Ask three ice pack suppliers how much gel a 20-litre box needs and you will get three numbers, usually 600 g, 800 g and 1,000 g. All three can be right. The missing variable is wall thickness, and it does more to the melt curve than the gel weight does. A 20-litre box built from 20 mm expanded polystyrene loses heat roughly twice as fast as the same box in 40 mm, which means the pack has to give up its cold twice as quickly to hold the same product band.
Start from internal volume, then correct for surface area
Internal volume is only a proxy. What actually drives heat gain is external surface area and the temperature difference across the wall. A tall 20-litre box of 30 × 30 × 22 cm has about 0.44 m² of external area. A flat 20-litre box of 40 × 30 × 17 cm has about 0.48 m². Same volume, 9 percent more surface, and at 30 °C ambient that shows up as a shorter hold by two to three hours.
A working starting point for gel weight, before any correction: 35 to 45 g of gel per litre of internal volume for a 24-hour hold at 25 °C ambient with 30 mm walls, and 55 to 65 g per litre for a 48-hour hold or a 35 °C ambient. For a 20-litre box that is 700 to 900 g for the first case and 1,100 to 1,300 g for the second. These are pre-correction figures and should be treated as a first guess, not a specification.
Wall thickness changes the clock, not just the weight
Every 10 mm of added insulation in a moulded EPS box cuts heat gain by roughly 25 to 30 percent at the same ambient, until the lid seam becomes the limiting path. That has two consequences for pack design.
- Doubling wall thickness from 20 to 40 mm can let you cut gel weight by a third and still gain hold time.
- Past about 40 mm, extra wall buys much less, because lid fit and seam leakage start to dominate.
Polyurethane boxes at 40 to 50 mm perform close to EPS at 60 mm for the same volume, with thinner walls and therefore more usable internal space — worth the cost when a courier charges by external dimensions. Vacuum-insulated panels reach the same hold at 20 to 25 mm, but they are rigid, expensive, and easily damaged at the corners, which opens a thermal bridge exactly where the box is handled most.
A worked example at 20 litres. Take a 20-litre box, 30 mm EPS, 0.44 m² external area, ambient 32 °C, product band 2 to 8 °C, target 36 hours. Start at 60 g per litre: 1,200 g. Test with three 400 g packs at −18 °C. The trial shows product at 9 °C by hour 30, so the box is short by roughly six hours. Options: add a fourth 400 g pack (plus 33 percent gel, plus 1.6 kg of shipping weight, plus freezer energy), or move to 40 mm walls (plus 8 mm per side, minus 12 percent internal volume, and the same four packs now hold past 44 hours). For most programs the wall change wins on cost per shipment after about 200 units.
Fill ratio matters alongside wall thickness. A box packed to 90 percent of internal volume holds cold far better than one at 60 percent, because the air gap becomes a convection cell that moves warm air across the product. If the payload is fixed at 12 litres in a 20-litre box, spend some of that space on void fill before spending it on gel.
Matching pack geometry to the box
A 1,000 g fill split into two 500 g packs at 25 mm thickness chills faster and freezes in 24 hours; a single 1,000 g pack at 50 mm needs 36 to 40 hours and holds cold longer once frozen. Thin packs spread across a face give more uniform surface temperature; thick packs give a longer tail. For boxes under 25 litres, two packs of 400 to 600 g placed on opposing walls beat one central pack of the same total weight by three to five hours of hold, simply because the warm air has less distance to travel before it meets cold surface.
Freezer capacity is the constraint that often decides this. A line running 2,000 boxes a day at 800 g needs 1.6 tonnes of gel frozen daily, and thin packs fit into racking more efficiently than blocks. Wall thickness, pack geometry and freezer throughput have to be chosen together — sizing the pack in isolation produces a number that works on paper and jams the freezer.
Absorption and retention figures for fills used in these weight ranges are listed in the SAP product data sheets.
Da maohua
Zhengzhou Flor New Materials Co., Ltd. specializes in SAP super‑absorbent polymer products. Serving personal hygiene, agriculture & horticulture, pet supplies and other industries, we leverage supply‑chain resources to provide particle‑size selection, performance matching and customized solutions based on clients’ practical requirements.
