A method of forming an article, the method comprising: (a) providing a mould having a first part and a second part, the first and second parts (12,14) having respective first and second cavity-forming surfaces (22a, 24a); (b) closing the mould thereby defining a cavity between the first and second cavity-forming surfaces (22a, 24a); (c) injecting a molten plastic composition comprising a polymer and a physical blowing agent into the cavity, wherein the physical blowing agent is a gas dissolved in the polymer, the molten plastic composition being injected at an injection pressure; (d) after the injecting step, applying a packing pressure to the cavity in a packing step, wherein during at least one or both of the injecting step (c) and the packing step (d), the injected plastic composition in contact with the first and second cavity-forming surfaces (22a, 24a) is cooled to form first and second solid skins (4, 6) respectively adjacent to and in contact with the first and second cavity-forming surfaces (22a, 24a), and wherein after the packing step (d) in at least one first region of the cavity at least some of the plastic composition between the first and second solid skins (4, 6) remains molten; (e) opening the mould before the molten plastic composition between the first and second solid skins (4, 6) has solidified in the at least one first region of the cavity, thereby allowing the molten plastic composition between the first and second solid skins (4, 6) to expand by foaming to form an expanded cellular foam, wherein the opening step comprises removing the first part of the mould so that the first solid skin is no longer in contact with the first cavity-forming surface, while maintaining the second solid skin in contact with the second cavity-forming surface, wherein during the injecting step (c) and the 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 the opening step (e), at least some of the molten plastic composition is exposed to an external pressure lower than the minimum pressure threshold to form in the article at least one first portion comprising expanded cellular foam formed from the plastic composition, and wherein 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 comprising a substantially homogeneous, solid phase, unexpanded plastic composition. (a) providing a mould having a first part and a second part, the first and second parts (12,14) having respective first and second cavity-forming surfaces (22a, 24a); (b) closing the mould thereby defining a cavity between the first and second cavity-forming surfaces (22a, 24a); (c) injecting a molten plastic composition comprising a polymer and a physical blowing agent into the cavity, wherein the physical blowing agent is a gas dissolved in the polymer, the molten plastic composition being injected at an injection pressure; (d) after the injecting step, applying a packing pressure to the cavity in a packing step, wherein during at least one or both of the injecting step (c) and the packing step (d), the injected plastic composition in contact with the first and second cavity-forming surfaces (22a, 24a) is cooled to form first and second solid skins (4, 6) respectively adjacent to and in contact with the first and second cavity-forming surfaces (22a, 24a), and wherein after the packing step (d) in at least one first region of the cavity at least some of the plastic composition between the first and second solid skins (4, 6) remains molten; (e) opening the mould before the molten plastic composition between the first and second solid skins (4, 6) has solidified in the at least one first region of the cavity, thereby allowing the molten plastic composition between the first and second solid skins (4, 6) to expand by foaming to form an expanded cellular foam, wherein the opening step comprises removing the first part of the mould so that the first solid skin is no longer in contact with the first cavity-forming surface, while maintaining the second solid skin in contact with the second cavity-forming surface, wherein during the injecting step (c) and the 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 the opening step (e), at least some of the molten plastic composition is exposed to an external pressure lower than the minimum pressure threshold to form in the article at least one first portion comprising expanded cellular foam formed from the plastic composition, and wherein 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 comprising a substantially homogeneous, solid phase, unexpanded plastic composition.