LISBON, Maine -&nbspThe shutdown of a fiberboard manufacturing plant in Lisbon has left about 80 employees out of work.

Knight-Celotex, the town’ s third-largest employer, was closed Friday morning. An attorney for the Chicago-based company says management hopes to reopen in a month, but the duration of the shutdown is considered indefinite and will be reviewed week to week.

Charles Micoleau linked the closing to the slow economy’ s impact on sales and the rise in the plant’ s operating costs, particularly for natural gas and electricity.

Micoleau emphasized that the closing was unrelated to an April vote by employees to be represented by the Machinists union.