Machine Screw type threads vs. thread forming threads

[kjustice]

I need some general information or direction in finding information about the use of machine screw threads vs. plastite type thread forming threads.

I have a customer that is insistant on using a #4-40 machine screw to thread into a molded plastic part. (This is causing assembly problems and breakage in finished goods.)

I bleive the correct thread for this application should be a plastite type thread forming thread.

What I can not seem to find is a source of infomation to validate either usage.

Can someone point me in the right direction?

Thanks,
Ken