What blown rubber is
Blown rubber starts from a conventional rubber compound with a chemical blowing agent added. During vulcanization the blowing agent decomposes and releases gas, which forms small cells inside the rubber. The result is a compound with lower density and lower hardness than the same recipe in solid form, but still with a rubber surface that contacts the ground.
How blown rubber outsoles are made
- Compounding — Base rubbers, fillers, oils, cure system and blowing agent are mixed, with care taken to keep temperatures low enough that the blowing agent does not react early.
- Molding — Blanks are vulcanized in a heated mold. Cure and expansion happen together, so their timing has to be balanced.
- Expansion and finishing — Parts expand as the mold opens and are then stabilised, trimmed and buffed for bonding.
The blowing agent level sets the density. More gas gives a lighter, softer and more flexible part, and lower wear resistance.
Grip: 6 / 8, High
The softer, more compliant surface of blown rubber conforms well to the ground, which gives good traction on roads and dry hard surfaces. Wet grip depends on the base compound and filler, just as with solid rubber. As blown rubber wears, its tread loses depth faster, so grip can drop sooner over the shoe's life.
Flex: 8 / 8, Max
Flex is where blown rubber leads. Lower density and hardness let the outsole bend easily with the foot, which keeps the forefoot natural and smooth through toe-off. It also adds a little cushioning compared with solid rubber.
Abrasion: 4 / 8, Medium
The gas cells that make blown rubber light and flexible also weaken it against wear. Abrasion loss measured to ISO 4649 is clearly higher than for solid or carbon rubber. That is why blown rubber is rarely used in the heel or on rough terrain, and why its thickness should allow for wear.
Where blown rubber is used
- Running shoe forefoots — flexible, grippy rubber where the foot bends and pushes off.
- Lightweight trainers — full blown rubber outsoles where every gram counts.
- Walking and casual shoes — soft, quiet and comfortable on pavements.
- Dual-compound outsoles — paired with carbon rubber in the heel.
Designing with blown rubber
Use blown rubber where flexibility and low weight matter more than maximum wear life.
- Combine it with harder compounds where the outsole wears fastest.
- Pair a blown rubber forefoot with a carbon rubber heel.
- Add extra thickness where you expect wear, especially under the big toe.
- Avoid it on rugged trail and work outsoles, where cuts and abrasion shorten its life.
- Specify density alongside hardness; both affect flex and wear.