Introduction to lithium battery production process, process flow, principle, formula, and parameters
Introduction to lithium battery production process, process flow, principle, formula, and parameters
01
Mixing process
1. Mixing process
(1) Function: The batching process is actually to mix the various components in the slurry in a standard proportion and prepare the slurry to facilitate uniform coating and ensure the consistency of the electrode.
(2) Slurry related parameters:
a. Viscosity: The resistance of a fluid to flow. Viscosity is a property of a fluid. When a fluid flows in a pipe, there are three states: laminar flow, transitional flow, and turbulent flow.
b. Solid content: The percentage of solid matter in the slurry
(3) Methods for testing the uniformity of stirring and mixing of solid-liquid suspension systems:
Direct method:
a. Viscosity method:
Take samples from different positions of the system and measure the viscosity of the slurry with a viscometer ; the smaller the deviation, the more uniform the mixing;
b. Particle size method:
Use particle size scraper to observe the particle size of the slurry; laser diffraction particle size tester
c. Specific gravity method:
Take samples from different positions of the system and measure the density of the slurry . The smaller the deviation, the more uniform the mixing.
Indirect method:
a. Solid content method: Take samples from different positions of the system, bake them at appropriate temperature and time, and measure the weight of the solid content. The smaller the deviation, the more uniform the mixing;
b. SEM: Take samples from different positions of the system, apply them to the substrate, dry them, and use SEM (electron microscope) to observe the distribution of particles or elements in the membrane after the slurry is dried; (the solid part of the system is usually a conductive material)
2. Positive electrode stirring process:
The positive electrode materials include: active materials, NMP (N-methylpyrrolidone, polar aprotic solvent), conductive agent (carbon black, connecting large active material particles to make good conductivity), PVDF (polyvinylidene fluoride, thickener and binder, can resist UV and climate change),
3. Negative electrode stirring process
The negative electrode materials include: graphite, deionized water, conductive agent, SBR (styrene-butadiene rubber, water-soluble latex, binder, an anionic polymer dispersion with good mechanical stability and operability, and high bonding strength), CMC (sodium carboxymethyl cellulose, thickener and stabilizer)
4. Viscosity characteristics of slurry
Variation curve of slurry viscosity with stirring time
As the stirring time increases, the slurry viscosity tends to a stable value and no longer changes (it can be said that the slurry is evenly dispersed).
4. Slurry formulation
1.1 Positive electrode formula : LiCoO2+conductive agent+binder+current collector (aluminum foil)
LiCoO2(10μm): 96.0%
Conductive agent (Carbon ECP) 2.0%
Adhesive (PVDF 761) 2.0%
NMP (increases adhesion): weight ratio of solid matter is 8:15
a) The positive electrode viscosity is controlled at 6000cps (temperature 25°C);
b) The weight of NMP must be properly adjusted to meet the viscosity requirements;
c) Pay special attention to the effects of temperature and humidity on viscosity
Positive active material:
Lithium cobalt oxide: positive electrode active material, lithium ion source, to improve the lithium source for the battery. Non-polar material, irregular shape, particle size D50 is generally 6-8 μm, water content ≤ 0.2%, usually alkaline, pH value is about 10-11.
Lithium manganate: non-polar substance, irregular shape, particle size D50 is generally 5-7 μm, water content ≤0.2%, usually weakly alkaline, pH value is about 8.
Conductive agent: chain-like substance, water content < 1%, particle size is generally 1-5 μm. Superconducting carbon black with excellent conductivity is usually used, such as Ketjen Carbon ECP and ECP600JD. Its functions : improve the conductivity of the positive electrode material , compensate for the electronic conductivity of the positive electrode active material; increase the electrolyte absorption of the positive electrode sheet, increase the reaction interface, and reduce polarization.
PVDF adhesive: non-polar substance, chain-like, molecular weight ranging from 300,000 to 3,000,000; molecular weight decreases and viscosity deteriorates after absorbing water. Used to bond lithium cobalt oxide, conductive agent and aluminum foil or aluminum mesh together. Common brands include Kynar761.
NMP: Weakly polar liquid, used to dissolve/swell PVDF and dilute slurry.
Current collector (positive electrode lead) : made of aluminum foil or aluminum tape.
1.2 Negative electrode formula: graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)
Negative electrode material (graphite): 94.5%
Conductive agent (Carbon ECP): 1.0% (Ketjen superconducting carbon black)
Binder (SBR): 2.25% (SBR = styrene butadiene rubber latex)
Thickener (CMC): 2.25% (CMC = sodium carboxymethyl cellulose)
The weight ratio of water: solid matter is 1600:1417.5
a) Negative electrode viscosity control 5000-6000cps (temperature 25 rotor 3)
b) The water weight needs to be adjusted appropriately to meet the viscosity requirements;
c) Pay special attention to the effects of temperature and humidity on viscosity
5. Positive and negative mixing
Graphite : Negative active material, the main substance that constitutes the negative electrode reaction; mainly divided into two categories: natural graphite and artificial graphite. Non-polar substances are easily contaminated by non-polar substances and easily dispersed in non-polar substances; they are not easy to absorb water and are not easy to disperse in water. Contaminated graphite is easy to re-agglomerate after being dispersed in water. The general particle size D50 is about 20μm. The particle shapes are diverse and mostly irregular, mainly spherical, flaky, fibrous, etc.
Conductive agent : Its functions are:
a) Improve the conductivity of the negative electrode sheet and compensate for the electronic conductivity of the negative electrode active material.
b) Improve reaction depth and utilization rate.
c) Prevent the formation of dendrites.
d) Use the liquid absorption capacity of conductive materials to improve the reaction interface and reduce polarization. (You can choose to add or not according to the distribution of graphite particle size).
Additives: reduce irreversible reactions, improve adhesion, increase slurry viscosity, and prevent slurry precipitation.
Thickener/anti-settling agent (CMC): a high molecular weight compound that is easily soluble in water and polar solvents.
Isopropyl alcohol: a weakly polar substance, which can reduce the polarity of the adhesive solution and improve the compatibility of graphite and the adhesive solution after being added; it has a strong defoaming effect; it can easily catalyze the network crosslinking of the adhesive and improve the bonding strength.
Ethanol: A weakly polar substance, which can reduce the polarity of the adhesive solution and improve the compatibility of graphite and the adhesive solution after being added; it has a strong defoaming effect; it can easily catalyze the linear crosslinking of the adhesive and improve the bonding strength (the effects of isopropyl alcohol and ethanol are essentially the same, and the cost factor can be considered when choosing which one to add during mass production).
Water-based adhesive (SBR) : Bond graphite, conductive agent, additives and copper foil or copper mesh together. Small molecule linear chain emulsion, easily soluble in water and polar solvents.
Deionized water (or distilled water) : diluent, added in appropriate amount to change the fluidity of the slurry.
Negative lead : Made of copper foil or nickel strip.
2.1 Positive electrode mixture:
2.1.1 Pretreatment of raw materials
1) Lithium cobalt oxide : Dehydration. Generally, it is baked at 120 °C and normal pressure for about 2 hours.
2) Conductive agent : Dehydrate. Generally, bake at 200 °C and normal pressure for about 2 hours.
3) Adhesive : Dehydration. Generally, it is baked at 120-140 °C under normal pressure for about 2 hours. The baking temperature depends on the molecular weight.
4) NMP: Dehydration. Use dry molecular sieve for dehydration or adopt special material taking facilities and use directly.
2.1.2 Material ball milling:
1) After 4 hours, separate the ball mill through sieve;
2) Pour LiCoO2 and Carbon ECP into the material barrel, add grinding balls (dry material: grinding balls = 1:1), and perform ball milling on a roller bottle with the speed controlled at more than 60rmp
2.1.3 Blending of raw materials:
1) Dissolution (at standard concentration) and heat treatment of the adhesive.
2) Ball milling of lithium cobalt oxide and conductive agent: The powder is initially mixed, and the lithium cobalt oxide and conductive agent are bonded together to improve the agglomeration effect and conductivity. After being prepared into slurry, it will not be distributed alone in the binder. The ball milling time is generally about 2 hours; in order to avoid mixing with impurities, agate balls are usually used as ball milling mediators.
2.1.4 Dispersion and wetting of dry powder:
Principle : Solid powder is placed in the air. Over time, it will absorb some air on the surface of the solid. After the liquid adhesive is added, the liquid and gas begin to compete for the solid surface. If the adsorption force between the solid and the gas is stronger than that between the solid and the liquid, the liquid cannot wet the solid. If the adsorption force between the solid and the liquid is stronger than that between the solid and the gas, the liquid can wet the solid and squeeze out the gas.
When the wetting angle is ≤90°, the solid is wetted. When the wetting angle is >90°, the solid is not wetted.
All members of the positive electrode material can be wetted by the binder solution, so the positive electrode powder is relatively easy to disperse.
The influence of dispersion method on dispersion:
1) Static method (long time, poor effect, but does not damage the original structure of the material);
2) Stirring method: rotation or rotation plus revolution (short time, good effect, but may damage the structure of some materials).
The influence of stirring paddle on dispersion speed: stirring paddles generally include snake-shaped, butterfly-shaped, spherical, paddle-shaped, gear-shaped, etc. Generally, snake-shaped, butterfly-shaped, and paddle-shaped stirring paddles are used to deal with materials or ingredients that are difficult to disperse in the initial stage; spherical and gear-shaped stirring paddles are used in the state where the dispersion difficulty is relatively low, and the effect is better.
The influence of stirring speed on dispersion speed. Generally speaking, the higher the stirring speed, the faster the dispersion speed, but the greater the damage to the material structure and the equipment.
The influence of concentration on dispersion speed. Generally speaking, the lower the slurry concentration, the faster the dispersion speed, but too thin will lead to material waste and aggravation of slurry precipitation.
The influence of concentration on bonding strength . The higher the concentration, the greater the flexibility and bonding strength; the lower the concentration, the lower the bonding strength.
The effect of vacuum degree on dispersion speed. High vacuum degree is conducive to the discharge of gas from the gaps and surfaces of materials, reducing the difficulty of liquid adsorption; the difficulty of evenly dispersing materials will be greatly reduced when the material is completely weightless or the gravity is reduced.
The effect of temperature on dispersion speed. At a suitable temperature, the slurry has good fluidity and is easy to disperse. If it is too hot, the slurry will easily form a crust, and if it is too cold, the fluidity of the slurry will be greatly reduced.
Dilution : Adjust the slurry to a suitable concentration for easy coating.
2.1.5 Operation steps
a) Pour NMP into a power mixer (100L) to 80°C, weigh PVDF and add it, then start the machine; parameter settings: speed 25±2r/min, stirring for 115-125min;
b) Turn on the cooling system, add the ground positive electrode dry material in four equal portions, with an interval of 28-32 minutes between each addition. Add NMP to the third addition as needed, and add NMP after the fourth addition. Power mixer parameter setting: speed 20±2r/min
c) After the fourth addition, high-speed stirring is performed for 30±2 minutes, and the time is 480±10 minutes; the parameters of the power mixer are set as follows: revolution is 30±2r/min, rotation is 25±2r/min;
d) Vacuum mixing: Connect the power mixer to vacuum, maintain the vacuum degree at -0.09Mpa, and stir for 30±2min; power mixer parameter settings: revolution 10±2min, rotation 8±2r/min
e) Take 250-300 ml of slurry and measure the viscosity using a viscometer; test conditions: rotor number 5, speed 12 or 30 rpm, temperature range 25°C;
f) The positive electrode material is taken out from the power mixer for colloid grinding and screening, and a label is attached to the stainless steel basin. After being handed over to the slurry pulling equipment operator, it can flow into the slurry pulling operation process.
2.1.6 Notes
a) Complete and clean the equipment and working environment;
b) When operating the machine, pay attention to safety and avoid hitting your head.
2.2 Negative electrode mixing
2.2.1 Pretreatment of raw materials:
1) Graphite:
A. Mix to homogenize the raw materials and improve consistency.
B. Bake at 300~400°C under normal pressure to remove oily substances on the surface, improve compatibility with water-based adhesives, and round the edges and corners of the graphite surface (some materials are not allowed to be baked to maintain surface characteristics, otherwise the performance will be reduced).
2) Water-based adhesive: dilute appropriately to improve dispersion ability .
2.2.2 Blending, wetting and dispersing:
1) Graphite and binder solution have different polarities and are difficult to disperse.
2) The graphite can be initially wetted with an alcohol-water solution and then mixed with the binder solution.
3) The stirring concentration should be appropriately reduced to improve dispersibility.
4) The dispersion process is to reduce the distance between polar and non-polar substances and increase potential energy or surface energy, so it is an endothermic reaction. The overall temperature decreases during stirring. If conditions permit, the stirring temperature should be appropriately increased to facilitate heat absorption, improve fluidity, and reduce the difficulty of dispersion.
5) If a vacuum degassing process is added to the stirring process to remove gas and promote solid-liquid adsorption, the effect will be better.
6) Dispersion principles, dispersion methods and related contents in positive electrode ingredients
2.2.3 Dilution:
Adjust the slurry to a suitable concentration for easy coating.
2.2.4 Material ball milling
1) Pour the negative electrode and KetjenblackECP into the material bucket and add the ball mill (dry material: grinding ball = 1:1.2) and perform ball milling on the roller bottle, and control the speed at more than 60rmp;
2) After 4 hours, separate the ball mill through sieve;
2.2.5 Operation steps
1) Heat purified water to 80°C and pour into a power mixer (2L)
2) Add CMC and stir for 60±2min; set the parameters of the power mixer: revolution at 25±2min, rotation at 15±2r/min;
3) Add SBR and deionized water and stir for 60±2min;
Power mixer parameter settings: revolution 30±2min, rotation 20±2r/min;
4) Add the negative electrode dry material in four equal batches, and add pure water at the same time, with an interval of 28-32 minutes between each addition; set the power mixer parameters: revolution at 20±2r/min, rotation at 15±2r/min;
5) After the fourth addition, high-speed stirring is performed at 30±2r/min for 480±10min;
Power mixer parameter settings: orbital revolution 30±2r/min, rotation 25±2r/min;
6) Vacuum mixing: Connect the power mixer to vacuum, maintain the vacuum degree at -0.09 to 0.10Mpa, and stir for 30±2min;
Power mixer parameter settings: revolution 10±2min, rotation 8±2r/min
7) Take 500 ml of slurry and measure the viscosity using a viscometer;
Test conditions: rotor number 5, speed 30 rpm, temperature range 25°C;
8) Take the negative electrode material out of the power mixer for grinding and sieving, and label it on the stainless steel basin. After handing it over to the slurry pulling equipment operator, it can flow into the slurry pulling process.
2.2.6 Notes
1) Complete and clean the equipment and working environment;
2) When operating the machine, pay attention to safety and avoid hitting your head.
Ingredients Notes:
1.Prevent other impurities from mixing in;
2.Prevent slurry from splashing;
3.The concentration (solid content) of the slurry should be adjusted gradually from high to low to avoid adding trouble;
4.Pay attention to scraping the sides and bottom during the mixing intervals to ensure uniform dispersion;
5.The slurry should not be left for a long time to avoid sedimentation or loss of uniformity;
6.The materials to be baked must be sealed and cooled before they can be added to avoid changes in the properties of the component materials;
7.The length of mixing time is mainly determined by equipment performance and the amount of material added;
8.The stirring paddle should be replaced according to the difficulty of slurry dispersion. If it cannot be replaced, the speed can be adjusted from slow to fast to avoid damage to the equipment;
9.Screen the slurry before discharging to remove large particles to prevent the belt from breaking during coating;
10.Strengthen the training of batching personnel to ensure that they have professional knowledge to avoid major disasters;
11.The key to batching is to disperse evenly and grasp the center. Other methods can be adjusted by yourself.
02
1. Coating process
The coating process is a process based on the study of fluid properties, in which one or more layers of liquid are coated on a substrate, which is usually a flexible film or backing paper. The coated liquid coating is then dried in an oven or cured to form a film layer with special functions. The main methods of coating lithium-ion battery pole pieces are currently comma roll transfer coating and slot extrusion coating.
2. Purpose of coating process
The coating process is to evenly apply the positive and negative electrode slurry on the base layer. The coating process is divided into positive and negative electrode pre-coating coating and positive and negative electrode slurry coating.
Pre-coating: Apply the pre-coating slurry evenly on the foil. Generally, a transfer coater is used to achieve a low viscosity slurry and a thin coating method with a thickness of 2-3μm.
Slurry coating: The purpose is to evenly apply the positive and negative electrode slurry on the foil. Use an extrusion coating machine to evenly apply the slurry with high viscosity on the foil with a certain thickness.
3. Notes on coating process:
1. Appearance
No particles, precipitation, impurities and bubbles, etc.
There are no scratches, material drop, edge warping, or foil exposure on the surface of the pole piece.
2. Copper, aluminum foil
Uniform thickness, no scratches or wrinkles
3. Viscoelasticity of coating solution
If the solution is squeezed out from the lip of the coating head with a large shear force, or coated at a faster speed, the solution will shrink when the pressure is released after coating, which will eventually lead to the formation of vertical stripes. In severe cases, the shrinkage of the solution will cause wrinkles and curling of the film.
4. Common problems in coating process
(1) “Gel” defect
Gels are mainly formed during the glue preparation process, and some polymer particles are not dissolved in the solvent.
Solution: Premix the solvent and polymer. A two-component solvent is sometimes beneficial to the dissolution of the polymer. The order of adding the components during the preparation of the glue must be strictly followed. Optimizing the temperature curve of the reaction is also the key to controlling the gel.
(2) Defects of surface tension
The properties of various surfaces/interfaces between the solid substrate and the coating liquid.
a. Causes of cellulite
The solvent in the coating solution evaporates, causing temperature differences in different areas. At the same time, the solvent content on the surface and bottom of the solution also has a concentration difference due to the different evaporation rates. The drying rate of the oven is too fast, or the hot air speed of the oven is too fast.
b. Causes of thick edges
After coating, the solvent at the edge of the coating solution evaporates faster than that in the inner area, so the coating solution in the low surface tension area flows to the edge and accumulates, making the edge too thick.
c. Pinhole and crater defects
If pollutant dust is added during the coating process, it will form a crater-like defect. If it is already on the substrate before coating, it will form a pinhole-like defect.
d. Regular vertical stripe defects
This defect is caused by the unstable force applied to the coating liquid at different locations.
Solution: Adjust process parameters. Dilute your coating solution to reduce solution viscosity. Add some surfactant.
e. Regular horizontal stripe defects
Pump reasons. Choose a pulse-free gear pump. If there is no pulse-free pump, you can install a buffer device on the pump to minimize the pulse.
Vibration disturbances in the coating equipment cause this defect.
03
(1) Rolling: Rolling the electrode sheets with positive and negative electrode materials attached to them can make the coated materials more compact, increase energy density, and ensure thickness consistency. It can also further control dust and humidity.
At present, lithium-ion battery manufacturers at home and abroad all use double-roller presses to roll the electrodes.
Necessity of rolling: After the electrode is coated and dried, the peel strength between the active material and the current collector foil is very low. At this time, it is necessary to roll it to enhance the bonding strength between the active material and the foil to prevent peeling during electrolyte immersion and battery use.
The purposes of roller pressing are as follows :
a. Ensure that the surface of the electrode is smooth and flat to prevent burrs on the coating surface from piercing the diaphragm and causing a short circuit;
b. Compact the electrode coating material to reduce the volume of the electrode to increase the energy density of the battery;
c. Make the active material and conductive agent particles contact more closely and improve the electronic conductivity;
d. Enhance the bonding strength between the coating material and the current collector, reduce the occurrence of powder loss of the battery pole during the cycle process, and improve the cycle life and safety performance of the battery.
(2) Basic mechanism of battery pole sheet rolling: Battery pole sheet rolling is powder rolling, which aims to improve the compaction density and uniformity of the active material of the battery pole sheet, improve the adhesion of the active material, and improve the surface roughness. The rolling process follows the law of weight invariance.
(3) The rolling processing performance of the electrode is mainly characterized by the parameters of the rolling process model, namely, the compaction impedance γ and the minimum porosity ε, the mechanical properties of the electrode (hardness and elastic deformability), the pore size distribution, the microstructure of the electrode coating, and the electrochemical properties such as the rate capability.
(4) Problems and solutions in the rolling process
a The lateral thickness of the pole piece is uneven.
When the thickness of the unrolled pole piece is consistent on the left and right sides, the rolling pressure needs to be adjusted left and right to ensure that the compaction density of the pole piece is consistent on the left and right sides after rolling.
b The longitudinal thickness of the pole piece is uneven.
The rebound of the pole piece is caused by excessive moisture inside the pole piece and too fast rolling speed. The pole piece rebound problem can be solved by using hot roller process and controlling the rolling speed.
c Dark stripes appear on the surface of the pole piece: This situation is mainly caused by vibration marks on the surface of the roller, large cylindricity error of the roller body, and small and uneven front tension.
d. The pole piece has curling: This is caused by excessive elongation of the pole piece. The solution is mainly to increase the roller diameter, reduce the compression of the pole piece, adjust the front and rear tension of the pole piece, etc.
e. Pits appear on the surface of the roller : This is fatigue pitting on the surface of the roller, which is mainly caused by uneven roller material and heat treatment metallographic structure, poor fatigue strength of the roller surface, and is also related to the surface roughness of the roller.
(5) Impact of roller pressing on lithium batteries
Impact on battery specific energy and specific power: According to Faraday's law, the amount of electricity that passes through the battery electrode is proportional to the mass of the active material. Pole sheet rolling directly affects the compaction density of the active material in the pole sheet, and directly affects the battery specific energy.
Impact on battery energy density and power density: Similarly, the compaction density of the active material in the electrode directly affects the energy density and power density of the battery.
Impact on battery cycle life: Pole rolling directly affects the adhesion of active materials on the battery collector, which directly affects the separation and shedding of active materials during battery charging and discharging, thus affecting the battery cycle life.
Impact on battery internal resistance: The compaction density and degree of shedding of the active material on the electrode greatly affect the ohmic internal resistance and electrochemical internal resistance of the battery, which directly affects the various performance of the battery.
Impact on battery safety: The compaction density uniformity of the active material on the pole piece, the surface roughness caused by rolling of the battery pole piece, etc. will directly affect the lithium deposition at the negative electrode, copper deposition at the positive electrode, and discharge at sharp corners of the battery, ultimately leading to safety accidents.
(6) Causes of belt breakage due to roller pressure;
1. Poor slurry compaction
2. There are small particles on the surface of the dressing
3. The dressing slurry is agglomerated and the active ingredients are unevenly distributed
4. The dressing has air bubble pits
5. Uneven thickness of the pole piece
6. The dressing is thinned to a thickness that exceeds the standard, and is extremely curled, wrinkled, bulging, or cracked.
7. Extreme Roll Slide
8. Extremely curled edges with blank space and abnormally wavy edges
9. Coating string
10. Leave white wrinkles on the edge of the extreme curl
11. Leave a blank gap on the edge of the extreme roll
12. The foil has insufficient ductility/tensile strength
13. There are pinholes in the foil
14. Foil thickness does not meet the standard
15. Poor consistency of foil surface density
16. Uneven foil tension
17. Dark marks on the surface of gravure foil
18. Improper adjustment of roller speed causes excessive speed changes
19. Particles on the rubber rollers are not cleaned in time
20. Stretch tension setting does not match
21. The number of layers of stretched Teflon adhesive does not match
22. Improper pasting position of stretched Teflon
23. Belt wearing error
24. The reception desk of the pole roll is not aligned
25. The pole piece is biased when feeding the roller and is not perpendicular to the roller
26. Unstable roller pressure
27. The roller is not adjusted to the level
28. The tape is not firmly attached
29. Abnormal tension before and after rolling
30. Improper operation by personnel
04
Lithium-ion battery cutting process introduction:
(1) After the porous electrode is coated and rolled, the electrode piece needs to be cut to achieve the required design structure and size.
(2) There are three main methods for cutting pole pieces: (1) disc cutting; (2) die stamping; (3) laser cutting;
(3) The traditional methods of disc slitting and die punching use tools or dies to cut the pole pieces based on the principle that the material will separate from each other after the material is subjected to plastic deformation due to stress and cracks.
Disadvantages: The tool or mold has high requirements in terms of strength, rigidity and precision
The incision formed after cutting generally has problems such as burrs or slag
(4) Advantages of laser cutting pole pieces:
1.The cutting edge burr can be stably controlled within 0.01mm;
2.Laser cutting only requires a one-time cost investment, and there is no cost for changing tools or molds;
3. Laser cutting has no processing stress and will not cause edge powder falling off.
(5) Disadvantages of laser cutting pole pieces:
The high-performance lasers used are expensive and require a large one-time investment, so most companies are unwilling to accept them.
2. Laser cutting will produce carbonized edges (also known as heat-affected zones), and the negative electrode material will also have a certain amount of copper exposure;
3. A large amount of dust will be generated during the laser cutting process, and the collection and treatment of dust is a considerable problem.
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