Have you ever needed to write on plastic and the ink didn't stick even when you used permanent marker. Well I work in a company that prints on plastic all the time. We prepare the plastic to get the ink to stick by oxidizing the surface first. In short we pass the plastic past a flame that lightly "burns" it. But not enough to see a discoloration.
Step 1: Writing With a Marker
So you want to write something. Try writing on plain plastic.
Step 2: Ink Comes Off
Rubbing a finger across the ink causes it to come off.
Step 3: Oxidize Surface
Step 4: Mark the Surface
Step 5: Rub the Ink
Rub on the area with marker and flame treated. It should resist coming off.
Step 6: Candle May Cause Soot
A candle may cause a soot mark like the one in the picture. Using a small propane torch instead of a candle might avoid this.