HEAT INSULATING PIPE

摘要

A method for manufacturing a heat insulating pipe having a foamed insulating layer around the outer surface of said pipe. The foamed insulating layer has an expansion ratio of at least 4. It has three integral sub-layers, said sub-layers including a cellular middle foamed sub-layer which is integral with a cellular inner foamed sub-layer and a cellular outer foamed sub-layer, said inner and outer sub-layer being on opposite sides of said middle sub-layer, said cellular inner foamed sub-layer being the innermost sub-layer with respect to said pipe and surrounding the outer surface of said pipe. The cells of said inner and outer sub-layers are smaller than the cells of said middle cellular sub-layer, the ratio mu 1/ mu 2 of the cell dimensions of said cellular middle sub-layer are between 0.3 and 0.9, wherein mu 1 is the average cell dimension measured in the radial direction of said pipe and mu 2 is the average cell dimension measured in the axial direction of said pipe. The method for producing said 3 sub-layered foamed insulating layer comprises passing a pipe heated to a temperature of between 60 DEG C. and 120 DEG C. through the nipple of a cross-head die and simultaneously extruding a mixture of a propylene-ethylene block polymer having an ethylene content of between 1 and 20% by weight and a Melt Index of between 0.1 and 4 together with a foaming agent over the outer surface of said pipe to cause said mixture to expand and form said 3 sub-layered foamed insulating layer on said pipe, the relationship between the overall thickness t of the 3 sub-layered foamed insulating layer, the expansion ratio M of the 3 sub-layered foamed insulating layer, and the die-nipple distance d of a cross-head die being wherein K=0.75 to 0.20.