# 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. - **Patent:** US 4490728 - **Original title:** Thermal ink jet printer - **Owner:** Hewlett Packard Co - **Granted:** 1984 - **Status:** Public domain (expired) - **Times cited:** 329 - **Field:** consumer_electronics, mechanical ## What it does 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 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. ## 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. ## Real-world examples 1. HP ThinkJet printer 2. HP DeskJet series thermal printheads 3. Canon Bubble Jet printheads using dual-pulse warming ## 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. ## 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. **Full plain-English explainer:** https://patentbrief.org/patent/us/4490728/thermal-inkjet-printing **Original patent:** https://patents.google.com/patent/US4490728 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Canon's Bubble Jet Printers Make Ink Droplets](https://patentbrief.org/patent/us/4723129/inkjet-bubble-jet-printing) — Canon's 1988 patent on bubble jet printing uses a tiny heater to instantly vaporize ink, creating a bubble that pushes out a droplet of ink from the printer head. - [How Piezoelectric Inkjet Printing Works](https://patentbrief.org/patent/us/3946398/drop-on-demand-inkjet) — A 1970 patent describing how to print images by using electrical pulses to bend a tiny crystal plate, squeezing individual ink drops out of a nozzle on demand. - [How Laser Printers Use Rotating Mirrors to Write Information](https://patentbrief.org/patent/us/3867571/laser-printer-starkweather) — A 1972 Xerox patent describing how to use a spinning mirror to scan a laser beam across a page, adjusting the speed of the data to keep the image sharp. - [How 3D Printers Build Objects Layer by Layer from Liquid](https://patentbrief.org/patent/us/4575330/stereolithography-3d-printing) — This patent describes the foundational method for 3D printing, where a machine builds a three-dimensional object layer by layer by hardening a liquid material with light or other energy. - [How Printers Save Toner by Dropping Light-Colored Pixels](https://patentbrief.org/patent/us/6476836/image-forming-apparatus-having-a-function-of-saving-developing-agent) — A Ricoh patent for printers that save toner by completely ignoring very light pixels and using a special dot pattern for darker ones.