Plastic impregnation is a method used to fill the interconnected porosity of a PM compact with a polyester resin to enhance machinability, allow plating and to create a pressure-tight part for a sealing application.
Here is a good article from Modern Machine Shop Magazine which discusses the benefits of plastic impregnation for a PM component:
https://www.mmsonline.com/articles/increased-machinability-and-improved-finish-of-powdered-metal-parts