Sparks from a drilling machine apparently spread to the explosive material stored at some distance and caused the explosion, police said.
The sound of explosion could be heard outside the 110-year-old factory, producing nitrocellulose, nitroglycerin and propellants for small arms and ammunitions, police said.
Of the injured, three workers were taken to Coimbatore, two others admitted in the nearby Wellington Military Hospital and the remaining in the factory hospital.
Five workers had lost their lives in a similar blast in the factory when mixture of explosive dough exploded in November 2010.