In the past, crushing was done between mill stones that later became steel rollers. Hence the factories are known as oil mills and the process as oil milling or oil crushing. Seeds with a high oil content, like rape seed and sunflower seed, are usually mechanically pressed in expellers after a preheating step in indirectly heated conditioners. The expeller cake (or pressed cake) will then be further treated in the extractor, as it might still contain up to 18 per cent of oil. In some cases, the expeller cake may not be further extracted but sold as such for feed purposes.

In the expeller, the oil-bearing material is fed into one end of a cylinder within which a power-driven worm conveyor forces the material to the other end of the cylinder and out against resistance. The pressure exerted in the process squeezes out the oil.