Actually I would say that 15mm is unnecessary. Anything from 8mm-12mm would suffice in my opinion. Carcinus uses 12mm HDPE for armour and I'm confident that the important bits are suitably protected. 12mm upwards is a waste of weight and starts to make the robot too bulky looking (again, just my opinion). If you really want thicker HDPE than that, then layer it up in localised areas - perhaps at points where you reckon your robot could be more prone to receiving impacts - rather than spreading it all over your robot.