Three workers were injured — one of them critically — in an explosion outside a plastics factory in northeast Minneapolis this morning. Assistant Fire Chief Cheri Penn says the men were replacing a cap on an exterior tank at the Interplastic Corporation when the blast occurred. One of the workers suffered second and third degree chemical burns and was taken to Hennepin County Medical Center in critical condition. A second worker suffered serious injuries. The cause of the blast is under investigation. None of the chemical in the tank spilled.