Gears made of plastics are growing larger, more precise, more complex in geometry, and more powerful. High-performance resins and long-fiber compounds are aiding this evolution.
Plastic gears have gone from curiosity to industrial mainstay in the past 50 years. Today they transfer torque and motion in products as diverse as cars, watches, sewing machines, building controls and missiles. Even with all the ground they've gained, their evolution is far from over as new and more demanding gear applications continue to emerge.

The strongest growth area has been the automotive sector. As amenities have become central to competitive success, automakers have sought to power a variety of vehicle subsystems with motors and gears rather than muscle, hydraulics and cables. This has introduced plastic gears into applications ranging from lift gates, seating and tracking headlights to break actuators, electronic throttle bodies and turbo controls.
Appliances also make broad use of plastic power gears. Some larger applications, like clothes-washer transmissions have pushed the limit on gear size, often as a replacement for metal. Plastic gears are present in many other areas, for example, damper drives in HVAC zone controls, valve actuators in fluid devices, automatic flushers in public restrooms, power screws that shape control surfaces on small aircraft, and gyro and steering controls in military applications.

The growth of plastic gears is based majorly on the advances in molding and materials that allow for larger, more precise and more powerful gears. Early plastic gears tended to be spur gears, typically less than 1 inch. across, that delivered no more than 0.25 hp. Now gears are made in many configurations and commonly operate at 2 hp in diameters of 4 to 6 in. Gears are molded with diameters as large as 18 inches. By 2010, power levels should rise to 10 hp or more.

Processors face many challenges in creating gear geometries that maximize power while minimizing transmission error and noise. Such gears call for great precision in molding concentricity, tooth geometry and other properties. Some gears, like helical types, can involve complex mould movements to release the finished product, while others need cored teeth in thicker sections to control shrinkage. Although the latest polymers, equipment and tooling put the next generation of plastic gears within reach of most moulders, the true challenge any processor faces is in adapting its entire operation for such high-precision.

Manufacturers of precision gears also need specialized measuring equipment to verify gear quality, such as double-flank roll checkers for quality control and computer-controlled inspection to evaluate gear teeth and other features. But having the right equipment must be coupled with adaptation of moulding environment to ensure that the gears are as uniform as possible from shot to shot and cavity-to-cavity. A focus on staff and operating procedures could therefore be the deciding factor in producing precision gears.

Moulders need good environmental controls in the moulding area because gear dimensions can be affected as temperature shifts from season to season and even by opening an outside bay door to permit passage of a forklift. Other factors needing attention include having a stable power supply, the right drying equipment to control polymer temperature and moisture level, and a consistent airflow over cooling parts. Some shops use robotics to remove gears from the mold and place them on conveyors the same way time and again to ensure uniform cooling.

The most common plastic gears are spur, cylindrical worm and helical gears, although nearly all gears made in metal have also been made in plastic. Gears are often made in split-cavity moulds. Tooling for helical gears calls for attention to detail because it must allow either the gear or the gear ring forming the teeth to rotate during ejection. Worm gears, which generate less noise than spur gears, are removed after molding either by being un­screwed out of the cavities or by using multiple slides. If slides are used, they must be highly precise to prevent leaving significant parting lines in the gear.

Today's slate of engineering thermoplastics gives processors more options for precision gears than ever before. Acetal, PBT, and nylon, the most common choices, create gear sets having good fatigue and wear resistance, lubricity, rigidity for high tangential forces, and toughness in shock-loaded situations such as in reciprocating motors. These crystalline polymers must be molded hot enough to promote full crystallinity. Otherwise, gear dimensions can shift if end-use temperature rises above the mold temperature and causes additional crystallization.

Acetal has been a primary gear material in automobiles, appliances, office equipment, and other applications for over 40 years. It provides dimensional stability, high fatigue and chemical resistance at temperatures up to 90 C. It has excellent lubricity against metals and plastics.
PBT polyester produces extremely smooth surfaces and has a maximum operating temperature of 150 C for unfilled and 170 C for glass-reinforced grades. It works well against acetal and other plastics, as well as against metal, and is often used in housings.
Nylons offer great toughness and wear well against other plastics and metals, often in worm gears and housings. Nylon gears operate to temperatures to 175 C for glass-reinforced grades and to 150 C for unfilled ones. But nylons are unsuitable for precision gears because their dimensions change as they absorb moisture and lubricants.
Polyphenylene sulfide (PPS) offers high stiffness, dimensional stability, and fatigue and chemical resistance at temperatures as high as 200 C. It is finding broad use in demanding industrial, automotive, and other end uses. Liquid-crystal polymers (LCP) offer great dimensional stability in small, precision gears. It tolerates temperatures to 220 C and has high chemical resistance and low mold shrinkage. It has been molded to tooth thickness of about 0.066 mm, or two-thirds the diameter of a human hair.
Thermoplastic elastomers help gears run quieter and make them more flexible and better able to absorb shock loads. A copolyester TP elastomer, for instance, is being used in lower-power, higher-speed gears because it allows them to tolerate inaccuracies and reduce noise while providing sufficient dimensional stability and stiffness. One such application involves gears in window-blind actuators.

Polyethylene, polypropylene, and ultra-high-molecular-weight PE have been used in gears at lower temperatures in aggressive chemical and high-wear environments. Other polymers have been considered for gears, but many impose severe limitations on gear function. Polycarbonate, for instance, has poor lubricity and resistance to chemicals and fatigue. ABS and LDPE generally cannot meet the fatigue endurance, dimensional stability, and heat and creep-resistance requirements of precision gears. Such polymers are most often found in basic, low-load or low-speed gears.