OK - your torquing a plastic screw into 18 gauge material - frankly, I am not surprised that you are having problems.
I know very little about screws made from plastic, but I am sure that it is much softer than your metal part - btw - what type of material are you using?
Could you have some burring on the threads of the metal part that is cuting into the plastic part?
Could the screws be cross threaded when inserted?
Have you checked both components with go/no-go gages? I would highly reccommend doing this, especially on a defective part.
You stated that you measured the screw - what did you measure? Major diameter, pitch diameter?
Have you considered replacing the threads in the metal part with a threaded insert such as a PEM type nut?
I know very little about screws made from plastic, but I am sure that it is much softer than your metal part - btw - what type of material are you using?
Could you have some burring on the threads of the metal part that is cuting into the plastic part?
Could the screws be cross threaded when inserted?
Have you checked both components with go/no-go gages? I would highly reccommend doing this, especially on a defective part.
You stated that you measured the screw - what did you measure? Major diameter, pitch diameter?
Have you considered replacing the threads in the metal part with a threaded insert such as a PEM type nut?