thumb|250px|An expeller used for expeller pressing
thumb|right|[[Coconut oil is expelled from copra at an oil mill in Thrippunithura, Kerala, India]]
Expeller pressing (also called oil pressing) is a mechanical method for extracting oil from raw materials. The raw materials are squeezed under high pressure in a single step. When used for the extraction of food oils, typical raw materials are nuts, seeds and algae, which are supplied to the press in a continuous feed. As the raw material is pressed, friction causes it to heat. In the case of harder nuts, which require higher pressure, the material temperature can exceed 120 °F (49 °C). "Expeller" is a trademarked term of Anderson International Corporation since 1900, although it has become genericized, is often confused with screw press equipment in general, and does not indicate whether oil extraction is done hot or cold.
Description
An expeller press is a screw-type machine that mainly presses oil seeds through a caged barrel-like cavity. Some other materials processed with an expeller press include meat by-products, synthetic rubber and animal feeds.
Raw materials enter one side of the press and waste products exit the other side.
Design
Continuous screw
The earliest expeller presses utilized a continuous screw design. Anderson observed that in the continuous flighting arrangement of a compression screw, there are tendencies for slippery materials either to co-rotate with the screw or to pass through with minimal dewatering. He wrote that "brewers' slops, slaughterhouse refuse" and other "soft and mushy" materials dewater poorly in continuous screw presses.
His invention consisted of putting interruptions in the flighting of a compression screw. It was much like having a hanger bearing in a screw conveyor: there is no flighting on the shaft at that point, so material tends to stop moving and pile up. It is only after solids accumulate in the gap that the downstream flighting catches material. When this happens, material is forced along its way. The result is better dewatering and thus a more consistent press cake.
Resistor teeth
After the 1900 patent, a major improvement was made with the addition of resistor teeth. Fitted into the gaps where there is no flighting, these teeth increase the agitation within the press, further diminishing co-rotation tendencies.
Expanded applications
Applications of the interrupted screw design expanded to conditions of constant feed, at constant consistency. If either the consistency or the flow rate diminished, squeezing would diminish until it was inadequate for proper moisture removal.