Method of forming an article from a molten plastic composition including a polymer and a blowing agent. The method provides that during an injecting step (c) and a packing step (d), the injection pressure and the packing pressure, respectively, are maintained above a minimum pressure threshold in at least one second region of the cavity to maintain the physical blowing agent as a gas dissolved in the polymer so that substantially no gas bubbles are formed in the at least one second region, during a mould opening step (e), at least some of the molten plastic composition is exposed to an external pressure lower than a minimum pressure threshold to form in the article at least one first portion having expanded cellular foam formed from the plastic composition, and prior to the opening step (e), the plastic composition in the at least one second region of the cavity has been cooled so as to be fully solidified to form in the article at least one second portion having a substantially homogeneous, solid phase, unexpanded plastic composition.