How to control the coating thickness of cigarette package UV varnish to balance the coating strength and the overall stiffness of the cigarette package?
Release Time : 2025-09-10
When controlling the coating thickness of UV varnish on cigarette packages, a fundamental adaptation logic must first be established based on the characteristics of the cigarette package substrate. This is because the stiffness of the substrate itself directly affects the balance between coating thickness and overall performance. Commonly used substrates for cigarette packages, such as white cardboard and aluminized paper, vary in their inherent stiffness. White cardboard has higher stiffness and is relatively tolerant of coating thickness; aluminized paper, due to the influence of the coating, has slightly lower stiffness. Excessively thick coatings can easily increase the substrate load, resulting in a decrease in the overall stiffness of the cigarette package. Therefore, a thickness range must be set based on substrate stiffness. When substrate stiffness is high, the upper limit of the thickness can be appropriately relaxed to improve coating strength while maintaining the same stiffness. When substrate stiffness is low, the lower limit of the thickness must be strictly controlled to avoid excessive coating weight that weakens the cigarette package stiffness. This establishes a "substrate-thickness" adaptation relationship from the very beginning.
The key basis for adjusting thickness is the difference in coating strength requirements, and the strength priority must be determined based on the actual cigarette package usage scenario. During transportation and stacking, the front patterned area of cigarette packages is susceptible to friction and scratching, requiring a sufficient coating thickness to form a protective layer against external damage. However, excessively thick coating in the creased areas of the package (such as the folds on the lid and bottom) can cause cracking during folding, reducing the protective effect and affecting the stiffness after folding. Therefore, a "regionally differentiated thickness" design is necessary: the UV varnish on the front patterned area can be appropriately thickened to enhance abrasion and scratch resistance, while the thickness can be reduced in the creased areas to ensure smooth folding. This approach meets the strength requirements of different areas without compromising the overall stiffness of the cigarette package.
The formulation characteristics of the UV varnish on the cigarette package have a critical impact on thickness control, and formulation optimization is necessary to minimize the negative impact of thickness on stiffness. High-solids UV varnish for cigarette package coatings can form a dense coating at a relatively thin thickness, achieving the desired strength. This eliminates the need for thicker coatings to improve performance and effectively avoids the loss of stiffness associated with overly thick coatings. Low-viscosity UV varnish for cigarette package coatings can easily sag if not properly controlled, leading to uneven thickness and, in turn, localized stiffness variations in the cigarette pack (e.g., excessively thick areas of sag causing localized softening). Therefore, when selecting a varnish, prioritize products with high solids content and stable viscosity. These formulations minimize thickness requirements, achieving high strength in thin coatings while ensuring uniformity and maintaining stable stiffness.
Precise control of coating process parameters is key to achieving consistent thickness. Process fluctuations that can lead to thickness variations and affect stiffness and strength must be avoided. When using a blade coating process, blade pressure, angle, and coating speed directly influence thickness. Excessive pressure and a steep angle result in a thin coating with insufficient strength; while insufficient pressure and a gentle angle result in a thick coating with reduced stiffness. Process parameters must be adjusted based on the varnish viscosity and substrate characteristics: When the viscosity is high, increase the blade pressure appropriately to avoid overly thick coatings; when the viscosity is low, reduce the pressure to prevent overly thin coatings. Coordinate the coating speed with the curing speed to ensure a uniform coating thickness before curing, avoiding uneven thickness and unbalanced stiffness caused by process fluctuations.
Thickness should be factored into the requirements of subsequent cigarette packaging processing to prevent inappropriate thickness from affecting post-process stiffness. Cigarette packaging undergoes die-cutting, hot stamping, and forming. Excessively thick UV varnish on cigarette packages can easily burr during die-cutting, resulting in irregular edges and impacting stiffness after forming. During the hot stamping process, an excessively thick coating increases the distance between the hot stamping layer and the substrate, reducing adhesion. Furthermore, uneven thickness can cause misalignment, indirectly compromising the stiffness of the packaging. Therefore, a "machining allowance" must be reserved: the coating thickness before die-cutting must be controlled within the range that the die-cutter can smoothly cut, avoiding excessive thickness that may result in incomplete die-cutting. The coating thickness in the hot stamping area must be compatible with the hot stamping foil to ensure that the coating and hot stamping layer jointly maintain the stiffness of the cigarette pack after hot stamping, without causing additional deformation.
The overall balance of stiffness requires consideration of the "coating-substrate" synergy, rather than relying solely on thickness adjustment. If the coating thickness approaches the upper limit and still fails to meet strength requirements, reinforcing agents (such as nanoparticles) can be added to the varnish to increase coating density and enhance strength without increasing thickness. If the coating thickness reaches the lower limit but cigarette pack stiffness is still insufficient, substrate pretreatment (such as light embossing) can enhance the substrate's inherent stiffness, reducing the coating's reliance on stiffness. This synergistic strategy of "coating optimization + substrate reinforcement" can overcome the "thickness-stiffness-strength" conflict, achieving a balance between the three without adjusting thickness.
Finally, actual testing is required to verify the appropriateness of the thickness, establishing a closed "test-and-adjust" loop. After coating, the coating strength (such as abrasion resistance test and scratch test) and cigarette pack stiffness (such as bending stiffness test) need to be tested simultaneously: if the strength is insufficient but the stiffness is sufficient, the thickness can be appropriately increased within the allowable range of stiffness; if the stiffness is insufficient but the strength meets the standard, the thickness needs to be reduced or the formula needs to be optimized; if both do not meet the standards, the combination of "substrate-formula-process" needs to be readjusted until the optimal thickness is found that meets the strength requirements while maintaining the stiffness of the cigarette pack, ensuring that the final product has both protective performance and a crisp appearance.