Plastic gears are moving towards larger sizes, more complex geometries, and higher strength. At the same time, high-performance resins and composite materials filled with long glass fibers play an important role in promoting.
Plastic gears have undergone a process of change from new materials to important industrial materials in the past 50 years. Today they have penetrated into many different application areas, such as automobiles, watches, sewing machines, structural control facilities and missiles, etc., play a role in transmitting torque and motion forms. In addition to existing application areas, new and more difficult-to-machine gear application areas will continue to emerge, this trend is still in deep development.

 

Automotive industry has become one of the fastest growing areas of plastic gears, and this successful change is encouraging. Automobile manufacturers are struggling to find various auxiliary systems for car driving. What they need is motors and gears instead of power, hydraulics or cables. This change makes plastic gears in-depth application in many applications, from lift doors, seats, tracking headlights to brake actuators, electric throttle segments, turbine adjustment devices and so on.
Application of plastic power gears is further broadened. In some applications where large size requirements are required, plastic gears are often used to replace metal gears, such as washing machine transmissions that use plastics, which changes application limit of gears in terms of size.
Plastic gears are also used in many other fields, such as damping drives for ventilation and air conditioning systems (HVAC), valve drives in mobile facilities, automatic sweepers in public restrooms, power spirals used in small aircraft to control surface stability, screw meters and control devices in military field.

Due to advantages of plastic gear molding, characteristics of larger, high-precision and high-strength molding, this is an important reason for development of plastic gears.
How to design a gear configuration to maximize transmission power while minimizing transmission errors and noise is also faced with many difficulties. This puts forward high machining accuracy requirements for concentricity, tooth profile and other characteristics of gear.
Some helical gears may require complex forming actions to manufacture final product, while other gears require core teeth in thicker parts to reduce shrinkage. Although many molding experts have used the latest polymer materials, equipment and processing technology to achieve ability to produce a new generation of plastic gears, for all processors, a real challenge will be how to cooperate with manufacture of this entire high precision product.

Allowable tolerances of high-precision gears are generally difficult to describe as "good" as explained by American Plastics Industry Association (SPI). But today, most molding experts use the latest molding machines equipped with processing control units to control accuracy of molding temperature, injection pressure and other variables to mold precision gears on a complex window. Some gear molding experts use more advanced methods. They place temperature and pressure sensors in cavity to improve consistency and repeatability of molding.
Manufacturers of precision gears also need to use professional testing equipment, such as double-tooth flank rolling detectors used to control quality of gears, computer-controlled detectors for evaluating gear tooth flank and other characteristics. But having right equipment is only beginning.
Those who try to enter precision gear industry must also adjust their molding environment to ensure that gears they produce are as consistent as possible in every injection and every cavity. Since behavior of mechanics is often decisive factor when producing precision gears, they must focus on training of employees and control of operation process.
Since size of gear is easily affected by seasonal temperature changes, even temperature fluctuations caused by opening door and letting a forklift pass can affect dimensional accuracy of gear, so mold manufacturers need to strictly control environmental conditions in molding area.
Other factors to be considered include: a stable power supply, suitable drying equipment that can control temperature and humidity of polymer, and a cooling unit with a constant airflow. In some occasions, automatic technology is used to move gear from forming position and place it on conveying unit through a repeated action to achieve same cooling method.

Comparing processing of high-precision parts with requirements of general forming processing, it is necessary to pay attention to more details and measurement technology required to achieve level of accurate measurement. This tool must ensure that molding temperature and cooling rate in cavity of each molding are same. The most common problem in precision gear machining is how to deal with symmetry cooling of gear and consistency between cavities.
Mold of precision gear generally does not exceed 4 cavities. Since the first generation of molds only produced one gear, there are few specific instructions, gear tooth inserts are often used to reduce cost of secondary cutting.
Precision gear should be injected from a gate at the center of gear. Multiple gates are easy to form fusion lines, change pressure distribution and shrinkage, affect gear tolerances. For glass fiber reinforced materials, since fibers are arranged radially along welding line, it is easy to cause eccentric "collision" in radius when using multiple gates.
A molding expert can control deformation of tooth space and obtain a product with a controllable, consistent and uniform shrinkage capacity based on good equipment, molding design, stretchability of material used, and processing conditions. During molding, precise control of temperature, injection pressure and cooling process of molding surface is required.
Other important factors include wall thickness, gate size and location, filler type, amount and direction, flow rate and internal stress of molding.
The most common plastic gears are spur gears, cylindrical worm gears and helical gears. Almost all gears made of metal can be made of plastic. Gears are usually formed by split mold cavities. When helical gear is processed, gear or gear ring forming teeth must be rotated during injection, so attention to its details is required.
Noise generated by worm wheel during operation is smaller than that of a straight tooth, and it can be removed by unscrewing cavity or using multiple sliding mechanisms after forming. If a sliding mechanism is used, it must be operated with high precision to avoid obvious seam lines on gears.

More advanced plastic gear forming methods are being developed. For example, secondary injection molding method, by designing an elastic body between wheel shaft and gear teeth, makes gear run more quietly. When gear suddenly stops running, it can better absorb vibration and avoid damage to gear teeth.
Axle can be re-molded with other materials, composite materials with better flexibility or higher value and better self-lubricating effect 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 accuracy of gear, and reduce internal stress.
In addition to gear itself, molding personnel also need to pay attention to design structure of gear. Position of gear shafts in structure must be linearly arranged 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, materials such as glass fiber reinforced materials or mineral-filled polymers should be used to make gear structures with a certain degree of rigidity.
Now, in the field of precision gear manufacturing, emergence of a series of engineering thermoplastics provides processing personnel with more choices than before. The most commonly used materials such as acetal, PBT and polyamide can produce gear equipment with excellent fatigue resistance, wear resistance, smoothness, high resistance to high tangential stress strength, and resistance to vibration loads such as reciprocating motor operation.
Crystalline polymer must be molded at a sufficiently high temperature to ensure full crystallization of material, otherwise material will undergo secondary crystallization when temperature rises above molding temperature during use, which will cause size of gear to change.
As an important gear manufacturing material, acetal is widely used in automobiles, appliances, office equipment and other fields, with a history of more than 40 years. Its dimensional stability, high fatigue and chemical resistance can withstand temperatures up to 90℃ or more. Compared with metals and other plastic materials, it has excellent lubricating properties.
PBT polyester can produce a very smooth surface, without filling modification, its maximum working temperature can reach 150℃, working temperature of glass fiber reinforced products can reach 170℃. Compared with products made of acetal, other types of plastics and metal materials, it works well and is often used in structure 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. When polyamide gear is not filled, operating temperature can reach 150℃, working temperature of glass fiber reinforced product can reach 175℃. However, polyamides have characteristics of absorbing moisture or lubricants to cause dimensional changes, making them unsuitable for use in the field of precision gears.
Temperature of high hardness, dimensional stability, fatigue resistance and chemical resistance of polyphenylene sulfide (PPS) can reach 200℃. Its application is going deep into application fields with demanding working conditions, automotive industry and other end uses.
Precision gear made of liquid crystal polymer (LCP) has good dimensional stability. It can withstand temperatures up to 220℃, has high chemical resistance and low molding shrinkage changes. 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 diameter of a human hair.
Thermoplastic elastomer can make gear run quieter, gear made of it has better flexibility and can absorb impact load 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 actuators.
Materials such as polyethylene, polypropylene and ultra-high molecular weight polyethylene have also been used in gear production in relatively low temperature, corrosive chemical environments or high-wear environments. Other polymer materials are also considered, but they are subject to many harsh restrictions in gear applications;
Polycarbonate has poor lubricity, chemical resistance, and fatigue resistance; ABS and LDPE materials generally cannot meet requirements of precision gears for lubrication, fatigue, dimensional stability, heat resistance, and creep resistance. Such polymers are mostly used in the field of conventional, low-load or low-speed gears.

Compared with plastic gears of same size, metal gears operate well and have better dimensional stability when temperature and humidity change. But compared with metal materials, plastics have many advantages in cost, design, processing and performance.
Compared with metal molding, inherent design freedom of plastic molding ensures more efficient gear manufacturing. Products such as internal gears, gear sets, worm gears, etc. can be molded with plastics, but it is difficult to mold them with metal materials at a reasonable price. Application field of plastic gears is wider than that of metal gears, so they promote development of gears to withstand higher loads and transmit more power.
Plastic gears are also an important material that meets requirements of low and quiet operation, which requires high-precision, new tooth profile and materials with excellent lubricity or flexibility.
Plastic gears generally do not require secondary processing, so compared with stamped parts and machined metal gears, 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.