Why is the back of the plank I cut badly charred?

Make sure you have raised the planks. If it is placed directly on the flat steel plate, the gap between the wood board and the flat steel plate is very small. When the laser passes through the wood board, the flat steel plate cannot absorb all the laser energy, and the remaining laser reflection will burn the wood board. Please use honeycomb laser bed or raise the board to keep the cutting position and the board hollow.