In the flexible packaging printing process, achieving a high-quality print result depends largely on the ink's saturation and coverage. Based on our experience in production, we have identified common issues such as poor ink saturation and uneven color application, especially in spot colors like green, pink, and purple. In this article, we aim to analyze these problems and share our insights with fellow professionals to help improve overall print quality.
**What is Ink Saturation?**
Ink saturation refers to the ability of the ink to produce a smooth, even, and complete coverage on the substrate during printing. When the ink lacks proper saturation, it appears dull, uneven, or patchy, which can significantly affect the visual appeal of the final product.
**Symptoms of Poor Saturation**
From a visual inspection, the printed surface may show uneven particle distribution, rough texture, and poor dispersion under magnification. This often indicates that the resin and pigment in the ink are not well mixed, leading to reduced gloss and poor adhesion.
**Common Causes and Solutions**
1. **Ink Viscosity**
Controlling ink viscosity is crucial for optimal performance. For high-speed printing machines, the ink should have a lower viscosity to ensure smooth transfer. At a speed of 150 meters per minute, the ideal viscosity range is around 16–18 seconds. If the speed increases, the viscosity can be reduced to 15 seconds to maintain good ink flow and prevent issues like ink flower.
2. **Impression Roller Selection**
Choosing the right impression roller is essential. A softer roller (around 60–65 Shore A) helps in better ink leveling and transfer, especially when printing large area spot colors. The pressure must be maintained between 0.18–0.2 MPa, and the roller should be kept clean to avoid contamination.
3. **Ink Quality and Handling**
Old ink may degrade over time due to oxidation, solvent loss, and moisture absorption, leading to poor performance. It’s best to mix old and new ink in a 7:3 ratio and ensure thorough stirring before use. Using a good agitator roller and, if necessary, polyurethane-based inks can enhance ink flow and leveling.
4. **Solvent Composition**
The solvent system plays a key role in ink performance. Fast-drying solvents can cause instability, leading to issues like knife marks or clogging. A balanced solvent blend with slow-drying components is recommended. Add solvents gradually through the filter sock rather than directly into the ink pan to maintain color consistency.
5. **Doctor Blade Angle and Pressure**
The doctor blade should be relatively soft, and its angle should allow for sufficient ink flow without causing dry-up or ink flower. Adjusting the gap between the blade and the plate roller can also help reduce these issues.
6. **Production Environment**
High humidity levels (above 50%) can lead to moisture absorption in the ink, reducing its solubility and transfer efficiency. Maintaining proper humidity levels and ensuring cleanliness in the workshop are important steps to avoid such problems.
7. **Plate Roller Condition**
A poorly engraved or dirty plate roller can hinder ink transfer. Cleaning the roller thoroughly after each run and using a mesh angle of 40–45 degrees can improve ink distribution and reduce the risk of ink flower.
8. **Raw Material Quality**
The corona treatment of the film must be adequate to ensure proper ink adhesion. Low corona intensity can lead to poor ink transfer, especially in fine screen printing, resulting in a lack of coverage and color inconsistency.
**Key Takeaways**
To achieve consistent and high-quality prints, focus on selecting the right impression roller, maintaining proper ink viscosity, managing the solvent system, adjusting the doctor blade angle, controlling the printing environment, and using quality raw materials. By addressing these factors, you can significantly improve ink saturation and overall print performance.
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