Plastic gears are continuing to displace metal gears in a widening arena of applications. Their unique characteristics are also being enhanced with new developments, both in materials and processing. In this regard plastics contrast somewhat dramatically from metals, in that the latter materials and processes are essentially fully developed and, therefore, are in a relatively static state of development.
Among the various methods of producing plastic gears, molding is unique in many respects. For that reason, it is singled out for in-depth treatment in this separate section.

20.1 General Characteristics of Plastic Gears

Among the characteristics responsible for the large increase in plastic gear usage the following are probably the most significant:

1. Cost effectiveness of the injection-molding process.
2. Elimination of machining operations; capability of fabrication with inserts and integral
   designs.
3. Low density: light weight, low inertia.
4. Uniformity of parts.
5. Capability to absorb shock and vibration as a result of elastic compliance.
6. Ability to operate with minimum or no lubrication, due to inherent lubricity.
7. Relatively low coefficient of friction.
8. Corrosion resistance; elimination of plating, or protective coatings.
9. Quietness of operation.
10. Tolerances often less critical than for metal gears, due in part to their greater
     resilience.
11. Consistency with trend to greater use of plastic housings and other components.
12. One step production; no preliminary or secondary operations.

At the same time the design engineer should be familiar with the limitations of plastic gears relative to metal gears. The most significant of these are as follows:

1. Less load-carrying capacity due to lower maximum allowable stress; the greater
    compliance of plastic gears may also produce stress concentrations.
2. Plastic gears cannot generally be molded to the same accuracy as high-precision
    machined metal gears.
3. Plastic gears are subject to greater dimensional instabilities due to their greater
    coefficient of thermal expansion and moisture absorption.
4. Reduced ability to operate at elevated temperatures; as an approximate figure,
   operation is limited to less than 250 degrees^o F. Also limited cold temperature
   operations.
5. Initial high mold cost in developing correct tooth form and dimensions.
6. Can be negatively affected by certain chemicals and even some lubricants.
7. Improper molding tools and process can produce residual internal stresses at the
    tooth roots resulting in over stressing and/or distortion with aging.
8. Cost of plastics track petrochemical pricing and thus are more volatile and increasing
    in comparison to metals
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