Abstract: A sachet or food container is formed of two plastics sheets joined to one another around the edges of an internal pocket of the sachet or container by welded seals. The compositions of the two plastics sheets differ one from the other and are selected such that the strength of the seals is controllable by varying at least one welding parameter selected from temperature, pressure and dwell time. The two sheets are welded to one another using a first set of welding parameters to produce strong seals in some of the seam regions bounding the internal pocket, while other seam regions bounding the pocket are welded using a second set of welding parameters to produce weak seals at which the sachet will burst open preferentially when the pocket is pressurised.