First Iron, Then Print The 'Secret Recipe'. Come And See!

With the significant improvement in people's living standards and the enhancement of aesthetic concepts, packaging printing technology is gradually developing toward refinement and high quality. Papers with special optical effects, such as laser paper, holographic position paper, and transfer laser film paper, have surged in popularity, greatly enriching packaging printing techniques. However, with increasingly strict national environmental protection requirements and heightened public environmental awareness, hot stamping has become the preferred printing process to highlight the main elements and patterns of packaging design, replacing laser paper, holographic position paper, and other heavy metal aluminum-containing papers.Hot stamping can be divided into cold stamping and hot stamping. Hot stamping has advantages such as good stability, efficient use of electroplated aluminum, strong adhesion of printed patterns to the stamping, easy control of pattern transfer levels, and high product yield. Therefore, it is a widely used stamping process. This article explores the 'stamp first, then print' process.The 'stamp first, then print' process involves traditional gold stamping followed by offset printing on the stamped patterns. The electroplated aluminum used in stamping includes plain electroplated aluminum, laser electroplated aluminum, and laser pillar electroplated aluminum. Among them, laser electroplated aluminum is most frequently used in design due to its high brightness, good optical effect, and dazzling appearance. Offset printing, with its advantages of realistic color reproduction, rich layering, delicate printed patterns, and clear dot recovery, is the most commonly used printing method.Precautions During Hot StampingWhen performing hot stamping, it is necessary not only to ensure the stamping effect but also to consider whether it meets printing suitability requirements. Therefore, attention should be paid to the following aspects during stamping.01Adhesion of Stamping FoilHot stamping requires that the stamped aluminum foil layer adheres firmly to the paper surface. Otherwise, during the offset printing process, the foil layer can easily be pulled up by the ink and stick to the blanket, causing problems such as foil peeling, dot loss, and incomplete patterns or text. In severe cases, the foil pulled off by the ink may transfer to the ink rollers, causing serious damage. To ensure strong adhesion of stamping foil, attention should be paid to the following factors.

02Surface Tension of Hot Stamping Aluminum FoilAfter hot stamping, the surface tension (Dain value) of the aluminum foil should be greater than 38. Typically, the Dain value of hot stamping aluminum foil can reach 40 and can be tested using a Dain pen. If the Dain value is below 38, the adhesion of ink on the aluminum foil after printing will be poor and can be easily rubbed off, indicating that the electrolytic aluminum is of substandard quality and needs to be replaced.03Brightness of Hot Stamping Aluminum FoilThe brightness of electrolytic aluminum foil varies as well.(1) Choose the appropriate hot-stamping foil according to the characteristics of different paper types. Commonly used papers in packaging printing include white cardboard, glass card paper, coated paper, glass-coated paper, laser transfer paper, and laser laminating paper. These papers differ significantly in terms of surface coating materials, absorbency, and surface tension. Hot-stamping foil is mainly adhered to the paper via an adhesive layer. Since thermoplastic resins vary in type and properties, the foil must be matched with the adhesive layer suitable for each type of paper.(2) During the hot-stamping process, it is crucial to control temperature, pressure, and speed, particularly ensuring that the pressure is applied evenly and not excessively. Excessive pressure during stamping can cause the paper surface to indent, leading to incomplete prints. Additionally, operators must follow standardized procedures, and the adhesion of the hot-stamping foil should be periodically checked. A common method for checking adhesion is the tape pull test, which is performed as follows: first, smoothly apply adhesive tape to the stamped design without bubbles or wrinkles. Then, quickly pull off the tape and check whether the foil is lifted. If the foil remains intact, it indicates firm adhesion, ensuring that ink will not pull off the foil during printing. Conversely, if the foil is lifted, it indicates weak adhesion, and the pressure needs to be readjusted.(3) Large-area hot stamping often faces adhesion issues, primarily due to insufficient air release between the stamping plate, foil, and paper during pressing. To address this, the stamping plate can be engraved so that the center is slightly higher than the edges, with a typical height difference of about 0.01mm for large designs. Additionally, rotary drum stamping (cylinder-to-cylinder stamping) can better solve large-area adhesion problems because the line-contact pressure effectively addresses the insufficient air release that occurs in flat-to-flat stamping, which can cause weak foil adhesion.

It will affect the color after printing. Electrolytic aluminum foil has a relatively high brightness, resulting in more vivid colors after printing and better dazzling visual effects; conversely, foil after stamping appears darker, reducing the color vividness after printing and yielding poorer visual effects. In addition, it is necessary to ensure that the stamping temperature is not too high, as excessive heat can lower the brightness of the stamped foil and affect the color of the pattern. Precautions for Offset PrintingDue to significant differences in ink absorption, adhesion, and drying properties of the foil surface compared to paper, the following points must be observed when performing offset printing after stamping:(1) More ink is generally used in offset printing after stamping. Typically, the ink coverage on glass card or coated paper is around 150%–200%, which is 1.3–1.5 times that of holographic paper. When printing large areas with high ink saturation, an appropriate amount of ink conditioner should be added to improve ink flow; otherwise, excessive ink on the roller can cause a "ssss" sound as the ink threads break during printing, and the printed colors will appear lighter with poor saturation. The proportion of ink conditioner added is usually 2%–6%.The amount of ink conditioner added should also be determined according to the ambient temperature. In higher temperatures, ink flows more easily, so a lower proportion may be used; conversely, at lower temperatures, a higher proportion is necessary. When the ambient temperature is below 15°C, ink flow decreases significantly, reducing the effectiveness of ink transfer and distribution. When the temperature is very low, the proportion of ink conditioner can be increased to around 8%; however, excessive addition can cause issues with plate smudging, which can be addressed by using inks with higher water resistance. In low-temperature printing, increasing the temperature of the transfer rollers to around 33°C can also improve ink transfer.(2) Additionally, when printing large areas with high saturation, attention should be paid to the ink leveling of the printed pattern. When using a high ink volume, the pattern may develop a "pitted" appearance, severely affecting print quality. This can be resolved by lowering the ink viscosity, using soft rubber blankets, increasing the number of impression cylinders, or using inks with higher color density to reduce ink usage and improve transfer, resulting in smooth, pit-free printed colors.

(3) The most common issue during the offset printing process is the pull-back of the stamping foil. When the adhesive strength of the hot stamping aluminum foil is insufficient, pull-back problems inevitably occur during offset printing. If the adhesive strength of the hot stamping aluminum foil is slightly insufficient, measures such as adding anti-stick agents to the ink or reducing the printing speed can be used to minimize pull-back issues. When the adhesive strength of the hot stamping aluminum foil is too low to carry out offset printing, the foil can be hot-pressed before printing. During hot pressing, it is important to ensure that the temperature is not too high, typically between 50°C and 60°C. Additionally, the stamping pressure should be managed during hot pressing, but there is no need to use vacuum metallization again.

(4) When performing offset printing after stamping, attention must also be paid to the ink's adhesion. Typically, UV ink dries in two stages: 'surface dry' and 'through dry.' The freshly printed ink cures under UV light, which constitutes the 'surface dry' stage. However, the 'through dry' at the interface between the ink layer and the substrate is not fully cured and requires at least 4 more hours for complete curing. Only then does the ink's adhesion fully manifest. Moreover, to ensure that the offset ink firmly adheres to the stamped aluminum foil surface, a UV varnish can be applied after offset printing to coat the printed surface.

The above are some insights from the author's experience with the stamp-then-print process. Naturally, to achieve high-quality stamp-then-print results, careful selection of paper, vacuum metallized aluminum foil, and ink, along with standardized stamping and offset printing procedures, meticulous operation by staff, and close coordination, are all necessary to ensure stable and controlled quality.