Threads can be molded into your plastic part on either the I.D. or O.D. surface.
An O.D. thread design must include a non-threaded surface, or land, extending from one end of the thread approximately one and one-half times the thickness of the thread. I.D. threads should be a maximum of 0.050 (1.27mm) long, and no finer than 32 threads per inch (12 threads/cm), as fine threads molded in plastic tend to be very weak.