A screw driven directly into plastic will strip after a few assemblies, so any part that will be opened more than once needs a metal thread. There are five ways to put one in, they need different things from the boss, and they differ enormously in pull out and torque strength, which is what the choice should be made on.
The five types
Heat set inserts are pushed in with a heated tip so plastic flows into the knurling. Ultrasonic inserts do the same with ultrasonic energy and are the production equivalent. Press in inserts rely on interference alone. Moulded in inserts are placed in the tool before the shot and are the strongest. Self tapping screws for plastics cut their own thread and are the cheapest where the joint is made once.
What each needs from the boss
Heat set and ultrasonic need a hole to the manufacturer's specified diameter and depth, with enough wall around it to take the hoop stress. Press in inserts need a tighter hole and more wall still. Moulded in inserts need the tool to locate them and enough plastic around them that shrinkage does not crack the boss. Self tapping screws need a boss designed for the specific screw's thread form.
Choosing on pull out and torque
Moulded in is strongest, then ultrasonic and heat set, then press in, with self tapping screws last for repeated assembly. Pull out resistance comes from the knurling and undercuts engaging melted plastic; torque resistance comes from flats, flutes or a hexagonal profile. If the joint sees vibration or repeated servicing, choose from the strong end of that list.
Practical mistakes
Using a hole size taken from a different manufacturer's table. Bottoming the insert on the floor of a blind hole so it stops proud. Installing into a boss with too thin a wall, which splits. Installing too fast so plastic does not have time to flow. And, on printed parts, orienting the boss so pull out acts along a layer boundary.
Questions people ask about threaded inserts for plastic
Which threaded insert is strongest?
Moulded in inserts, because the plastic forms around them under pressure. Ultrasonic and heat set are next, then press in, with self tapping screws weakest for repeated assembly.
What hole size should I use?
The one published by the manufacturer of the specific insert, for the specific material family. Insert tables are not interchangeable between makers.
Can I use inserts in 3D printed parts?
Yes, and heat set inserts are the standard answer. Allow a slightly more generous boss and orient it so pull out is not along a layer boundary.
What stops an insert spinning?
Flats, flutes or a knurled profile that the plastic keys into. Pull out and torque resistance come from different features, so check the insert offers both.