Offset printing and cold stamping process for metal sheet

Abstract

The invention discloses an offset printing and cold stamping process for a metal sheet, and relates to the technical field of printing and surface decoration. The process sequentially comprises the following steps: carrying out atmospheric pressure plasma treatment on the surface of a metal sheet; coating an epoxy-acrylic acid composite resin bottom coating and carrying out infrared leveling; printing patterned UV glue through offset printing; attaching electrochemical aluminum foil and pressing; firstly stripping the base film in an uncured state of the UV glue, so that the aluminum-plated layer is transferred to the surface of the glue pattern; and finally carrying out UV illumination curing. In the using process, the interface bonding force is enhanced through plasma treatment and the special prime coat, the technology of first stripping and then curing is innovatively adopted, the problem of adhesive force reduction caused by curing stress is solved, and the adhesion firmness and durability of the hot stamping pattern on the metal sheet are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to printing and surface decoration, in particular to a process for offset cold stamping of metal sheets. BACKGROUND

[0002] Metal materials are widely used in packaging containers, household appliance housings, architectural decorations, etc. due to their excellent mechanical strength and durability. In order to improve the aesthetic appearance and brand value of products, pattern decoration treatment is usually performed on the metal surface. Traditional metal decoration processes include screen printing, heat transfer printing, spraying, etc., but these processes have the disadvantages of low production efficiency, poor pattern fineness, poor environmental friendliness, etc.

[0003] Offset cold stamping technology, as a new type of decoration process, has achieved good results on paper materials in recent years. Through the steps of printing special glue, attaching electrochemical aluminum foil, ultraviolet curing and peeling, etc., the offset cold stamping technology can form a metal luster pattern on the surface of the printing substrate. Compared with traditional hot stamping processes, offset cold stamping has the advantages of low energy consumption, high efficiency, good pattern fineness, etc.

[0004] However, when the existing offset cold stamping process is directly applied to metal sheets, a series of technical problems will occur due to the non-absorbing and high surface energy characteristics of the metal surface. The most prominent problem is the insufficient adhesion of the stamped pattern, which manifests as quality defects such as pattern edge lifting, easy peeling, and poor scratch resistance. The root cause of these problems lies in the insufficient interfacial bonding strength between the metal surface and the polymer glue layer, and the inability to effectively release the internal stress generated during the curing process.

[0005] Although there are some methods in the prior art to improve the adhesion of the metal surface, such as mechanical polishing, chemical treatment, etc., these methods either damage the surface of the substrate or have the risk of environmental pollution. Some technologies attempt to improve the adhesion by improving the glue formula, but the effect is limited and the cost is high. Therefore, developing an offset cold stamping process specifically for the characteristics of metal sheets, which can ensure the quality of the pattern and has good environmental friendliness and economic efficiency, has become a technical problem to be solved in the field. SUMMARY

[0006] The main purpose of the present application is to provide an offset cold stamping process for metal sheets, which aims to solve the technical problems of insufficient adhesion and easy peeling of the stamped pattern on the metal sheets in the prior art.

[0007] To achieve the above-mentioned purpose, the present application provides an offset cold stamping process for metal sheets.

[0008] The process first involves atmospheric pressure plasma treatment of the metal sheet surface, with the treatment power controlled within the range of 800-1500W and the treatment speed maintained at 10-30m / min. This step forms an active surface layer with high surface energy and high roughness on the metal surface, laying a solid foundation for subsequent process steps.

[0009] After surface activation, an epoxy-acrylic composite resin primer is applied to the plasma-treated metal sheet surface. By precisely controlling process parameters, the dry film thickness of the primer is maintained between 2-5 μm, and infrared leveling is performed at 60-80°C. This primer layer not only forms a strong bond with the activated metal surface but also provides an ideal adhesion base for the upper adhesive layer.

[0010] Next, patterned UV-curable adhesive is printed onto the base coating using offset printing. During this process, the wet film thickness of the adhesive is strictly controlled within the range of 3-8 μm, while the printing environment temperature is stably maintained within the range of 23±2℃. This precise parameter control ensures that the adhesive pattern has excellent leveling and shape accuracy.

[0011] After the adhesive printing is completed, the aluminized layer of the electroplated aluminum foil is bonded to the surface of the metal sheet printed with UV adhesive, and then pressed together using an elastic pressure roller with a Shore A hardness of 60-80 degrees under a pressure of 0.3-0.6 MPa. This step ensures full contact between the aluminized layer and the adhesive layer while avoiding damage to the intricate pattern.

[0012] A key innovation of this process lies in peeling the base film of the electroplated aluminum foil from the metal sheet before the UV adhesive has cured. This peeling-then-curing sequence effectively avoids the adverse effects of internal stress caused by adhesive curing shrinkage on interfacial bonding, which is present in traditional processes.

[0013] Finally, the metal sheet with the transferred pattern is cured by UV light, with the curing energy controlled within the range of 400-800 mJ / cm². This step allows the UV adhesive to fully cross-link and cure, permanently and firmly fixing the aluminum plating layer to the surface of the metal substrate.

[0014] In a preferred embodiment, atmospheric pressure plasma treatment uses a mixture of nitrogen and argon as the working gas, with argon accounting for 40%-60% of the volume. This gas ratio can further improve the uniformity and effectiveness of plasma treatment.

[0015] In another preferred embodiment, the epoxy resin component and the acrylic resin component in the epoxy-acrylic composite resin are mixed in a mass ratio of 1:0.5-2. This optimized ratio ensures both the adhesion strength of the base coat to the metal substrate and compatibility with the top-coat adhesive.

[0016] To further enhance process performance, UV-curable adhesives contain 10%-25% by weight of microencapsulated photoinitiators. These special photoinitiators provide more uniform curing and reduce curing shrinkage stress.

[0017] During the base film peeling process, by controlling the peeling angle within the range of 90°-135° and the peeling tension within the range of 0.5-2.0 N / cm, it is possible to ensure that the aluminum plating layer is transferred completely and accurately to the adhesive pattern surface.

[0018] In the UV curing process, a two-step curing method can be adopted: first, pre-curing is performed at a low energy of 50-100 mJ / cm², followed by main curing at an energy of 300-500 mJ / cm². This staged curing method helps to further release internal stress and improve the quality of interfacial bonding.

[0019] Depending on product requirements, after the hot stamping pattern has cured, a color overprinting process can be carried out. Color inks are precisely overprinted on the cured hot stamping pattern area using offset printing, and a protective varnish is applied to the entire overprinted surface to enhance the decorative effect and durability.

[0020] This process is suitable for a variety of metal materials, including aluminum, aluminum alloys, tinplate, and stainless steel. The primer coating can be applied using a doctor blade or micro-wire bar coating method to ensure uniformity and consistency in coating thickness. UV curing can be performed using a metal halide lamp or an LED-UV light source, depending on specific requirements.

[0021] The technical solution provided in this application may include the following beneficial effects:

[0022] This application employs atmospheric pressure plasma treatment with specific parameters to form an active surface layer with high surface energy and high roughness on the metal sheet surface. This improves the wettability and mechanical interlocking between the substrate and subsequent coatings, providing a solid foundation for the entire process system. The epoxy-acrylic composite resin base coating applied on this basis can effectively penetrate and anchor within the active surface layer, while its functional groups can form strong chemical bonds with the upper layer material, constructing a strong and robust transition bridge. By precisely controlling the wet film thickness of the UV adhesive and the printing environment temperature, the adhesive pattern is ensured to have optimal volumetric morphology and leveling, providing a uniform and sufficient adhesive medium for the subsequent transfer of the electroplated aluminum layer.

[0023] Using elastic rollers with specific hardness and optimized pressure parameters for pressing ensures full contact while avoiding damage to intricate patterns. Innovatively, the base film is peeled off before the UV adhesive cures, avoiding the risk of reduced adhesion caused by internal stress from UV adhesive curing shrinkage directly acting on the "adhesive-aluminized layer" interface, as is common in traditional processes. Finally, UV curing of the transferred pattern allows the UV adhesive to complete its cross-linking reaction under unconstrained conditions, permanently fixing the temporarily stored aluminized layer to the metal sheet with extremely high strength.

[0024] These steps work together to form a complete technical system, ensuring that the final hot stamping pattern has excellent adhesion and durability on the metal sheet, solving the technical problem of hot stamping patterns easily peeling off. At the same time, this process also improves the fineness and edge sharpness of the pattern, providing a superior decorative effect for metal products. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a flowchart of the offset printing and cold foil stamping process for metal sheets in this application. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0028] Please see Figure 1 The figure shows the overall flow chart of the offset cold foil stamping process for metal sheets in this application. As shown in the figure, this process is performed in the following six core steps in sequence:

[0029] First, the surface of the metal sheet is treated with atmospheric pressure plasma, with the treatment power controlled within the range of 800-1500W and the treatment speed maintained at 10-30m / min. This treatment creates an active surface layer with high surface energy and high roughness. Next, an epoxy-acrylic composite resin primer is coated onto the activated surface layer, with the dry film thickness precisely controlled between 2-5μm, and infrared leveling is performed at 60-80℃. Subsequently, patterned UV-curable adhesive is printed onto the primer using offset printing, with the wet film thickness controlled within the range of 3-8μm, and the printing environment temperature maintained at a stable state of 23±2℃. Then, the aluminized layer of the electroplated aluminum foil is bonded to the metal sheet surface printed with UV adhesive using an elastic pressure roller with a Shore A hardness of 60-80 degrees under a pressure of 0.3-0.6MPa. While the UV adhesive is still uncured, the base film of the electroplated aluminum foil is peeled off from the metal sheet, allowing the aluminum plating layer to be precisely transferred and temporarily retained on the uncured adhesive pattern surface. Finally, the metal sheet with the transferred pattern is UV-cured at an energy level of 400-800 mJ / cm², ensuring complete curing of the UV adhesive and firm adhesion of the aluminum plating layer to the metal substrate. These steps are sequentially linked, each with specific process parameters and quality control standards, collectively forming a complete technical system.

[0030] Example 1

[0031] This embodiment uses a 0.3mm thick 1060 aluminum sheet as the substrate and implements the offset cold foil stamping process of this application to form a highly adhesive brand logo pattern on the enterprise's products.

[0032] In the surface pretreatment stage, aluminum sheets are fixed on a continuous conveyor and their surfaces are treated using atmospheric pressure plasma treatment equipment. High-purity nitrogen is used as the main process gas, with a processing power of 1200W, a processing speed of 15m / min, and a distance of 1.5mm between the treatment head and the sheet surface. After this treatment, the surface properties of the aluminum sheet change significantly. The surface energy increases from 30-40mN / m before treatment to 70-75mN / m, and the surface roughness Ra value increases from 0.1-0.2μm to 0.8-1.2μm, forming a highly reactive surface layer.

[0033] In the primer treatment stage, a precision wire rod coater is used to evenly apply an epoxy-acrylic composite resin primer to the treated aluminum sheet surface. The primer is a 1:1 mass ratio of epoxy resin and acrylic resin, with an appropriate amount of solvent added to adjust the viscosity to 25-30s. Precise control ensures a dry film thickness of 3±0.2μm. The sheet is then placed in an infrared leveling tunnel and leveled at 65±2℃ for 30s, allowing the coating to surface dry and reach a semi-cured state. This step creates a primer coating that not only forms a strong bond with the metal surface but also provides a good adhesion base for subsequent adhesives.

[0034] In the adhesive printing stage, a high-performance offset printing press is used to print the company logo pattern with UV-curable adhesive on the base coating. The selected UV adhesive has a viscosity of 1200±50 cP at 25°C and contains a specific ratio of oligomers, reactive diluents, and photoinitiators. The wet film thickness of the adhesive is precisely controlled to 5±0.3 μm using an anilox roller. Throughout the printing process, the ambient temperature is stably controlled at 23±1°C, and the relative humidity at 50±5%. These stringent control conditions ensure that the adhesive pattern has excellent shape accuracy and surface quality.

[0035] During the material lamination stage, the aluminum plating layer of high-quality electroplated aluminum foil is immediately aligned and bonded to the printed surface. A silicone pressure roller with a Shore A hardness of 70±2 is used for lamination under a linear pressure of 0.4±0.05MPa, with the lamination speed controlled at 10-15m / min. During the lamination process, it is crucial to ensure uniform pressure distribution to avoid air bubbles or poor bonding.

[0036] During the substrate separation stage, while the UV adhesive is uncured, the base film of the electroplated aluminum foil is uniformly peeled from the aluminum sheet surface at a peel angle of 120±5° and a tension of 1.0±0.1 N / cm, with the peeling speed controlled at 8-12 m / min. In this process, the aluminum plating layer is precisely transferred and temporarily fixed to the uncured adhesive pattern surface, with a transfer rate exceeding 98%. This peel-then-curing arrangement effectively avoids the adverse effects of curing stress on interfacial bonding.

[0037] During the curing and shaping stage, the transferred aluminum sheet is fed into a UV curing device and irradiated with a metal halide lamp of specific power at an energy of 600±20mJ / cm². The irradiation distance is maintained at 15cm, and the conveying speed is set to 5m / min. After irradiation, the UV adhesive is fully cured, firmly adhering the aluminum plating layer to the surface of the aluminum sheet to form a durable decorative pattern.

[0038] Example 2

[0039] This embodiment uses 0.25mm thick tinplate as the substrate to demonstrate the complete decoration process of this application in the field of packaging containers, which is particularly suitable for high-end packaging products such as food cans and gift boxes.

[0040] In the surface pretreatment stage, an argon-nitrogen mixed gas is used as the plasma working medium, with argon accounting for 50% of the volume. The processing power is increased to 1500W, and the processing speed is adjusted to 10m / min to ensure that the tin-plated surface achieves sufficient activation. This parameter optimization is specifically designed for the surface characteristics of tinplate, effectively breaking down the surface oxide layer and forming a more uniform active surface.

[0041] In the primer treatment stage, the mass ratio of epoxy resin to acrylic resin was adjusted to 1:1.5 to enhance the affinity for the tinplate layer. Microwire coating technology was employed to control the dry film thickness at 4±0.2μm. The leveling process parameters were optimized to level at 70±2℃ for 25s. This condition ensures sufficient leveling of the coating while avoiding the negative impact of excessive heating on the tinplate layer.

[0042] In the printing process, a special UV adhesive is selected, with viscosity controlled at 800±30 cP and wet film thickness adjusted to 6±0.3 μm to meet the requirements of more detailed patterns and the characteristics of tinplate surfaces. The ambient temperature is strictly controlled at 23±1℃ to ensure the stability of the adhesive's leveling properties.

[0043] During the transfer lamination stage, a fluororubber pressure roller with a Shore A hardness of 65±2 degrees is used, and the pressure parameter is adjusted to 0.5±0.05MPa. Fluororubber material has better chemical resistance and dimensional stability, which can ensure uniform pressing even on curved materials, and is particularly suitable for complex shapes of packaging containers.

[0044] During the peeling process, the peeling angle is adjusted to 135±5°, and the tension is controlled at 0.8±0.1 N / cm to suit the rigidity and surface characteristics of the tinplate substrate. This parameter optimization effectively avoids potential pattern damage or incomplete transfer during the peeling process.

[0045] During the curing stage, a two-stage curing process is employed. First, pre-curing is carried out at an energy of 80±5 mJ / cm² using a specific light source to avoid overheating and affecting the tinplate. Then, final curing is completed at an energy of 450±20 mJ / cm² using a metal halide lamp to ensure sufficient curing depth. This tiered curing method ensures curing quality while avoiding thermal impact on the tinplate substrate.

[0046] After the hot stamping pattern has cured, the color overprinting process continues. A precision offset printing machine is used to accurately overprint the four colors of ink on the hot stamping pattern area, with the overprinting accuracy controlled within ±0.1mm. Finally, a protective treatment is applied by coating the entire decorative surface with a high-performance UV varnish, with a coating thickness of 8-10μm, and curing it at an energy of 400±20mJ / cm² to form a durable protective layer.

[0047] Comparative Example 1

[0048] This comparative example uses the exact same process parameters and materials as Example 1, the only difference being that the surface pretreatment step is omitted, and only the aluminum sheet is cleaned by routine alcohol wiping. Other process parameter settings are consistent with Example 1.

[0049] Experimental results showed that during the substrate separation stage, approximately 40% of the aluminum plating layer transfer was incomplete, resulting in noticeable pattern defects and blurred edges. Systematic testing of the final product revealed extremely poor adhesion, with a cross-cut adhesion test rating of level 4 and a peeling area exceeding 65%. This indicates that a lack of effective surface pretreatment severely impacts the transfer effect and final adhesion of the aluminum plating layer.

[0050] Comparative Example 2

[0051] This comparative example uses the same material parameters as Example 1, but the process flow sequence is adjusted, reverting to the traditional process of curing first and then peeling. That is, after the material is laminated, a curing treatment is performed first, followed by the peeling of the base film. Other process parameters are exactly the same as in Example 1.

[0052] Experimental results show that although the aluminum plating layer can be basically transferred, the edges of the hot stamping pattern exhibit uniform lifting, and microscopic observation reveals obvious stress cracks at the interface. The adhesion test rating is level 2, with a peeling area between 10-15%, and the pattern edges are prone to peeling after being scratched with a fingernail. This indicates that the traditional process sequence prevents the effective release of curing stress, thus affecting the interfacial bonding strength.

[0053] To quantitatively evaluate the technical advantages of the process in this application, systematic tests were conducted on samples prepared in Examples 1 and 2 and two comparative examples. The test environment was 23±2℃ and the relative humidity was 50±5%. The results are shown in Table 1:

[0054] Table 1. Performance test results of samples under different process conditions

[0055] Test item Test standard Example 1 Example 2 Comparative Example 1 Comparative Example 2 Adhesion grade GB / T 9286-1998 0 grade 0 grade 4 grade 2 grade Wear resistance (times) GB / T 1768-2006 >1000 >1200 150 450 Alcohol resistance (times) ASTM D4752 >500 >600 80 200 Gloss (GU) GB / T 9754-2007 95.2 96.8 85.6 90.3 Salt spray resistance time (h) GB / T 10125-2012 >240 >240 72 120 Pattern shrinkage rate (%) Micro measurement <0.5 <0.3 2.8 1.2

[0056] As can be seen from the test data in Table 1, the process scheme provided in this application is significantly superior to the comparative example in all performance indicators, demonstrating its technological advancement and practical value. The optimized design and parameter coordination of each process step jointly contribute to the excellent overall performance of the final product.

[0057] The surface pretreatment step involves atmospheric pressure plasma treatment to form a highly active surface layer on the metal surface, which is fundamental for achieving good adhesion. Plasma treatment not only cleans the surface but also increases surface roughness and surface energy through physical and chemical processes, providing more bonding sites for the base coating. Optimization of treatment power and speed ensures the uniformity and stability of the treatment results.

[0058] The primer treatment step, through the application of epoxy-acrylic composite resin, establishes an effective transition layer between the metal substrate and the adhesive layer. Thickness control and leveling processes of the primer layer ensure the uniformity and integrity of the coating, preventing localized defects. Optimized formulation of the composite resin allows it to form a strong bond with the metal surface while maintaining good compatibility with the upper adhesive layer.

[0059] Parameter control in the adhesive printing process is crucial to the quality of the pattern. Optimizing the adhesive viscosity ensures the stability of the printing process and the accuracy of the pattern, controlling the wet film thickness ensures the appropriate amount of adhesive, and maintaining a stable ambient temperature avoids fluctuations in the adhesive properties. These meticulous control measures provide a solid foundation for subsequent processes.

[0060] The material lamination process, through optimization of roller hardness and pressure, achieved full contact between the aluminum plating layer and the adhesive layer while avoiding damage to the intricate patterns. Control of the lamination speed ensured the stability of the lamination process, creating favorable conditions for subsequent peeling operations.

[0061] The innovative arrangement of the substrate separation step is key to this process. By peeling off the substrate before the adhesive has cured, the impact of curing stress on the interfacial bonding is effectively avoided. Optimization of the peeling angle and tension ensures the integrity and accuracy of the aluminum plating transfer, providing an ideal interfacial state for subsequent curing.

[0062] Energy control during the curing process ensures full cross-linking of the adhesive while avoiding brittleness caused by over-curing. The two-stage curing process used in Example 2 further improves the curing quality and is particularly suitable for heat-sensitive tinplate substrates.

[0063] Through comparative analysis of the above embodiments and comparative examples, it is fully demonstrated that this application solves the technical problems of insufficient adhesion, poor durability, and low pattern accuracy in traditional cold stamping processes for metal sheets by forming an active surface layer through atmospheric pressure plasma treatment and combining it with an innovative process of peeling followed by curing. This process not only improves product quality but also has advantages such as process stability, environmental protection, and energy saving, making it particularly suitable for surface decoration of high-end metal products.

[0064] The successful implementation of this process benefited from the meticulous design and parameter optimization of each step. Surface pretreatment laid the foundation for the entire process, the primer layer played a crucial bridging role, and the innovative process sequence effectively solved the internal stress problem. The synergistic effect of these technical measures resulted in a final product exhibiting significant advantages in adhesion, abrasion resistance, corrosion resistance, and pattern accuracy.

[0065] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the protection scope of this application. Various modifications and improvements made by those skilled in the art to specific embodiments based on the teachings of this application without departing from the essential spirit of this application should fall within the protection scope claimed by this application.

Claims

1. A cold foil stamping process for metal sheets, characterized in that, Implement the following steps: Atmospheric pressure plasma treatment is applied to the surface of metal sheets, with a treatment power of 800-1500W and a treatment speed of 10-30m / min. An epoxy-acrylic composite resin base coating is applied to the surface of a plasma-treated metal sheet, with the dry film thickness controlled at 2-5 μm, and infrared leveling is performed at 60-80℃. A patterned UV-curable adhesive is printed on the base coating by offset printing, with the wet film thickness of the adhesive controlled at 3-8 μm and the printing environment temperature maintained at 23±2℃. The aluminum-plated layer of the electroplated aluminum foil is bonded to the surface of a metal sheet printed with UV adhesive, and then pressed together under a pressure of 0.3-0.6 MPa using an elastic pressure roller with a Shore A hardness of 60-80 degrees. While the UV adhesive is still uncured, the base film of the electroplated aluminum foil is peeled off from the metal sheet. The metal sheet with the transferred aluminum-coated pattern is cured by UV light with a curing energy of 400-800 mJ / cm².

2. The process according to claim 1, characterized in that, The atmospheric pressure plasma treatment uses a mixture of nitrogen and argon as the working gas, with argon accounting for 40%-60% of the volume.

3. The process according to claim 1, characterized in that, In the epoxy-acrylic composite resin, the mass ratio of the epoxy resin component to the acrylic resin component is 1:0.5-2.

4. The process according to claim 1, characterized in that, The UV-curable adhesive contains 10%-25% by mass of microencapsulated photoinitiator.

5. The process according to claim 1, characterized in that, When peeling off the base film, the peeling angle is controlled to be 90°-135° and the peeling tension is 0.5-2.0 N / cm.

6. The process according to claim 1, characterized in that, The UV curing process is carried out in two steps: first, pre-curing is performed at an energy of 50-100 mJ / cm², and then the main curing is completed at an energy of 300-500 mJ / cm².

7. The process according to claim 1, characterized in that, After UV curing, the following steps are also included: Color inks are then overprinted onto the cured hot stamping pattern area using offset printing. Apply a protective varnish to the entire surface after overprinting.

8. The process according to claim 1, characterized in that, The metal sheet material includes aluminum, aluminum alloy, tinplate, or stainless steel.

9. The process according to claim 1, characterized in that, The base coating is applied using a doctor blade coating or a micro-wire bar coating method.

10. The process according to claim 1, characterized in that, The UV curing process uses a metal halide lamp or an LED-UV light source.

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