Plastic gears are moving towards larger sizes, more complex geometries, and higher strengths, while high-performance resins and long glass fiber-filled composites have played an important role in promoting this.
Plastic gears have undergone a transformation from new materials to important industrial materials in the past 50 years. Today they have penetrated into many different application fields, such as automobiles, watches, sewing machines, structural control facilities, and missiles, playing the role of transmitting torque and motion. In addition to existing application fields, new and more difficult gear application fields will continue to emerge, and this trend is still developing in depth.
Automotive industry has become one of the fastest-growing areas for plastic gears, and this successful change is encouraging. Automakers are working hard to find various auxiliary systems for vehicle drives. What they need is motors and gears instead of power, hydraulics or cables. This change has led to in-depth application of plastic gears in many application areas, from liftgates, seats, and tracking headlights to brake actuators, electric throttle segments, and turbine adjustment devices.
Application of plastic power gears has been further expanded. In some applications with large size requirements, plastic gears are often used to replace metal gears, such as plastic washing machine transmissions, which changes application limit of gear size.
Plastic gears are also used in many other fields, such as vibration damping drives in ventilation and air conditioning systems (HVAC), valve transmissions in mobile facilities, automatic sweepers in public lounges, power screws for control surface stability on small aircraft, gyroscopic instruments and control devices in military field.

Due to advantages of plastic gear molding and ability to mold larger, high-precision and high-strength features, this is an important reason for development of plastic gears.
How to design a gear configuration that maximizes transmission power while minimizing transmission errors and noise still faces many challenges. This puts high processing precision requirements on concentricity, tooth shape and other characteristics of gear.
Some helical gears may require complex molding operations to create final product, while others may require core teeth in thicker sections to reduce shrinkage. While many molders have used the latest polymer materials, equipment and processing technologies to achieve ability to produce next generation of plastic gears, a real challenge for all molders is how to coordinate manufacturing of this entire high-precision product.

Tolerances allowed for high-precision gears are generally difficult to describe as "good" as described by Society of Plastics Industry (SPI). But today most molders use the latest molding machines equipped with process control units to control accuracy of molding temperature, injection pressure and other variables within a complex window to mold precision gears. Some gear molders use more advanced methods, placing temperature and pressure sensors in cavity to improve molding consistency and repeatability.
Precision gear manufacturers also need to use specialized testing equipment, such as double-flank rolling testers to control gear quality and computer-controlled testers to evaluate gear tooth flanks and other features. But having right equipment is just beginning.
Molders seeking 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 cavity to cavity. Since behavior of technicians is often determining factor when producing precision gears, they must focus on employee training and process control.
Because gear dimensions are easily affected by seasonal temperature changes, or even temperature fluctuations caused by opening a door to allow a forklift to pass through, molders need to strictly control environmental conditions in molding area.
Other factors to consider include: a stable power supply, suitable drying equipment that can control polymer temperature and humidity, and cooling units with constant airflow. In some cases, automation technology is used to remove gear from molding position and place it on a transfer unit in a repetitive motion to achieve a consistent cooling pattern.

Processing of high-precision parts requires more attention to detail and measurement technology to achieve level of precision measurement required compared to general molding operations. This tool must ensure that molding temperature and cooling rate in cavity are same from mold to mold. The most common problem in precision gear processing is how to deal with gear cooling symmetry and consistency between mold cavities.
Precision gear molds generally do not exceed 4 cavities. Since the first generation of molds only produce one gear, there are few specific instructions, and gear tooth inserts are 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 forming fusion lines, changing pressure distribution and shrinkage, and affecting gear tolerances. For glass fiber reinforced materials, since fibers are arranged radially along weld line, using multiple gates is prone to causing eccentric "collisions" of radius.
A molding expert can control deformation of tooth groove and obtain a product with controllable, consistent, and uniform shrinkage ability based on good equipment, molding design, material stretching ability and processing conditions. During molding, precise control of molding surface temperature, 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 gears, cylindrical worm gears, and helical gears. Almost all gears made of metal can be made of plastic. Gears are often molded in split mold cavities. Helical gears require attention to detail because gear or gear ring that forms teeth must be rotated during injection.
Worm gears produce less noise than spur gears when they are running. After molding, they are removed by rotating out of cavity or using multiple slide mechanisms. If slide mechanisms are used, they must be operated with high precision to avoid obvious parting lines on gear.

More advanced plastic gear molding methods are being developed. For example, two-shot injection molding, which uses an elastomer between axle and gear teeth, makes gears run quieter, can better absorb vibrations and avoid gear damage when gear stops suddenly.
Axle can be re-molded with other materials, composite materials with better flexibility or higher value and better self-lubricating effects can be selected. Gas-assisted and injection compression molding are also studied as a way to improve gear tooth quality, overall gear accuracy, and reduce internal stress.
In addition to gear itself, molders also need to pay attention to design structure of gear. Position of gear shaft in structure must be arranged linearly to ensure that gear runs 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 has provided processors with more options than before. The most commonly used materials such as acetal, PBT and polyamide can produce excellent fatigue resistance, wear resistance, smoothness, high tangent stress strength resistance, and gear equipment that can withstand vibration loads such as reciprocating motor operation.
For crystalline polymers, they must be molded at a sufficiently high temperature to ensure that material is fully crystallized. Otherwise, when used, material will undergo secondary crystallization due to temperature rising above molding temperature, resulting in changes in gear size.
As an important gear manufacturing material, acetal has been widely used in the fields of automobiles, appliances, office equipment, etc. for more than 40 years. Its dimensional stability and high fatigue and chemical resistance can withstand temperatures up to 90 ℃ or more. Compared to metal and other plastic materials, it has excellent lubricity.
PBT polyester can produce very smooth surfaces and has a maximum operating temperature of 150℃ without filler modification and 170℃ with glass fiber reinforced products. It works well compared to acetal, other types of plastics and metal materials and is often used in gear structures.
Polyamide materials, compared to other plastic materials and metal materials, have good toughness and durability and are often used in applications such as turbine transmission design and gear frames. Polyamide gears can operate at temperatures of up to 150℃ when unfilled and 175℃ when reinforced with glass fiber. However, polyamides have characteristic of dimensional changes caused by moisture absorption or lubricants, making them unsuitable for precision gears.
High hardness, dimensional stability, fatigue resistance and chemical resistance of polyphenylene sulfide (PPS) can reach temperatures of 200℃. Its application is deepening into applications with demanding working conditions, automotive industry and other end uses.
Precision gears made of liquid crystal polymer (LCP) have good dimensional stability. It can withstand temperatures up to 220℃, has high chemical resistance and low mold shrinkage variation. Gears with a tooth thickness of about 0.066 mm, equivalent to 2/3 diameter of a human hair, have been molded using this material.
Thermoplastic elastomers can make gears run quieter, make gears more flexible, and absorb shock loads well. For example, a low-power, high-speed gear made of a copolyester thermoplastic elastomer can allow some deviation during operation while ensuring sufficient dimensional stability and hardness, and can reduce 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 temperatures, corrosive chemical environments or high wear environments. Other polymer materials have also been considered, but are subject to many stringent requirements in gear applications;

 

Polycarbonate has poor lubricity, chemical resistance and fatigue resistance; ABS and LDPE materials generally cannot meet lubricity, fatigue resistance, dimensional stability, heat resistance, creep resistance and other performance requirements of precision gears. Such polymers are mostly used in conventional, low-load or low-speed gear applications.

Metal gears perform well and have good dimensional stability over temperature and humidity changes compared to plastic gears of same size. However, plastics have many advantages over metals in terms of cost, design, processing and performance.
Inherent design freedom of plastic molding allows for more efficient gear manufacturing than metal molding. Plastics can mold internal gears, gear sets, worm gears, etc., which are difficult to mold at a reasonable price using metal materials. Plastic gears have a wider range of applications than metal gears, so they are driving development of gears to withstand higher loads and transmit more power.
Plastic gears are also an important material for meeting low-noise requirements, which require high precision, new tooth shapes and excellent lubricity or flexibility.
Plastic gears generally do not require secondary processing, so they can be reduced by 50% to 90% in cost compared to stamped and machined metal gears. Plastic gears are lighter and more inert than metal gears and can be used in environments where metal gears are susceptible to corrosion and degradation, such as water meters and chemical equipment controls.
Compared with metal gears, plastic gears can deflect and deform to absorb shock loads, can better distribute local load changes caused by shaft deflection and misalignment. Inherent lubricity of many plastics makes them ideal gear materials for printers, toys and other low-load operating mechanisms, not including lubricants. Except for operating in a dry environment, gears can also be lubricated with grease or oil.

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