A printing system for forming a three dimensional object is disclosed. The laser diodes are comprised in standard packages including a base plate serving as cooling surface of the laser diode, a metal housing arranged on the base plate to protect the laser diode, and at least two driving pins which extend from the laser diode through the base plate and which are used for driving the laser diode within the package. A stair-climber incline is extended slantingly downward and rearward from near a front of a bottom of the cart basket to the wheels.