The Injection Molding Process and Material Analysis of Plastic Gear

Plastic gears are moving towards larger sizes, more complex geometries, and higher strength nowadays. At the same time, high-performance resins and composite materials filled with long glass fibers play an important promoting role for this movements.

Plastic gears have undergone a process of change from new materials to an important industrial material 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. And they play a role in transmitting torque and motion forms. In addition to the existing application areas, new and more difficult-to-machine gear application areas will continue to emerge, and this trend is still in deep development.

The automotive industry has become one of the fastest growing areas of plastic gears, and this 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 has made plastic gears deeply applied to many fields, from lift doors, seats, tracking headlights to brake actuators, electric throttle sections, turbine adjustment devices, and so on.


The application of plastic power gears is further broadened. In some application areas with large size requirements, plastic gears are often used to replace metal gears, such as washing machine transmissions that use plastics, which changes the 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, and power spirals that control surface stability on small aircraft and the military equipment including screw gauges and control devices.

For Large-size, High-strength Plastic Gear

The advantages of plastic gear molding and the characteristics of realizing the larger, high-precision and high-strength molding, this is an important reason for the development of plastic gears.

The issue about 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 the concentricity, tooth profile and other characteristics of the gear.

Some helical gears may require complex forming operations to manufacture the 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 the ability to produce a new generation of plastic gears. But for all processors, a real challenge will be how to coordinate the manufacture of this entire high precision product.

Difficulties of control

The allowable tolerances of high-precision gears are generally more than the “good” explained by the American Plastics Industry Association (SPI). But today, most molding experts use the latest molding machines equipped with processing control units. Mold the precision gears by controlling the accuracy of molding temperature, injection pressure and other variables. Some gear molding experts use more advanced methods. They place temperature and pressure sensors in the cavity to improve the consistency and repeatability of the molding.

Manufacturers of precision gears also need to use professional testing equipment, such as double-tooth flank rolling detectors used to control the quality of gears, and computer-controlled detectors for evaluating gear tooth flank and other characteristics. But having the right equipment is just the beginning.

Those manufacturers who try to enter the precision gear industry must also adjust their molding environment to ensure that the gears they produce are as consistent as possible in every injection molding and every cavity of the mold. Since the action of operator is often the decisive factor when producing precision gears. We must focus on the training of employees and the control of the operation process.

The size of the gear is easily affected by seasonal temperature changes, even the temperature fluctuations caused by opening the door and letting a forklift pass can affect the dimensional accuracy of the gear. Therefore, mold manufacturers need to strictly control the environmental conditions in the molding area.

Other factors to consider:

In some occasions, automatic technology is used to move the gear out from the mold cavity and place it on the conveying belt through a repeated action to achieve the same cooling method.

Important forming cooling steps

Comparing with the requirements of general forming processing, the processing of high-precision parts needs to pay attention to more details and the measurement technology required to achieve the level of accurate measurement. This tool must ensure that the molding temperature and cooling rate in the cavity of each molding are the same. The most common problem in precision gear machining is how to deal with the symmetry cooling of the gear and the consistency between the cavities.

The 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. And gear tooth inserts are often used to reduce the cost of secondary cutting.

The precision gear should be injected from a gate at the center of the gear. Multiple gates are easy to form a fusion line, change the pressure distribution and shrinkage, and affect the gear tolerance. For glass fiber reinforced materials, since the fibers are arranged radially along the welding line, it is easy to cause eccentric “collision” in the radius when using multiple gates.

If a molding expert can control the deformation of the tooth space and obtain a product with a controllable, consistent and uniform shrinkage ability. That must base on the good equipment, molding design, stretchability of the material used, and processing conditions. During molding, precise temperature control, injection pressure and cooling process of the 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 instead of plastic. Gears are usually formed by split mold cavities. When the helical gear is processed, the gear or the gear ring forming the teeth must be rotated during injection, so attention to its details is required.

The noise generated by the worm wheel during operation is smaller than that of a straight tooth. And it can be removed by unscrewing the 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 the gears.

New process and new resin

More advanced plastic gear forming methods are being developed. For example, the secondary injection molding method, by designing an elastic body between the wheel shaft and the gear teeth, makes the gear run more quietly. When the gear suddenly stops running, it can better absorb vibration and avoid damage to the gear teeth.

The axle can be re-molded with other materials, and composite materials with better flexibility or higher value and better self-lubricating effect can be selected. Meanwhile, the gas-assisted method and the injection compression molding method are studied as a method to improve the quality of the gear teeth, the overall accuracy of the gear, and reduce the internal stress.

In addition to the gear itself, the molding personnel also need to pay attention to the design structure of the gear. The position of the gear shafts must be in a linear arrangement so that it can ensure the gears run in a straight line, even when the load and temperature change. Therefore, the dimensional stability and accuracy of the 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, the 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 the following characteristics:

  • Excellent fatigue resistance, wear resistance, smoothness,
  • Resistance to high tangential stress strength
  • Ability to withstand the vibration load caused by the operation of reciprocating motors, etc.

The crystalline polymer must be molded at a high enough temperature to ensure the full crystallization of the material. Otherwise the material will undergo secondary crystallization when the temperature rises above the molding temperature during use. And it will cause the change of the gear size.

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℃. Besides, the working temperature of glass fiber reinforced products can reach 170℃ with the glass fiber reinforced. Compared with products made of acetal, other types of plastics and metal materials, it works well and is often used in the structure of gears.

Polyamide materials, compared with other plastic materials and metal materials, have good toughness and durability. They are often used in applications such as turbine transmission design and gear frames. When the polyamide gear is not filled, the operating temperature can reach 150℃. In addition, the working temperature of the glass fiber reinforced product can reach 175℃. However, polyamides have the characteristics of absorbing moisture or lubricants to cause dimensional changes, making them unsuitable for use in the field of precision gears.

The temperature of high hardness, dimensional stability, fatigue resistance and chemical resistance of polyphenylene sulfide (PPS) can reach 200°C. Its application is going deep into the application fields with demanding working conditions, the automotive industry and other end uses.

The precision gear made of liquid crystal polymer (LCP) has good dimensional stability. It can withstand temperatures up to 220°C, 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. It is equivalent to 2/3 of the diameter of a human hair.

The thermoplastic elastomer can make the gear run more quietly. And the gear made of it is more flexible, and can well absorb the impact load. For example, a low-power, high-speed gear made of copolyester thermoplastic elastomer can not only allow some deviations during operation, but also reduce operating noise while ensuring sufficient dimensional stability and hardness. 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 the requirements of precision gears for lubrication, fatigue resistance, dimensional stability, heat resistance and creep resistance. Most of these polymers are used in the field of conventional, low-load or low-speed gears.

Advantages of using plastic gears

Compared with plastic gears of the same size, metal gears perform 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, the inherent design freedom of plastic molding ensures more efficient gear manufacturing. Internal gears, gear sets, worm gears and other products can be molded with plastic. But it is difficult to mold with metal materials at a reasonable price. Plastic gears have wider applications than metal gears. So they promote the development of gears to withstand higher loads and transmit greater power.

Plastic gears are also an important material that meets the requirements of low and quiet operation. And that requires the appearance of materials with high precision, new tooth profile and excellent lubricity or flexibility.

Plastic gears generally do not require secondary processing. Therefore, compared with stamped parts and machined metal gears, the cost is guaranteed to be reduced by 50% to 90%. Plastic gears are lighter and more inert than metal gears. Besides, they can be used in environments where metal gears are prone to corrosion and degradation, such as the control of water meters and chemical equipment.

Compared with metal gears, plastic gears can deflect and deform to absorb impact loads. And they can better disperse local load changes caused by shaft deflection and misaligned teeth. The inherent lubricating characteristics of many plastics make them ideal gear materials for printers, toys, and other low-load operating mechanisms. Lubricants are not included here. In addition to running in a dry environment, gears can also be lubricated with grease or oil.

Reinforcement of materials

In the description of gears and structural materials, the important role of fibers and fillers on the performance of resin materials should be considered. For example, when the acetal copolymer is filled with 25% short glass fiber (2mm or less) filler, its tensile strength increases by 2 times at high temperature, and its hardness increases by 3 times.

The use of long glass fiber (10 mm or smaller) filler can improve the strength, creep resistance, dimensional stability, toughness, hardness, wear performance, etc. and other more properties. Because it can obtain the required hardness and good controllable thermal expansion performance. The long glass fiber reinforced materials are becoming an attractive candidate material in large-size gears and structural applications.

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