An apparatus for moulding a thermoplastic package part(s) to an end portion of a container body includes inner and outer mould elements between which a mould cavity per package part is formed. At least one of these mould elements is at least partly freely suspended. When one package part is moulded with the mould elements, plastic melt is injected in a first number ≧1 of points in the cavity, an island of plastic melt for each point is formed inside the cavity, and the distribution of the points causes a force centre to pass through a surface defined by a parallel projection of each first number of islands on a plane orthogonal to a direction of the compression force, and when the first number >1, a second number of imaginary straight lines interconnecting a third number of parallel projections such that the surface is at its maximum.