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.
Original patent title: “Thermal ink jet printer”
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. Granted to Hewlett Packard Co in 1984 with 5 claims and 329 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
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.
The gap
What does this patent NOT 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
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.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
HP ThinkJet printer
HP DeskJet series thermal printheads
Canon Bubble Jet printheads using dual-pulse warming
Why it matters
The bigger picture
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.
Filed
September 7, 1982
Granted
December 25, 1984
Market context
Who's building on this
Companies in this space
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.
Market impact
This technology transformed home and office printing by making high-resolution color printers cheap and quiet compared to noisy dot-matrix printers. It triggered decades of patent litigationlitigationA lawsuit over patent infringement. Litigated patents often signal commercial importance.Read more → and cross-licensing deals between HP and Canon over thermal printhead designs.
Claim 1 — Plain English
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.
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.
What it does not 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.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Moderate
Citation count
40/40
Highly cited
Claim breadth
3/20
Moderate scope
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$32K – $104K
Midpoint $65K · expired or expiring · industry ×0.9
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
The original legal language
Original claims
5 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Vaught, J. L., Cloutier, F. L., Donald, D. K., Meyer, J. D., Tacklind, C. A., & Taub, H. H. (1984). How Thermal Inkjet Printers Use Two-Step Heating to Shoot Ink (U.S. Patent No. 4,490,728). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/4490728/thermal-inkjet-printing
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
Frequently Asked Questions
What does How Thermal Inkjet Printers Use Two-Step Heating to Shoot Ink cover?
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.
Who owns patent US 4490728?
Hewlett Packard Co owns this patent, granted in 1984.
When does this patent expire?
This patent has expired and is now in the public domain — anyone can use the invention freely.
What is patent US 4490728 cited by?
This patent has been cited by 329 later patents that build on its ideas.
What problem does this patent solve?
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.
What does this patent NOT cover?
Does not cover inkjet printers that use piezoelectric elements to mechanically squeeze ink out of the nozzle.
Patent monitoring