I am carving letters in plastic, what speed and feed do I need to use so I wont melt the plastic.
Speed through a material depends on the material itself, depth of cut, size and type of bit. I can only recommend a single flute or spiral up-cut but, and moderately fast feed rate. You’re wanting to ‘chip’ the plastic out, as the chips carry the heat away.
Create a journal of your trials: material, bit, feed rate, depth, etc. Keep trying, you’ll get it dialed in.