Choosing a polystyrene box for chilled shipping
Anyone shipping chilled goods — medicines, meat, fish, fresh produce or lab samples — sooner or later ends up with a polystyrene box. Rightly so: EPS (expanded polystyrene) insulates well, weighs very little and protects the contents against knocks. But choosing a polystyrene box is more than ordering a box that looks big enough. Wall thickness, internal dimensions and the combination with gel packs together determine whether your product arrives at the right temperature. Below are the considerations as they come up in practice.
Why polystyrene in the cold chain
EPS consists largely of still air, trapped in closed cells. That air is the actual insulator: heat only passes through it slowly. The material also absorbs no moisture, is hygienic and cushions knocks — important for glass, sample tubes and delicate fresh goods. And because the box itself weighs next to nothing, you mainly pay to transport your product, not your packaging. A good polystyrene box is moulded in one piece, with a lid that fits snugly into the rim. Every gap is a heat leak.
Choosing a polystyrene box: three questions
How long do the contents need to stay at temperature?
Wall thickness is the main dial you can turn. A thicker wall keeps the contents at temperature for longer, but makes the box bigger on the outside — and that costs shipping volume. So don't base your calculation on the ideal transit time, but on the real one: including the time a parcel may sit on the recipient's doorstep or in a depot. A shipment that spends a day longer in transit than planned needs to be able to take it. Choose your wall thickness for that scenario, not for the best day.
What internal dimensions do you really need?
Empty space in the box is your biggest enemy. Air that can circulate exchanges heat and lets the contents warm up faster. The right box therefore fits as closely as possible around the product plus gel packs. Buying oversized because everything will always fit is an expensive habit: you need more gel packs to achieve the same result, and you pay the carrier for extra volume. With the external dimensions, also watch your carrier's size limits — one centimetre too many can push you into a more expensive rate band.
What do you cool with?
The box insulates, but it doesn't cool. That's the job of gel packs or other cooling elements. A few rules of thumb: place gel packs on top of or around the contents, because cold air sinks. Match the conditioning to your product: fully frozen packs for frozen goods, chilled packs for fresh products that must not freeze. Frost-sensitive contents — think vaccines, fresh vegetables or some lab samples — must never sit directly against a frozen pack; use a separating layer or position the pack at a distance.
The mistakes we see most often
- Product not pre-chilled. Polystyrene holds temperature, it doesn't make anything cold. What goes into the box warm arrives warm.
- Gel packs not fully conditioned. A half-frozen gel pack delivers only part of its capacity. Build the freezing time into your process.
- Too much empty space. Filling up with void fill or paper only partly solves it; it barely insulates. Better to choose a well-fitting size.
- Lid not properly closed. Check that the lid sits in the rim all the way round and seal the box with tape. One gap does more harm than a centimetre of wall thickness can make up for.
- No outer box used. A cardboard outer box protects the EPS against transport damage and gives a flat, reliable surface for shipping labels — on bare polystyrene, labels come off more easily.
- Shipping just before a weekend or public holiday. Transit time then becomes longer than your packaging is designed for. Plan chilled shipments early in the week.
Test first, then scale up
Don't rely blindly on specifications. Do a test shipment with a temperature logger in the box, under the toughest conditions you expect: the longest route, warm weather, the smallest amount of product (little mass warms up faster). Only read out the logger at the moment the recipient would normally unpack, not on arrival at the door. Repeat the test at the change of season — a configuration that comfortably passes in March can fall short in July.
When polystyrene isn't the best choice
A polystyrene box is the standard for boxes that ship as parcels. If you ship at pallet level, a thermal pallet cover over the whole pallet is often more logical. For small, flat shipments such as samples or medication in small quantities, an insulated envelope or insulated bag can be enough, combined with a suitable gel pack. The question is always the same: how much mass are you shipping, how long is it in transit and what temperature limit applies.
Not sure about the right size or wall thickness for your shipment? Request a sample or browse the range of polystyrene boxes in our webshop. That way you simply test it with your own product, on your own route.
