Abstract: A device for producing hollow, undercut projections on a packing case of thermoplastic sheet material in a deep drawing mold which comprises at least one projecting stem at whose free end a core having a laterally projecting nose may be pivoted around a transverse pin and is elastically supported against the action of a spring. The stem and core are covered by the heated plastic sheet during the deep drawing process to provide the undercut projection, and during the removal of the finished packing case the core is pivoted around its pin to a position in which its laterally projecting nose releases the undercut part of the projection on the packing. The hollow undercut projections on the packing case thus produced are hook shaped and a hollow shaft projecting from one of the packing halves changes to a hollow projection extending almost vertically from the shaft and in the locked position this projection intermeshed with a similar projection of a hollow shaft on the other packing half.