The defect rate dropped by about 60%, and there's actually a new solution for double printing on cigarette packs!
In the production of cigarette packs, not only exquisite craftsmanship is required, but also very strict quality requirements, of which overprinting accuracy is one of the core indicators to measure the printing quality of cigarette packs. At present, the domestic cigarette pack gravure production generally adopts the photoelectric scanning automatic registration system (hereinafter referred to as "traditional follow-up"), the principle of which is to scan the edge of the printing pattern or a specific cross mark line, and feedback the signal to adjust the plate roller phase.
However, there are still many pain points in the actual production of multi-brand, small-batch, and quick-change cigarette packs: the design of cigarette packs of different brands is very different, and the overprinting color sequence, roller circumference, and layout are different; Widely used substrates such as transfer silver cardboard and laser cardboard have complex surface characteristics, such as high reflection and uneven aluminum plating layer. These factors often make it difficult for traditional benchmarking systems to achieve stable tracking, causing periodic or random overprinting runs, and in severe cases, the overprinting error can reach more than ±0.15mm (as shown in the overprinting mark pointed to by the arrow in Figure 1), far exceeding the internal control standards of enterprises (usually ≤± 0.10mm), resulting in a large number of scrap products and downtime and debugging time. To solve this problem, a stable overprinting reference scheme that is not disturbed by the layout pattern and the surface of the substrate is urgently needed.
Analysis of key factors affecting the stability of overprinting
Inaccurate overprinting is a systemic problem, rooted in the dynamic balance of the printing process, and the following three factors are particularly critical.
01/ Transfer the impact of printing tension fluctuations on silver cardboard
Tension is the "lifeline" of gravure printing machines and has a decisive impact on overprinting.
(1) Influence of material properties: transfer silver cardboard has high stiffness and strong hygroscopicity, and is prone to slight expansion and contraction during the drying process. If the tension between unwinding, winding and unit is improperly set or fluctuating, it will directly lead to the change of the length of the paper path and cause inaccurate longitudinal overprinting.
(2) Tension matching requirements: The tension gradient between each printing unit must be accurately matched. If the tension is too small, the paper will be loose and easy to drift horizontally; Excessive tension, stretching of the paper, and retraction after drying will lead to inaccurate overprinting. For multi-color gravure printing, the response speed and stability of the tension system are crucial.
02/ Accuracy and condition of printing plate rollers
The printing plate roller is the carrier of graphic and text transfer, and its own quality directly affects the accuracy of overprinting.
(1) Radial runout and dynamic balance: If the radial runout of the new roll or the repaired plate roller exceeds the standard (usually required to ≤0.02mm) or the dynamic balance is not good, periodic vibration will occur at high speed of 250m/min, which will be directly converted into periodic overprinting error.
(2) Plate roll installation and thermal expansion: The installation is not centered, the keyway is worn or the thermal expansion of the plate roller is uneven due to friction heat generation in continuous printing, which will change the effective circumference of the plate roller and affect the longitudinal registration accuracy, and this effect will gradually accumulate with the running time.
03/ Printing quality and recognition impact of mark marks
The marker of the traditional method is printed directly on the substrate, and its quality is the "eye" of the registration system.
(1) Printing effect: On silver cardboard, ink adhesion and drying speed will affect the edge clarity of the marker. If the marking is phantom, fluffy or dirty, the photoelectric eye cannot capture stably, resulting in frequent correction or even malfunction of the system.
(2) Position interference: If the marker is located in an area with complex graphics or strong reflection, background interference will greatly reduce the signal-to-noise ratio and make the tracking unstable.
"Follow-up overprinting" innovative solution
The core idea of this scheme is to transfer the registration reference mark, that is, the "marker", from the substrate to a set of independent, high-precision mechanical discs, as shown in Figure 2, so as to completely avoid the interference of the surface state of the substrate material and the printing pattern.
01/ System Composition
(1) Special follow-up: the disc is made of aluminum alloy or composite materials with high rigidity and low thermal expansion coefficient, usually with a diameter of 150~200mm (customized by visual model), and a thickness of 15~20mm. The end face runout is required to ≤ 0.01mm, and the surface should be matted.
(2) High-precision markers: use high-contrast, sharp edges of special reflective or light-transmitting markers.
(3) Mounting base: Design a stable fixture to install the follow-up plate on the stationary parts of the printing unit header (near the driving side) to ensure that it is not directly mechanically linked with the plate roller system, but the position is fixed.
02/ Specific Steps
(1) Step 1: Installation and calibration
(1) Firmly install the follow-up plate on the rack near the plate head of the first color group (reference color group) through a customized fixture to ensure that the disc plane is parallel to the plate roller axis.
(2) At the precise position of the circumference of the follow-up disk, 0°, 90°, 180°, 270° marks are usually made, and high-precision markers are pasted. Use the dial indicator to calibrate the end face and radial runout of the follow-up disc to ensure that its comprehensive runout < 0.02mm.
(3) Adjust the photoelectric sensor of each color group or add a special sensor so that it can scan the mark on the corresponding tray, not the mark on the paper stably and clearly.
(2) Step 2: System parameter setting
(1) Tension parameters (taking a brand of 10-color gravure printing machine with a quantitative capacity of 125g/m² transfer silver cardboard as an example): the unwinding tension is set to 180~220N; The paper feed force is set to 200~240N; The tension increment between each color group is set to 8~12N (incremental). The winding tension is set to 260~300N, and it needs to be fine-tuned according to the specific paper and speed, and the goal is to pass the paper smoothly without vibration.
(2) Registration system parameters: the scanning mode is switched to the "follow-up" mode; The fault-tolerant window can be appropriately reduced to improve the sensitivity according to the accuracy of the follow-up mark. The response speed is medium to high to avoid excessive sensitivity and shock.
(3) Step 3: Closed-loop control operation
After the machine is turned on, the registration system continuously tracks the fixed mark on the tracking plate. Because the position of the follow-up is absolutely fixed, such as the phase change of any plate roller relative to the machine reference caused by vibration and thermal expansion, it will be detected by the system in real time, and the plate roller phase motor will be driven to compensate, forming a stable closed-loop control with the follow-up plate as the absolute reference.
Comparison of application effects and parameters
Since the implementation of this scheme on a major model, the effect has been very remarkable.
01/ Overprinting accuracy is significantly improved
The overprinting error of the traditional method fluctuates greatly, especially in the reel change and acceleration stage, the error can reach ±0.12mm to ±0.18mm; After follow-up overprinting, the overprinting accuracy is stable within ±0.08mm for a long time, and the best state can reach ±0.05mm, as shown in Figure 3, which is better than the industry advanced standard.
02/ Enhanced Stability and Adaptability
Completely resolves tracking loss issues caused by reflective silver cardboard, barcode patterns, and other layout interferences; highly adaptable to rollers of different brands and circumferences, with average initial registration adjustment time reduced by about 40% after plate changes.
03/ Production Efficiency and Cost Effectiveness
The defect rate caused by registration issues decreased by approximately 60%; consumption of auxiliary materials such as paper and ink due to registration adjustments was reduced; the effective operating rate of equipment increased significantly.
For the registration drift problem in tobacco package gravure printing caused by material and roller variability, the proposed "tracking disc registration" technical solution shifts the registration reference from the changing substrate to a fixed high-precision mechanical disc, isolating the main interference factors at the source. By finely controlling tension, ensuring roller quality, and using an independent tracking disc to provide stable signals, the registration system transforms from "passive following" to "active stabilization." Practical data demonstrate that this solution can effectively control registration accuracy to a high level of ±0.08mm, significantly improving production stability and economic benefits, and has great potential for widespread application in the high-end packaging printing industry. In the future, further exploration can integrate tracking discs with roller temperature monitoring and online tension feedback systems to achieve more intelligent pre-compensation control.