1. Solution spinning method

Solution spinning is the most commonly used method of forming hollow fiber ultrafiltration membrane. The basic principle of its pore formation is that the solvent and the non-solvent undergo double diffusion during the entire coagulation process of the filament, which makes the polymer solution into a thermodynamically unstable state, resulting in liquid-liquid or solid-liquid phase separation, and the polymer is solidified in rich phase. The bulk of the membrane is formed, while the polymer-poor phase forms the so-called pore structure.
The formed hollow fiber ultrafiltration membrane usually has a tight layer on the inner and outer surfaces of the membrane, and a finger-like pore structure inside as a support point.
It is divided into dry/wet phase inversion method and wet/wet phase inversion method. The difference of the spinneret, the difference of the material of the support layer and the surface dense layer.

2. Melt spinning-drawing method

The melt-spinning-stretching method is suitable for the preparation of gas separation membranes, and the ultrafiltration membrane pore size is formed according to the evaporation of the supersolvent.
The key to this process is melt extrusion of the polymer under high stress. During the post-stretching process, the lamellar structures of the polymer material arranged in parallel perpendicular to the extrusion direction are pulled apart to form micropores, and then the pore structures are fixed according to the heat-setting process.
This method is basically bulk extrusion of molten polymer and does not require solvents and recycling. The processing technology is simple and the productivity is high, but the regulations are strict and the technical operation is difficult.

3. Thermally induced phase separation method

This method is a spinning process and also belongs to melt spinning, but its pore-forming principle is very different from that of melt spinning.
It combines polymers with some high-melting, small-molecule water chemicals to form a homogeneous liquid state at a sustained high temperature. During the whole cooling process, solid-liquid or liquid-liquid phase separation occurs in the film-forming system, and then the diluent is removed by means of extraction, etc., so as to obtain polymer raw materials with a microporous plate structure.
Since the whole process of pore formation by phase separation of hollow fiber ultrafiltration membrane is driven by temperature change, this method is called thermally induced phase separation method.