The appliance industry is engaged in a classic love-hate relationship with plastic. Some engineers lust after the material's light weight, design aesthetics and recycling attributes. At the same time, others lament the volatile cost of plastic, and question its durability and structural integrity.

Thermoplastics such as acrylonitrile-butadiene-styrene (ABS), nylon, polyethylene, polypropylene and polystyrene have been widely used in the appliance industry for years. In fact, most blenders, can openers, coffee makers, vacuums and other small appliances use large amounts of plastic.

Even large appliances feature numerous internal components made from plastic, such as gears, pulleys, pump housings, shelves, trays and tubs. Exterior plastic components are not as common, but include control panels, exhaust vents, handles, knobs and nameplates.

Traditionally, air conditioners, dishwashers, dryers, freezers, furnaces, refrigerators, stoves, washing machines and other large household appliances have been assembled with metal components. But, appliance manufacturers are taking a new look at the benefits of reinforced plastic, especially for large structural components, such as doors, frames and panels.

"Compounders are working with a tremendous variety of additives and are expanding the performance of polymers that have been around for many years," says Paul Friedhoff, a materials chemist at Maytag Co. (Newton, IA). It is an ongoing process, with many appliance applications still being tried. Areas in which glass, metal and other materials are still being used are subject to examination.

The long-term goal is to develop white goods made entirely from lightweight material. In fact, all-plastic refrigerators are currently available in Europe, where manufacturers have been forced to think outside the box on product design. By using plastic instead of metal, the traditional white box has been transformed into a curvaceous piece of furniture that is available in thousands of color choices.

An all-plastic refrigerator is not an entirely new concept. In fact, one debuted almost 40 years ago. In 1967, the Marbon Chemical div. of BorgWarner Inc. (Auburn Hills, MI) unveiled a prototype refrigerator that was formed using ABS plastic. But, the product was too expensive to mass-produce.

As appliance manufacturers strive to offer consumers new products that appeal to quickly changing tastes and new home design trends, thermoplastics have become an attractive alternative to metal. Materials such as polypropylene help engineers improve performance and reduce production costs. Plastic is lightweight, versatile and flexible, allowing new technological innovation and design freedom.

"Plastic allows engineers great freedoms in their designs," says Ben Matuska, North America consumer durables, appliances and electronics market manager for plastics at the Dow Chemical Co. (Midland, MI). For instance, plastic allows greater curvatures and three-dimensional shapes. In addition, the high consistency of plastic produces smoother surfaces and has fewer irregularities than metal or fiberglass.

"New grades of polypropylene deliver lower overall costs than metal materials when all manufacturing and processing costs are taken into account," adds Matuska. "As manufacturers rely increasingly on aesthetics to differentiate their products to discerning consumers, plastic content of appliances is increasing. Plastic enables more design flexibility and delivers lower overall costs than metal, which was traditionally used."

According to the American Plastics Council (APC, Arlington, VA), plastic accounts for almost 25 percent of material content in appliances by weight. However, plastics account for 60 percent of material content by volume. Without plastics, the APC estimates that today's major appliances would cost at least 25 percent more and use 30 percent more energy.

A major shift to plastic took place in the appliance industry more than 15 years ago. But, plastics continue to evolve in terms of better strength, better luster, color options and heat resistance. Analysts at Mastio & Co. (St. Joseph, MO) report that resin consumption for injection-molded appliance parts has been climbing steadily, from 600 million pounds in 1997 to 1 billion pounds in 2006.

Refrigerator parts comprise the largest share of the injection-molded appliance market, accounting for more than one-third of overall resin consumption. Laundry machine components account for 22 percent of demand.

Adhesives such as hot melts, silicones, cyanoacrylates and anaerobics are used to assemble plastic refrigerator cabinets, doors, liners, and shelves. Photo courtesy Henkel Corp.

Numerous Benefits

"The white goods industry is highly competitive and is continually striving to increase productivity and reduce costs," says Werner Posch, development engineer for business unit engineering applications at Borealis A/S (Linz, Austria). "Besides the lightweight benefits of [plastic, the material] allows for part integration, lowering the number of manufacturing steps, and also provides a reduction in finishing requirements. In addition, the need for separate heat and sound insulation is minimized thanks to a decrease in thermal and acoustic transmission."

Many appliance manufacturers prefer polyethylene because it is chemically inert and does not interact with additives and fillers. Due to its molecular structure, it also offers good long-term heat resistance that can be enhanced with additives.

Borealis has developed a line of polypropylene that can replace stainless steel in dishwasher and washing machine interiors. "Parts molded from polypropylene offer significant cost benefits over traditional stainless steel materials," claims Posch. "This new solution lowers production costs and increases production efficiency, while delivering improved, long-term performance."

Benefits include lack of corrosion, high stiffness, outstanding impact resistance and uniform shrinkage. The material has been tailored to provide excellent resistance to the aggressive environment found in these appliances. Interior tubs and dishwasher basements made of polypropylene won't discolor or degrade despite frequent exposure to high temperatures and detergents.

Another advantage of using plastic in appliances is the ability to consolidate parts and reduce weight. "We have seen an increase in the usage of plastic as a metal replacement," says Ron Dugas, applications development engineer at Ticona Polymers Inc. (Florence, KY). "You can do things cheaper in plastic than metal.

"For instance, it's possible to eliminate parts and components," explains Dugas. "One plastic-molded part can replace five or six metal parts, while achieving up to a sevenfold weight savings. We're constantly looking at new opportunities to replace metal with plastic."

Plastic also helps engineers address noise and vibration issues. "There are advantages to running plastic-on-plastic gears or bearings vs. metal-on-metal," says Maytag's Friedhoff, who specializes in laundry products. "Often, we realize a reduction in horsepower to drive the system and a reduction in noise."

Despite its advantages, plastic presents many challenges to appliance engineers, such as flame resistance, dimensional stability, color stability and structural strength. However, the biggest obstacle to wider use of plastic is volatile market conditions that affect the price of raw materials.

Many plastic resin suppliers have raised their prices during the last 6 months, attributing some of the increases to capacity constraints caused by last fall's severe hurricanes along the Gulf Coast. They are also faced with unprecedented operating costs, such as skyrocketing energy prices.

Borealis, a leading supplier of polyethylene and polypropylene to the appliance industry, recently announced that it was increasing its prices by 150 euros per ton. "Over the last 2 years, we have experienced a structural shift in the cost of oil-based feedstock," says Paul Turner, vice president of business unit engineering applications. "The price increase is necessary to keep [our] businesses profitable."

Those decisions are being felt by appliance manufacturers such as Whirpool Corp. (Benton Harbor, MI). The company reported that its third-quarter operating profit was hampered by $110 million in higher material and oil-related costs vs. the same period a year earlier.

Light Applications

Traditionally, most plastic applications in large appliances have focused on internal drive components, such as gears and pulleys. For example, most washing machines use a large pulley to transmit torque from the motor to spin the inner drum.

Several years ago, engineers at Merloni Elettrodomestici UK Ltd. (Peterborough, England) decided to replace the metal pulley in a Hotpoint washing machine. The plastic pulley offers a cost-effective alternative to its cast aluminum counterpart. The injection-molded pulley, made of glass-reinforced nylon resin, costs less than a casting and there are no compromises in mechanical properties.

Handles, knobs and latches are also popular applications for plastic. When engineers at Gorenje (Velenje, Slovakia) recently developed a new washing machine, they focused on producing a cost-competitive product. The key was to reduce the number of components and assembly operations. By molding a plastic door handle as a single part, they simplified the latching system and decreased production time by replacing several metal parts.

According to Maytag's Friedhoff, "it is [traditionally] difficult for plastic to compete with sheet metal on cost, strength and appearance in parts like cabinets, tops and bases." However, engineers in Europe are pushing the envelope, because they are looking for designs that meet the strict new RoHS (Restriction of Hazardous Substances) directive, which takes effect this July. The RoHS bans the use of certain chemicals and heavy metals in products.

Another environmental initiative, the Waste Electrical and Electronic Equipment (WEEE) directive took effect last year. It requires manufacturers to establish and provide for an "end-of-life" recycling program. It specifies 10 equipment categories, including large household appliances. As a result, more attention is now being given to disassembly.

"Recycling concerns and environmental initiatives are the key factors for the increased use of plastics within major appliances in Europe," explains Borealis' Posch.

For example, European manufacturers are experimenting with using plastic to make chassis for large appliances. Engineers at BSH Bosch und Siemens Hausgerate GmbH (Munich, Germany) replaced the metal base of a tumble dryer with plastic. The chassis is the main part of the unit, with the pump, motor and height adjustment all integrated. By replacing metal with polypropylene, material usage was reduced and recycling ability was enhanced. Both parts of the base were injection molded and then welded together.

"There are virtually no limits as to what plastic can do in the field of appliances," claims Dow's Matuska. "As resins continue to evolve, it is entirely realistic to think that appliances will be made solely out of plastic.

"[Traditionally] large plastic parts [such as refrigerator doors] were hampered by stiffness and toughness issues, while also suffering from comparisons to previously less expensive metal or fiberglass solutions," adds Matuska. "However, manufacturing large parts out of plastic is becoming simpler with new advances in resins and molding technology. High melt strength polypropylene resins are especially suited for thermoforming, an ideal process for making large parts. These new polymers exhibit less sag or warpage, allowing for new wall thickness options."

Consumers in Europe are already able to purchase some appliances built completely out of molded-in sheets of plastic. Manufacturers such as Arcelik (Istanbul, Turkey) and Gorenje offer full-plastic freezers and refrigerators that are available in 8,000 different colors, which ironically includes brushed aluminum and stainless steel.

"The color spectrum is a big selling point with consumers," says Dietmar Plobst, general manager of Senoplast Klepsch & Co. GmbH (Piesendorf, Austria), which produces the ABS acrylic sheets used to make the refrigerators. "It offers consumers much more variety over traditional products.

"And, production changeover is not a problem," adds Plobst. "Making 200 refrigerators in one color and 200 in a totally different color is something you simply can't do easily and cost-effectively with metal."

Painting is not necessary on the high-gloss surface, which shaves off manufacturing time and expense. In addition, an all-plastic refrigerator weighs 50 percent lighter than its steel counterpart.

"Plastic sheets are easy to process," claims Plobst. "We use a fully automated edge-bending machine with a flat sheet of plastic to make the doors and outer shells." According to Plobst, the process is similar to thermoforming and is done at 360 F. Average cycle time is around 60 seconds, depending on the color and thickness of the plastic sheets.

BC40: Best of Both Worlds Manufacturers and plastic companies are scrambling to develop hybrid materials that combine the benefits of plastic and metal. Several companies are currently assembling structural parts using plastic-metal composite technology. While most activity is focusing on automotive applications, such as car doors, tailgates and engine hoods, some of the developments are expected to trickle down to the appliance industry.

Bayer Polymers (Pittsburgh) has developed a way to strengthen U-shaped sheet metal profiles by using plastic ribbing. The metal provides strength and stiffness, while the plastic ribbing provides the support necessary to prevent premature buckling without adding significant weight to the structure.

Recent trials carried out by engineers at Lanxess AG (Leverkusen, Germany) have shown that injecting punched steel with polyamide can be a more effective and more economical joining technique than traditional spot welding. "In tensile fatigue tests carried out on specimens of steel and polyamide 6, which is 30 percent glass fiber reinforced, the resultant hybrid assembly points had higher resistance to sustained loads than comparable welded points," says Ulrich Dajek, an engineer in the semicrystalline products business unit.

To produce hybrid assembly points, also known as in-mold assembly points, different-sized holes are first punched with collars in two or more pieces of metal. The metal sheets are then placed in the injection molding tool so that the collars of adjacent pieces fit into each other. The collars and holes are then provided with a frictional rivet head of thermoplastic by injection molding. If a tensile load is applied, the main forces are held by the collars, which are inside each other. The injected rivet heads distribute the load and prevent the pieces of metal from sliding apart.

"One advantage of this construction is that far narrower tolerances can be achieved than with spot welding," says Dajek. "The injection mold serves as a precision assembly gauge." And, since the hybrid component is a single-product solution, it offers recycling benefits.

To extend the application and performance potential of the hybrid technique even further, Dajek and his colleagues are replacing steel with organic sheets. "What we basically then have is a hybrid part made completely of plastic," he points out. "It is lighter than its counterpart of metal and also has higher surface stiffness and much higher strength data. This also gives us an opportunity to integrate add-on parts like reinforcements, fixing points, guides and clips by simply molding them in place."

New nanotechnology applications will eventually allow appliance makers to dramatically improve the structural performance of plastic. By manipulating atoms and molecules, it's possible to create materials that are lighter, stronger and able to withstand a wide range of temperatures.

"Nanotechnology will be used more in the future to address structural integrity issues," says Ticona's Dugas. "Plastics such as polypropylene will benefit from the technology. We're paying close attention to what's going on in the auto industry."