Screw Terminology

I am specifying screws to hold together a plastic enclosure with plastic bosses.

Are the descriptions for “Self Tapping” and “Thread Forming” screws the same and used interchangeably?

In otherwords are they 2 ways to say the same thing?

I believe there are different kinds of self-tapping.

Self-tapping alone just means that you don’t have to tap them to drive them in. Thread-forming screws forms descent threads meant for a screws that is to be removed and reinstalled. Thread-cutting screws have a taper on the end similar to a bottoming tap so they can cut beefier machine screw threads. I have used generic production enclosure to do mock-ups and I have seen them come with all kinds. The cheaper enclosures come with self-tapping screws that make threads that don’t wear well.

Mcmaster-Carr is a good illustrated reference… McMaster-Carr

Different kinds of plastics handle the stresses of metals screws in different ways. There are screws designed with a cutter at the bottom to remove plastic for the following threads, and some designed with different configurations to deform in a thread. It is best to look up a manufacturers design guide for the type of plastic you are working with and they will specify the type of screw needed. Some plastics such as grade of PC react really strangely to screw or fasteners.