The internal tub of a dishwasher is made of one of two basic materials – stainless steel or plastic. Tubs were once made of sheet metal covered with enamel paint. Problems with rust and corrosion caused by paint chipping and cracking led to the replacement of sheet metal with plastic. Plastic tubs are strong and durable, usually outlasting the dishwasher itself; and they don't rust. Stainless steel tubs, introduced later, offered additional benefits over plastic tubs.
Durability and Stain Resistance
Plastic tubs offer resistance to cracking and rusting, and hold up well over the lifetime of the machine. The newer stainless steel dishwasher tubs, however, provide a heavier and thicker wall than plastic tubs, and greater resistance to staining and discoloration. Light-colored plastic can become discolored. A gray-speckled plastic, developed later, provides improved resistance to staining.
Tub Material and Noise Levels
Plastic tubs tend to be rather noisy, transferring the rush of water and vibration of the mechanism through the thin plastic tub walls. Thicker, heavier stainless steel tub walls transmit less sound from inside the machine. The heavier walls also damp vibrations and tend to be quieter when the cleaning cycle is running.
Water Temperature and Sanitization
Thick stainless steel walls withstand higher temperatures than plastic. As a result water inside a stainless steel dishwasher can reach higher temperatures without damaging or weakening the tub material. Hotter water enhances the cleaning power of the dishwasher detergent. Stainless steel units can heat water to more than 160 degrees compared with the 140 degree maximum of a plastic unit. Water that reaches nearly 165 degrees not only cleans but sterilizes the dishes; something a plastic-tub dishwasher cannot do as well.
Stainless steel tubs hold in heat better than plastic tubs do. The metal walls act as sources of heat during the drying cycle. This means the unit uses less power during the drying cycle, saving electricity over time. Heating elements in plastic-tub dishwashers use 600 to 700 watts of power to heat the water. Consumer Reports tests show that stainless steel dishwashers average 450 watts of energy during an average cycle, saving 150 to 250 watts of energy for every use.
Stainless steel dishwashers cost more than equivalent plastic tub models, up front. When you study the energy ratings, however, you will find that the energy costs for models with stainless steel tubs give you significant savings. Because stainless steel dishwashers are more durable, combining the energy savings with the longer unit life may add up to a lower overall cost per load.
Cleaning and Maintenance
Shiny stainless steel tubs can quickly show stains, especially if you live in an area that has hard water. Calcium deposits show up most quickly on stainless steel walls. You have to decide for yourself whether this is a disadvantage or not. The tendency of stainless steel to show stains, means you are aware of them quickly and can clean them off. While plastic may hide deposits, it also doesn't warn you that you might need to clean the inside of the dishwasher.