Preforms for blow moulding

A preform for blow moulding to form a container, the preform being formed of a thermoplastic material, the preform comprising a neck finish at an open end of the preform, an elongate body section and a closed end, wherein the body section has elongate, longitudinally extending inner and outer surfaces each centered about a longitudinal axis of the preform, the inner and outer surfaces defining therebetween a wall of the body section, wherein the wall of the body section has two locally thickened portions along the body section, each locally thickened portion being provided at a respective one of two longitudinally spaced locations along the body section, wherein a first locally thickened portion is located at a lowermost third of the length of the body section adjacent to the closed end and a second locally thickened portion is located at an uppermost third of the length of the body section adjacent to the open end, each locally thickened portion being thicker than adjacent first and second thinner portions at respective opposite longitudinal ends of the respective thickened portion, each thickened portion having an inner surface portion which is nearer to the longitudinal axis than respective first and second inner surface portions of the first and second thinner portions, wherein the wall thickness changes between the locally thickened portion and the adjacent first and second thinner portions are smoothly continuously inclined in the longitudinal direction, with first and second inclined inner faces, each at a respective first and second blend angle, relative to the longitudinal axis, for the respective first and second inner surface portions extending away from the locally thickened portion, wherein each locally thickened portion defines a ridge on the inner surface, wherein the ridge is linear and extends orthogonally to the longitudinal axis along an angular segment of less than 90° of the inner surface, and wherein at least one of the first and second thinner portions adjacent to each locally thickened portion comprises a part of a longitudinally extending thickened element which extends between the two locally thickened portions and, along its length, is thicker than laterally adjacent regions of the wall on respective opposite lateral sides of the element, and wherein the element comprises a radially inwardly directed angular segment and at least a part of the longitudinally extending thickened element has a planar chordal inner surface.