Ink for technical pens is designed to be an extremely opaque black. The original purpose was to make drawn lines on transparent paper and film as opaque as possible to speed up the diazo copying process by which large drawings were mainly duplicated.
This opacity was achieved by using soot as the black pigment and binding this in a mixture of shellac and an adhesive liquid.
The Rotring cartridges for the Rapidograph Pen and the eyedropper bottle for the Rapidograph have ink designed for use on tracing paper. Tracing paper has an absorbent surface that assists drying time.
The Staedtler M2 ink is formulated for paper and drafting film. The basis of film is polyester and is totally non absorbent so an ink that adheres and dries on this surface in a timely manner is required.
The compromise with faster drying film ink is that extra care is required to stop it drying up in the pen nib.

Rapidograph Ink Cartridges

Replacement Capillary Ink Cartridges for Rotring Rapidograph Technical Pens. The Cartridge Incorporates the Breathing Helix, Resulting in a Low Maintenance Pen with Reliable Ink Flow.

Rotring Drawing Ink for Paper

Rotring 23ml Drawing Ink for Tracing Paper - Black Archival Quality Pigment Ink, Finely Processed to Flow Through Technical Pen Nibs.

Staedtler Drawing Ink for Film

745 M2-9 22ml Ink Bottle Suitable for both Tracing Paper & Polyester Based Drafting Film. Black Archival Quality Pigment Ink. Finely Processed to Flow Through Technical Pen Nibs.

Koh I Noor Ink

Koh-I-Noor Ink is available in 22ml bottles with convenient tapered tip for refilling Rotring Isograph and Staedtler Mars Matic Technical Pen ink reservoirs.