After pre-heating and melting, the test sample is pressed with a defined flow speed through a capillary nozzle.
http://www.thermo.com/com/cda/technology/detail/1,2165,10000...

In the piezo method, the print head is made up of a printing chamber, a piezo crystal which influences the printing chamber through the vibration plate, an ink channel into the printing chamber and a capillary nozzle through which the ink is fired.
http://www.sbac.edu/~printrp/methods.htm