Joining self-tapping screw

This has a polypropylene (PP) seat, with welded tubular-steel legs. Use a Phillips screwdriver to remove the four screws that attach seat to the legs. These self-tapping screws (Fig. 1.5) with sharp, widely spaced threads, are much longer than the typical 4 mm thick seat. When screwed into a moulded cylindrical boss on the hidden side of the seat, the threads cut grooves in the initially smooth plastic. It is supported by four or more buttresses, to prevent bending loads causing failure, where the boss joins the seat. Measure the thickness of the buttresses. 

Reduction of the number of product components, usually achieved by combining functions. Typical examples include box and cover assemblies joined with live hinges, the use of self-tapping screws to eliminate nuts, and staking of molded plastic bosses instead of metal rivets or screws. 

A large variety of mechanical fasteners can be used for joining plastic parts to themselves and to other materials. These include machine screws, self-tapping screws, rivets, spring clips, and nuts. In general, when repeated disassembly of the product is anticipated, mechanical fasteners are used. 

Because of the lower ductility of the fluorocarhon materials (e.g., TFE, CTFE, FEP), snap-fit and press-fit joints are seldom nsed. Rivets or studs can be used in forming permanent mechanical joints. These can be provided with thermal techniques on the melt-processable grades. Self-tapping screws and threaded inserts are used for many mechanical joining operations. In holted connections, some stress relaxation may occur the first day after installation. In such cases, mechanical fasteners should he tightened thereafter, stress relaxation is negligible. 

Polysulfone parts can be joined with aU the processes described in this chapter. Because of their inherent dimensional stability and creep resistance, polysulfone parts can be press fitted with ease. Generally, the amount of interference wUl be less than that required for other thermoplastics. Self-tapping screws and threaded inserts have also been used. 

The principal means of assembling polyurethane RIM parts are fasteners and adhesives. For self-tapping screws, the pilot holes can be molded-in, providing dense skin surface for a better grip. The thermoplastic materials can be joined by fasteners and adhesives as well as all the welding techniques. 

Polyimide parts can be joined with mechanical fasteners. Self-tapping screws must be strong enough to withstand distortion when they are inserted into the polyimide resin which is very hard. Polyimide parts can be bonded with epo adhesives. Only abrasion and solvent cleaning is necessary to treat the substrate prior to bonding. Tbe plastic part will usually have a hi er thermal rating than the adhesive. Thermosetting polyimides carmot be heat welded or solvent cemented. 

Screw Assembly. Self-tapping screws are used for joining ETFE parts. For maximum holding power, the boss diameter should be about double the screw diameter, and the engagement length about 2.5 times the screw diameter lubricants should be avoided. Threaded inserts can be molded in place, pressed in, or driven in ultrasonically. 

Mechanical joining can be done conveniently with self-tapping screws. This method requires drilling or molding a pilot hole into the part. It is important that a close tolerance ( 0.001 in.) to the supplier s recommended pilot hole size be maintained for optimum properties. 

Polyimide parts can be joined with mechanical fasteners. Self-tapping screws must be strong enough to withstand distortion when they are inserted into the polyimide resin that is very hard. 

When two pieces of plastic must be joined together in service but must also be suited to simple disassembly, threaded joints are possible. Such joints may be plastic to plastic, as in toothpaste tubes and caps, where male and female threads are molded. If metal screws are to be used, female threads can be molded in a plastic part to accept the metal screw. If such a threaded application does not have to withstand much tension in the screw, a selftapping screw may be turned into a molded hole of appropriate diameter. Self-tapping screws designed for plastics applications are better suited than those for metal appUca-tions. Metal female-threaded inserts may be molded into plastic parts to accept metal screws in a subsequent assembly operation. Metal female-threaded inserts may also be staked into a molded hole of appropriate size, sometimes mechanically, often with heat ( heat staking ) to soften the thermoplastics at the interface so as to achieve more positive bonding (Figs. 9.16 and Fig. 9.17). 

Thin wahed brass tubes are easily joined togs er with silver solder. Aluminum tubing can t be soldered but is easily held together with screws-because the wall thickness Is sufficient to hold threads. The holes can be drilled and tapped In the conventional manner or simply drilled with a hand drill and threaded with self tapping machine screws. 

---