Gear Molding: Where It's Headed
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 arena. 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 brought plastic gears into uses ranging from lift gates, seating, and tracking headlights to brake 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.
Larger, stronger gears
Much of the reason for the growth of plastic gears rests on 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 in. 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 in. 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 mold 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 molders, the true challenge any processor faces is in adapting its entire operation for such high-precision work.
Focus on tight control
Tolerances needed for precision gears generally go well beyond those defined as “fine” by The Society of the Plastics Industry (see graph below). However, today’s molding machines with the latest process controls provide the accuracy to hold mold temperature, injection pressure, and other variables within a tight enough window to allow most processors to mold precision gears. Some gear molders go a step further and place pressure and temperature sensors in mold cavities to improve consistency and repeatability.
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 is just the start. Those who want to join the ranks of precision gear molders must also adapt their molding environment to ensure that the gears they make are as uniform as possible from shot to shot and cavity to cavity. They must focus on their staff and operating procedures, because worker’s actions are often the deciding factor in producing precision gears.
Molders need good environmental controls in the molding 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.
Mold cooling is a key
Tooling for high-precision components demands great attention to detail and an accuracy level approaching that of gauge making, which is well beyond that usually found in standard molding tools. The tool must keep mold temperature and cooling rate the same from cavity to cavity. The most common problem in tooling for high-precision gears deals with the ability to cool symmetrically across a gear and uniformly among multiple cavities.
Tools for precision gears usually contain no more than four cavities. Since a first-generation tool rarely creates a gear to specification, inserts for teeth are often used to reduce re-cutting costs.
Precision gears should be filled from one gate at the center of the gear. Multiple gates create weld lines and can affect tolerances by varying the pressure distribution and shrinkage. With fiber-reinforced materials, multiple gates often cause radial run-out “bumps” as fibers align themselves radially along the weld line.
A molder’s ability to keep warpage at bay and gain controlled, consistent, and uniform shrinkage begins with good part and mold design and extends to the material used and its processing conditions. In molding, this calls for close control of mold surface temperature, injection pressure, and cooling. Other key factors include wall thickness, gate size and location, filler type, level and orientation, flow, and molded-in stress.
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 molds. 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 un screwed 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.
New processes & resins
A variety of advanced molding methods are being explored to improve plastic gear fabrication. Two-shot molding, for instance, can place an elastomer between hub and teeth to make gears quieter or better able to absorb shock so they suffer less tooth damage in hard stops. Hubs can be overmolded with teeth made of a more flexible polymer or a higher-cost, internally lubricated compound. Gas-assist and injection-compression molding are also being investigated as ways to improve tooth quality and overall accuracy and keep residual stress low.
In addition to the gears themselves, molders also need to focus on the gear housing. Housing dimensional stability and precision are extremely important because the locations of gear axes in housings must line up to hold gears in alignment, even as loading and temperature changes. For this reason, housings are made relatively stiff by using fiber-reinforced and mineral-filled polymers.
SPI’s classification for “fine” molding tolerances fall short of those needed for common plastic gears that meet the American Gear Manufacturers Association’s Q7 quality level and have a 12 or 24 diametral pitch (DP)—i.e., 12 or 24 teeth/in. of gear diameter. AGMA Q10 level gears, which are in the realm of possibility for many gear molders, require even tighter concentricity.
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 autos, appliances, office equipment, and other applications for over 40 years. It provides dimensional stability and 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 thicknesses 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 en-durance, 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.
Reinforcements play role
Material specification for gears and housings should take into account the dramatic effects fibers and fillers have on resin performance. For instance, when acetal copolymer is loaded with 25% short-glass fiber (2 mm or less), its tensile strength more than doubles at elevated temperatures and its stiffness more than triples. The use of long-glass fiber (10 mm or more) boosts strength, creep resistance, dimensional stability, toughness, rigidity, wear, and other properties even further. This makes long-fiber reinforcement attractive for use in large gears and housings to gain needed stiffness and better control of thermal expansion.
Dr. Zan Smith is a senior staff engineer and Ticona’s technology leader for gears. He has been involved in development of plastic gears and gear materials for more than 25 years. He can be reached at (859) 372-3162. David Sheridan is a senior design engineer and has been involved with design of plastic gears for the past 10 years. He is an active member of the American Gear Manufacturers Association’s plastic gear standards committee. He can be reached at (248) 340-7485.
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