After setting a reasonable tension, a minimum change in tension should be maintained. The tension instability is mainly caused by the failure, damage or unstable air pressure of the control components of the automatic tension control system. In addition, the causes of tension changes are: 1, Changes in temperature and humidity (changes in storage environment). 2, The time-dependent change of the substrate. 3, The thickness of the substrate changes. 4Speed changes during operation (caused by the process of acceleration and deceleration, unstable motor operation, unstable changes in load, etc.) 5, The change of the friction coefficient of the contact surface between the film and the relevant guide roller.