Two popular techniques exist that get the ink from the cartridge to the paper with a large degree of precision—thermal and piezoelectric printheads. Both achieve the same final result, using two different delivery systems.
Thermal printheads utilize the heat property, which explains how increased temperature will expand the volume of a substance if all other conditions are kept constant. There are hundreds of microscopic pores in a printhead, which heat is generated through electricity. Then creates an expansion in the ink and forces the ink out of the printhead. This simultaneously creates a vacuum in the printhead that pulls ink from the cartridge to fill the printhead once more. The limiting factor in the number of vacuum holes is how close they are to one another, as the thermal energy cannot be transferred from one to the other, as this would cause imprecise dot placement. Canon was the first company to develop and utilize a thermal printing system, which is still one of their more popular methods.
Piezoelectric printhead delivery systems provide the same result, using a different method to extract the ink out of the printhead. Instead of a thermal pulse physically expanding the ink and forcing it out, an electric charge expands a piezoelectric crystal in the printhead. This forces ink out of the printhead and when the crystal returns to its previous size, creates a vacuum that draws ink out of the cartridge and fills up the printhead with ink once more. The advantages to this method are: there is no thermal charge, which can affect the chemical properties of the ink and make it dry more quickly and there is not heat damage to the delivery system. Epson uses a piezoelectric printing system in their printers.
Both printing systems utilize the same principle of expansion to drive ink out of a cartridge. This method has the capability of controlling the quantity and speed of the ink leaving the cartridge through the electrical or thermal strength of the charge. While the properties of both are very similar, piezoelectric printing does have the advantage of not having to rely on the expansion of liquids with varying densities, meaning one type of ink will not expand at the same rate as another, so the thermal pulses must be calibrated for every different type of ink. With a piezo system, the crystal expands at one constant rate, therefore the type of in used in not a factor, the quantity of ink exerted out will remain constant.