Thread-cutting 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-cutting screws are used in harder, less ductile plastics. Thread-cutting screws remove material as they are installed, thereby avoiding high stress. However, these screws should not be installed and removed repeatedly. 

ABS parts can be designed for snap-fit assembly using a general guideline of 5% allowable strain during the interference phase of the assembly. Thread-cutting screws are frequently recommended for nonfoeuned... 

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... 

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. 

For the purposes of producing threads in most thermoplastics, a standard metalwork tap and die can be used but it is recommended that the drilled hole be countersunk prior to tapping. Thread cutting screws are most suitable for use with thermoplastics, and screws with thread pitches of standard machine screws are to be recommended. 

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