The problem with which this invention is concerned is the molding of a hollow plastic container such as a large tub for use in a washing machine. In a particular washing machine design, the tub has an open top and a closed bottom, and it tapers toward the open top so that the diameter of the opening is smaller than the diameter of the rest of the tub. Such re-entrant shapes present certain difficulties in the design of molding equipment, because during the molding process it is necessary for the mold core to have a maximum diameter as large as that of the tub bottom, yet this core must subsequently be withdrawn through the smaller diameter at the opening of the tub.
In addition, this particular washing machine tub has a circular transverse cross-section, which is more difficult to mold when the shape is re-entrant.
In U.S. Pat. Nos. 3,865,529 and 3,905,740, which are assigned in common with this application, a technique is shown for withdrawing a mold core from a re-entrant plastic tub. The core is made in separate segments which move relative to each other. After the molding step is completed, this movement of the segments causes the core to collapse radially inwardly so that its effective diameter is reduced, permitting the core then to be withdrawn from the narrower diameter interior of the tub. In these prior art patents, there is a central core segment and a number of peripheral core segments which slide on the outer faces of the central core segment. Core collapse is achieved because the sliding motion of the peripheral core segments has a radially inward component which reduces the core diameter. The outer faces of the central core segment must be flat to permit the sliding motion of the peripheral core segments relative thereto. That shape does not lend itself readily to the molding of a circular cross-section.
In U.S. Pat. No. 3,865,529 the relative motion between the peripheral and central core segments is achieved by extending the peripheral core segments toward the tub as they collapse, thereby moving the tub away from the central segment.
In U.S. Pat. No. 3,905,740 this relative motion is achieved by withdrawing the central core segment from the tub, leaving the collapsed peripheral core segments within the tub interior.