A roller having a multiplicity of open moulding cavities on its surface, the roller being hollow with its cavity being partially filled with liquid nitrogen, characterised in that: A vacuum insulation compartment is located on each side of the roller； The vacuum insulation compartment, in radial cross section, presenting an outside generally plane and circular wall in the form of an annular disc, joint to a roller side wall by two portions having: a general sigmoid cross section for the portion at the peripheral part of the vacuum panel； a general sigmoid cross section for the portion at the central part of the vacuum panel.
