Waterproof screw fastener
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a screw, more particularly to an improved waterproof screw fastener. 
     2. Description of the Related Art 
     Screw fasteners are used to join building materials such as plastic roofing boards. Referring to FIGS. 1 and 2, a conventional screw fastener (1) includes a head (11), a threaded shank (10) extending downwards from the head (11), and a sealing washer (12) sleeved onto the threaded shank (10) below the head (11). The head (11) has an upper hexagonal portion provided with six flat side faces (13). The sealing washer (12) is made of an elastic material. A wrench (20) having a bore with six inner faces (21) is used to hold the upper hexagonal portion of the head (11) and to turn the screw fastener (1) joining two separate members (22, 23). The sealing washer (12) is pressed by the head (11) in order to prevent water from seeping through the engaging holes of the members (22, 23). When the sealing washer (12) is pressed by the head (11), the periphery of the sealing washer (12) extends outwards and is not covered or protected by the head (11). The periphery of the sealing washer (12) is easily exposed to the elements, thereby easily and wearing the sealing washer (12) and shortening its effective service life. In addition, the screw fastener (1) does not have a good outward appearance. 
     The inner faces (21) of the wrench (20) gradually wear after a long period of use. The engagement between the flat side faces (13) of the head (11) and the inner faces (21) of the wrench (20) therefore becomes loose. Thus, when the wrench (20) is used to turn the screw fastener (1), the wrench (20) does not properly engage and easily detaches from the screw fastener (1). 
     SUMMARY OF THE INVENTION 
     Therefore, the objective of the invention is to provide a waterproof screw fastener having an improved construction. The sealing washer of the waterproof screw fastener has a longer effective service life. The wrench is able to tightly engage the head of the waterproof screw fastener and faces no risk of untimely detachment. 
     Accordingly, a waterproof screw fastener of this invention includes a head having a top and a bottom end, and a threaded shank downwardly extending from the bottom end of the head. The bottom end of the head has an indentation formed thereon around the threaded shank. A sealing washer is sleeved onto the threaded shank and received in the indentation of the bottom end of the head. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which: 
     FIG. 1 is a perspective view of a conventional screw fastener. 
     FIG. 2 shows that the conventional screw fastener is used to join two separate members. 
     FIG. 3 is an exploded sectional view of a preferred embodiment of a waterproof screw fastener of this invention. 
     FIG. 4 is a sectional view of the preferred embodiment. 
     FIG. 5 is a sectional view of the preferred embodiment which is used to join two separate members. 
     FIG. 6 is a sectional view of another preferred embodiment of this invention which is used to join two separate members. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 3 and 4, a waterproof screw fastener (3) of this invention includes a head (30) having a top and a bottom end (30&#39;, 30&#34;), and a threaded shank (33) downwardly extending from the bottom end (30&#34;) of the head (30). The head (30) of the waterproof screw fastener (3) is made of a rigid plastic material. The bottom end (30&#34;) of the head (30) has an annular indentation (32) formed thereon around the threaded shank (33). The indentation (32) has a concaved surface (321). The top end (30&#39;) of the head (30) has six side faces (35) each of which has a radially projecting and axially extending rib (34) formed thereon. The threaded shank (33) is made of a metal material and has an annular flange (31) and a pair of upward projections (31&#39;) encapsulated in the plastic head (30). The waterproof screw fastener (3) further includes an annular elastic sealing washer (40) and an annular metal member (41) disposed above the sealing washer (40). The sealing washer (40) and the metal member (41) each have a convex outer surface to fit in the indentation (32) and abut on the concaved surface (321) of the indentation (32). The sealing washer (40) and the metal member (41) are sleeved onto the threaded shank (33) and received in the indentation (32) of the bottom end (30&#34;) of the head (30). The sealing washer (40) can be made of a rubber material having good abrasion resistance. 
     Referring to FIG. 5, a wrench (5) having a bore with six inner faces (52) is used to hold the head (30) and turn the waterproof screw fastener (3) joining two separate members (511, 512). The sealing washer (40) is pressed by the head (30) attached to the member (511) and received in the indentation (32) to prevent water from seeping through the engaging holes of the members (511, 512). The sealing washer (40) is generally covered and protected by the head (30) and there is only little clearance (61) between the bottom end (30&#34;) of the head (30) and the member (511). The sealing washer (40) is no more unduly exposed to the elements, so it is no more easily worn out and has a longer effective service life. In addition, the waterproof screw fastener (3) has a good outward appearance. 
     Due to the ribs (34) of the side faces (35) of the head (30), when the wrench (5) is used to turn the waterproof screw fastener (3), the wrench (5) always properly engages and does not easily detach from the waterproof screw fastener (3) although the inner faces (52) of the wrench (5) are subject to wear after a long period of use. 
     Referring to FIG. 6, each of the side faces (35) of the head (30) has a projecting stub (36) formed thereon instead of the rib (34) in FIG. 3. 
     The plastic head (30) of the waterproof screw fastener (3) of this invention can be made of a variety of colors as desired to satisfy the customers&#39;s needs. 
     While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment, but is intended to cover various arrangements included within the spirit and scope of the broadest interpretations and equivalent arrangements.