Laser marking of plastic


I’m having my snapmaker since a few weeks and I’m very happy with it.
I even were successfully in engraving wood, paper and plastic. Furthermore I used the 1.6W laser module for cutting wood and paper.

But now I’m getting stuck with the laser.

I know that plastic materials can be marked with the help of lasers: frothing (getting a light / brown color on dark materials) and carbonization (getting a dark color on bright materials).

For my tests I used a light gray (RAL 7035) PS plastic plate and a black ABS plastic plate.
I’ve played with the power and speed parameters for a long time but I couldn’t generate any laser marking. Either nothing happens or the plastic get engraved.

Can anyone give me a hint, how the parameters should be set (fast speed, low power, …)?
Or can the snapmaker laser module not be used for laser marking operations?

Thanks for our help in advance!