Methods of forming an article from a molten plastic composition comprising a polymer and a blowing agent. The methods include injecting the molten plastic composition into a mould, allowing the plastic composition to form a first solid skin adjacent to an in contact with a first cavity-forming surface of the mould and a second solid skin adjacent to and in contact with a second cavity-forming surface of the mould, and then opening the mould before the molten plastic composition between the first and second solid skins in at least one portion of the circumference of a region of the mould cavity defining an annular cross-section of the cavity has solidified.