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20.17 Elimination of Gear Noise

In complete conjugate action and/or excessive backlash are  the source of noise. Plastic-molded gears are generally less accurate than their metal counterparts. Furthermore, due to the presence of a larger TCE there is more backlash built into the gear train.
To avoid noise, more resilient material, such  as Urethane can be used. Figure 1.63 shows several gears made of Urethane which in mesh with Delrin gears produce a practically noiseless gear train. The face width of the Urethane gears must be increased correspondingly to compensate for lower load carrying ability of this material.

20.18 Mold Construction

Depending on the quantity of gears to be produced a decision has to be made to make one single cavity or a multiplicity of identical cavities. If more than one cavity is involved these are used as "family molds" inserted in mold bases which can accommodate a number of cavities for identical or different parts. Since special terminology will be used we shall first describe the elements shown in Figure 1.64.

1 — LOCATING RING is the element which assures the proper location of the mold on the platen with respect to the nozzle which injects the molten plastic.
2— SPRUE BUSHING is the element which mates with the nozzle. It has a spherical or flat receptacle which accurately mates with the surface of the nozzle.
3— SPRUE is the channel in the sprue bushing through which the molten plastic is injected.
4— RUNNER is the channel which distributes material to different cavities within the same mold base.
5-CORE PIN is the element which by its presence restricts the flow of plastic; hence, a hole or void will be created in the molded part.
6-EJECTOR PINS are pins-operated by the molding machine. These have a relative motion with respect to the cavity in the direction which will cause ejection of the part from the mold.
7-FRONT SIDE is considered the side on which the sprue bushing and the Nozzle are located.
8- GATE is the orifice through which the molten plastic enters the cavity.
* - VENT is a miniscule opening through which the air can be evacuated from the cavity as the molten plastic fills it. The Vent is configured to let air escape, but does not fill up with plastic.
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 *Not visible due to its small size.