5. Process stability

The equations written in the previous paragraphs to model the spinning process are stationary (time-independent) and allow us to calculate speed, cross-section and stress fields that are constant over time. In reality, however, an industrial process is always subject to minor flow or temperature instabilities, due for example to imperfect control systems. The process is viable if these faults remain limited, or are damped out at best. However, in the case of hydrodynamic instability, these faults are amplified and a periodic flow (itself perfectly regular in this case) takes the place of the stationary flow. This leads to variations in solidified wire diameter of up to 100%, which is of course unacceptable for this process.