You can freely build on How Thermal Inkjet Printers Use Two-Step Heating to Shoot Ink

What this patent 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 is now free to use

All 0 claims of US 4490728 are in the public domain. Specifically:

The 0 dependent claims add narrowing limitations and are also free.

What is NOT covered

Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.

• 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.

Who is building on this today

Hewlett-Packard built its entire consumer printing empire on this thermal inkjet foundation. Competitors like Canon and Lexmark developed similar thermal technologies, while Epson chose to focus on competing piezoelectric printheads.

Products built on expired version of this technology

How to cite this patent in your documentation

Hewlett Packard Co. US Patent 4490728. Thermal ink jet printer. Granted 1984, expired 2002. Now in the public domain.

Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.

PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.