When printing on flexible packaging substrates, gravure printing requires careful attention to several key properties of the film, including surface tension, heat shrinkage, surface smoothness, deformation resistance, high-temperature stability, and solvent solubility. These characteristics significantly impact the quality of the printed output, making their proper control essential for successful printing.
Here are some important considerations for different types of films:
1. **BOPP (Biaxially Oriented Polypropylene)**
BOPP is one of the most commonly used materials in flexible packaging. It's important to ensure proper surface tension, as low tension can lead to poor ink adhesion. Additionally, BOPP should have a smooth surface without fine pits, which could affect the print quality, especially in fine mesh areas. The printing tension and embossing force should be moderate, and drying temperatures should not exceed 80°C to avoid damage.
2. **BOPET (Biaxially Oriented Polyethylene Terephthalate)**
PET films are typically thin, around 12 microns, and prone to wrinkling during printing. They require higher tension during the process and are more selective when it comes to ink compatibility. Using standard inks may result in peeling or poor adhesion. Specialized inks are recommended. Static electricity is also common, leading to issues like plate blocking, fluffing, and scratch marks. Printing in a high-humidity environment can help reduce static, and BOPET can tolerate higher drying temperatures compared to other films.
3. **BOPA (Biaxially Oriented Polyamide)**
BOPA is known for its high moisture absorption and susceptibility to deformation. This makes it particularly sensitive during the printing process. Key points to consider include:
- After moisture absorption, BOPA can change dimensions, so it should be unpacked and used immediately. Any remaining film must be resealed promptly.
- Preheating and dehumidification for 1–2 color runs at 60–90°C in air can help reduce moisture content.
- Avoid printing nylon films when ambient humidity exceeds 85%.
- Printed BOPA should be moved quickly to the next stage, such as lamination, and stored for no more than 24 hours if not processed immediately.
- Maintain appropriate printing tension and pressure to prevent distortion.
4. **K-Coated Film**
K-coated films, whether roll-coated or spray-coated, often have uneven surfaces and inconsistent thickness, which can affect ink transfer. The PVDC layer is brittle, so excessive printing tension or pressure should be avoided. Overprinting may be challenging, and light screen printing can result in missing dots. A high-hardness pressure roller is recommended. Additionally, these films are sensitive to solvents, and improper use can cause coating dissolution. High residual solvent levels can lead to sticking, so adequate drying and cooling are crucial.
5. **Matting Film**
Matting films can be printed using an OPP process on their glossy side. However, the matting layer is not heat-resistant, so drying temperatures must be carefully controlled to avoid damage.
6. **Pearlescent Film**
For pearlescent films, surface printing with transparent inks is ideal. When printing heat-shrinkable films, low drying temperatures are necessary. Special heat-shrinkable inks should be used to ensure that the ink shrinks along with the film without peeling off. For PVC heat-shrinkable films, the solvent compatibility with the film must also be considered.
7. **CPP and CPE (Cast Polypropylene and Cast Polyethylene)**
Unstretched PP and PE films have minimal tension during printing, making overprint registration difficult. When designing patterns, potential deformation due to the material’s behavior during printing must be taken into account to ensure accurate final results.
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