Method for making mobile phone rear cover glass with gradient color and luster based on composite salt bath ion exchange coloring technology

Abstract

The invention discloses a method for manufacturing color gradient of mobile phone rear cover glass based on a composite salt bath ion exchange coloring technology. According to the preparation method, by designing a specific composite salt bath formula, an innovative workpiece loading mode and precise process control, an ion concentration gradient is formed on the surface of the glass through ion exchange, so that the gradient color effect which is soft in transition, rich in color and high in binding force is prepared through a one-step method; and the glass naturally forms an ion concentration gradient at the interface of the fused salt and the air, so that the steps of adjusting and converting for multiple times in the traditional process are omitted. The production process is simplified, the processing time of a single product can be shortened, and the overall production efficiency is improved; and meanwhile, the equipment investment and the consumption of consumables are reduced, the manufacturing cost is effectively reduced, and the method is more suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of glass surface treatment technology, specifically a method for producing color gradient on mobile phone back cover glass based on composite salt bath ion exchange coloring technology. Background Technology

[0002] As consumer electronics products such as smartphones become increasingly high-end and personalized, their appearance design has become a crucial competitive factor. Glass back covers are widely used due to their superior texture, colorability, and signal penetration. Traditional glass coloring methods, such as overall coloring, spraying, and coating, suffer from problems such as monotonous colors, easy peeling, poor texture, or poor environmental performance. Ion exchange technology can form a stable and wear-resistant coloring layer on the glass surface.

[0003] However, while single-type ion exchange can achieve a single color in existing technologies, it is difficult to create natural and rich gradient color effects. Multi-step ion exchange or masking processes are complex and costly, and the color transitions are abrupt, failing to meet market demands for high-quality gradient colors. Furthermore, mobile phone glass has extremely high requirements for color uniformity, consistency, and durability, which ordinary ion exchange processes cannot simultaneously satisfy.

[0004] Therefore, developing a simple, highly controllable method for coloring mobile phone back cover glass that can achieve soft and natural gradients and high color durability has significant industrial value. Summary of the Invention

[0005] The purpose of this invention is to provide a method for producing color gradient on mobile phone back cover glass based on composite salt bath ion exchange coloring technology in order to solve the problems mentioned above.

[0006] The technical solution adopted in this invention is as follows: A method for producing color gradient on mobile phone back cover glass based on composite salt bath ion exchange coloring technology, the method comprising the following steps: S1: Clean and dry the aluminosilicate system mobile phone back cover glass substrate to remove surface contaminants and prepare a clean substrate for subsequent ion exchange steps; S2: Prepare a composite molten salt using sodium nitrate as the base salt, add silver nitrate and copper nitrate as colorants, and add 0.5%-3% zinc nitrate as a color stabilizer. The ratio of copper nitrate to zinc nitrate is 1:0.2 to 1:5. At the same time, add 0.1% cerium nitrate as a color modifier. Heat the composite molten salt to a working temperature of 380℃-420℃ to provide a suitable reaction medium for the subsequent gradient ion exchange step. S3: The glass back cover treated in S1 is placed at an angle of 15°-75° in a special immersion device. The control device immerses the glass back cover part into the composite molten salt bath prepared in S2 at a speed of 0.5-5 mm / s. The immersion depth is 20%-80% of the glass length. The ion exchange is completed while keeping the glass in a static state. The exchange time is 30 minutes to 2 hours, so that the coloring ions form a continuously changing concentration gradient on the glass surface. S4: After the glass back cover has undergone ion exchange in S3, remove it and clean off any residual salts. Then, place it in a muffle furnace at 480℃-550℃ for heat treatment for 10-30 minutes to reduce the diffused ions and aggregate them into nanoparticles for color development, while also enhancing the color durability of the glass. S5: Polish and clean the glass back cover after S4 heat treatment to remove surface impurities, and apply an oleophobic coating or anti-fingerprint coating as needed to complete the entire gradient coloring process.

[0007] In a preferred embodiment, in step S1, a mixture of neutral cleaning agent and deionized water is used for cleaning, and the glass surface is treated with an ultrasonic cleaning device at a frequency of 40 kHz for 15 minutes to thoroughly remove oil, dust, and other contaminants. After cleaning, the glass substrate is placed in a hot air drying oven, set to 80°C, and kept there for 20 minutes until the surface is completely dry and free of watermarks.

[0008] In a preferred embodiment, in step S1, the aluminosilicate glass substrate for the mobile phone back cover is first cleaned. A mixture of neutral cleaning agent and deionized water is used for cleaning, and the substrate is treated with an ultrasonic cleaner at a frequency of 40 kHz for 15 minutes to thoroughly remove contaminants from the glass surface. After cleaning, the glass substrate is placed in a hot air drying oven at 80°C for 20 minutes until the surface is completely dry and free of watermarks.

[0009] In a preferred embodiment, the quality of the pretreatment in step S1 directly determines the uniformity of the subsequent ion exchange. If residual contaminants are present on the glass surface, they will form a barrier layer during ion exchange, preventing the uniform diffusion of coloring ions and ultimately resulting in localized color loss or spots. Therefore, the cleaning process must strictly follow a multi-step procedure: first, decompose oil stains with a neutral cleaning agent; then, use ultrasonic cleaning to remove microparticles; and finally, use hot air drying to ensure no moisture residue remains, providing an ideal clean substrate for ion exchange.

[0010] In a preferred embodiment, in step S2, when preparing the composite molten salt, sodium nitrate is used as the base salt, and silver nitrate (a first colorant) and copper nitrate (a second colorant) are added. The amount of silver nitrate added is controlled between 1.5% and 2.0%, and the amount of copper nitrate added is controlled between 0.8% and 3.0%. Simultaneously, zinc nitrate (a color stabilizer) is added at 0.5% to 3% of the total mass, and the ratio of copper nitrate to zinc nitrate is maintained between 1:0.2 and 1:5. Then, 0.05% to 0.1% of cerium nitrate is added as a color modifier. All components are placed in a crucible and heated to a working temperature of 380°C to 420°C, and continuously stirred until completely melted and homogeneous.

[0011] In a preferred embodiment, in step S2, the formulation ratio of the composite molten salt needs to be precisely controlled according to the target gradient hue. The content of silver nitrate affects the depth of the final color, the proportion of copper nitrate determines the hue bias, and the addition of zinc nitrate can effectively inhibit the aggregation of coloring ions, making the gradient transition smoother. The trace addition of cerium nitrate can optimize the hue stability and prevent fading after long-term use. During the melting process of the molten salt, it is necessary to maintain uniform stirring to ensure that all components are evenly dispersed, avoid uneven color caused by excessively high local concentrations, and at the same time control the heating rate to prevent the decomposition of the base salt from affecting the ion exchange effect.

[0012] In a preferred embodiment, in step S3, the gradual ion exchange stage, the pretreated glass back cover is first fixed at an angle on a specially designed immersion device, with the angle set between 15° and 75°. Then, the immersion device is controlled to immerse the glass back cover portion into the composite molten salt bath at a slow speed of 0.5 mm / s to 5 mm / s, with an immersion depth of 20% to 80% of the glass length. After immersion, the glass back cover is kept stationary in this position, and the ion exchange process begins.

[0013] In a preferred embodiment, in step S3, under static conditions, silver and copper ions exchange with sodium ions in the glass surface immersed in molten salt. The portion not immersed in molten salt is exposed to air or a protective atmosphere. Due to the combined effects of concentration gradient and thermal convection, coloring ions form a unique diffusion field near the gas-liquid-solid three-phase interface. This diffusion field causes a continuous non-linear decrease in ion exchange concentration from the immersed end to the unimmersed end, laying the foundation for subsequent gradient color development. The exchange time needs to be controlled between 30 minutes and 2 hours; too long an exchange time will result in an excessively wide gradient region, while too short a time will prevent the formation of a clear gradient transition.

[0014] In a preferred embodiment, in step S4, the heat treatment color development and strengthening stage, the glass back cover after ion exchange is first removed, and the surface is cleaned with deionized water to remove any residual molten salt, ensuring no salt residue remains. Then, the glass back cover is placed in a muffle furnace, and the furnace temperature is set to 480°C to 550°C for 10 to 30 minutes. A suitable atmosphere, such as a nitrogen-hydrogen mixture or pure nitrogen, is selected based on the target gradient color.

[0015] During heat treatment, silver and copper ions that diffuse into the glass surface are reduced and aggregate into nanoparticles. The size and concentration of these nanoparticles change along a gradient direction, resulting in a continuous and smooth gradient effect. Simultaneously, heat treatment strengthens the mechanical properties of the glass, improving scratch resistance and durability. A nitrogen-hydrogen mixture helps reduce silver ions to metallic silver particles, producing a warm tone; pure nitrogen, on the other hand, facilitates the formation of specific compounds from copper ions, producing a cool tone. Different atmospheric conditions directly influence the final color bias.

[0016] In a preferred embodiment, in step S5, the post-processing stage first involves polishing the heat-treated glass back cover. Polishing fluid and a polishing pad are used to finely polish the glass surface, controlling the polishing pressure and rotation speed to ensure the removal of minor surface imperfections and residual traces. After polishing, the glass surface is rinsed again with deionized water to remove any polishing fluid residue, and then air-dried to ensure the surface is dry and clean.

[0017] The final step in post-processing is applying an oleophobic or anti-fingerprint coating. This coating process uses vacuum evaporation to control the coating thickness within a suitable range, ensuring uniformity and adhesion. After coating, the glass back cover surface will have excellent anti-fingerprint and hydrophobic properties, while enhancing the translucency of the gradient colors and improving the overall aesthetic appeal. Post-processing requires strict control of each step to ensure the final product meets the appearance requirements of high-end consumer electronics, with no surface defects or coating peeling.

[0018] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. In this invention, the one-step method for creating gradient effects eliminates the need for additional masking or multi-step dyeing operations, as well as complex technologies such as laser assistance, throughout the coloring process. By using an inclined immersion method, an ion concentration gradient naturally forms at the interface between the molten salt and air in the glass, eliminating the need for multiple adjustments and conversions in traditional processes. This not only simplifies the production process but also shortens the processing time for individual products, improving overall production efficiency. Simultaneously, it reduces equipment investment and consumable usage, effectively lowering manufacturing costs and making it more suitable for large-scale industrial production.

[0019] 2. In this invention, a nonlinear concentration gradient based on diffusion kinetics allows for a continuous and natural transition of color from the immersed end to the unimmersed end. This transition lacks obvious boundaries or abrupt color bands, visually resembling the gradient effect of natural light and shadow, meeting the high demands of high-end consumer electronics products for aesthetics. The smooth transition enhances the product's luxurious feel, allowing the phone's back cover to present rich visual layers from different angles, thus strengthening the product's market competitiveness.

[0020] 3. In this invention, by adjusting the ratio of silver ions to copper ions in the composite salt bath, the basic color tone bias of the gradient can be changed; adding trace amounts of color-adjusting ions can fine-tune the warmth or stability of the color tone; changing the tilt angle, immersion depth, and exchange time can control the range, transition speed, and color depth of the gradient, thus allowing manufacturers to flexibly design a variety of gradient schemes according to market demands, from warm-toned sunset purple to dawn gold, to cool-toned deep sea blue to glacier silver, to meet the personalized appearance requirements of different brands and product series.

[0021] 4. In this invention, the coloring layer formed by ion exchange penetrates deep into the glass surface rather than adhering to it, thus exhibiting stronger wear and scratch resistance and being less prone to peeling or wear during daily use. Subsequent heat treatment further stabilizes the coloring nanoparticles, making them less susceptible to fading under ultraviolet radiation during long-term use, ensuring color retention and enhancing the user experience of the product.

[0022] 5. In this invention, the entire coloring process does not require the use of organic dyes or coatings, avoiding the emission of organic pollutants in traditional processes. The molten salt component can be recycled, reducing raw material waste; at the same time, the generated waste liquid has a simple composition, is easier to treat, and has less impact on the environment. Thus, this method not only conforms to the current development trend of green manufacturing, but also reduces the environmental treatment costs for enterprises, achieving a win-win situation for both economic and environmental benefits. Attached Figure Description

[0023] Figure 1 This is a schematic diagram illustrating the process principle of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] Example 1: Reference Figure 1 The gradient changes from rosy purple to golden yellow at dawn. First, a Corning Gorilla Glass phone back cover is selected for cleaning. The purpose of cleaning is to thoroughly remove oil, dust, and other contaminants from the glass surface, ensuring that coloring ions react evenly with the glass surface during the subsequent ion exchange process, avoiding uneven coloring or defects caused by surface impurities. After cleaning, the glass back cover must be kept dry in preparation for the next step.

[0026] Next, a composite molten salt for ion exchange is prepared. Sodium nitrate is used as the base salt, and 2.0% silver nitrate, 0.8% copper nitrate, 1.2% zinc nitrate, and 0.1% cerium nitrate by mass are added. Silver nitrate and copper nitrate, as the main colorants, provide the basic hue for the gradient effect; zinc nitrate, as a color stabilizer, inhibits the aggregation of coloring ions on the glass surface, ensuring a smooth gradient transition; cerium nitrate, as a hue adjuster, is used to fine-tune the warmth or coolness of the gradient or improve color stability. After mixing these components, the mixture is heated to 400℃ to melt uniformly, forming a stable composite molten salt system.

[0027] Then, a gradient ion exchange process is performed. The cleaned and dried glass back cover is fixed at a 45° angle to a specialized device, and the control device slowly immerses 60% of the glass back cover's length into the prepared composite molten salt at a speed of 2 mm / s. This immersion is maintained for 60 minutes, and the entire process is carried out in an air atmosphere. The tilted immersion method utilizes the unique diffusion field at the molten salt-air interface to create a continuously varying concentration gradient of coloring ions on the glass surface, thus laying the foundation for the subsequent gradient effect.

[0028] After ion exchange, the glass back cover is removed from the molten salt and cleaned to remove any residual molten salt from the surface. Subsequently, the glass back cover is heat-treated in a nitrogen-hydrogen mixture at 520°C (5% hydrogen content) for 20 minutes. During the heat treatment, silver and copper ions that diffuse into the glass surface are reduced and aggregate into nanoparticles. The size and concentration of these nanoparticles change gradually, ultimately exhibiting a smooth transition from rosy purple to dawn gold.

[0029] Finally, the heat-treated glass back cover is polished to remove any minor surface imperfections and improve its light transmittance and texture. After polishing, it is washed and air-dried again to obtain a mobile phone glass back cover with a gradient effect from sunset purple to dawn gold. The color transition is smooth and natural, and the overall color is luxurious and layered.

[0030] Example 2: Refer to Figure 1 Deep sea blue → Glacier silver gradient: First, select a Corning Gorilla Glass phone back cover for cleaning. The cleaning process must ensure the glass surface is free of any oil, dust, or other contaminants to provide a clean substrate for the subsequent ion exchange process and prevent surface impurities from affecting the uniformity of the coloring effect. After cleaning, the glass back cover must be kept dry until the next step.

[0031] Next, the composite molten salt is prepared. Using sodium nitrate as the base salt, 1.5% silver nitrate, 3.0% copper nitrate, 1.0% zinc nitrate, and 0.05% cerium nitrate by mass are added. Silver nitrate and copper nitrate act as colorants, with the higher proportion of copper nitrate providing the blue base for the gradient effect; zinc nitrate acts as a stabilizer, helping to maintain the uniform distribution of coloring ions; and cerium nitrate acts as a hue adjuster, fine-tuning the hue stability of the gradient. After mixing these components, the mixture is heated to 410℃ to melt and homogenize, forming a composite molten salt system suitable for deep-sea blue gradients.

[0032] Then, gradient ion exchange is performed. The cleaned and dried glass back cover is fixed at a 30° angle, and the control device immerses 50% of its length in the composite molten salt, maintaining this position for 80 minutes. Adjusting the tilt angle and immersion depth affects the diffusion gradient of coloring ions on the glass surface, thus controlling the range and speed of the gradient transition. A longer holding time allows the coloring ions to more fully exchange with the glass surface, forming a more pronounced color gradient.

[0033] After ion exchange, the glass back cover is removed and residual molten salt is cleaned. The mixture is then placed in a pure nitrogen atmosphere at 500°C for 25 minutes for heat treatment. The pure nitrogen atmosphere promotes the formation of more Cu ions. 2 The addition of [+] enhances the blue hue, making the gradient effect lean more towards deep sea blue. During the heat treatment process, ions are reduced and aggregated into nanoparticles, and the continuous changes in size and concentration create a smooth transition from deep sea blue to glacial silver.

[0034] Finally, the heat-treated glass back cover undergoes polishing, cleaning, and air drying. Polishing removes minor surface imperfections, improving the smoothness and texture of the glass; cleaning and drying ensure no residual impurities remain on the surface. The resulting phone glass back cover exhibits a gradient effect from deep sea blue to glacial silver, with a smooth and natural transition, luxurious colors, and a high-end visual appeal.

[0035] From the above, we can conclude that: In this invention, the one-step gradient fabrication design eliminates the need for additional masking or multi-step dyeing operations, as well as complex technologies such as laser assistance, throughout the coloring process. By using an inclined immersion method, an ion concentration gradient naturally forms at the interface between the molten salt and air in the glass, eliminating the need for multiple adjustments and conversions in traditional processes. This not only simplifies the production process but also shortens the processing time for individual products, improving overall production efficiency. Simultaneously, it reduces equipment investment and consumable usage, effectively lowering manufacturing costs and making it more suitable for large-scale industrial production.

[0036] In this invention, a nonlinear concentration gradient based on diffusion kinetics allows for a continuous and natural transition of color from the immersed end to the unimmersed end. This transition lacks sharp boundaries or abrupt color bands, visually resembling the gradient effect of natural light and shadow, meeting the high aesthetic requirements of high-end consumer electronics products. The smooth transition enhances the product's luxurious feel, allowing the phone's back cover to exhibit rich visual layers from different angles, thus strengthening the product's market competitiveness.

[0037] In this invention, by adjusting the ratio of silver ions to copper ions in the composite salt bath, the basic hue bias of the gradient can be changed; adding trace amounts of color-adjusting ions can fine-tune the warmth or stability of the hue; changing the tilt angle, immersion depth, and exchange time can control the range, transition speed, and color depth of the gradient, thus allowing manufacturers to flexibly design a variety of gradient schemes according to market demands, from warm-toned sunset purple to dawn gold, to cool-toned deep sea blue to glacier silver, to meet the personalized appearance requirements of different brands and product series.

[0038] In this invention, the coloring layer formed by ion exchange penetrates deep into the glass surface rather than adhering to it, thus exhibiting stronger wear and scratch resistance and being less prone to peeling or wear during daily use. Subsequent heat treatment further stabilizes the coloring nanoparticles, making them less susceptible to fading under ultraviolet radiation during long-term use, ensuring color retention and enhancing the user experience.

[0039] In this invention, the entire coloring process does not require the use of organic dyes or coatings, avoiding the emission of organic pollutants in traditional processes. The molten salt component can be recycled, reducing raw material waste; at the same time, the generated waste liquid has a simple composition, is easier to treat, and has less impact on the environment. Thus, this method not only conforms to the current development trend of green manufacturing, but also reduces the environmental treatment costs for enterprises, achieving a win-win situation for both economic and environmental benefits.

[0040] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the term "comprising" or any other variations thereof is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0041] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for fabricating color gradient on mobile phone back cover glass based on composite salt bath ion exchange coloring technology, characterized in that: The method includes the following steps: S1: Clean and dry the aluminosilicate system mobile phone back cover glass substrate to remove surface contaminants and prepare a clean substrate for subsequent ion exchange steps; S2: Prepare a composite molten salt using sodium nitrate as the base salt, add silver nitrate and copper nitrate as colorants, and add 0.5%-3% zinc nitrate as a color stabilizer. The ratio of copper nitrate to zinc nitrate is 1:0.2 to 1:

5. At the same time, add 0.1% cerium nitrate as a color modifier. Heat the composite molten salt to a working temperature of 380℃-420℃ to provide a suitable reaction medium for the subsequent gradient ion exchange step. S3: The glass back cover treated in S1 is placed at an angle of 15°-75° in a special immersion device. The control device immerses the glass back cover part into the composite molten salt bath prepared in S2 at a speed of 0.5-5 mm / s. The immersion depth is 20%-80% of the glass length. The ion exchange is completed while keeping the glass in a static state. The exchange time is 30 minutes to 2 hours, so that the coloring ions form a continuously changing concentration gradient on the glass surface. S4: After the glass back cover has undergone ion exchange in S3, remove it and clean off any residual salts. Then, place it in a muffle furnace at 480℃-550℃ for heat treatment for 10-30 minutes to reduce the diffused ions and aggregate them into nanoparticles for color development, while also enhancing the color durability of the glass. S5: Polish and clean the glass back cover after S4 heat treatment to remove surface impurities, and apply an oleophobic coating or anti-fingerprint coating as needed to complete the entire gradient coloring process.

2. The method for producing a color gradient on a mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S1, a mixed solution of neutral cleaning agent and deionized water is used for cleaning. The glass is treated with an ultrasonic cleaning device at a frequency of 40kHz for 15 minutes to thoroughly remove oil, dust and other contaminants from the glass surface. After cleaning, the glass substrate is placed in a hot air drying oven, the temperature is set to 80℃ and maintained for 20 minutes until the surface is completely dry and free of watermarks.

3. The method for producing a color gradient on a mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S1, the aluminosilicate system mobile phone back cover glass substrate is first cleaned. During cleaning, a mixed solution of neutral cleaning agent and deionized water is used, and the glass substrate is treated with an ultrasonic cleaning device at a frequency of 40kHz for 15 minutes to thoroughly remove contaminants from the glass surface. After cleaning, the glass substrate is placed in a hot air drying oven, the temperature is set to 80℃, and it is kept for 20 minutes until the surface is completely dry and free of watermarks.

4. The method for producing a color gradient on a mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S1, the cleaning process must strictly follow a multi-step procedure. First, the oil stains are decomposed by a neutral cleaning agent, then the tiny particles are removed by ultrasonic cleaning, and finally, hot air drying is performed to ensure that no moisture remains, providing an ideal clean substrate for ion exchange.

5. The method for fabricating color gradient on mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S2, all components are placed in a crucible and heated to a working temperature of 380°C to 420°C, and stirred continuously until completely melted and homogeneous.

6. The method for producing a color gradient on a mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S2, the molten salt needs to be stirred at a constant speed during the melting process to ensure that all components are evenly dispersed and to avoid uneven color caused by excessively high local concentrations.

7. The method for producing a color gradient on a mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S3, during the gradual ion exchange stage, the pretreated glass back cover is first fixed at an angle on a specially designed immersion device, with the angle set between 15° and 75°. Then, the immersion device is controlled to immerse the glass back cover part into the composite molten salt bath at a slow speed of 0.5 mm / s to 5 mm / s, with an immersion depth of 20% to 80% of the glass length. After immersion, the glass back cover is kept stationary in this position, and the ion exchange process begins.

8. The method for producing a color gradient on a mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S3, under static conditions, the surface of the glass immersed in molten salt will exchange silver ions, copper ions and sodium ions in the glass; the part not immersed in molten salt is exposed to air or protective atmosphere, and due to the combined effect of concentration gradient and thermal convection, the coloring ions form a unique diffusion field near the gas-liquid-solid three-phase interface.

9. A method for fabricating color gradient on mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S4, the heat treatment color development and strengthening stage, the glass back cover that has completed ion exchange is first removed, and the surface residual molten salt is washed with deionized water to ensure that there is no salt residue; then the glass back cover is placed in a muffle furnace, the furnace temperature is set to 480℃ to 550℃, and the holding time is 10 minutes to 30 minutes; a suitable atmosphere environment is selected according to the target gradient color, such as a nitrogen-hydrogen mixture or pure nitrogen.

10. A method for fabricating color gradient on mobile phone back cover glass based on composite salt bath ion exchange coloring technology as described in claim 1, characterized in that: In step S5, vacuum evaporation is used to coat the oleophobic coating or anti-fingerprint coating to control the coating thickness within a suitable range and ensure coating uniformity and adhesion. After coating, the glass back cover surface will have good anti-fingerprint performance and hydrophobicity, while enhancing the transparency of the gradient color and improving the overall appearance and texture. Post-processing requires strict control of each step of the operation to ensure that the final product meets the appearance requirements of high-end consumer electronics products and is free from any surface defects or coating peeling.

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