The technically correct term for closed cell sponge rubber is expanded rubber. The cells are like balloons, holding gas and not allowing moisture to pass through. The raw compound is mixed on the same way as solid rubber compounds, but a chemical blowing agent is also added. Under the heat and pressure of a pre-cure or pre-forming process, this additive decomposes and mixes with the rubber compound and generates nitrogen gas.

As this reaction is occurring, the mixed batch is heated and allowed to expand into a bun. The bun of closed cell sponge rubber is then aged and split into sheets.