1. Field of the Invention
The present invention relates to a process for production of foamed articles in a mold (hereinafter referred to as "foamed articles") of polypropylene resins which are employed as thermal insulators, shock absorbing packaging materials, reusable containers, core materials for bumpers, etc.
2. Description of the Prior Art
Foamed articles of polypropylene are excellent in resistance to chemicals, resistance to heat, strain recovery after compression, etc. as compared to foamed articles of polystyrene. Further the foamed articles are also excellent in resistance to heat, compressive strength, etc. even when compared with foamed articles of polyethylene so that they have been widely used as shock absorbing packaging materials, reusable containers, core materials for bumpers, etc. To produce such foamed articles of polypropylene, there are known the following methods.
(A) Method which comprises treating pre-expanded beads of polyolefin with an inorganic gas under pressure to impregnate the heads with the inorganic gas, gradually releasing the pressure, filling the beads in a mold capable of closing but incapable of sealing while the inner pressure of the beads is kept at 1.18 atms. or more and thermally fusing with steam, etc. to prepare a molded article in the shape of the mold (U.S. Pat. No. 3,953,558).
(B) Method which comprises filling pre-expanded beads of polyolefin in a mold capable of closing but incapable of sealing, thermally fusing with steam or the like, withdrawing the molded article from the mold and, thermally curing while the volume of the molded article is 70 to 110% of the volume of the mold to prepare the molded article in the shape of the mold (U.S. Pat. No. 4,631,159).
(C) Method which comprises compressing pre-expanded beads of cross-linked polyolefin to 80% or less of an apparent bulk volume of the raw beads by gaseous pressure, filling the compressed beads in a molding mold and thermally fusing to give a molded article in the shape of the mold (Published Examined Japanese Patent Application No. 33996/1978).
However, Method (A) described above involves a defect that initial investment becomes large because installation for the compressive treatment with inorganic gas is of a large scale and Method (B) has also a defect that products having a complicated shape are inferior in visual appearance such as sink mark, surface property, etc. Further, Method (C) is not sufficiently satisfactory in surface property and dimensional accuracy.