Most technical membranes for gas and liquid filtration with filter grades in the sub-micron and the molecular range are made from polymers. Producing flat sheet membranes is quite similar to foil production, while hollow fibre membranes are made by spinning through a ring spinneret by means of a central air or water jet. The chemical composition of the casting and spinning solution respectively, as well as the process parameters, influence the pore size distribution of the membrane. Colloids or solid particles in the solution must be avoided in order to exclude membrane defects (like outsized pores) or embeddings that affect mechanical stability. Leading membrane producers use Seebach filters for pre-treating polyethersulphon solutions prior to the production of ultrafiltration membranes.