There are technical documents filed by the present applicant, which disclose techniques for blow-molding a cylinder-shaped molten resin into a skin-covered panel (see, for example, Patent Document No. 1: Japanese Laid-Open Patent Publication No. 10-235720).
In Patent Document No. 1, by means of the blowing pressure during blow molding, a skin material is heat-welded to an outer surface of one wall portion, and the other wall portion is formed with inner ribs protruding to contact an inner surface of the one wall portion, thereby obtaining the skin-covered panel.
However, where the cylinder-shaped molten resin is used as in Patent Document No. 1, the molten resin is stretched in areas where the inner ribs are formed, and it is therefore necessary to increase the wall thickness of the cylinder-shaped molten resin in order to prevent formation of pinholes. This results in an increased weight of a final-molded-product skin-covered panel. Therefore, where the cylinder-shaped molten resin is used as in Patent Document No. 1, it may be difficult to reduce the weight and the thickness of the final-molded-product resin panel.
Normally, the thickness (meaning “wall thickness”) of the cylinder-shaped molten resin (meaning “cylindrical parison”) is uniform. When split molds are clamped together, the pressing force for pressing the cylindrical parison against the split molds will be uniform across the entire surface of the cylindrical parison. Therefore, the parison pressed against one split mold for forming depressed portions such as inner ribs is stretched because of the blow ratio in accordance with the depressed portions, thereby resulting in locally thinned portions. Since no depressed portions are formed on the other split mold, no thinned portions will be produced. As a result, the thickness of the cylindrical parison needs to be set taking into account the thinned portions to be produced on the one split mold side on which the depressed portions are to be formed, thereby resulting in an unnecessary thickness on the other split mold side. Therefore, where a cylindrical parison having a necessarily uniform wall thickness is used, the wall thickness of the wall portion where no depressed portions are formed is greater than the wall thickness of the wall portion where depressed portions are formed after blow molding, and it is therefore difficult to reduce the weight and the thickness of the final-molded-product resin panel.
Note that Patent Document No. 2 (Japanese Laid-Open Patent Publication No. 2010-155583) discloses a technique for obtaining a luggage board having a good mechanical strength property and rigidity even when the wall thickness of a resin panel of the luggage board is reduced for weight reduction.
In Patent Document No. 2, a rear wall is formed with a plurality of inner ribs integrally welded to a rear surface of a front wall, and a plurality of circular indentation-shaped round ribs integrally welded to the rear surface of the front wall are formed between the inner ribs of the rear wall. This makes it possible to obtain a required mechanical strength property and rigidity even when the average wall thickness of a resin wall of the luggage board is reduced to 1.5-2.5 mm for weight reduction. The average wall thickness of the resin wall of the luggage board is the average value between the wall thickness of the front wall and the wall thickness of the rear wall of the luggage board.