In 1994 all 300-dpi thermal ink jet pens manufactured by HP had firing chambers with a single inlet. With this design, particles are a major contributor to yield loss and quality degradation. Particles can lodge in an inlet channel, block ink from entering the chamber, and result in a scrapped pen. Particles also float in and out of channels and cause intermittent defects in print quality.The authors were challenged to design a pen that would be robust to particles with no degradation to manufacturing yield, pen fluidic performance, or print quality. The project started with simple hydraulic modeling, concept development, and peer review. Then design proliferation began. Since the pen was in manufacturing, the challenges normally present in an R&D project (no printer, no ink, no print quality standards, no printer drivers) were absent. Approximately 50 designs were created, built and tested in 6 months, leading to the successful design, which was ultimately patented. Since then, about ten other particle-tolerant architectures have been designed and introduced into manufacturing. The design concept has proved successful over drop weights from 5 to 150 nanograms. The concept not only guards against particles but facilitates control of drop trajectory and pen operating frequency.