Two ways to make a thread
A thread-forming screw displaces material. It pushes plastic aside as it enters, and the plastic cold-flows around the thread and then relaxes against it. The result is a joint with no material removed and, in a thermoplastic, a strong and slightly elastic thread.
A thread-cutting screw removes material. It has cutting edges that carve a thread as it is driven, and the chip has to go somewhere. That suits brittle materials and highly filled resins where displacement would crack the boss rather than flow.
Why forming is the default for thermoplastics
Thermoplastics flow under stress, which is exactly what forming needs. Because no material is removed, the thread has full engagement, and because the plastic relaxes against the screw, the joint has a built-in resistance to loosening that a cut thread does not have.
The limit is stress. Hoop stress in the boss rises with the amount of displaced material, so the boss wall thickness and the pilot hole diameter have to be designed for the screw rather than assumed.
When cutting is the right answer
- Highly glass-filled resins, where the filler makes the material brittle and displacement cracks it
- Thermosets, which do not flow and cannot be formed
- Very thin walls where the displaced volume has nowhere to go
- Situations where a small amount of chip is acceptable and boss cracking is not
The design rule that matters most
Aim for three to five full threads of engagement. Fewer than three and the joint strips under load; more than five adds driving torque and boss stress without adding much strength, because the load is carried by the first few threads in any case.
And remember the service question. A formed thread degrades each time the screw is re-driven, because each drive cuts slightly deeper into the plastic. If the product will be opened in service, the position needs a machine screw and an insert, not a better self-tapping screw.