What challenge did the Bíró brothers face when adapting newsprint ink for their ballpoint design?

Answer

The thick newspaper ink was too viscous to flow reliably through the ball-and-socket mechanism

The brothers had to develop a specific, viscous, oil-based ink formulation that was thick enough to resist drying out in the reservoir but fluid enough to be distributed correctly by the rolling ball.

What challenge did the Bíró brothers face when adapting newsprint ink for their ballpoint design?
Historyinventorpatentballpoint penwriting instrument