York Region fire crews were called to the scene around 10 a.m. Wednesday (in the area around Mulock Drive and Bayview Avenue).
The explosion happened at the Dortec Industries plant, which is a division of Magna International Inc. (TSX:MG).
Magna says the incident took place on a manufacturing line and five employees were taken to hospital for "further evaluation." None of them were in life-threatening condition.
The plant's 200 employees were evacuated.
Deputy Fire Chief Paul Leslie says there was a small explosion in an area of the plant using machinery with pressurized parts.
He says there was no extensive damage to the inside or the outside of the plant.
The Ministry of Labour and the Ontario Fire Marshal's office have been called in to investigate.
(CFRB, The Canadian Press)