One of the injured was hospitalized in critical condition while the others were treated at local hospitals and released.
The blast at the Oremet-Wah Chang plant, located about 65 miles south of Portland, apparently struck a department where up to 30 employees work the night shift.
There was no release of any toxic materials reported.
Oremet-Wah Chang manufactures titanium, along with other high-quality production metals. Its parent company is Los Angeles-based Teledyne Inc.
The explosion occurred during the titanium melting process and was followed by a smaller explosion, said Fire Department spokeswoman Wanda Omdahl.