Sounds like you need to play with the focus.
The charring indicates you have too wide of beam or are going too slow. Possibly both. I had similar until I got the focus set properly.
There is no fixed focal length. You have to focus by hand. That is what the "weak" button is for. It is easier to focus using BenCutLaser. It has a focus function in the second tab from the left. It cuts a line then you readjust, cut again, until you get the smallest line possible. You need to do this with a piece the same thickness as the piece you are cutting. It will take several runs to cut through 3mm. You will need to change the focus deeper for each run. There is tool in the forum that connects to the focus ring so you can focus easier during work. @Ralph the writer of BenCutLaser and the Admin of the forum designed the tool.
From what I read some plastics melt back and reattach. A fan may help to cool the plastic during cutting. There should be more info in the forum about cutting for you to read. I am new to this too. Until I played with the focus I had problems too.