1. Field of the Invention
The field of the invention relates to processes for manufacturing molded articles such as bathtubs and the like.
2. Brief Description of the Prior Art
In the development of bathtubs, porcelain-cast iron fixtures have gradually been replaced by lighter and more resilient component structures. One of the difficulties with the porcelain-cast iron and enamelled fixtures has been their susceptibility to impact damage and their extreme weight which makes movement and installation of large fixtures such as bathtubs and whirlpool tubs difficult. The porcelain-cast iron fixtures did have the advantage of having a very solid feel and high weight bearing capability.
Initial attempts by the industry to replace these porcelain-cast iron fixtures proved difficult. Thin stainless steel fixtures were lighter than the porcelain-cast iron fixtures, but did not have the solid feel or structural strength required for large articles such as bathtubs and whirlpool tubs. The early composite structures have a plastic, hollow feel and would tend to deform, crack, chip or delaminate when subjected to impact, thermal shock or the weight of the typical bather. Additionally, their production required the use of a large amount of polymeric material which increased the weight of a tub and increased the cost of manufacture. Furthermore, certain polymeric materials or resins shrink upon curing. This resulted in bowing, warpage and distortion of the tubs.
Reinforced grids have been used to add strength and rigidity to the surfaces of bathtubs. For example, U.S. Pat. No. 2,820,228 describes the reinforcement of a bathtub by a gridwork secured to the bottom of the tub. The gridwork may also extend to the sides of the tub in order to deaden vibrations in the sides of the tub. Although the gridwork adds strength, it also adds weight and requires additional steps in the manufacturing process.
Various methods have been described for creating a cavity in a molded article. For example, U.S. Pat. Nos. 3,610,563, 3,674,394 and 3,368,239 describe methods for forming articles, such as tanks and molded hollow articles. During the molding operation, a balloon or tube expands resulting in the production of a hollow article. None of these patents, however, relate to the formation of composite baths or whirlpools, but rather describe the formation of a cavity for the purpose of creating a hollow object, such as a tank.
German Patent DT 2951091 teaches the formation of a plastic bathtub which has parallel tubes extending the length of a flange, which tubes may be filled with foam or other materials. The purpose of the tubes is to add rigidity to the tub and reinforce the structure, taking the place of heavy steel rods which were formerly used to reinforce the tub.
The tubes are only useful in areas having a large cross-section, and may be difficult to insert during the molding process.
Commonly assigned U.S. Pat. No. 4,664,982 to Genovese et al. and patent application Ser. No. 07/400,280 to Kuszaj et al., both of which are incorporated by reference herein, describe polymeric foam backed enameled-carbon steel or stainless steel plumbing fixtures that are resistant to chipping, cracking, delamination or deformation when subjected to impact from either the finish or non-finish side. The use of the aforementioned composite molded structures results in fixtures which have the feel of porcelain-cast iron and enamel fixtures, high impact strength, and resistance to delamination. These composite structures possess excellent physical and mechanical properties as a result of the chemical bonding of the reinforced foam polymeric layer to the enameled steel or stainless steel shell. A problem with such fixtures, however, is that a large amount of polymeric material is used in certain portions of the tubs, resulting in increased weight and increased costs. In addition, in the portions having increased cross-sectional areas of polymeric material, the fixtures may suffer problems due to distortion, bow and warpage from the shrinkage of the polymeric material as it cures. They also may be subject to distortion, bow and warpage in these thicker portions due to the inability of the polymeric material to release heat and gas produced during the curing process
Another approach was to replace the enamelled-steel shell with a polymeric-cosmetic surface layer and binding that layer directly to a foamed plastic substrate to provide a high impact strength, delamination-resistant structure. This approach is disclosed in commonly assigned U.S. Pat. Nos. 4,844,944 and 4,844,955, both to Graefe et al., the disclosures of which are incorporated by reference herein. These approaches suffer from the same problems described above with regard to U.S. Pat. No. 4,664,982 and application Ser. No. 07/400,280.
A still further approach is described in commonly assigned application Ser. No. 07/499,188 filed Mar. 26, 1990. This application discloses a method wherein a bathtub shell is loaded into a mold receptor such that a void is provided between the shell and mold receptor. A male mold member exerts pressure upon the shell as a hardenable material is introduced under pressure into the void. Once this material has hardened, the now-coated bathtub shell is removed from the mold receptor.
Since there is considerable pressure on the bottom of the bathtub shell as the hardenable material enters the void, it may tend to deflect even though the male mold member bears against the top surface of the shell. This is due to the fact that all shells ar not precisely alike, and voids are created between the male mold member and the shell where deformation may occur.