How to Control the Uniformity of Lithium Battery Coating?

Controlling the uniformity of lithium battery coating involves multiple factors across various aspects such as personnel, machinery, materials, methods, and environment. However, the fundamental factors are directly related to the coating process conditions, including the coating substrate, adhesive, coating steel roller/rubber roller, and the laminating machine.

1.    Coating Substrate: Key factors are the material type, surface properties, thickness, and its uniformity.

2.    Adhesive: Main considerations are its working viscosity, affinity, and adhesion to the substrate surface.

3.    Coating Steel Roller: As the direct carrier of the adhesive and the supporting benchmark for the coating substrate and rubber roller, it is the core of the entire coating mechanism. Factors like geometric tolerance, rigidity, dynamic and static balance quality, surface quality, temperature uniformity, and heat deformation affect coating uniformity.

4.    Coating Rubber Roller: The rubber roller is an essential variable for coating quality. Its material (e.g., durability of the rubber layer), hardness, geometric tolerance, rigidity, dynamic and static balance quality, surface quality, and heat deformation also influence coating uniformity.

5.    Laminating Machine: It serves as the foundational platform for coating. In addition to the precision and sensitivity of the coating steel roller and rubber roller pressing mechanism, it also encompasses the design operating speed and overall machine stability.

The influencing factors for coating uniformity differ in the lateral and longitudinal directions, necessitating different control measures. These measures relate to both the design and manufacturing of the machine, as well as operational and process control.

In practice, ensuring uniform coating involves both technical adjustments to the machinery and precise operational procedures.

Lateral Uniformity of Coating and Its Control

The uniformity of coating in the width direction is primarily influenced by the following factors:

1. Geometric tolerance of steel rollers and rubber rollers.
2. Contact pressure between steel rollers and the substrate.
3. Lateral distribution of the adhesive, primarily referring to the viscosity of the adhesive and its corresponding transfer rate. This is comprehensively determined by the first two factors and the working viscosity of the adhesive.

The primary benchmark for ensuring coating accuracy is the geometric tolerance and rigidity of the steel roller. During operation, the geometric tolerance, rigidity, balance precision, assembly precision, and built-in heating and insulation system of the steel roller directly affect the state of the adhesive, thereby influencing the transfer rate and coating uniformity. Therefore, designing and selecting high-precision, wear-resistant steel rollers is a fundamental task.

Rubber rollers, especially the coating rubber roller, are the primary influencing factor or variable for lateral coating uniformity. The material, geometric tolerance, rigidity, hardness of the rubber layer, dynamic balance of the rubber roller, support structure, pressure adjustment, and other factors significantly impact coating uniformity. As a slender shaft, the rigidity of the rubber roller is the most pronounced influencing factor. From a usage perspective, the cleanliness of the rubber roller surface is also an essential production variable affecting the uniformity of rubber roller coating. Therefore, the quality and maintenance of rubber rollers require meticulous control.

As evident from the above analysis, the wider the substrate or coating width, the more difficult it becomes to ensure coating uniformity. When the coating width reaches a certain value, coating uniformity may become the primary challenge to the precision of the entire solvent-free laminating machine.

Longitudinal Uniformity of Coating and Its Control

The longitudinal uniformity of coating, also known as the uniformity along the feed direction of the substrate, is determined by various factors, including:

• Substrate type, surface quality, and its consistency;
• Machine operating speed, the number and frequency of speed changes;
• Viscosity condition of the adhesive, particularly the viscosity consistency during prolonged operation;
• Quality of the rubber roller, especially its elasticity, wear resistance, and heat resistance;
• Control mechanism and structure of the adhesive application volume in the solvent-free laminating machine;
• Design of the glue dispensing system in the laminating machine;
• Environmental factors, among others.

Under normal conditions, when the machine operates at a stable speed, the transfer rate of the adhesive remains consistent, leading to a uniform coating. However, during acceleration or deceleration, the transfer rate of the adhesive may undergo slight to significant changes, impacting coating uniformity. Therefore, the machine's design must incorporate corresponding pre-processing to minimize the effects of frequent startups or speed changes on coating uniformity.

The pressing of steel and rubber rollers not only results in bending and compression deformation but also thermal deformation after continuous operation. These deformations affect the pressure conditions and contact width in the contact area, subsequently influencing the transfer rate and coating uniformity. The material and hardness of the rubber roller sometimes significantly affect coating uniformity. Thus, the impact of these deformations must be thoroughly considered during machine design, emphasizing the selection and design of high-quality, durable rubber rollers.

The duration of downtime during the process also affects the viscosity of the adhesive in the glue tank and on the rubber and steel rollers of the coating system, potentially influencing coating uniformity. Therefore, it is crucial to control downtime to prevent excessive viscosity increases in the adhesive.

The control mechanism and structure for adhesive application volume in solvent-free laminating machines are vital yet often overlooked factors affecting longitudinal coating uniformity. The method of adjusting and controlling the adhesive volume significantly impacts its control precision and response speed.

For instance, modern solvent-free laminating machines typically employ multi-roller transfer coating methods, controlling the adhesive volume through speed differences. However, some machines adjust the adhesive volume by modifying the metering intervals, which usually results in slow response speeds and low coating precision and uniformity.

Certain solvent-free laminating machines cannot continuously or arbitrarily set the glue application width, or the glue dispensing width differs from the application width, leading to dead zones (areas near glue dams) within the glue tank. Over time, the concentration of adhesive near the glue dams may increase or turn cloudy, reducing the transfer rate of the adhesive locally (at the edges), thereby directly affecting transfer uniformity. This results in lower adhesive volume at certain areas, typically the edges compared to the central region. This type of unevenness, primarily manifesting as lateral non-uniformity, is caused by continuous operation and may repeat based on the timing of adhesive addition. Thus, it is essential to select laminating machines with rational adhesive volume control and the ability to arbitrarily set the glue dispensing width.

Environmental factors become more evident during prolonged continuous production, as changes in relative humidity and temperature can affect the adhesive coating conditions to a certain extent.

In summary, only when both lateral and longitudinal uniformity are ensured can overall coating uniformity be achieved. While lateral uniformity is often more influenced by the machine itself, longitudinal uniformity is more challenging to control due to the multitude of factors affecting it during long-term or mass production.

Of course, the primary factors influencing coating uniformity vary across short, medium, and long timeframes, necessitating corresponding measures based on different scenarios.

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