How do I fix kitchen sink drain elbow that comes loose?

It is a double sink and there is a connection underneath each drain, with a nylon washer. I am able to bunt it on but obviously that is not tight enough in the long run. It is a SS sink and so probably vibrates. Anyway, if it will not tighten does that mean the nylonwasher is bad or what? I have some teflon... show more It is a double sink and there is a connection underneath each drain, with a nylon washer. I am able to bunt it on but obviously that is not tight enough in the long run. It is a SS sink and so probably vibrates.
Anyway, if it will not tighten does that mean the nylonwasher is bad or what? I have some teflon tape, would wrapping the pipe make a tighter fit? What doe I need for a permanent cure. I know this is a common problem but don't know the cure. It is PVC pipe.
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