Grasp these 6 key factors, and hot stamping will be no problem.
With the continuous development and application of foil stamping technology in the printing and packaging industry, foil stamping plays a finishing touch role on printed packaging products, enhancing visual accents. Currently, there are three main types of foil stamping processes: the traditional hot stamping process; a more mature cold stamping process; and in recent years, the gradually developing screen printing or digital inkjet heat transfer foil stamping process.
Overall, foil stamping technology is widely applied in the printing and packaging field, playing a crucial role in surface decoration and visual communication of printed products. Foil stamping is extensively used in cigarette pack printing, but some quality and technical issues may arise during the process, such as foil graininess, air bubbles, edge roughness, plate smearing, or incomplete patterns. To effectively address these quality issues in foil stamping, control and avoid technical risk factors that affect foil stamping quality, and stabilize product quality, this article studies the mechanism of how the performance of the printed product's surface ink layer affects the suitability for aluminum foil stamping, and shares the findings.
Case Study of Foil Stamping Quality Issues
During the foil stamping process of red checkered aluminum foil and gold aluminum foil on Yuxi (soft) cigarette pack cartons, severe graininess was encountered on the stamped areas, affecting the appearance quality of the stamping. The possible reasons were analyzed: firstly, the halftone layer of the diamond grid on the gravure plate was too heavy (100%), and the red frame ink layer was thick, affecting the release and adhesion of the red checkered aluminum foil. Secondly, there were compatibility issues between the release layer and the adhesive layer of the red checkered aluminum foil and the printed surface ink and varnish layers.
Subsequently, the halftone layer of the gravure plate's diamond grid was reduced from 100% to 70%, as shown in Figure 1, and after adjusting the heat resistance of the red ink, machine testing for foil stamping showed a significant improvement in the graininess of the red aluminum foil, meeting product appearance quality requirements. In response to the above foil stamping issues, the company established a project research team to explore the mechanism of how the surface ink layer performance affects the suitability of aluminum foil stamping.
Figure 1: The halftone layer of the intaglio diamond-shaped plate was reduced from 100% to 70%Factors Affecting Hot Stamping QualityThrough comprehensive analysis, the main factors affecting the quality of hot stamping are as follows:01 Hot Stamping TemperatureThis should be determined based on the stamping suitability of aluminized foil, generally within the temperature range of 85°C to 130°C.02 Hot Stamping PressureThe appropriate stamping pressure should be determined based on factors such as the thickness of the paper of the stamped product and the transfer performance of the aluminized foil. Too high or too low pressure is not suitable.03 Hot Stamping SpeedThe speed should be chosen according to the transfer performance of the aluminized foil. While ensuring stable stamping quality, the stamping speed can be higher.For flat-to-flat hot stamping machines, the maximum stamping speed is generally controlled at around 5000 sheets per hour, while for rotary hot stamping machines, the optimal speed is around 80–110 meters per minute.04 Effect of Aluminized Foil Stamping Suitabilitya. Effect of the release layer on stamping quality:During the hot stamping process, under certain temperature conditions (e.g., 120°C), the release layer of the aluminized foil vaporizes, causing other layers of the foil to separate from the base film layer. If the release layer is too tight, the vaporization reaction is incomplete, which may cause quality defects such as hot stamping sandiness. Conversely, if the release layer is too loose, the vaporization reaction is more complete, and the base film layer separates fully from other layers, which is essential for good foil transfer. However, it may cause defects such as smudging and rough edges, so the release layer should have a moderate tightness.b. Effect of the adhesive layer on hot stamping quality:During the stamping process, under certain temperature conditions (e.g., 120°C), the adhesive layer of the foil softens or liquefies, allowing the color and aluminum layers of the foil to adhere to the substrate. If the adhesive layer softening point is too high, the layer does not easily liquefy or only liquefies partially, resulting in weak adhesion to the substrate and quality defects such as sandiness or missing strokes. Conversely, if the adhesive layer softening point is too low, it easily liquefies, and the foil can stick to the substrate more readily, which may lead to defects such as smudging and rough edges. Generally, the adhesive layer should be used in conjunction with the release layer to match properly.
05Factors Affecting the Hot Stamping Suitability of Electrolytic Aluminuma. Dynamic and Static Friction Coefficients: This index indirectly reflects the smoothness of the printed surface. Generally, surfaces that are too smooth are not conducive to hot stamping and adhesion.b. Surface Tension: The surface tension of paper is usually measured using a dyne pen. For gravure printing, the surface tension of ordinary paper generally meets production requirements. When performing UV printing on non-absorbent substrates, it is recommended that the surface tension reaches above 38 dyn/cm. For hot stamping, the surface tension generally needs to be above 36 dyn/cm.c. Ink Layer Thickness: Generally, the thinner the ink layer in the hot stamping area, the easier it is for hot stamping and adhesion.d. Substrate Surface Properties: Generally, non-absorbent papers (such as metallized paper) are more difficult to hot stamp than absorbent papers (such as coated paper or white card paper).e. Thermal Conductivity of the Paper: Hot-stamped products with high thermal conductivity are harder to stamp than those with low thermal conductivity.f. Ink Dot Percentage in the Hot Stamping Area: Hot stamping areas with solid colors are easier to stamp than screened areas (e.g., 50% halftone) which are more difficult.06Hot Stamping Base Platea. Material: Common hot stamping base plates include hard plastic sheets, soft rubber sheets, metal sheets, and paper-based materials.b. Material Hardness: Higher hardness of the base plate is more favorable for stamping fine lines.Impact of Printed Surface Ink Layer Properties on the Suitability of Electrolytic Aluminum Hot StampingTesting and Verification of the Influence Mechanism01Technical Route(1) Analyze the factors affecting the quality of traditional hot stamping, explore the relationship between printed surface ink layer properties and the suitability of electrolytic aluminum hot stamping, and identify relevant control technologies, parameters, and methods.(2) Conduct machine tests and verification, standardize the parameters for using hot stamping plates and gravure printing plates, and determine printing control parameters under the condition of "stamping-suitable" surfaces to stabilize hot stamp product quality.(3) Apply the relevant control technologies and methods to cigarette labels to achieve stable mass production of hot stamping and promote their application in the industry.02Implementation Process(1) The "stamping-suitable" parameters for hot stamping plates were determined: copper plate thickness 7 ± 0.10 mm; the finest fine line not less than 0.1 mm, minimum engraving depth not less than 0.25 mm, and minimum hardness of hot stamping plate not less than 300 HV.(2) The "stamping-suitable" parameters for gravure plates were determined: screen line count, screen angle, flow channel, well depth, chromium layer hardness, and smoothness. See Table 1 below.Table 1: "Stamping-Suitable" Gravure Plate Parameters
(3) Through testing and trials, the surface characteristic parameters of heat transfer products that are 'suitable for hot stamping' after printing include: smoothness, static friction coefficient, dynamic friction coefficient, surface tension, ink layer thickness, thermal conductivity efficiency, and the dot area rate of the color ink plate at the stamping position. See Table 2 below.Table 2 Surface characteristic parameters of heat transfer products (after printing) that are 'suitable for hot stamping'
(4) Surface characteristic parameters of the base paper for hot stamping products that are 'suitable for stamping': smoothness, static friction coefficient, dynamic friction coefficient, surface tension, thermal conductivity efficiency. See Table 3 below. Table 3 Surface characteristic parameters of the base paper for hot stamping products that are 'suitable for stamping'
(5) Determine the optimal printing control parameters under the surface property characteristics of 'suitable hot stamping' products: plate making parameters, printing speed, drying temperature, ink brand, ink viscosity on the press, varnish brand, varnish viscosity on the press, etc. See Table 4 below. Table 4 Optimal Printing Control Parameters under the Surface Property Characteristics of 'Suitable Hot Stamping' Products
By measuring and analyzing the surface characteristic parameters of paper, and simultaneously selecting suitable foil stamping process control parameters for machine testing, multiple adjustments were made to identify the optimal printing control parameters under the 'suitable-for-stamping' surface characteristic parameters of the product. These parameters include plate-making parameters, printing speed, drying temperature, ink brand, ink viscosity on the press, varnish brand, varnish viscosity on the press, etc., to meet the quality requirements of the foil-stamped products and ensure that there are no sand defects, air bubbles, or adhesive patterns in the stamped areas.In short, after machine testing and verification, experiments and tests were conducted on the interconnected factors including the designed foil stamping plates, gravure plates parameters, 'suitable-for-stamping' surface characteristic parameters of the product, and printing control parameters under 'suitable-for-stamping' surface conditions. This effectively solved quality issues in the foil stamping process such as sand defects, air bubbles, edge burrs, blocking, and broken strokes, improving the pass rate of products in the stamping process and bringing significant economic benefits to the company. It plays an important role in controlling and stabilizing the quality of foil-stamped products. This has enabled the mature application of foil stamping techniques in products such as Yuxi small cigarette boxes, Hongtashan small cigarette boxes, and Ashima small cigarette boxes, demonstrating industry promotion and application value.