Application of Numerical Control Technology of Reshaping with Profiled Easy-opening Mark

Introduction

Easy-opening lids are essential components in the food canning industry, designed to allow consumers to open cans with ease. Despite the complexity of the production process, it typically involves several key steps: sheet metal processing, basic lid forming, pull ring assembly, post-processing, visual inspection, and final packaging. There are two main types of easy-opening lids: circular and non-circular. Non-circular lids, such as oval, square, or horseshoe-shaped, are specifically referred to as "easy-open" lids due to their unique opening mechanism.

The function of an easy-opening lid is not only to seal the can but also to ensure a smooth and safe opening experience. To achieve this, the lid surface is pre-scored with a specific depth of grooves, and a pull ring is riveted onto the lid. When pulled, the lid separates along the score line, providing a clean and controlled opening. For food safety, these lids must remain free from rust or corrosion throughout the storage and consumption period. Therefore, tinplate materials used for easy-opening lids are coated with a layer of corrosion-resistant paint before the scoring process. However, the scoring operation inevitably damages the coating, which is why the coating is applied after the score lines are created.

To maximize the opening size, the score line usually follows the shape of the lid body, creating a similar path inward. This ensures that the lid can be fully removed without tearing or breaking. The traditional methods of applying coatings to score lines have limitations, especially when dealing with irregularly shaped lids. These challenges have led to the development of more advanced technologies, such as numerical control (NC) systems, to improve efficiency and precision in the coating process.

1. Traditional Coating Technology

Traditional coatings for score lines include phenolic resin, epoxy-phenolic resin, acrylic resin, and aluminum pastes. These materials offer excellent corrosion resistance, adhesion, flexibility, and are non-toxic and tasteless, meeting strict food safety standards. They are also suitable for storage and transportation, requiring high corrosion resistance and easy cleaning. However, due to the directional nature of the coating spray, which often forms a fan-like pattern rather than a cylindrical one, achieving uniform coverage on curved surfaces can be challenging. In circular lids, the coating tends to form a more even ring due to the rotation of the cap during application.

For irregularly shaped lids, the coating process becomes more complex. Conventional methods like manual brushing or purely mechanical approaches are often used. Manual brushing, while simple, lacks consistency and efficiency, making it suitable only for small-scale production. Purely mechanical methods involve multiple steps, including capping, positioning, and spraying, requiring precise synchronization of various mechanical components. While effective for large-scale production, these systems are complex, costly, and difficult to adapt to different lid shapes.

2. Application of Numerical Control Techniques for Irregularly Shaped Easy-Open Lids

Combining mechanical systems with numerical control technology offers a more efficient and flexible solution. The mechanical part handles tasks like capping and positioning, while the NC system controls the precise movement of the spray gun. This allows for accurate trajectory tracking, ensuring consistent coating along the score lines. For example, in the case of an oval-shaped lid (T501), the X, Y, and Z axes are controlled by servo motors, enabling three-dimensional motion for precise coating application.

2.1 Project Design

In the T501 example, the X-axis moves horizontally, the Y-axis vertically, and the Z-axis controls the spray gun's vertical position. The NC system coordinates these movements to follow the exact path of the score line. After completing the coating, the system returns to the origin, ready for the next lid. This approach significantly improves accuracy, reduces waste, and enhances productivity.

2.2 Selection of CNC System

A Delta PUTNC-H6C CNC system was chosen for its high-speed response, open architecture, and support for G-code programming. It also includes features like PLC logic control and parameter settings, making it versatile for industrial automation. The AC servo drive (ASD-AB) ensures smooth acceleration and deceleration, improving overall performance. With a resolution of 100,000 pulses per revolution, the system achieves high precision, even for fast-moving, short-stroke applications.

2.3 Advantages of Numerical Control Systems

Compared to traditional methods, the NC system offers faster production cycles, easier setup, higher accuracy, and lower maintenance costs. It also allows for quick changes between different lid types, reducing downtime and increasing efficiency. Table 1 summarizes the key differences between traditional and NC-based processes, highlighting the significant improvements in speed, accuracy, and cost-effectiveness.

3. Conclusion

The integration of numerical control systems into the coating process has transformed the manufacturing of easy-opening lids. By replacing traditional mechanical cam systems with high-precision servos, the process becomes more efficient, reliable, and adaptable. This advancement not only improves product quality but also supports the growing demand for flexible and automated production solutions in the food packaging industry.

Excerpt from: Packaging and Food Machinery, 2011-04