This invention describes a staged process in which successive dynamic vulcanizations of plural curable rubber blend components are based on condensation reactions of a functional group (reactive site) carried by each curable rubber. The process is carried out in three stages during each of which gases evolved during the condensation reactions are removed. It is essential to form an intermediate fully dense hard blend in the second stage. This intermediate hard blend contains a relatively large amount of plastic (relative to curable rubbers) and is therefore much harder than the desired final blend. In the third stage, additional rubber and curative for uncured rubber are added to achieve the desired softness. This process, carried out in an extruder, results in fully dense pellets which are injection moulded to produce articles having essentially the same specific gravity as that of the pellets.