Thread forming screws

A number of thread-forming fasteners are especially designed for use with plastics (Figure 7.8). The type used will generally depend on the modulus of the plastic and the specific requirements of the assembly operation. Characteristics of several common and specialty thread-forming screws are summarized in Table 7.3. 

TABLE 7.3 Characteristics of Various Types of Thread-Forming Screws... 

Similar to thread-forming screws, thread-cutting screws should also not be installed and removed repeatedly. If frequent removal and reinstalling of the screw must occur, the boss should be large enough to allow replacement of the screw with the next larger size. 

AB for thread forming screws, type BF or BT/25 for thread cutting screws. These screws provide for more plastic between each metal thread than most of the other types. Specially designed screws for plastics are also available. 

Thread-forming screws are used in the softer, more ductile plastics with moduli below 1380 MPa (200,000 Ih/in ). There are a number of fasteners specially designed for use with plastics (Fig. 9.10). Threadforming screws displace plastic material during the threading operation. This type of screw induces high stress levels in the part and is not recommended for parts made of weak resins. 

ABS, and thread-forming screws for foamed grades. Depending on the application, the use of bosses and boss caps may be advantageous. 

Table 5-6 presents the optimum pilot hole diameter, and driving and stripping torques of self-tapping screws for 30% glass-reinforced polyester. Both thread forming screws and thread cutting screws can be used. 

Figure 26.2 Deformation of plastic around threads for thread-forming screw.

Thread forming screws displace the plastic material in the hole so that it plastically flows around the screw thread thereby cold forming the thread in the plastic [6-8], as shown in Fig. 26.2. Since no material is removed inside the hole, it is important to leave a sufficient gap between the thread root and the inside diameter of the hole in the plastic for the displaced material. Otherwise, the internal stresses in the hole would be large and may lead to premature failure... 

Fig. 26.3 shows examples of standard thread forming screws, Trilobe screw, and Hi-Lo screw. Thread forming screws are typically used with soft plastics that have a flexural modulus of 200,000 psi or lower, but in some cases, with sufficient clearance they can also be used with plastics that have a flexural modulus of 400,000 psi or lower [6]. 

FIGURE 12.14 Three-lobed thread forming screw. 

---