Hitherto, a sandwich molded product including a surface layer, and an inner layer included in the surface layer has been known. As an injection molding method for molding the sandwich molded product (co-injection method), mainly two methods of a multi-stage molding method and a simultaneous molding method have been known. The multi-stage molding method is a method that injects and fills a surface layer molten resin into a mold cavity, and then injects and fills an inner layer molten resin into the surface layer molten resin to fill the interior of the mold cavity with the two molten resins (see Patent Literature 1). The simultaneous molding method is a method that injects and fills the surface layer molten resin into the mold cavity, and then injects and fills the surface layer molten resin and the inner layer molten resin into the surface layer molten resin injected earlier in a laminar flow state in which the surface layer molten resin is disposed on an outer circumferential side and the inner layer molten resin is disposed in the center thereof, thereby filling the interior of the mold cavity with the two molten resins (see Patent Literature 2).
Further, hitherto, a hollow molded product having a hollow portion therein has been known. As an injection molding method for molding the hollow molded product, for example, a hollow injection molding method (gas assist injection molding method) has been known. The hollow injection molding method is a method that injects and fills the molten resin into the mold cavity, then pours the pressurized gas into the molten resin to form a hollow portion, and discharges the poured pressurized gas (see Patent Literature 3). In the hollow injection molding method, generally, molding is performed at a constant volume of the mold cavity, but a method for expanding the volume of the mold cavity in conjunction with pouring of the pressurized gas has also been known (see Patent Literature 4). The method for expanding the volume of the mold cavity in conjunction with pouring of the pressurized gas is referred to as a high hollow molding method separately from the general hollow injection molding method, and is a method which is capable of increasing the volume of the hollow portion of the interior of the hollow molded product.