How Are Bathtubs Made?
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The History of Bathing
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Bathing has evolved over the centuries from the religious or social experience in Roman times and before, to the hygiene-based bathing of today. Bath tubs began as hand-filled wooden tubs. After World War I, they became more common as permanent installations with running water. Today, enamel, plastic and fiberglass bathtubs are found in almost every residence.
Cast Iron Enamel Tubs
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The cast iron is poured into a mold, heated to 1500 degrees F, then blasted with chilled iron grit. After the cleaning process is complete, acid pickling begins. Hydrochloric acid and sulfuric acid are used to descale the metal and build a strong bond for the enamel. After this, a dip of nickle sulfate and Boric acid to further strengthen the enamel bond is often used. Then the tub is coated and dipped again in a neutralizing solution made from soda ash and borax to remove the acids and prevent rust. After all these rinses, the cast iron tub is dried quickly and enameled.
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Sheet Metal Enamel Tubs
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Metal tubs are made from sheets of steel that are formed into the desired shape rather than poured into a mold. The tub is then heated to extremely high temperatures and shot-blasted with cold metal grit. After this, the tub is cleaned first with an organic solution then an alkaline one designed to clean off the grease deposits and the film left by the cleaner. Next the tub is acid pickled, nickle dipped and neutralized to prepare for the enamel process.
The Enameling Process
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For dry enamel, the tubs are heated to the melting point of the enamel, which is then dusted onto the tub in powder form. It melts on contact, then is fired in a furnace to form a slick finish similar to porcelain.
For wet enamel, tubs are dipped or sprayed with liquid enamel and then fired. The first coat is a ground coat, and after firing the tub is dipped or sprayed and fired again for the cover coat.
Plastic and Fiberglass Tubs
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The methods of production for plastic tubs are more varied and depend on the type of plastic being used. All plastics begin with a petroleum and natural gas base that is then mixed with other chemicals such as dyes, fillers and plasticizers. This substance is mixed and melted into pellets or powder, depending on the future use of the polymer.
Some plastic tubs are then formed by injection molding, where the polymer base is heated and injected into a closed mold.
Fiber reinforced tubs are formed with compression molding. Fibers, polyester resin, pigments, fillers and other additives are mixed together and pressed to form a compressed sheet that is then cut and placed in a mold. Inside the mold, the material is heated activating a binding catalyst. This completes the transformation from pellets and glass fibers into a uniform plastic reinforced tub.
Fiberglass tubs can be built using a spray lay-out process in which chopped glass fibers and liquid plastic are simultaneously sprayed into a mold. The plastic is allowed to cure at room temperature and then removed from the mold. Clearly this process is cheaper and faster than that of enamel coated metal tubs, which probably accounts for its prevalence in modern home construction.
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