Continuous preparation method and production line for multi-layer hollow coating of steel drum bottom cover

Abstract

The invention discloses a continuous preparation method of a multi-layer hollow coating of a steel drum bottom cover and a production line. The preparation method comprises the steps of metal strip pretreatment and patterned coating transfer printing and curing, wherein firstly, surface cleaning, drying, chemical conversion treatment and conversion film drying are conducted on a continuously-moving metal strip; and then at least one functional coating is accurately transferred to the area, corresponding to the finished steel drum bottom cover, of the metal strip through at least one intaglio printing unit and independently cured, the coating transfer printing area is strictly limited in the outline of the bottom cover, and the metal area outside the outline is kept clean. The production line sequentially comprises an uncoiler, a strip pretreatment section, at least one printing curing module and a coiler in the strip advancing direction. The utilization rate of the coating reaches 95% or above, leftover materials can be directly recycled in a high-quality mode, the coating is excellent in performance and suitable for continuous high-speed production, a multi-layer coating can be flexibly prepared, and high-quality, low-cost and environment-friendly manufacturing is achieved.

Description

Technical Field

[0001] This invention belongs to the field of steel drum packaging manufacturing technology, specifically a method and production line for continuous preparation of multi-layer perforated coating on steel drum bottom caps. Background Technology

[0002] As a common industrial packaging container, steel drums require their bottom lids to possess excellent corrosion resistance to ensure the safety of the contents, while also often needing a certain degree of decorative appeal for brand identification. Currently, the mainstream coating processes for steel drum bottom lids in the industry are mainly divided into two types, both of which have significant defects.

[0003] The first method is post-coating, which involves stamping the metal sheet into individual bottom caps before spraying or roller coating. The inherent drawbacks of this method are: firstly, the paint utilization rate is extremely low, with a large amount of paint wasted as overspray, and the uniformity of the coating is difficult to control precisely; secondly, both spraying and roller coating cover the entire surface of the bottom cap, but areas that will be rolled and wrapped during barrel manufacturing do not require coating protection. This coating is functionally redundant, resulting in ineffective paint consumption and cost waste.

[0004] The second method is the pre-coating process, which involves coating the entire roll of metal strip first, and then stamping out individual bottom covers using a die. While this method can improve coating continuity to some extent, it brings a more serious problem: the metal scraps generated after stamping typically account for 30%-50% of the material area, and these scraps are completely covered by the coating. Scraps contaminated with organic coatings are classified as hazardous waste during recycling and smelting, significantly reducing their recycling value and incurring high hazardous waste disposal costs, which seriously violates the principles of green manufacturing and the circular economy.

[0005] Therefore, there is an urgent need in this field for an innovative coating preparation technology that simultaneously meets the following core requirements: 1) achieving precise application of coatings and significantly saving costs; 2) ensuring the cleanliness of stamping scraps for efficient recycling; 3) integrating necessary pretreatment for chemical conversion to ensure coating performance; 4) adapting to high-speed continuous industrial production; and 5) possessing flexibility to prepare multi-layer coating structures that meet different needs. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the present invention aims to provide a method and production line for continuous preparation of multi-layer hollow coating of steel barrel bottom cover. The core objective is to fundamentally solve the problems of paint waste and waste pollution while ensuring the final performance of the coating, and to achieve high-quality and high-efficiency continuous production.

[0007] To achieve the above objectives, the present invention employs the following technical means: A method for continuous preparation of a multi-layer perforated coating on a steel drum bottom cover includes the following steps: S1: Metal strip pretreatment: The continuously moving metal strip is surface cleaned, dried after cleaning, chemically converted and the conversion film is dried, thereby forming a chemical conversion film on the surface of the metal strip. S2: Patterned Coating Transfer and Curing: At least one gravure printing unit is used. Each gravure printing unit includes a steel gravure printing roller with a closed pattern engraved on its surface, which is consistent with the shape and size of the finished steel drum bottom cover, and a transfer roller that presses against the steel gravure printing roller. Through the at least one gravure printing unit, at least one layer of functional coating is sequentially transferred to the area of ​​the finished steel drum bottom cover on the metal strip with the chemical conversion film treated in step S1, and each layer of coating is cured independently. In step S2, the transfer areas of all functional coatings are strictly limited to the outline of the finished steel drum bottom cover, thereby forming a multi-layer coating structure based on chemical conversion film on the metal strip that is only located within the outline of the steel drum bottom cover, and the metal area outside the outline of the steel drum bottom cover remains clean without any organic coating covering throughout the process.

[0008] Preferably, the chemical conversion treatment in step S1 is any one of phosphating, chromating, or chromium-free chemical conversion treatment.

[0009] Preferably, the multilayer coating structure described in step S2 consists of two or more coating layers.

[0010] Preferably, when the multi-layer coating structure consists of two or more coatings, the transfer is performed sequentially by two or more gravure printing units, and the pattern of the steel gravure printing roller in the subsequent gravure printing unit must be precisely aligned with the pattern of the steel gravure printing roller in the first gravure printing unit.

[0011] Preferably, the engraving depth of the steel gravure printing roller is 10μm to 100μm.

[0012] Preferably, the Shore hardness of the transfer roller is A50 to A85, and the linear pressure between the transfer roller and the steel gravure printing roller is 50 N / cm to 400 N / cm.

[0013] Preferably, the functional coating includes a primer, a color paint, a barrier coating, or a transparent protective varnish; the curing treatment is hot air curing, ultraviolet curing, or infrared curing.

[0014] Preferably, the undercoat is an epoxy, polyurethane, polyester, silicone, silicone-modified, or fluorocarbon protective paint; the topcoat is a transparent varnish, the type of which matches the undercoat.

[0015] A continuous production line for a method of continuously preparing a multi-layer perforated coating for steel drum bottom covers, the production line comprising, sequentially along the metal strip travel direction: Unwinder; The strip pretreatment section includes, in sequence, a strip cleaning device, a first drying device, a chemical conversion treatment device, and a second drying device; At least one printing and curing module, each of the printing and curing modules consisting of a gravure printing unit and a curing device immediately following it; The gravure printing unit includes a steel gravure printing roller engraved with a pattern of a steel barrel bottom cover and a transfer roller that works with it. Winding machine; The number N of the printing and curing modules is not less than 1, and matches the number of layers of the multilayer coating structure to be prepared.

[0016] Preferably, when the number N of the printing curing modules is greater than or equal to 2, a pattern alignment control system is provided between each of the gravure printing units.

[0017] The present invention has the following beneficial effects: 1. Extreme material conservation: The coating is applied only to the bottom cover area of ​​the steel drum that becomes the final product, with a utilization rate of over 95%. This avoids complete waste on scraps and ineffective coating of the rolled edges in the post-coating process, significantly reducing raw material costs.

[0018] 2. Thorough waste cleanliness and high recycling value: All scrap areas generated after stamping retain the original state of the metal substrate and chemical conversion coating, free from any organic coating pollution. These scraps can be directly recycled as clean, high-quality scrap steel. The recycling process is simple, valuable, and completely eliminates hazardous waste treatment costs and environmental burden.

[0019] 3. Superior coating performance: This invention integrates standard chemical conversion pretreatment and a complete multi-layer coating system to ensure that the coating has excellent adhesion, mechanical properties and corrosion resistance, fully meeting or even exceeding the quality requirements of steel drum products.

[0020] 4. High process flexibility and product adaptability: By configuring different numbers of printing-curing modules, steel drum bottom caps with double, triple, four or more coating layers can be produced flexibly and conveniently, adapting to different market demands from ordinary industrial drums to high-end decorative drums.

[0021] 5. Excellent continuous production efficiency: The entire process, from uncoiling, pretreatment, multiple printing and curing to rewinding, is completed online continuously. The production speed can be perfectly matched with high-speed stamping lines, and the efficiency is far higher than that of traditional single-piece post-coating processes, which is conducive to realizing large-scale industrial production. Attached Figure Description

[0022] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a schematic diagram of the production line modules of the present invention. Detailed Implementation

[0023] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] like Figure 1-2 As shown, the present invention provides a method for continuous preparation of a multi-layer perforated coating on a steel drum bottom cover, comprising the following steps: S1: Metal Strip Pretreatment: The continuously moving metal strip undergoes surface cleaning, drying, chemical conversion treatment, and conversion film drying to form a chemical conversion film on its surface. This pretreatment step removes oil and impurities from the metal strip surface, while the resulting chemical conversion film significantly improves the adhesion between subsequent coatings and the substrate, ensuring the protective performance of the coating.

[0025] S2: Patterned Coating Transfer and Curing: At least one gravure printing unit is used. Each gravure printing unit includes a steel gravure printing roller with a closed pattern engraved on its surface that matches the shape and size of the finished steel drum bottom cover, and a transfer roller that presses against the steel gravure printing roller. Through at least one gravure printing unit, at least one layer of functional coating is sequentially transferred to the area of ​​the finished steel drum bottom cover on the metal strip with chemical conversion film treated in step S1, and each layer of coating is cured independently.

[0026] The key design feature is that in step S2, the transfer area of ​​all functional coatings is strictly confined within the outline of the finished steel drum bottom cover. This creates a multi-layer coating structure based on a chemical conversion film on the metal strip, located only within the outline of the steel drum bottom cover, while the metal areas outside the outline remain clean and free of any organic coating throughout the process. This design prevents paint waste in non-product areas from the outset and ensures that the scrap generated during stamping is free of organic coating contamination.

[0027] Furthermore, the chemical conversion treatment mentioned in step S1 can be any one of phosphating, chromating, or chromium-free chemical conversion treatment, which can be flexibly selected according to product performance requirements and environmental protection requirements.

[0028] Furthermore, the multi-layer coating structure described in step S2 consists of two or more coating layers, and the precise superposition of the multi-layer coatings can be achieved through multiple sets of gravure printing units. When the multi-layer coating structure consists of two or more coating layers, the transfer is performed sequentially by two or more gravure printing units, and the pattern of the steel gravure printing roller in the subsequent gravure printing unit must be precisely aligned with the pattern of the steel gravure printing roller in the first gravure printing unit to ensure precise adhesion of the multi-layer coatings and guarantee the overall performance of the coating.

[0029] Furthermore, the cell engraving depth of the steel gravure printing roller ranges from 10μm to 100μm, which can be precisely adjusted according to the coating thickness requirements to ensure coating thickness uniformity. The Shore hardness of the transfer roller is A50 to A85, and the linear pressure between the transfer roller and the steel gravure printing roller is 50N / cm to 400N / cm. This parameter range ensures that the functional coating is efficiently and completely transferred to the surface of the metal strip, while avoiding damage to the strip and conversion film.

[0030] Furthermore, functional coatings include primers, color paints, barrier coatings, or transparent protective varnishes; curing treatments include hot air curing, ultraviolet curing, or infrared curing, with the appropriate curing method selected based on the characteristics of the coating material to ensure sufficient curing and stable performance. The base coat is preferably an epoxy, polyurethane, polyester, silicone, silicone-modified, or fluorocarbon protective paint, possessing excellent corrosion resistance and adhesion; the top coat is a transparent varnish, its type matching the base coat, enhancing the coating's gloss, weather resistance, and decorative properties.

[0031] On the other hand, the present invention provides a continuous production line for implementing the above method, comprising, in sequence along the metal strip travel direction: Uncoiler: Used to smoothly unwind coiled metal strips to provide raw materials for continuous production; The strip pretreatment section includes, in sequence, a strip cleaning device, a first drying device, a chemical conversion treatment device, and a second drying device, which respectively realize the surface cleaning of the metal strip, drying after cleaning, preparation of chemical conversion film, and curing of conversion film. At least one printing and curing module: Each printing and curing module consists of a gravure printing unit and a curing device that follows it; the gravure printing unit includes a steel gravure printing roller engraved with a pattern of a steel barrel bottom cover and a transfer roller that works with it to achieve precise transfer of the functional coating, and the curing device achieves timely curing of the coating after transfer. Winding machine: Used to wind up metal strips that have completed multi-layer coating into coils, facilitating subsequent stamping processing, storage, and transportation.

[0032] The number of printing and curing modules, N, is not less than 1 and matches the number of layers in the multilayer coating structure to be prepared. The number of coating layers can be flexibly adjusted by increasing or decreasing the number of printing and curing modules. When the number of printing and curing modules, N, is greater than or equal to 2, a pattern alignment control system is set between each gravure printing unit to ensure accurate alignment of the multilayer coating transfer. Example

[0033] Three-layer coating structure for the bottom lid of 208L steel drum 1. Raw materials and equipment Metal strip: Cold-rolled steel sheet coils with a thickness of 1.2mm; Pretreatment agents: alkaline degreasing agent, zinc phosphating solution; Coating materials: epoxy primer (gray), polyester paint (blue), fluorocarbon clear varnish; Main equipment: The continuous production line described in this invention is equipped with three printing and curing modules, and a pattern alignment control system is set between each printing and curing module.

[0034] 2. Process Steps S1: Pre-treatment of metal strip Uncoiling and Cleaning: The steel coil is smoothly unwound on the uncoiler and enters the strip cleaning device. The surface oil and iron powder are thoroughly removed by alkaline spraying and mechanical brushing, and then it is rinsed clean with deionized water. Drying after cleaning: The strip enters the first drying device and is completely dried by 80℃ hot air drying mode to avoid residual moisture affecting the effect of subsequent chemical conversion treatment. Chemical conversion treatment: The dried strip enters the chemical conversion treatment device, and zinc phosphate solution is applied evenly by spraying. The treatment temperature is controlled at 45°C and the reaction time is 60 seconds, forming a uniform and dense gray phosphate film on the surface of the strip. Conversion film drying: The strip enters the second drying device and is dried with hot air at 100°C for 2 minutes to completely cure the phosphating film and improve the adhesion between the phosphating film and the substrate.

[0035] S2: Patterned coating transfer and curing First layer (primer layer) transfer and curing: The strip enters the first printing and curing module. The surface of the steel gravure printing roller in this module is engraved with a closed pattern that matches the shape of the bottom cover of the 208L steel drum, with a cell depth of 35μm. Using epoxy primer, the epoxy primer is precisely transferred to the bottom cover outline on the phosphated strip surface through a transfer roller (Shore hardness A70, linear pressure between the transfer roller and the steel gravure printing roller is 200N / cm). The strip immediately enters the first curing device and is cured in a hot air tunnel at 180℃ for 2 minutes to form a firm primer layer.

[0036] Second layer (color paint layer) transfer and curing: The strip enters the second printing and curing module, where the pattern alignment control system ensures that the pattern on the steel gravure printing roller of this module is precisely aligned with the pattern of the first printing and curing module. The cell depth of the steel gravure printing roller is 25μm. Using polyester blue color paint, it is precisely transferred and completely covers the cured primer layer under a transfer pressure of 200N / cm. Then the strip enters the second curing device and is cured with hot air at 160℃ for 2 minutes, so that the color paint layer is fully cured and tightly bonded to the primer layer.

[0037] The third layer (varnish layer) is transferred and cured: The strip enters the third printing and curing module, where the pattern alignment control system ensures that the pattern on the steel gravure printing roller is precisely aligned with the outline of the bottom cover, with a cell depth of 20μm; fluorocarbon transparent varnish is used to accurately transfer and completely cover the paint layer; the strip enters the third curing device and is cured with hot air at 150℃ for 3 minutes to form a high-gloss, highly weather-resistant transparent protective layer.

[0038] S3: Rewinding and Subsequent Processing The metal strip with all coatings completed is wound into a coil by a winding machine, and then the coil is transported to a high-speed stamping production line where individual steel drum bottom covers are stamped out using molds.

[0039] 3. Implementation Results Each 208L steel drum bottom cover produced by stamping has a three-layer composite coating structure of "phosphating film + epoxy primer + polyester paint + transparent varnish". After testing, the coating adhesion meets the Class 1 standard specified in GB / T 1720-1979, and the salt spray corrosion resistance meets the requirement of more than 500 hours specified in GB / T 10125-2021, which fully meets the usage requirements of high-end steel drum bottom covers. At the same time, all the mesh scraps produced by stamping have only a clean phosphating film on the surface, without any organic coating residue. After being packaged, they can be sold directly to steel mills for recycling and smelting as high-quality scrap steel.

[0040] 4. Conclusion This embodiment, through innovative process design, successfully combines the precision of gravure printing with the multi-layer protection requirements of industrial coating, simultaneously achieving high product quality, low production cost, and green manufacturing on a continuous production line. This verifies the feasibility and superiority of the technical solution of this invention and provides a transformative solution for the steel drum manufacturing industry. Example

[0041] The bottom cap of a 100L steel drum is manufactured using a double-layer coating structure. 1. Raw materials and equipment Metal strip: Cold-rolled steel sheet coils with a thickness of 1.0 mm; Pretreatment agents: neutral degreasing agent, chromium-free chemical conversion solution; Coating materials: polyurethane primer (black), polyester clear varnish; Main equipment: The continuous production line described in this invention is equipped with two printing and curing modules, and a pattern alignment control system is set between the two modules.

[0042] 2. Process Steps S1: Pre-treatment of metal strip Uncoiling and Cleaning: The steel coil is smoothly unwound on the uncoiler and enters the strip cleaning device. The surface oil and impurities are thoroughly removed by a combination of neutral degreasing agent spraying, ultrasonic cleaning and mechanical brushing. Then it is rinsed with deionized water until neutral. Drying after cleaning: The strip enters the first drying device and is completely dried by a 75℃ hot air drying mode to avoid residual moisture affecting the chemical conversion effect; Chemical conversion treatment: The dried strip enters the chemical conversion treatment device, and a chromium-free chemical conversion solution is applied by dip coating. The treatment temperature is controlled at 35°C and the reaction time is 40 seconds, forming a uniform and dense colorless conversion film on the surface of the strip. Conversion film drying: The strip enters the second drying device and is dried with hot air at 90°C for 1.5 minutes to completely cure the conversion film and improve its adhesion to subsequent coatings.

[0043] S2: Patterned coating transfer and curing First layer (primer layer) transfer and curing: The strip enters the first printing and curing module, where the surface of the steel gravure printing roller is engraved with a closed pattern consistent with the shape of the bottom cover of a 100L steel drum, with a cell depth of 40μm; using polyurethane primer, the primer is precisely transferred to the bottom cover outline of the treated strip surface through a transfer roller (Shore hardness A75, linear pressure between the transfer roller and the steel gravure printing roller is 250N / cm); the strip immediately enters the first curing device and is cured in a hot air tunnel at 170℃ for 2.5 minutes to form a firm primer layer.

[0044] Second layer (varnish layer) transfer and curing: The strip enters the second printing and curing module, where the pattern alignment control system ensures that the pattern on the steel gravure printing roller of this module is precisely aligned with the pattern of the first module, with a cell depth of 22μm; using polyester transparent varnish, it is precisely transferred and completely covers the cured primer layer under a transfer pressure of 250N / cm; then the strip enters the second curing device and is cured with hot air at 150℃ for 3 minutes to form a smooth, weather-resistant transparent protective layer.

[0045] S3: Rewinding and Subsequent Processing The metal strip with all coatings completed is wound into a coil by a winding machine, and then the coil is transported to a high-speed stamping production line to stamp out individual 100L steel drum bottom caps using a die.

[0046] 3. Implementation Results Each 100L steel drum bottom cover produced by stamping has a double-layer composite coating structure of "chromium-free conversion film + polyurethane primer + polyester transparent varnish". After testing, the coating adhesion meets the Class 1 standard specified in GB / T 1720-1979, and the salt spray corrosion resistance meets the requirement of more than 400 hours specified in GB / T 10125-2021, which fully meets the usage requirements of ordinary industrial steel drum bottom covers. The surface of the scrap produced by stamping is only a clean chromium-free conversion film with no organic coating residue, and can be directly recycled and smelted as high-quality scrap steel.

[0047] 4. Conclusion This embodiment, through the adoption of a double-layer coating structure and chromium-free chemical conversion treatment, verifies the flexibility of the invention in simplifying the coating structure, adapting to the production of medium-sized steel drums, and meeting environmental protection requirements. The process is continuous and stable, with a coating utilization rate of over 96%, significantly reducing production costs and achieving the goal of green manufacturing. Example

[0048] A four-layer coating structure was used to prepare the bottom cover of a 20L small steel drum (including a barrier coating). 1. Raw materials and equipment Metal strip: Cold-rolled steel sheet coils with a thickness of 0.8mm; Pretreatment agents: alkaline degreasing agent, chromating solution; Coating materials: epoxy primer (gray), polyester paint (red), polyamide barrier coating, fluorocarbon clear varnish; Main equipment: The continuous production line described in this invention is equipped with four printing and curing modules, and each module is equipped with a pattern alignment control system; the curing device includes two modes: hot air curing and ultraviolet curing.

[0049] 2. Process Steps S1: Pre-treatment of metal strip Uncoiling and cleaning: The steel coil is smoothly unwound on the uncoiler and enters the strip cleaning device. The surface oil and iron powder are removed by high-pressure spraying of alkaline degreasing agent and mechanical brushing, and then rinsed clean with deionized water. Drying after cleaning: The strip enters the first drying device and its surface is completely dried using an 85℃ hot air drying mode. Chemical conversion treatment: The dried strip enters the chemical conversion treatment device, and the chromating treatment solution is applied by spraying. The treatment temperature is controlled at 50°C and the reaction time is 30 seconds, forming a uniform yellow chromating film on the surface of the strip. Conversion film drying: The strip enters the second drying device and is dried with hot air at 105°C for 1 minute to completely cure the chromate film.

[0050] S2: Patterned coating transfer and curing First layer (primer layer) transfer and curing: The strip enters the first printing and curing module, where a steel gravure printing roller is engraved with a closed pattern that matches the shape of the bottom cover of a 20L steel drum, with a cell depth of 30μm; using epoxy primer, the pattern is precisely transferred into the outline of the bottom cover using a transfer roller (Shore hardness A65, linear pressure 180N / cm); the strip enters the first curing device and is cured with hot air at 180℃ for 2 minutes to form the primer layer.

[0051] Second layer (color paint layer) transfer and curing: The strip enters the second printing and curing module. After being precisely aligned by the alignment control system, the polyester red color paint is transferred and covered on the primer layer by a steel gravure printing roller with a cell depth of 25μm. The transfer pressure is 180N / cm. Then it enters the second curing device and is cured with hot air at 160℃ for 2 minutes.

[0052] The third layer (barrier coating) is transferred and cured: The strip enters the third printing and curing module. After precise alignment, the polyamide barrier coating is transferred and covered on the paint layer by a steel gravure printing roller with a cell depth of 15μm. The transfer pressure is 200N / cm. The strip then enters the third curing device and is cured for 30 seconds using ultraviolet curing mode (wavelength 365nm, light intensity 800mW / cm²) to form a dense barrier layer.

[0053] Fourth layer (varnish layer) transfer and curing: The strip enters the fourth printing and curing module. After precise alignment, the fluorocarbon transparent varnish is transferred and covered on the barrier coating by a steel gravure printing roller with a cell depth of 20μm. The transfer pressure is 200N / cm. The strip then enters the fourth curing device and is cured with hot air at 150℃ for 3 minutes to form a highly weather-resistant and high-gloss protective layer.

[0054] S3: Rewinding and Subsequent Processing The metal strip with the coating completed is wound up by a winding machine and transported to the stamping production line to stamp out individual 20L steel drum bottom covers.

[0055] 3. Implementation Results The stamped 20L steel drum bottom cover has a four-layer composite coating structure consisting of "chromium coating + epoxy primer + polyester paint + polyamide barrier layer + fluorocarbon varnish". Testing shows that the coating adhesion meets the Class 0 standard specified in GB / T 1720-1979, its salt spray corrosion resistance exceeds 600 hours as specified in GB / T 10125-2021, and its barrier performance against organic solvents meets the Class 1 standard specified in GB / T 19789-2005. It is suitable for use in small steel drums containing highly corrosive contents. The stamping scraps are clean and free of coating contamination, resulting in high recycling value.

[0056] 4. Conclusion This embodiment, by adding a barrier coating and a UV curing process, verifies the superiority of the present invention in preparing multi-layered complex coating structures, adapting to the production of small special steel drums, and meeting high-performance protection requirements. The process can flexibly integrate different curing modes, achieving high coating precision and providing an efficient solution for the manufacturing of high-end special steel drums.

[0057] The examples provided in this invention are not intended to limit the implementation. Those skilled in the art will recognize that various variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of this invention.

Claims

1. A method for continuously producing a multilayered hollowed-out coating of a steel drum bottom cover, characterized by, The method comprises the following steps: S1: metal strip pretreatment: surface cleaning, drying after cleaning, chemical conversion treatment and drying of the conversion film on the continuously running metal strip, so as to form a layer of chemical conversion film on the surface of the metal strip; S2: patterned coating transfer and curing: at least one gravure printing unit is used, each of which comprises a steel gravure printing roller with a closed pattern engraved on the surface, which is consistent with the shape and size of the steel drum bottom cover product, and a transfer rubber roller matched with the steel gravure printing roller; through the at least one gravure printing unit, at least one layer of functional coating is transferred to the area corresponding to the steel drum bottom cover product on the metal strip with the chemical conversion film treated by step S1, and each layer of coating is independently cured; Wherein, the transfer area of all functional coatings in step S2 is strictly limited within the outline of the steel drum bottom cover product, so as to form a multi-layer coating structure based on the chemical conversion film only within the outline of the steel drum bottom cover on the metal strip, and the metal area outside the outline of the steel drum bottom cover remains clean without any organic coating covering during the whole process.

2. The method of claim 1, wherein, The chemical conversion treatment in step S1 is any one of phosphating treatment, chromizing treatment or chromium-free chemical conversion treatment.

3. The method of claim 1, wherein, The multi-layer coating structure in step S2 is composed of two or more layers of coating.

4. The method of claim 3, wherein, When the multi-layer coating structure is composed of two or more layers of coating, the transfer is carried out by two or more gravure printing units in turn, and the pattern of the steel gravure printing roller in the subsequent gravure printing unit needs to be accurately aligned with the pattern of the steel gravure printing roller in the first gravure printing unit.

5. The method of claim 1, wherein, The engraving depth of the steel gravure printing roller is 10-100 μm.

6. The method of claim 1, wherein, The Shore hardness of the transfer rubber roller is A50-A85, and the linear pressure between the transfer rubber roller and the steel gravure printing roller is 50-400 N / cm.

7. The method of claim 1, wherein, The functional coating includes primer, color paint, barrier coating or transparent protective varnish; the curing treatment is hot air curing, ultraviolet curing or infrared curing.

8. The method of claim 1, wherein, The primer coating is epoxy, polyurethane, polyester, silicone, silicon-modified or fluorocarbon protective paint; the topcoat is transparent varnish, which is matched with the primer coating.

9. A continuous production line for implementing the method according to any one of claims 1 to 8, characterized in that, The production line comprises, in sequence along the direction of the metal strip, an uncoiler, a strip pretreatment section comprising, in sequence, a strip cleaning device, a first drying device, a chemical conversion treatment device and a second drying device, at least one printing and curing module, each of which is composed of a gravure printing unit and a curing device following it, the gravure printing unit comprising a steel gravure printing roller engraved with the pattern of the steel drum bottom cover and a transfer rubber roller matched with it, and a coiler. The number N of the printing and curing modules is not less than 1 and is matched with the number of layers of the multi-layer coating structure to be prepared. When the number N of the printing and curing modules is greater than or equal to 2, a pattern alignment control system is arranged between each of the gravure printing units. ​ ​ ​ ​ 10. The continuous production line according to claim 9, characterized in that ​

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