HHO Cell End Cap Design: Materials and Sealing for Leak-Free Operation

End Cap Material Options

HDPE (High-Density Polyethylene): chemical resistance to KOH and NaOH, machineable, low cost, slight moisture absorption over long term. Best for thick (12–20mm) end caps where dimensional stability is important. Polypropylene: similar to HDPE, slightly better chemical resistance, lower moisture absorption, higher temperature resistance. First choice for serious builders. Acrylic (PMMA): visually transparent (allows seeing electrolyte level and bubble activity), but brittle and chemical-attacked by strong KOH over time. For short-term demonstration builds, not long-term service.

Port Placement

The electrolyte fill port and gas outlet port should be on opposite ends of the cell for best flow characteristics. Electrolyte in at bottom (if cell mounts vertically), gas out at top (buoyancy assists gas movement). If the cell mounts horizontally, position gas outlet at the highest point of the end cap to ensure gas collection and not electrolyte.

Bolt Pattern

Use stainless steel (316) bolts with nylon-insert locking nuts. Six bolts minimum for cells up to 6"×6"; eight bolts for larger cells. Torque bolts in a cross pattern (like lug nuts) to even compression across the gasket faces. Target bolt torque: 2–3 Nm for thin gaskets, 4–5 Nm for thicker gaskets — just enough to compress the gasket without cold-flow deformation.