The printing bed is kept adjustable to accommodate for the varying thickness of different materials. Although we’re not quite sure about the maximum thickness that flatbed printers are capable of adjusting to but that number may go as high as 4-5inches. It can change, however, depending on the type and build of the printer. The more expensive ones may have more thick material compatibility and vice versa.
Having known that flatbed printers are capable of printing stuff on unconventional materials; unconventional in the sense that most people didn’t know that printing was possible on such materials, it’s not very far to comprehend that they are mostly deployed for commercial and industrial applications. This is because only one in a thousand people would be having a flatbed printer at home and even fewer people would be using it for domestic purposes.