The calibrate_pp file itself can be used to clean the print heads
(you should do it if left the printer unused for a long time). Those two lines
can be used to do it. The first one cleans the print heads of the color cartridge,
and the second line the ones of the black one.
calibrate_ppa --clean | pnm2ppa -p -i - -o - | lpr -l calibrate_ppa --clean | pnm2ppa --bw -i - -o - | lpr -l