Choosing a polystyrene box: what to look for when shipping refrigerated goods
A polystyrene box remains the most widely used solution for shipping refrigerated products: medicines, meat, fish, fruit and vegetables, meal boxes and laboratory samples. The material is light, insulates well and can withstand impact. But not every box suits every shipment. The wrong size or wall thickness will cost you cooling capacity, filler material and shipping costs. Below you'll find what to look for, how to weigh your options and which mistakes occur most often in practice.
What a polystyrene box does — and what it doesn't
Polystyrene (EPS, expanded polystyrene) consists mostly of still air. That air slows down heat transfer. It's important to understand this clearly: the box doesn't cool, it slows down. The temperature inside the box is determined by what you put in it — the product itself and the gel packs. The box only ensures that temperature stays as long as possible.
That also means: a product that goes into the box at room temperature won't get cold on the way. Whoever ships refrigerated starts with a pre-cooled product and frozen gel packs. Without these two things, even the best box won't help you.
What to look for in a polystyrene box
Wall thickness
The thicker the wall, the longer the insulation lasts. The consideration is simple: a thicker wall means a longer cooling chain, but also larger external dimensions and thus a bigger package for the carrier. For a shipment delivered within a day, a thinner wall often suffices; if you need to account for a day's delay or hot summer days, choose thicker. Calculate for the worst realistic scenario, not the ideal delivery time.
Interior dimensions and filling
Choose the box based on interior dimensions, not exterior ones. And fill it as much as possible. Air in the box is your enemy: every empty space warms up faster and lets the product shift around. A smaller, well-filled box performs better than a large half-full one. If there's still space left, fill it — with extra gel packs if temperature is critical, otherwise with filler material.
Fit with the outer carton
You almost always ship a polystyrene box in a cardboard carton. That combination needs to work: the carton protects the polystyrene from breaking and gives you a neat package with a well-printed or labelable exterior. A box that rattles around in a carton that's too big breaks more easily and insulates poorly at the break points.
Gel packs: half the work
The box and the cooling are one system. A few practical rules:
- Freeze packs completely. A partially frozen gel pack delivers only a fraction of cooling capacity. Plan freezing time into your process — large packs need plenty of time to freeze solid.
- Put cold on top. Cold air sinks. Packs at the top of the shipment cool the entire contents; packs only on the bottom let the top layer warm up.
- Protect frost-sensitive products. Medicines that must stay between 2 and 8 degrees must not lie directly against a frozen pack: the outer packaging will then freeze and that's just as bad as too warm. A layer of cardboard or paper between pack and product solves that.
- Test your own combination. Put a temperature logger in a test shipment and see what really happens along the route your packages take. That tells you more than any table.
Common mistakes
- Not pre-cooling the product. The gel packs must then first bring the product to the right temperature and are then depleted.
- Too little cooling mass for the season. What works in March doesn't work in July. Scale up the number of packs in summer.
- Shipping on Friday. A package that spends the weekend at a depot rarely makes it. Align shipping days with the carrier's delivery days.
- Choosing a box that's too large. Lots of empty space, lots of warming, higher shipping costs. Measure your average order and choose accordingly.
- The lid doesn't close properly. A gap in the seal is a cold leak. Check that the lid fits tight all the way around before the carton closes.
Polystyrene box or thermal pallet cover?
Not every refrigerated shipment needs a box. A thermal pallet cover or insulation pouch in a regular box is lighter, can be delivered flat and takes up almost no storage space. For short routes and less critical products it's often enough. The polystyrene box wins on three points: longer insulation duration, solid protection of fragile products and fixed interior dimensions so gel packs and product stay in place. For medicines, fish and shipments that need to withstand a day's delay, the box is the safe choice. If you're unsure, let the temperature profile of your product and the delivery time of your shipment decide — not the price per package.
If you want to know whether a specific size suits your product and process, request a sample or check out the range of polystyrene boxes in our webshop. Then you simply test the combination of box, gel packs and carton with your own shipment.
