1. Field of the Invention
The present invention relates to a molding apparatus for use in plastic injection molding and, more particularly, to a molding apparatus which produces molded plastic articles having internal undercuts.
2. Description of the Related Art
Many articles may be economically constructed of injection molded plastic. A problem exists, however, when the article has undercuts which would prevent the article from being removed from the mold. One such article is shown in FIGS. 1 and 2. A molded plastic tank 10 for containing liquid is made of injection molded plastic top and bottom halves 12, 14 joined by flanges 16, 18 around the center of the tank. As best shown in FIG. 2, the tank halves are formed with internal walls, such as wall 20, which are provided to strengthen and rigidify the tank and to lessen liquid sloshing. In order to allow the liquid to flow through the internal walls, the walls are formed with openings such as opening 22. These openings are undercut, meaning that the axis through an opening extends transversely to the direction, represented by arrow 24, in which the article would be removed from the male die of an injection mold.
When the tank half 14 is formed in an injection mold, the undercut openings are formed by protrusions of the mold extending into the voids of the male mold which form the internal walls. Once the injected plastic has hardened, the tank half is locked into the male mold by the interengagement between the protrusions and the openings. The tank half cannot be removed from the mold unless the protrusions are disengaged from the openings.
In the past, molds have been provided with movable pins or rods which extend through the mold obliquely or perpendicularly to the direction in which the article is removed from the mold. The ends of the pins or rods protrude into the mold voids to form the undercuts. The pins or rods are withdrawn obliquely or transversely to release the molded article. This approach is not suitable, however, when a large number of undercuts are required. FIG. 2 shows three undercuts adjacent to a single wall intersection: It may be desired to mold a much larger tank with a great many wall intersections and undercut openings. In such a situation, it would be extremely expensive and difficult, if not impossible, to construct a mold having enough oblique or transverse pins to form all the required undercuts.