PURPOSE: To provide a manufacturing method of multi-ply polyester prefoam providing a multi-ply polyester bottle which is superior in heat resistance, by performing co-injection molding of a resin composite of polyethylene terephthalate and polyarylate which is used as a core layer of the multi-ply stretched polyester bottle while the melt viscosity of the resin composite is being limited to a fixed range.
CONSTITUTION: A skin layer and core layer are cast within a cavity through co-injection molding at the same time or in order within the range satisfying an expression (1) by melt viscosity of a resin composite of the core layer. 0.5ηs≤ηc≤3ηs (1), wherein the ηs and ηc show respectively melt viscosity of polyethylene terephthalate composing of the skin layer and that of the resin composite of the polyethylene terephthalate and polyarylate composing of the core layer. To adjust the melt viscosity within such a range it is desirable that intrinsic viscosity is within a range of 0.45W0.60 and a weight ratio between the polyethylene terephthalate and polyarylate is within that of 80:20W40:60. It is desirable that the intrinsic viscosity of the polyethylene terephthalate forming the skin layer is more than 0.70 and less than 1.20.