The two-alarm blaze erupted at the Plaskolite West plant in the 2200 block of East Del Amo Boulevard around 6:50 p.m.
A crew of 70 firefighters knocked the fire down in about 90 minutes.
Three civilians and a firefighter were taken to local hospitals for treatment.
One civilian was treated for burns while another suffered from smoke inhalation. No information on the third person's injuries was released.
The firefighter was treated for minor injuries and released.
The fire reached some chemicals used in plastic production, County Fire Inspector Tony Akins said, but these were regarded as an irritant more than as a danger.
The cause of the fire remains under investigation.
CNS contributed to this report.