How to design injection-moulded gears

Update:31-03-2021
Summary:

Compared with machined gears, injection-moulded gears h […]

Compared with machined gears, injection-moulded gears have almost nothing in common except for the use of involute conjugate to transmit motion. These two types of gears are essentially different. Machining gears are cut to a specified size on a special gear machine designed for specific processing tasks; injection-moulded gears are injection moulded in a gear cavity, which is usually processed by wire-cut electrical discharge machining machines (EDMs) . The size of the cavity of the injection gear can ensure that the injection gear that has been cooled and shrunk after injection has the correct dimensional tolerance. Millions of injection gears can be processed with one cavity.

The task of gear cutting manufacturers is to cut every gear according to tolerance requirements. The task faced by injection gear manufacturers is to create a nearly perfect gear cavity, and then use this cavity to process all gears that meet the tolerance requirements. This seemingly small but significant difference led to many other changes. This difference is already formed once the decision to use injection-moulded gears is made.

Design of injection gear

There is no doubt that the injection gear must be formed in the mould cavity. This fact has important consequences. Note: It is difficult for the mould cavity and the shaft parts in it to have the precise tolerances that the mechanical transmission mechanism can provide. The cavity and gear may shrink or expand at different rates with changes in humidity and temperature. With the different local conditions, the strength, hardness and even transmission efficiency of the injection gear will change. Under load conditions, the temperature of the surface of the gear teeth will increase, which will affect the characteristics of the plastic. Because of these variables and other factors, it is necessary to customize the design of the gear teeth.

The advantage of injection gear design is reflected in the application. Most injection gear drives are unique. A gear can be precisely designed to perform its designated function only when it meshes with another paired gear. In addition, the optimal design and manufacture of injection gears hardly need to consider tool factors.

The accuracy of the cavity made by the wire-cut EDM machine depends on the accuracy of the computer-aided design. The tolerance of the gear cavity can reach the micron level. In fact, traditional hobs are no longer needed, and the diametral pitch or modulus is no longer an important technical parameter. The base circle of the involute becomes an important variable. The pressure angle can be adjusted in an analog manner to balance the relationship between the strength and the height when the gear teeth are engaged. Compared with standard gears, custom-designed gears have greatly improved performance, quietness and allowable tolerances.

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