【In-depth Analysis】Precision Application of Film Slitting Machine in the Field of Lithium Battery Separator: How to Achieve Slitting Accuracy of ± 1μm

Lithium-ion batteries are the core of today's new energy era, and the performance of the separator is directly related to the safety, energy density and cycle life of the battery. The production of separators is an extremely precise process, in which slitting is a key process in the back of slitting a wide raw film into a specific width coil, and its accuracy requirements have reached the almost demanding ±1μm (micron) level. This is achieved through a system engineering that combines mechanical engineering, materials science, automatic control and sensing technology.

1. Why does lithium battery separator slitting require such high precision?

Understanding the "why" is the prerequisite for understanding the "how". ± 1μm accuracy requirement is not groundless, but is determined by the function of the separator and the process of battery manufacturing:

1. Ensuring battery safety: The core function of the separator is to isolate the positive and negative electrodes to prevent short circuits, while also closing the hole to block the current during overheating. Burrs, dust, or edge defects generated during the slitting process can puncture the diaphragm, leading to micro-short circuits and even thermal runaway during battery charging and discharging, posing serious safety hazards.

2. Improve battery energy density: In wound or laminated cells, the width of the separator must be highly consistent with the positive and negative electrode sheets. An overly wide diaphragm wastes space and reduces energy density; A diaphragm that is too narrow cannot completely isolate the positive and negative poles, resulting in a short circuit at the edge. ± accuracy of 1 μm is a prerequisite for maximizing the use of the internal space of the battery.

3. Meet high-speed automated assembly: Modern battery production lines are continuous at high speed. The slitting diaphragm roll must have a very high winding neatness (i.e., "rewinding meters"), otherwise problems such as correction failure and belt breakage will frequently occur on the subsequent automatic winding machine, which will seriously affect the production efficiency and yield rate.

2. Core technical barriers and solutions to achieve ± 1μm slitting accuracy

To achieve this extreme accuracy, the following five core barriers must be overcome and the corresponding top solutions must be given:

Barrier 1: Extremely stable tension control system

The slitting process is a dynamic process of unwinding, pulling, slitting, and winding. Small fluctuations in tension can cause the film belt to shake and tensile deformation, which directly destroys the slitting accuracy.

•Solution:

◦ Full closed-loop servo tension control: Uses a high-response servo motor to directly drive the rollers, replacing the traditional magnetic powder clutch/brake. The system detects the actual tension in real time through the tension sensor, compares it with the set value, and instantly adjusts the torque output of the servo motor through the PID algorithm to achieve fine-tuning of the tension at the milliNewton (mN) level.

◦ Multi-stage tension zone control: divides the entire slitting path into multiple independent tension control zones, such as unwinding zone, traction zone, slitting zone, and winding zone. The "floating rollers" or "dancer rollers" are used as cushions and sensors between the zones to achieve smooth transitions and decoupling of tensions to avoid interference with each other.

◦ Taper tension control: When winding, as the coil diameter increases, maintaining constant tension will cause the inner film to be squeezed and deformed. The system needs to automatically reduce the tension according to the specific curve (taper) according to the change of the coil diameter to ensure that the tightness of the winding is consistent and eliminate the phenomenon of "cabbage heart".

Barrier 2: Nanoscale Dynamic Guidance System (EPC)

The tape will inevitably produce micron-level deviations during high-speed operation, and must be corrected in real time.

•Solution:

◦ High-precision sensor: CCD (charge-coupled device) line scan camera or laser scanning sensor is used, and its detection accuracy can reach ±0.1μm. It captures the position of the tape edge or wire in real time.

◦ High-speed and high-precision actuator: The sensor transmits the position signal to the controller, which immediately drives the linear motor or piezoelectric ceramic-driven correction mechanism. The linear motor has a fast response speed and high positioning accuracy, and can be adjusted at the micron level in milliseconds, ensuring that the tape always follows the preset absolute path.

Barrier 3: Ultra-precision slitting tool system

The tool is the part that performs the slitting directly, and its condition determines the quality of the cut.

•Solution:

◦ Tool design and material: Circular cutter slitting method is adopted. The upper and lower cutterheads are made of ultra-precision ground high-hardness alloy steel or ceramic materials, and the sharpness, roundness, and straightness of the cutter edge must reach the micron level. The end and radial jumps of the tool must be strictly controlled within 1 μm.

◦ Tool engagement control: Overlap and Gap of the upper and lower blades are the core parameters. Through high-precision servo motors and closed-loop control systems, online micron-level adjustment and locking of these two parameters can be realized to adapt to different thicknesses and materials of diaphragms, achieving "clean and neat" shearing instead of "tearing" or "squeezing", eliminating the generation of burrs and dust from the source.

◦ Online knife inspection and cleaning: Integrated knife edge visual inspection system and vacuum suction device to monitor the knife edge status in real time and remove trace debris generated.

Barrier 4: All-round vibration reduction and thermal management

Vibrations from the device itself and fluctuations in ambient temperature are accuracy killers.

•Solution:

◦ Mechanical structure optimization: The high-strength cast iron base and frame are used for good vibration absorption and thermal stability. The critical rollers need to be dynamically balanced (G1.0 or higher) corrected to ensure minimal vibration at high speeds.

◦ Thermal expansion control: The drive unit, bearings and other heat sources and the main frame are thermally isolated. A constant temperature cooling system is installed in parts with extremely high precision requirements (such as tool holders) to control temperature fluctuations within ±0.5°C to avoid precision drift caused by thermal expansion and contraction.

◦ Environmental isolation: High-end slitting machines are even installed in constant temperature and humidity clean workshops, and are equipped with air floating vibration isolation platforms to completely isolate external environmental interference.

Barrier 5: Intelligent integrated control system

All of these subsystems do not work in isolation and require a powerful "brain" to work together.

•Solution:

◦ Integrated platform: Adopt an integrated control system based on PC-based or high-performance PLC, connect all servo drives, sensors and actuators through high-speed industrial Ethernet buses such as EtherCAT to achieve nanosecond synchronization of data and high-speed issuance of instructions.

◦ Advanced algorithms: The control system has built-in advanced control strategies such as adaptive algorithms, feedforward compensation, and disturbance observers, which can predict and compensate for various potential disturbances, such as changes in material properties and speed jumps.

◦ Digital twin and big data analysis: By collecting and analyzing equipment operation data, process parameters, and slitting quality in real time, a digital model is built to optimize process parameters, predictive maintenance, and quality traceability, and continuously improve the stability and yield of the slitting process.

3. Summary: Accuracy is the ultimate embodiment of system capabilities

To achieve a slitting accuracy of 1μm ± lithium battery separator, it does not rely on a single point breakthrough of a certain "black technology", but a high degree of integration and system integration of precision mechanical design, cutting-edge sensing technology, high-speed servo drive, advanced control algorithms and strict environmental management. It represents a country's highest technical level in the field of high-end CNC equipment.

Every micron improvement is a challenge to the limits of fundamental physics, materials science and engineering, and behind it is countless iterations, debugging and innovation. It is this unremitting pursuit of ultimate precision that lays a solid foundation for the large-scale, high-safety, and low-cost manufacturing of high-performance lithium batteries, ultimately promoting the vigorous development of the entire new energy industry.