As name suggests, expellers are mechanical machines used to expel / separate oil from the oil-bearing seeds. Now a days most of the expellers consists of screw type shafts enclosed in a perforated cylindrical chamber, hence the expeller is also called as Screw Press.
In a typical screw press, the feed material fed at the one end of the screw shaft and the De-oiled cake/Press Cake is collected the opposite end of the screw shaft, as the feed material travels in the annular space between screw shaft and perforated oil chamber, the oil seeds are subjected to high mechanical pressure which results in rupture of the oil cells. The oil separated from the seed, seeps through the perorations in the oil chamber and the defatted/pressed cake is collected at the opposite end of the shaft.