Self-tapping inserts

Metal Mechanical anchor During or after moulding, self-tapping inserts... 

Self-tapping inserts available in thread-cutting or thread-forming types. The advantages and drawbacks of this process for the thread-forming type are ... 

Self-tapping inserts. Fig. 14.4(a), cut their own threads, are available in brass, case-hardened steel and stainless steel from M3 to M16 and are used in the more brittle thermoplastics and in thermosets. 

Bosses for threaded inserts. Bosses for threaded inserts are very similar to those for self-tapping screws. The inside diameter is determined by the diameter of the insert the exact size for heat, press-in, or ultrasonic postmolded inserts are those recommended by the insert manufacturer for a given plastic who will also provide the lead diameter. (For self-tapping inserts, follow the same inside diameter criteria as for self-tapping screws.) In this case, however, the lead is an 8° included angle instead of a step. The outside diameter of the boss is 2 to 3 times the diameter of the insert and the minimum depth of the hole is the insert length plus 0.030 in. 

The holding power of screws can be affected by the size of the pilot drill-hole, the depth of screw engagement, the outside diameter of the screw, and quality of the bone [Cochran, 1982 DeCoster et al., 1990], Therefore, the selection of the screw type should be based on the assessment of the quality of the bone at the time of insertion. Under identical conditions, self-tapping screws provide a slightly greater holding power than non-self-tapping screws [Tencer et al., 1993]. 

Inserts, which have a tapped iimer hole to accept a standard threaded screw, can be self-tapping, expansion, heat or ultrasonic and can be headed or unheaded. 

Post-molded inserts come in four types press-in, expansion, self-tapping, and thread forming, and inserts that are installed by some method of heating (e.g., ultrasonic). Metal... 

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. 

Stripping torque n. (1) Of a self-tapping screw, the twisting moment in Nm required to strip the threads formed by the screw in a softer material. (2) Of a molded-in insert, the torque required to break the mechanical bond between the plastic and the insert s knurled surface. 

The boss in Fig. 8.19a will not supply the strength necessary for structural applications like threaded metal inserts and self-tapping screws. Structural applications require much stronger boss walls like those (f) in Fig. 8.196. As the circle in the lower left comer illustrates, the 25% maximum wall increase rule is exceeded by a considerable... 

The requirement for assembly of castings is limited since casting can make complex shapes. The most common methods are adhesives and fasteners. Self-tapping screws can be used with most of the materials used for casting. Threaded inserts can be molded in place, however they are difficult to salvage in the event of a reject moldment because the material is a thermoset and the inserts cannot be melted out. Threaded inserts can also be glued or tapped in place. 

It is possible to strip undercuts for parts molded of these processes in some cases and threads can be molded-in. However, side actions or split cavities, as would be required for details A and G, are more commonplace. Threaded inserts, like those at E and H, can also be molded-in. However, it may be more advantageous to emplace them after molding with adhesives since the molding cycle will be shorter and the risk of one misplaced insert ruining an entire moldment is ehminated. The other common methods of assembling compression, transfer, and thermoset injection-molded parts are self-tapping screws (holes can be tapped), adhesives, snap-fits, press-fits, and the usual screw and bolt techniques. None of the plastics welding techniques can be used with thermoset parts. 

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. 

The high modulus and creep resistance of reinforced PPS compounds make them more suitable than many other plastic materials for fastening with bolts, screws, or rivets. For one-time bolted assemblies, tapped or molded-in threads with bolts screwed in three to four diameters may be used. However, with bolted assemblies where repeated assembly and disassembly of parts are expected, or if torque retention at elevated temperature is a concern, ultrasonic or molded-in metal inserts may be used. Molded-in metal inserts should be preheated to the mold temperature before being placed in the tool for molding. For assembly with self-tapping screws, BF-type or BT-type steel thread cutting screws work well, using hole sizes as recommended for phenolformaldehyde thermosets by the... 

Used for inserting screws of all types, including machine, self-tapping, self-drilling and tapping... 

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. 

A self-tapping screw with boss cap can be torqued 40-100% higher than pressed-in or molded-in inserts. It will also limit thermal expansion of the boss. 

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). 

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