Crews were called to Charleywood Road, Kirkby, at 23:00 BST on Tuesday.
A large amount of plastic is on fire, with firefighters expected to stay at the scene for a number of hours.
Merseyside Fire and Rescue Service said the blaze has been contained, preventing it from spreading to other businesses in the area.
A large fire is being tackled by 70 firefighters at a Merseyside industrial unit, with local residents urged to shut windows due to smoke.