a flexible plastics container for loose products in bulk , e . g . 500 to 2000 liters , and having relatively thick walls , e . g . about 1 mm , is rendered flexible by providing the container with at least one plane of symmetry and forming preliminary fold lines , preferably of a w - shape cross - section , in the immediate vicinity of and along the line of intersection of the plane of symmetry with the walls . the two halves defined by the plane of symmetry can then be folded into each other . in a four - walled container a further folding of the two overlying sets of walls gives a flat article . the container is ideally formed by a blow moulding process in which the mould has an impression which forms the preliminary folds simultaneously with the walls . the mould may be provided with means for increasing and reducing pressure against different walls of the container while within the mould so that the folding operation can be carried out in the mould .