A method of forming an article (2; 102; 250), the method comprising: (a) providing a mould having an outer part (12; 112) and an inner part (14; 114), the outer part having a first cavity-forming surface and the inner part having a second cavity-forming surface; (b) closing the mould thereby defining a cavity between the first and second cavity-forming surfaces wherein, at least one region of the cavity defining an annular cross-section of the cavity between outer and inner circumferential edges of the cavity, the first cavity-forming surface is such that a distance between the first cavity-forming surface and the second cavity-forming surface alternates between a first distance (D1; D101) and a second distance (D2; D102) around at least a portion of the circumference of the region, the first distance being greater than the second distance; (c) injecting a molten plastic composition comprising a polymer and a blowing agent into the cavity; (d) allowing the plastic composition to form, in the said at least one region, a first solid skin (4; 104; 240) adjacent to and in contact with the first cavity-forming surface and having a first circumferential length, and a second solid skin (6; 106; 242) adjacent to and in contact with the second cavity-forming surface and having a second circumferential length, wherein the plastic composition between the first and second solid skins remains molten, to form, respectively where the distance between the first cavity-forming surface and the second cavity-forming surface alternates between the first distance and second distance, alternating first and second thicknesses of the plastic composition, each first and second thickness comprising the first and second skins with molten plastic composition therebetween; (e) opening the mould before the plastic composition between the first and second solid skins has solidified, thus allowing the plastic composition between the first and second solid skins, and the molten plastic composition at the alternating first and second thicknesses of the plastic composition, to expand by foaming, wherein said opening comprises removing the outer 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. (a) providing a mould having an outer part (12; 112) and an inner part (14; 114), the outer part having a first cavity-forming surface and the inner part having a second cavity-forming surface; (b) closing the mould thereby defining a cavity between the first and second cavity-forming surfaces wherein, at least one region of the cavity defining an annular cross-section of the cavity between outer and inner circumferential edges of the cavity, the first cavity-forming surface is such that a distance between the first cavity-forming surface and the second cavity-forming surface alternates between a first distance (D1; D101) and a second distance (D2; D102) around at least a portion of the circumference of the region, the first distance being greater than the second distance; (c) injecting a molten plastic composition comprising a polymer and a blowing agent into the cavity; (d) allowing the plastic composition to form, in the said at least one region, a first solid skin (4; 104; 240) adjacent to and in contact with the first cavity-forming surface and having a first circumferential length, and a second solid skin (6; 106; 242) adjacent to and in contact with the second cavity-forming surface and having a second circumferential length, wherein the plastic composition between the first and second solid skins remains molten, to form, respectively where the distance between the first cavity-forming surface and the second cavity-forming surface alternates between the first distance and second distance, alternating first and second thicknesses of the plastic composition, each first and second thickness comprising the first and second skins with molten plastic composition therebetween; (e) opening the mould before the plastic composition between the first and second solid skins has solidified, thus allowing the plastic composition between the first and second solid skins, and the molten plastic composition at the alternating first and second thicknesses of the plastic composition, to expand by foaming, wherein said opening comprises removing the outer 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.