A device for forming a rolled rim at the unrimmed mouth of the body of a container having a body which is fabricated from a sheet of an expanded thermoplastic material, the device including a first member with a first annular rim-forming surface, and a second member which is reciprocable relative to the first member. The second member has a second annular rim-forming surface which forms a generally closed rim-forming chamber with the first annular rim-forming surface when the second member is in a first position relative to the first member, the second member being reciprocable to a second position in which the first annular surface and the second annular surface are separated to permit the withdrawal of a container with a rolled rim at the mouth from the first member. The device further includes a transfer mechanism for inserting a container with an unrimmed mouth over a frustoconical portion of the first member to position such container to have a rim rolled in the mouth thereof in such rim-forming chamber and to withdraw a container with a rolled rim at the mouth from the first member. The rim-forming chamber is heated and there is no relative rotation between the rim-forming chamber and the container during the forming of a rolled rim in the container.