Inkjet Printing — How a Hot Wire Discovered the Bubble Jet

Canon's 1977 bubble jet patent describes the thermal inkjet process — where a tiny heater vaporizes ink to form a bubble that ejects a droplet — discovered accidentally when a researcher touched a syringe of ink with a hot soldering iron.

What it covers

This patent describes a method of propelling ink droplets from a nozzle using thermal energy. A small resistive heater element (a few microns across) sits inside the ink channel just behind the nozzle opening. When a brief electrical pulse (a few microseconds) heats the element to over 300°C, the ink in contact with it vaporizes instantly, forming a rapidly expanding bubble. The bubble's expansion pushes ink out through the nozzle as a droplet. When the heater cools, the bubble collapses and fresh ink is drawn into the channel by capillary action. The cycle repeats thousands of times per second for each nozzle. By precisely controlling which nozzles fire and when, the print head creates patterns of ink droplets that form characters and images.

What it doesn't cover

• —Piezoelectric inkjet (Epson's approach) — uses mechanical pressure from a piezo crystal rather than heat to eject droplets
• —Laser printing — uses static electricity and toner powder, not liquid ink
• —Continuous inkjet (CIJ) — a different approach where ink streams continuously and unwanted drops are deflected away
• —UV-cured or solvent-based industrial inkjet — different ink chemistries for industrial printing

The clever bit

The bubble jet was discovered independently and simultaneously at Canon in Japan and Hewlett-Packard in the U.S. in 1979 — both teams had the same accident. At Canon, researcher Ichiro Endo accidentally touched a syringe full of ink with a hot soldering iron; the heat vaporized ink and shot a droplet out of the needle. At HP, a similar accident occurred in the same year. Both companies filed patents, and the result was a cross-licensing agreement. HP called it 'thermal inkjet'; Canon called it 'bubble jet' — same physics, different name. The technology reduced the cost of a printer by a factor of 10 compared to daisy-wheel and dot-matrix printers, ultimately making inkjet the dominant home printing technology for 30 years.

Why it matters

Inkjet printing democratized color printing. Before it, color printing required professional offset presses or expensive laser systems. The bubble jet / thermal inkjet made it possible to print photographs at home for a few cents per page. HP's ink business became one of the most profitable in the company — selling ink at roughly $8,000 per liter for standard cartridges. The business model of cheap hardware and expensive consumables, pioneered by inkjet, has been applied to razors, coffee machines, and printers of all types.

Real-world examples

• 1.HP's ThinkJet (1984) was the first commercial thermal inkjet printer — it sold for $495 and used replaceable cartridges, establishing the 'razor and blades' business model of cheap printers with expensive ink
• 2.Canon's BJ-80 (1985) was the first bubble jet printer; Canon and HP's cross-licensing agreement allowed both companies to build on each other's patents
• 3.Modern inkjet print heads fire droplets at 18,000–30,000 per second per nozzle, with droplet volumes as small as 1 picoliter — enabling photographic-quality output from consumer printers

Glossary