All the injured persons were working on a machine when the incident took place, police said.
The cause of the blast is yet to be ascertained, SHO Khanna Sadar police station, Bhupinder Singh said.
However, the fire was controlled in time.
The injured were identified as Baldev Singh, Hans Raj, Balwinder Singh, Gurmeet Singh and Ravinder Singh.
Three of them who were stated to be critical were rushed to a Ludhiana hospital.