And the difference between metal gears
(1) One of the fundamental differences between molded plastic gears and metal gears is the difference in manufacturing methods. Almost all of the metal gears are cutting, grinding and processing, and molded plastic gear is made of mold processing. The use of wire cutting, can be spur gear cavity to achieve the accuracy of 2.5μm, but because the method is not developed into processing, the cutting error in any position may occur, it must be the entire internal gear cavity detection, Machined metal gears only need to detect several representative teeth. On the other hand, since the shrinkage of each part of the molded plastic gear is different and any part of the mold is likely to have an abnormal mold value, it is necessary to detect the parts of the gear. The advantage of molded plastic gears is that any special gear can be molded as long as it can be drawn with CAD. What is challenging is how to measure and adjust the shrinkage of molded plastic gears and the abnormalities of molding. Metal gears may also benefit from molded plastic gears in the application of full tooth profile detection technology and the advantages associated with the development method.

(2) plastic gears and metal gears due to differences in processing methods exist. Metal gear is cut or ground, is the rotary processing, it has a high degree of coaxial, diameter accuracy is also easier to guarantee, in the manufacture without contraction compensation. Plastic gears are molded and coaxial is more difficult to assure, but the profile is more accurate than metal gears because the gear cavity accuracy of the wire cutting process is higher than that of the gear cavity for machining with the roller electrode.

(3) plastic gear than metal gear strength is poor, but with the lack of metal gear self-lubricating, light weight, low noise and so on. The large, continuous and repeatable shrinkage characteristics of the engineering plastics in the mold cavity shall be taken into account and compensated when molding the gears. Under normal circumstances, the plastic gear diameter tolerance than the metal gear.

Plastic gear tolerance and transmission ratio are based on the structure of the metal gear, recommended, but these standards for plastic gear is unreasonable, because it does not accurately predict the function and life of plastic gear, even if the distribution of resin materials The plastic properties provided by the manufacturer can not accurately determine the true parameters of the material when the plastic gear enters or exits the meshing at high speed. The characteristics of traditional plastics are obtained in long-term practice.

Design and advantages
Usually the metal gear is designed according to the basic rack principle of cutting process, many plastic gear designers use a similar approach. The gear pitch defined by the metal gear describes the mounting distance between the gear and its cutting tool, and the tooth top modification refers to the additional trim characteristics of the cutting tool to produce the desired tooth profile. The full depth of the gear is actually the tool How much to enter the gear rough. However, these concepts are not required for plastic gears, and they often cause confusion and misunderstanding.

The biggest advantage of the basic rack approach is that the cut gears are allowed to be properly aligned with each other, while plastic gears are typically designed for high volume applications. The design should be as far as possible so that the gear vice device is more solid, strong, rather than the gear can adapt to a certain range of applications. The method listed below is the design method that achieves the specific transmission requirements and maximizes the gear function.
  
The key step in the manufacture of plastic gears is mold design. At this stage, it is necessary to estimate the shrinkage of the geometry of the plastic gear, otherwise it will cause many of the gears that have been tested to be abnormal or do not work at all. Plastic gear shrinkage is complex, can be divided into two aspects: macro and local. The main components of the gear base and the simple symmetrical gears are basically the same, including the outer diameter of the gear, the diameter of the tooth root, the base circle and the pitch circle. The local shrinkage of a single tooth is completely different, and the tooth thickness and other parameters are almost no shrinkage. In some cases, this may be particularly evident in hollow crystalline materials such as nylon and acetal, as local effects may also expand.