Patent Application
20060058105
The present invention relates to molding of plastic parts; more particularly, to overmolding of plastic parts onto substrates; and most particularly, to method and apparatus for overmolding a plastic gear onto a gear shaft.
It is well known to drive the rotation of a shaft by means of a gear, pulley, or lever arm (referred to herein collectively as a “gear”) attached to the end of the shaft. For a simple example, the rotary valve shaft of an electronically controlled air flow valve for admitting air to an internal combustion engine typically includes a gear mounted onto the end of the shaft. For minimizing manufacturing cost and finished weight of the assembly, it is known to form the gear from a injection-moldable polymer such as nylon, and further, to overmold the gear directly onto the end of the shaft. To keep the gear locked onto the shaft and prevent it from loosening and rotating on the shaft during its working lifetime, the shaft typically is provided with features in the overmolding region, such as a pair of opposed flats, also known in the art as “double D's” in reference to the appearance of the flats in a shaft cross-sectional view.
A weakness is known to arise in such a prior art gear when overmolded onto a shaft in this fashion. Typically, the mold is provided with an axial gate for injecting liquid polymer onto the end of the shaft. The polymer flows around the end surface to fill the double D voids and then flows primarily from the double D voids circumferentially around the shaft to form the remainder of the gear hub and gear teeth. Where the opposing flows from the double D voids meet, knit lines are formed which differ in polymeric structure and strength from the rest of the injection. The inventors believe that, although there is some longitudinal flow of polymer in the regions between the double D voids, most of the material therein enters circumferentially from the double D voids, thus producing pronounced longitudinal knit lines at the interfaces between the opposed circumferential flows. Testing has shown that such knit lines may have up to 50% less tensile strength as compared to casting formed from free-flowing polymer. In use, prior art gears may crack along the knit lines, when loaded torsionally against the shaft, resulting in catastrophic failure of the shaft/gear assembly.
What is needed in the art is a method and apparatus for overmolding a polymeric gear onto a shaft without creating significant zones of structural weakness in the molded component.
It is a principal object of the present invention to provide an assembly comprising an overmolded gear on a shaft which maintains the structural integrity of the assembly over its working life.
Briefly described, a shaft for receiving a polymer overmolding of a gear from an axial single-gate mold is provided with a plurality of features for preventing rotation of the molded gear on the shaft. The shaft features are in the form of a plurality of axial-direction flats, the number of such flats being greater than two (double D) and preferably four (referred to herein as “quadral D”). Finite element analysis shows that in a four-flat mold and shaft arrangement having a single, axial mold gate, longitudinal knit lines between the D's are virtually non-existent, and polymer in these areas is substantially as strong as polymer elsewhere in the casting.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
The benefits of a shaft and overmolded gear in accordance with the invention may be better appreciated by first considering a prior art shaft and overmolded gear.
Referring to
Referring to
As is known in the art, knit lines 24a,24b can represent zones of weakness in the resulting casting. Referring to
Referring to
When a polymer gear (not shown but understood to be similar to gear 22) is formed on portion 112 of shaft 110 by conventional injection overmolding using a hub and gear shaped mold (not shown) having an axial gate on axis 115, liquid polymer enters the mold via the axial gate, flows over and covers end 116, and axially enters the four cylindrical (quadral D) voids 120a,120b,120c,120d formed by cooperation of the cylindrical hub mold and shaft flats 114a,114b,114c,114d. The inventors believe that major amounts of polymer also flow axially from end 116 into inter-flat regions 118a,118b,118c,118d such that those regions are filled predominantly by such axial flows, thereby preventing the formation of longitudinal knit lines from circumferential flow as occurs in the prior art molding.
Although this explanation should not be relied on for patentability of the invention, from the surprising lack of longitudinal knit lines when using a quadral D shaft, it appears to the inventors that the quadral D arrangement of flats prevents the formation of knit lines by slowing down the filling of the D voids and conversely accelerating the axial filling of the inter-flat regions from end 116. Such relative filling rates, of course, are also a function of the radial dimension of the annular hub-forming space between shaft end 112 and the inner wall of the mold. By optimizing the axial fill rates of D voids and inter-flat regions to prevent circumferential filling of the inter-flat regions from the D voids, other shaft end configurations having a plurality of flats are possible, for example, a shaft end 212 having three flats 214a,214b,214c forming treble D voids 220a,220b,220c, as shown in
While this invention is depicted in the figures as a gear molded onto a shaft, as mentioned previously, it is intended that the invention be applied equally to other members molded to a shaft such as, for example, levers, pulleys and the like.
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
