You get a flame and put it to the plastic
Bascially there is something in the plastic that resists most screen inks. However, purchase one of those little gas welders that use the cannisters, and then run the flame up and down the bottle three or four times from a distance of about 4 inches before printing. This melts the plastic enough so that the ink welds into the plastic when it is printed instead of sitting on top.
Once printed and air cooled you should find that the ink will not scratch off.
Not sure on the exact ink we used, but Julie from Spectrum Screen Inks sell excellent inks and I would say it came from them.
Flaming in general works for a whole host of items. As another example I was once told that no one in the UK can print the silicon wrist bands you see everywhere. Apparently the ink rubs off after 24 hours. Not so, again, ink from Spectrum Screen Inks sell an ink that works once the item is flamed. I am not sure on the exact ink but if you are interested I can find out from Julie. Could also give you a contact of someone who orders loads of these and also the prices they currently pay. And would love to see someone get in there and get one over my ex business partner scum..... (Have I gone off topic

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USING: Whatever it takes to get the job done...