Patent ID: 11904579
Assignee: JSP CORPORATION
Field: Other special machines (Mechanical engineering)
Classification: CPC B  C | IPC B

Claim 8:
9. A method for producing a polypropylene-based resin expanded bead, comprising:
a granulating step of forming a polypropylene-based resin bead comprising:
a core layer, in a non-foamed state, comprising a polypropylene-based resin having a flexural modulus Mc of 800 MPa to 1200 MPa; and
a covering layer, which covers the core layer, comprising a polyethylene-based resin having a flexural modulus Ms of 250 MPa or less, wherein a mass ratio between the core layer and the covering layer is 95:5 to 88:12;

a first expanding step of impregnating the polypropylene-based resin bead dispersed in a dispersion medium in a sealed container with an inorganic physical blowing agent, and then releasing the polypropylene-based resin bead and the dispersion medium from the sealed container to a pressure lower than that in the sealed container, thereby obtaining a first-step expanded bead comprising:
a core layer, in a foamed state, comprising the polypropylene-based resin; and
a covering layer, which covers the core layer, comprising the polyethylene-based resin; and

a second expanding step of impregnating the first-step expanded bead with an inorganic gas in a pressure-resistant container to increase a pressure in cells of the first-step expanded bead, and then heating the first-step expanded bead taken out from the pressure-resistant container under a pressure lower than that in the cells to further expand the first-step expanded bead, thereby obtaining a polypropylene-based resin expanded bead having a bulk ratio (II) of 55 times to 90 times.