How Thermal Inkjet Printers Use Two-Step Heating to Shoot Ink

Hewlett-Packard's 1982 patent on a two-stage electrical pulse method that preheats ink before vaporizing it, allowing thermal inkjet printers to reliably eject precise droplets without clogging.

What it covers

The patent describes a method for ejecting ink droplets from a tiny nozzle using a heating resistor. Instead of hitting the resistor with a single blast of electricity, the system sends two distinct pulses. First, an electrical precursor current pulse preheats the ink to just below its boiling point without creating bubbles. This precursor pulse decreases over time proportional to the square root of the inverse of time. Second, a rapid nucleation pulse spikes the temperature to near the ink's superheat limit, instantly vaporizing a tiny layer of ink. This explosive bubble expansion acts like a piston, forcing a single, clean droplet of liquid ink out of the nozzle.

What it doesn't cover

• —Does not cover inkjet printers that use piezoelectric elements to mechanically squeeze ink out of the nozzle.
• —Does not cover thermal inkjet systems that use a single, uniform electrical pulse rather than a two-part precursor and nucleation sequence.
• —Does not cover preheating methods where the precursor pulse current does not vary substantially as the square root of the inverse of time.
• —Does not cover systems where the vaporized ink itself is allowed to escape through the nozzle orifice.

The clever bit

The clever bit is using a precursor pulse shaped specifically to the square root of the inverse of time. This exact mathematical curve matches the natural thermal diffusion of the ink, warming it up perfectly evenly without accidentally triggering premature boiling.

Why it matters

This patent solved a major reliability issue in early thermal inkjet development: thermal shock and inconsistent bubble formation. By splitting the electrical pulse, HP could control the ink's viscosity and thermal state right before ejection, paving the way for the commercial success of the HP ThinkJet and subsequent DeskJet lines that dominated home printing for decades.

Real-world examples

• 1.HP ThinkJet printer
• 2.HP DeskJet series thermal printheads
• 3.Canon Bubble Jet printheads using dual-pulse warming