5 steps to improve the quality of roll-to-roll hot stamping

In May 2010, our company introduced the Evergrande MK3920SW three-station roll-to-roll hot stamping machine to start the production of hot-stamped cigarette packs. Over more than four years, we continuously worked on optimizing the process design, equipment performance, material quality, and online inspection systems. These efforts eventually led to significant improvements. Based on the experience gained, we introduced the Evergreen MK820SW double-station roll-to-roll hot stamping machine in October 2014, which is now fully operational. In this article, I would like to share some of the challenges and solutions we encountered while working on improving the quality of our cigarette packaging.

Process Design Optimization

The process for producing the "Hongtashan (Classic 100)" hard-packed small box at our company includes gravure printing, followed by holographic and red anodized aluminum hot stamping using the MK3920SW three-station roll-to-roll machine, then die cutting, indentation, and embossing with the XTMQ8203 round die cutting machine. This process theoretically saves 2.5% of paper. However, due to the complexity of the roll-to-roll production, careful planning of the workflow is essential, along with considering the specific requirements of each piece of equipment. Tension control during gravure printing is particularly critical. During the proofing stage, it's important to set the tension values for each device and avoid large adjustments during normal production, as this can lead to misregistration issues. Also, in pre-press typesetting, the registration marks and inspection lines from different machines must be carefully aligned to ensure accuracy. It’s best to use shared overprint marks and similar colors to maintain consistency throughout the process.

Equipment Optimization

Our company and Evergreen collaborated to optimize several aspects of the MK3920SW three-station roll-to-roll hot stamping machine to better suit the production of our cigarette packaging. Here are some key improvements:

1. Surface Treatment of Paper Guide Roller

When producing the "Hongtashan (Century)" hard-packed small box using transfer gold cardboard, we noticed irregular hot stamping patterns. Adjustments to the equipment tension, registration system, and gravure settings did not fully resolve the issue. After analysis, we found that the smooth surface of the 225g/m² transfer gold paper caused slippage during feeding, leading to misalignment. To address this, we treated the surface of the paper guide roller with a high-friction material, reducing the chance of slipping and solving the problem. This solution also helped with the "Hongtashan (Classic 100)" product using laser transfer paper with a similar surface finish.

2. Optimization of Expansion Shaft Installation Position

During the production of the "Hongtashan (Classic)" soft case, excessive wrinkling occurred after winding, increasing scrap rates. The thin 97g/m² single-sided coated paper, combined with long rolls of up to 8,400 meters, caused instability during winding. We optimized the installation position of the expansion shaft, added a new rotating support frame, and eliminated mechanical clearance, effectively solving the wrinkling issue.

3. Addition of Adhesive Roller

We experienced a small amount of gold powder residue on the surface of the "Hongtashan (Classic)" soft box after hot stamping. Despite adjustments with the electrochemical aluminum supplier, the issue wasn't fully resolved. We added an adhesive roller to the machine, which adsorbs excess gold powder without affecting product quality, significantly improving the final result.

4. Installation of Platen Roller on Registration Scan Head

During paper feeding, the paper would bounce, causing unstable scanning of the overprint marks and resulting in misalignment. By installing a platen roller on the registration scan head, we ensured stable paper feeding before hot stamping, improving scanning accuracy and overall hot stamping quality.

5. Installation of Movable Baffle Under Electrochemical Aluminum

Dust from the end face of the anodized aluminum occasionally fell onto the product, causing contamination. Although adjustments were made to the cutting process, the issue persisted. We installed a movable baffle under the electrochemical aluminum installation position, preventing foreign particles from damaging the equipment or the hot stamping plate, and effectively resolving the problem.

Optimized Adjustment of Hot Stamping Performance of Raw and Auxiliary Materials

The multi-station roll-to-roll hot stamping method allows for multiple hot stampings in one pass, but this leads to longer paper feeding lengths and higher downtime costs. For example, each stop results in 30 meters of waste, equivalent to about 800 small boxes. To minimize this, we adjusted the length of the electrochemical aluminum based on the paper size and the jump size of the material, enabling efficient production and reducing unnecessary downtime, thus improving both efficiency and product quality.

Optimal Control of Online Quality Inspection

To ensure consistent quality, we equipped the bronzing equipment with a BST static camera system and jointly developed an online quality inspection system with Fourier. Operators use the BST camera to check each hot stamping pattern individually, adjusting parameters when necessary. Once the machine is running smoothly, the inspection system monitors the product in real-time, detecting defects such as misregistration, misapplication, sleeve stamping errors, and color deviations. It alerts operators promptly, helping reduce waste and improve overall product quality.

The Importance of Preparing for Startup

Before starting the machine, the following steps are crucial:

1. Conduct a full analysis of the roll product's quality status and adjust the hot stamping temperature and pressure according to sample conditions.
2. Check the registration status of the hot stamping pattern and determine if any adjustments to the hot stamping plate are needed.
3. When installing or replacing electrochemical aluminum, verify its length, joint positions, and tightness. Any material not meeting quality standards should not be used.
4. Clean the hot stamping plate when the machine is stopped.

Additionally, the operator should inspect the electrochemical aluminum conveying device, hot stamping unit, and rewinding/unwinding devices to ensure everything is functioning properly. This helps maintain stable product quality. Currently, our company's average pass rate for roll-to-roll hot stamping is 99.60%, which is 0.25% higher than that of sheet-fed hot stamping.

The above content is reproduced in the "Printing Technology" official WeChat (Printech1957), authorized by the company to publish, copyright, piracy will be investigated!

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