Plastic gears are moving toward larger sizes, more complex geometries, and higher strength, with high-performance resins and long-glass-filled composites playing an important role.
Plastic gears have undergone a transformation from novel materials to important industrial materials over past 50 years. Today they have penetrated into many different application fields, such as automobiles, watches, sewing machines, structural control facilities and missiles, etc., which play role of transmitting torque and forms of motion. In addition to existing application areas, new, more difficult-to-machine gear applications will continue to emerge, and this trend is still developing.

 

Automotive industry has become one of the fastest growing areas for plastic gears, and this successful change is encouraging. Automakers are struggling to find all kinds of auxiliary systems that drive cars, they need motors and gears rather than power, hydraulics or cables. This change has allowed plastic gears to penetrate into many applications, from liftgates, seats, and track headlights to brake actuators, power throttle segments, turbo trims, and more.
Application of plastic power gears is further expanded. In some application fields that require large size, plastic gears are often used to replace metal gears, such as washing machine transmissions using plastic, which changes application limit of gears in size.
Plastic gears are also used in many other applications, such as vibration damping drives in ventilation and air conditioning systems (HVAC), valve drives in mobile facilities, automatic sweepers in public lounges, and power screws to control surface stabilization on small aircraft Instruments, screw instruments and control devices in military field.

This is an important reason for development of plastic gears due to advantages of plastic gear molding and ability to mold larger, high-precision and high-strength features.
How to design a gear configuration that maximizes transmission power while minimizing transmission errors and noise remains a challenge. This places high demands on machining accuracy of concentricity, tooth profile and other characteristics of gears.
Some helical gears may require complex forming actions to make final product, others require use of core teeth in thicker sections to reduce shrinkage. While many forming specialists have used the latest polymeric materials, equipment and processing techniques to achieve ability to produce a new generation of plastic gears, a real challenge for all processors will be how to match this overall high precision product.

Tolerances allowed by high-precision gears are generally difficult to describe as "good" as stated by Society of Plastics Industry (SPI). But today most molding specialists use the latest molding machines with process control units that control accuracy of molding temperature, injection pressure, and other variables over a complex window to mold precision gears. Some gear forming specialists use a more advanced approach, placing temperature and pressure sensors in cavity to improve forming consistency and repeatability.
Manufacturers of precision gears also need to use specialized inspection equipment, such as double flank roll detectors to control gear quality, computer-controlled detectors to evaluate gear flanks and other characteristics. But having right equipment is just the beginning.
Molders trying to enter precision gear industry must also adjust their molding environment to ensure that gears they produce are as consistent as possible from shot to shot and from cavity to cavity. Since behavior of mechanic is often decisive factor in production of precision gears, they must focus on training of employees and control of operating process.
Because gear dimensions are susceptible to seasonal temperature changes, even temperature fluctuations caused by opening a door and letting a forklift pass through can affect gear dimensional accuracy, molders need to strictly control environmental conditions in molding area.
Other factors to consider include: a stable power supply, suitable drying equipment to control polymer temperature and humidity, cooling units with constant airflow. In some cases, automated techniques are used to remove gear from forming position and place it on transfer unit in a repeated action to achieve a consistent cooling method.

Compared with requirements of general forming and processing, processing of high-precision parts requires more attention to details and measurement technology required to achieve accurate measurement levels. This tool must ensure that cavity molding temperature and cooling rate are same for each molding. The most common problem in precision gear machining is how to deal with symmetrical cooling of gear and consistency between cavities.
Molds for precision gears generally do not exceed 4 cavities. Since the first generation of molds produced only one gear, there was little specification, and tooth inserts were often used to reduce cost of secondary cutting.
Precision gears should be injected from a gate at the center of gear. Multiple gates are prone to form fusion lines, change pressure distribution and shrinkage, and affect gear tolerances. For glass fiber reinforced materials, due to radial arrangement of fibers along welding line, it is easy to cause an eccentric "collision" of radii when using multiple gates.
A forming expert can control deformation of tooth space and obtain a product with controlled, consistent and uniform shrinkage capacity, which is premised on good equipment, forming design, stretchability of material used, and processing conditions. During molding, precise control of temperature of molding surface, injection pressure and cooling process is required.
Other important factors include wall thickness, gate size and location, filler type, amount and direction, flow rate and molding internal stress.
The most common plastic gears are spur, cylindrical worm and helical gears, and almost all gears made of metal can be made of plastic. Gears are often formed by split cavities. When machining helical gears, attention must be paid to details because gear or gear ring forming teeth must be rotated during injection.
Noise generated by worm gear during operation is smaller than that of spur gear, and after forming, it can be removed by unscrewing cavity or using multiple sliding mechanisms. If a sliding mechanism is used, it must be operated with high precision to avoid visible parting lines on gears.

 

More advanced plastic gear forming methods are being developed. For example, secondary injection molding method, by designing an elastic body between axle and gear teeth, makes gear run more quietly. When gear suddenly stops running, it can better absorb vibration and avoid damage of gear teeth.
Axles can be re-moulded with other materials, and a more flexible or more valuable, self-lubricating composite can be selected. At the same time, gas-assisted method and injection compression molding method are studied as a method to improve quality of gear teeth, the overall precision of gear and reduce internal stress.
In addition to gear itself, molder also needs to pay attention to design of gear. Position of gear shafts in structure must be in a linear arrangement to ensure that gears run in a straight line, even under load and temperature changes, so dimensional stability and accuracy of structure are very important. Considering this factor, a gear structure with a certain rigidity should be made using materials such as glass fiber reinforced materials or mineral-filled polymers.
Now, in the field of precision gear manufacturing, emergence of a range of engineering thermoplastics provides processors with more options than ever before. The most commonly used materials, such as acetal, PBT and polyamide, can produce excellent fatigue resistance, wear resistance, smoothness, resistance to high tangential stress strength, and can withstand vibration loads such as reciprocating motor operation.
For crystalline polymers, it must be molded at a high enough temperature to ensure sufficient crystallization of material. Otherwise, due to temperature rising above molding temperature, secondary crystallization of material will cause the gear size to change.
As an important gear manufacturing material, acetal has been widely used in automobiles, appliances, office equipment and other fields with a history of more than 40 years. Its dimensional stability properties and high fatigue and chemical resistance can withstand temperatures up to 90℃ or more. It has excellent lubricating properties compared to metals and other plastic materials.
PBT polyester can produce a very smooth surface, and its maximum working temperature can reach 150℃ without filling and modification, and working temperature of glass fiber reinforced products can reach 170℃. It works well compared to products from acetal, other types of plastics and metals and is often used in construction of gears.
Polyamide materials, compared with other plastic materials and metal materials, have good toughness and durability, are often used in applications such as turbine transmission design and gear frames. Operating temperature of unfilled polyamide gears can reach 150℃, and operating temperature of glass fiber reinforced products can reach 175℃. But polyamides have characteristics of dimensional changes caused by moisture absorption or lubricants, making them unsuitable for precision gear applications.
High hardness, dimensional stability, fatigue resistance and chemical resistance of polyphenylene sulfide (PPS) can reach temperatures up to 200℃. Its application is deepening into applications with demanding operating conditions, automotive industry and other end uses.
Precision gears made of liquid crystal polymer (LCP) have good dimensional stability. It can tolerate temperatures up to 220℃ with high chemical resistance and low molding shrinkage variation. Using this material, a shaped gear with a tooth thickness of about 0.066 mm has been made, which is equivalent to 2/3 of the diameter of a human hair.
Thermoplastic elastomers make gears quieter, resulting in more flexible gears that absorb shock loads well. For example, a low-power, high-speed gear made of copolyester thermoplastic elastomer can allow some deviations during operation while ensuring sufficient dimensional stability and hardness, while reducing operating noise. An example of such an application is gears used in curtain drives.
Materials such as polyethylene, polypropylene and ultra-high molecular weight polyethylene have also been used in gear production in relatively low-temperature, aggressive chemical environments or in high-wear environments. Other polymeric materials are also considered, but are subject to many severe constraints in gear applications;
Polycarbonate has poor lubricity, chemical resistance and fatigue resistance; ABS and LDPE materials usually cannot meet performance requirements of precision gears such as lubricity, fatigue resistance, dimensional stability, heat resistance, and creep resistance. Such polymers are mostly used in conventional, low-load or low-speed gear applications.

 

Compared to plastic gears of same size, metal gears perform well and provide better dimensional stability with changes in temperature and humidity. But compared to metal materials, plastics offer many advantages in cost, design, processing and performance.
Inherent design freedom of plastic molding ensures more efficient gear manufacturing compared to metal molding. Internal gears, gear sets, worm gears, etc. can be molded from plastic, which is difficult to mold from metal at a reasonable price. Plastic gears have a wider application area than metal gears, so they push gears toward higher loads and more power.
Plastic gears are also an important material for low-quiet operation, which requires high precision, new tooth profiles and materials with excellent lubricity or flexibility.
Gears made of plastic generally do not require secondary processing, so compared to stamped parts and metal gears of machined parts, cost is guaranteed to be reduced by 50% to 90%. Plastic gears are lighter and more inert than metal gears, can be used in environments where metal gears are prone to corrosion and degradation, such as control of water meters and chemical equipment.
Compared with metal gears, plastic gears can deflect and deform to absorb impact loads, can better disperse local load changes caused by shaft deflection and misaligned teeth. Inherent lubricating characteristics of many plastics make them ideal gear materials for printers, toys, and other low-load operating mechanisms, not including lubricants. In addition to operating in dry environments, gears can also be lubricated with grease or oil.

In description of gear and structural materials, important role of fibers and fillers on resin material properties should be considered. For example, when acetal copolymer is filled with 25% short glass fiber (2mm or less) filler, its tensile strength increases by 2 times and hardness increases by 3 times at high temperature.
Using long glass fiber (10 mm or less) fillers can improve strength, creep resistance, dimensional stability, toughness, hardness, wear properties, and more. Long glass fiber reinforcements are becoming an attractive candidate for large gear and structural applications because of required hardness and good controlled thermal expansion.