The die-cutting of hot melt adhesive label materials is one of the common challenges faced by label printing factories. In comparison to commonly used water-based adhesive materials, when labels with the same outer dimensions, waste width, and liner thickness are processed simultaneously on the same die-cutting equipment, water-based adhesive materials run smoothly, while hot melt adhesive label materials often encounter issues. This leads to low efficiency, significant waste, and the eventual necessity to increase waste width or switch to water-based adhesive materials. Some have attempted to improve the die-cutting process of hot melt adhesive materials by altering the adhesive formula. While this improved die-cutting performance, it lowered the initial viscosity of the adhesive, overshadowing the advantages of hot melt adhesive.
This article primarily analyzes the die-cutting characteristics and temperature adaptability of hot melt adhesive label materials. It provides insights into the issues encountered during die-cutting hot melt adhesive label materials in customer interactions and production practices, along with their corresponding solutions.
Die-cutting characteristics of hot-melt self-adhesive materials
The die-cutting principle of hot melt adhesive label materials is similar to that of water-based adhesive materials, as shown in Figure 1. For the die-cutting of hot melt adhesive label materials, it is crucial that both the facestock and adhesive are completely severed when the facestock is fully cut. This is because the adhesive in hot melt adhesive is a rubber-based adhesive with strong cohesive forces (similar to chewing gum, difficult to tear). Failure to cut it completely during die-cutting may result in waste edge fractures or the waste edge peeling off along with the label, leading to die-cutting quality issues. In contrast, the die-cutting of water-based adhesive materials is different; as long as the facestock is cut, the adhesive, even if not completely cut, will not hinder the normal waste removal at the paper edge.
Temperature Adaptability of Hot Melt Adhesive Label Materials:
The resistance generated when separating the facestock from the liner in hot melt adhesive label materials is referred to as peel force or release force. Release force is a crucial parameter affecting the performance of hot melt adhesive label materials. Excessive release force increases waste removal resistance, causing waste edge fractures. On the other hand, insufficient release force reduces the bonding strength between the label and the liner, resulting in issues during die-cutting waste or automatic labeling, such as label detachment or flying labels. Therefore, it is essential to control the release force of adhesive materials within a reasonable range. The viscosity of hot melt adhesive varies with temperature changes, known as temperature adaptability. The change in viscosity is a critical factor influencing the variation in release force of label materials. Another crucial factor affecting release force is the surface energy of the liner. Adjusting the surface energy of the liner, i.e., altering the silicone oil formula, can regulate the release force of the entire adhesive material, facilitating stable die-cutting and application characteristics. The relationship between release force, adhesive viscosity, and liner surface energy for hot melt adhesive label materials is illustrated in Figure 2. The yellow line represents release force, which ideally should remain constant as a straight line under temperature changes. The red line represents the initial viscosity change of the adhesive, increasing with temperature. The blue line represents the surface energy of the liner, decreasing with temperature. It can be observed that only when the surface energy of the liner changes with the viscosity of the adhesive can the final stability of the release force of label materials be ensured. This, in turn, stabilizes the die-cutting and application characteristics of label materials. Therefore, hot melt adhesive label material suppliers must adjust the release agent coating formula on the liner to meet the temperature variations in different seasons.
Due to the different physical and chemical properties of hot melt adhesive label materials compared to water-based adhesive materials, the die-cutting of hot melt adhesive label materials imposes strict requirements on the uniformity of liner paper, stability of surface energy, precision of die-cutting equipment, angle and sharpness of die-cutting blades, design of label waste edge size, selection of facestock strength, die-cutting speed, and environmental temperature. Compliance with these requirements, along with proper equipment adjustments, is essential to ensure effective die-cutting quality and production efficiency of hot melt adhesive label materials.
Common Die-Cutting Issues and Solutions for Hot Melt Adhesive Label Materials:
During the die-cutting process of hot melt adhesive label materials, common issues include waste edge fractures after die-cutting, label detachment during waste removal, and labels peeling off together with waste. Understanding the causes of these problems and identifying corresponding solutions is crucial for die-cutting operators.
Waste Edge Fractures During Waste Removal:
- Uneven edge of the paper roll, appearing jagged with small cracks, leading to reduced strength and easy tearing of the paper edge.
- Solution: Replace the label material, improve cutting quality, ensure a neat cross-section, and reduce die-cutting speed.
- Snake-like movement of label material during die-cutting, causing changes in waste edge size. Narrowing of the waste edge can lead to easy tearing of the paper edge.
- Solution: Replace the label material, improve cutting quality, avoid snake-like movement of the label material, and lower die-cutting speed.
- Excessively narrow waste edge or low tensile strength of label material.
- Solution: Appropriately increase waste edge size and reduce die-cutting speed.
- Overall low tensile strength of label material.
- Solution: Replace with label material with higher tensile strength and lower die-cutting speed.
- Changes in friction force or diameter of the waste rewind roller.
- Solution: Timely observe and adjust the friction force of the rewind roller, correctly control waste edge tension, and reduce die-cutting speed.
- Influence of label shape and column number.
- Solution: Increase label rounded corners, enhance waste edge tensile strength by increasing the number of waste columns.
Label Detachment During Waste Removal:
- Decreased release force due to a temperature decrease.
- Solution: Preheat label materials before die-cutting or perform in-line heating on the die-cutting machine to raise adhesive viscosity and accelerate release force increase.
- Excessively low release force leading to material deformation.
- Solution: Minimize waste removal distance on the die-cutting machine to prevent label loosening and prompt label winding after die-cutting.
Labels Peeling off with Waste Edges:
- Use the “liner dyeing method” to confirm die-cutting accuracy, inspect adhesive drawability, and ensure adhesive cutting.
- Shorten the winding distance as much as possible.
- Utilize new die-cutting tools with smaller angles and maintain proper sharpness.
- Adjust the die-cutting machine or change label materials prone to label and waste edge detachment.
As the use of hot melt adhesive label materials continues to expand, encountering various issues in printing factories becomes more common. This article aims to provide assistance to the label industry by offering insights into these challenges.