The present invention relates to a seat frame structure for a vehicle seat and, more particularly, to a seat frame structure for a vehicle, which comprises a seat frame main body integrally formed by blow molding of a synthetic resin.
In a conventional vehicle seat, as disclosed in Japanese Utility Model Publication No. 61- 3406, a structure for coupling a seat cushion as a seating portion of a passenger and a seat back as a backrest portion by a hinge knuckle mechanism is known. In a vehicle seat of this type, the seat back and the seat cushion respectively have seat frames having a predetermined stiffness. An outer portion of each seat frame is covered with a trim having a cushion member.
In order to meet a requirement for reducing the weights of vehicles, an attempt is made to form, using a lightweight synthetic resin, a seat frame, which is conventionally made of a metal pipe, as described in the above prior art. For this purpose, as disclosed in , e.g., Japanese Utility Model Application Laid-Open No. 62-75916, a technique for forming a seat frame by blow molding of a synthetic resin has been proposed. According to this blow molding technique, since the sectional shape of a hollow seat frame can be formed into a bag-like shape, the stiffness of the entire seat frame can be increased. According to this blow molding technique, since a backrest portion of a seat back, and a seating portion of a seat cushion can be formed to have recessed states, a so-called cushion function can be obtained, thus providing a comfortable seat. In this manner, various advantages can be provided when a seat frame is formed by blow molding.
In general, although a synthetic resin material is advantageous since it is light in weight, it suffers from a relatively low stiffness. For this reason, when a seat frame is formed by a synthetic resin, if a portion of a synthetic resin seat frame is simply fastened to a hinge plate of a metal hinge knuckle mechanism by bolts when a seat cushion and a seat back are coupled to each other through the hinge knuckle mechanism, a sufficient coupling strength between the hinge knuckle mechanism and the seat frame cannot be assured. Thus, such a seat frame in this state cannot be put into practical applications.
For example, when a seat frame formed by blow molding is to be reinforced, it is basically impossible to entirely embed a reinforcement plate in a resin. For this reason, a stiffening plate may be arranged on the inner surface side of the seat frame. However, in order to arrange the stiffening plate, after the seat frame is blow-molded using only a synthetic resin, a portion thereof must be cut out, and the stiffening plate must be inserted and fixed inside the seat frame. In this manner, a conventional reinforcing operation of a seat frame is troublesome, resulting in poor workability.
In order to fix a mounting member for mounting a hinge plate to a seat frame formed by blow molding of a synthetic resin, it is preferable to fix this mounting member to the outer surface of a resin molded body in an exposed state in consideration of workability and coupling reliability. In this case, however, it is difficult to fix the mounting member to the surface of the seat frame. In addition, as the surface area of the mounting member is increased, not only the peripheral portion of the mounting member but also its central portion must be fixed to the seat frame. For this reason, a demand has arisen for developing a reliable and firm method of fixing the mounting member to the seat frame.
As a method of solving a problem unique to a synthetic resin seat frame, i.e., a decrease in coupling strength with a hinge knuckle mechanism, as disclosed in, e.g., Japanese Utility Model Laid-Open No. 53-74304, an insert molding technique is known. In this technique, a seat frame is formed by injection molding, and a reinforcement plate is entirely embedded in the synthetic resin during injection molding, thereby increasing a stiffness of a portion of a seat frame, which portion is fastened to a hinge plate by bolts.
However, in the technique disclosed in this prior art, a synthetic resin is interposed between the hinge plate and the reinforcement plate. As a result, when a bolt is fastened, the interposed synthetic resin is plastically deformed, and the fastened bolt may be loosened. If the bolt is fastened too strong, the interposed synthetic resin may crack. In this manner, although the stiffness of a portion of the seat frame to be fastened to the hinge plate by bolts can be increased, a coupling strength cannot be ultimately increased. Thus, the insert molding technique using injection molding cannot be applied to formation of the seat frame in practice.