare available?
- Is the product sold as leakproof out of the box, between compartments, or both?
- Does any part contain stainless steel?
- Which food-contact reports are available for your market?
Five questions, asked before the order, remove most of the complaints we see on lunch boxes.