The higher the flex modulus the more brittle or rigid the material will be and the less creep it will normally have. Thus the flex modulus can be used as a general guide as to what type fastener is needed to make your threaded joint in plastic. Generally you can 'guesstimate' the flexural modulus with a sharp pocketknife. If you cut along a sharp edge of the material and get a nice curl the flexural modulus is around 200,000 or less and you can usually use a thread former. Also most plastic manufacturers do not have a good understanding of the relationship between threaded fasteners and plastic parts.
Most assemblers use self-tapping screws to assemble plastic parts.
There are two major categories of self-tapping screws. They are thread cutting and thread forming. These screws produce the best results in plastics having a flex modulus over 400,000. They can be used with some success in plastics with a flex modulus of 200,000 - 400,000. Thread forming screws move, or deform, the plastic material to produce the threads in the plastic part.