Method of Manufacturing Steering Wheel

1. Field of the Invention

The present invention relates to a method of manufacturing a steering wheel, and more particularly relates to a method of manufacturing a steering wheel, in which a coating layer is molded of a synthetic resin on a steering wheel core including an annular portion having a groove extending in the circumferential direction of the portion.

2. Description of the Prior Art

The cross section of at least the annular portion of the core of a steering wheel is U-shaped in order to reduce the weight of the core, namely, the weight of the wheel as a whole, as disclosed in the Japan Utility Model Application (OPI) No. 16559/87 (the term "OPI" as used herein means an "unexamined published application"). When such a steering wheel is to be manufactured, the core thereof is set in a molding cavity 54 defined by an upper molding die and a lower molding die 101 and 102, so that the outermost part 53 of the groove 52 of the annular portion 51 of the core is located as the top of the groove, as shown in FIG. 9. A synthetic resin 56 such as urethane, polypropylene and vinyl chloride is then injected into the cavity 54 through a gate 55 provided at the separation surfaces 112 of a molding die unit 100 consisting of the molding dies 101 and 102, so that a coating layer is molded of the resin on the core.

However, in such a method of manufacturing the steering wheel, if the injection of the synthetic resin 56 is completed in a short time, the synthetic resin injected through the gate 55 runs onto a wall 58 facing the gate and included in the annular portion 51 including another wall 57 and having the groove 52 defined between the walls 57 and 58 and gravitates on the wall 58 so that the resin starts filling the cavity 54 from the bottom thereof, the surface of the resin rises over the wall, and the resin flows down into the groove. In that case, since the resin 56 flows down into the groove 52 while involving much air, the coating layer is likely to have a cavity therein. Besides, the speed of the filling of the cavity 54 with the resin from the bottom of the cavity changes at the time of the inflow of the resin into the groove 52 so that the outside surface of the coating layer is likely to have a luster nonuniformity. Since the coating layer is likely to have such molding defects, it is difficult to properly manufacture the steering wheel in a short time. This is a problem.

The present invention was made in order to solve the problem mentioned above. Accordingly, it is an object of the present invention to provide a method of manufacturing a steering wheel in a short time so that a molding defect such as a cavity is less likely to be caused.

In the method, a coating layer is molded of a synthetic resin, by using a molding die unit, on at least the annular portion of a steering wheel core including a boss, the annular portion located around the boss and having a groove extending in the circumferential direction of the annular portion, and spokes coupling the boss and the portion to each other. The method of the present invention is characterized by comprising a die opening step in which an upper and a lower molding die, which have a gate formed at the separation surfaces of the dies and communicating with the molding cavity of the molding die unit, are opened from each other; a core setting step in which the steering wheel core is set in the molding cavity so that the outermost part of the groove of the annular portion having walls defining the groove between them is located as the top of the groove, one of the walls, which faces the gate and has a notched part notched to be smaller in height than the other of the walls, faces the gate at the notched part, and the top of the notched part is located below the separation surfaces of the dies; a coating layer molding step in which the dies are closed on each other and the synthetic resin is injected into the molding cavity through the gate so that the coating layer is molded of the resin on the core; and a steering wheel takeout step in which the die are opened from each other and the wheel is taken out from the dies.

In the coating layer molding step, the synthetic resin first flows into the groove of the annular portion of the core through the notched part of the portion, and thereafter starts filling the molding cavity from the bottom thereof gradually. Since the groove is thus filled with the resin before the cavity is filled with it, the resin does not flow from the cavity into the groove over the wall. For that reason, air is prevented from being involved by the resin into the groove to cause a molding defect such as a cavity and a luster nonuniformity when the coating layer is molded of the resin. It is thus made possible to inject the resin and mold the coating layer in a short time.

FIG. 1 is a bottom view of the steering wheel core on which a coating layer is molded of a synthetic resin in a method which is an embodiment of the present invention;

FIG. 2 is an enlarged partial perspective view of the core;

FIG. 3 is a partial sectional view of the core along a line III--III shown in FIG. 2 ;

FIG. 4 is a sectional view showing that the core is set in a molding die unit;

FIG. 5 is a sectional view showing the interior of the cavity of the molding die unit at the end of the molding of the coating layer;

FIG. 6 is a bottom view of a steering wheel having the coating layer molded;

FIG. 7 is an enlarged partial perspective view of a major part of the annular portion of a steering wheel core on which a coating layer is molded in a method which is another embodiment of the present invention;

FIG. 8 is a partial sectional view of the major part along a line VIII--VIII shown in FIG. 7; and

FIG. 9 is a partial sectional view showing that a coating layer is molded of a synthetic resin on a conventional steering wheel core in a conventional method.

Embodiments of the present invention are hereafter described in detail with reference to the drawings attached hereto.

FIGS. 1, 2, 3, 4, 5 and 6 are for describing a method which is one of the embodiments and in according to a steering wheel 15 is manufactured. In the method, the core 9 of the steering wheel is composed of a boss 19, an annular portion 1 located around the boss and having a groove 2 having a nearly U-shaped cross section and extending in the circumferential direction of the annular portion, and spokes 29 coupling the boss and the annular portion to each other, as shown in FIG. 1. The annular portion 1 has two walls 7 and 8 defining the groove 2 between them. The wall 8 faces a gate 5 provided at the separation surfaces 13 of a molding die unit 12 consisting of an upper and a lower molding dies 10 and 11, and has a notched part 8a which is notched to be smaller in height than the other part 8b of the wall and is located near the gate 5, as shown in FIG. 2. When the core 9 is set in the molding cavity 4 of the molding die unit 12, the top of the notched part 8a of the wall 8 is located below the separation surfaces 13 of the upper and the lower molding dies 10 and 11.

The method of manufacturing the steering wheel, in which a coating layer 14 is now molded on the core 9, is described step by step. In the die opening step of the method, the upper and lower molding dies 10 and 11 of the molding die unit 12, which has the gate 5 formed at the separation surfaces 13 of the dies and communicating with the molding cavity 4, are opened. In the core setting step of the method after that, the core 9 is set in the lower molding die 11 so that the outer most part 3 of the groove 2 of the annular portion 1 of the core is located as the top of the groove, the notched part 8a of the wall 8 of the annular portion faces the gate 5 of the molding die unit 12, and the top of the notched part is located in the molding cavity 4 and below the separation surfaces 13 of the die unit, as shown in FIGS. 3 and 4. In the coating layer molding step of the method after that, the upper and the lower molding dies 10 and 11 are closed on each other, a urethane resin 6 is injected into the molding cavity 4 through the gate 5 so that the coating layer 14 is molded of the urethane resin on the core 9. In that step, since the top of the notched part 8a of the wall 8 facing the gate 5 is located below the separation surfaces 13 of the molding die unit 12, the liquefied urethane resin hardly comes into contact with the wall 8 but passes over the wall and comes into contact with the other wall 7 facing the boss 19, so that the resin gravitates into the groove 2, flows in the groove at the bottom thereof from the notched part in the circumferential direction of the annular portion 1 of the core 9 and overflows the notched part, and most of the resin overflowing the notched part fills the cavity 4 from the bottom gradually. In the coating layer molding step of the method after that, all of the cavity 4 is thus filled with the urethane resin 6 so that the coating layer 14 is molded of the resin on the core 9, as shown in FIG. 5. In the steering wheel takeout step of the method after that, the upper and the lower molding dies 10 and 11 are opened from each other, and the steering wheel 15 with the core 9 and the coating layer 14 molded thereon is taken out from the molding die unit 12, as shown in FIG. 6.

The present invention is not confined to the embodiment described above, but may be practiced or embodied in other various ways without departing from the spirit or essential character thereof. For example, in a steering wheel manufacturing method which is another embodiment of the invention, the annular portion 1 of the core of a steering wheel is made of three walls 7, 8 and 16, as shown in FIGS. 7 and 8. The wall 8, which faces the gate 5 of a molding die unit 12, and the central wall 16 have notched parts 8a and 16a, which are located near the gate. The parts 8a and 16a are notched to be smaller in height than the other wall 7 facing the boss of the core. It is preferable that the notched part 16a of the central wall 16 is made smaller in height than that 8a of the wall 8 but larger in circumferential length than the latter to cause a synthetic resin 6 to more efficiently flow into the narrow grooves 2 of the annular portion 1 of the core.