Plastic film gravure printing is a complex process that requires careful control of various factors to ensure high-quality results. One common issue during this process is the "back sticking" phenomenon, where the printed side of the film becomes sticky after printing, especially during storage. This can lead to the ink layer being damaged or even cause the product to be scrapped. While high temperatures often exacerbate this problem, it is not the sole cause. Other factors such as the type of plastic used, ink formulation, solvent selection, air circulation, light exposure, and printing speed also play significant roles.
A major contributor to back sticking is incomplete evaporation of solvents in the ink. In gravure printing, solvents are used to dry the ink layer by evaporation. If the solvent doesn’t fully evaporate, residual liquid remains in the ink, which can cause adhesion between layers. This is particularly problematic in high-temperature seasons or when there’s insufficient drying time between printing stations. To prevent this, it's essential to adjust the solvent volatility based on environmental conditions like temperature, humidity, and airflow. For example, if the ink dries too quickly, adding a slower-evaporating solvent like 74-92 slow-drying thinner can help. Alternatively, using a mixed solvent of xylene, ethanol, and isopropanol (in equal parts) can improve performance.
The choice of solvents is also crucial. Common solvents used in plastic film gravure printing include alcohols like ethanol and isopropanol, as well as aromatic solvents such as toluene and xylene. Esters like ethyl acetate and butyl acetate may also be added depending on the ink formulation. It’s important to ensure complete solvent evaporation before winding the film. Slowing down the machine speed and increasing air flow in the drying system can help achieve this. After printing, the film should be stored loosely in a wire basket to allow continued drying and reduce pressure on the ink layer.
Another issue that can lead to sticking is poor ink quality. Some inks contain binders with low melting points, making them prone to sticking under high temperature and humidity conditions. To mitigate this, maintaining proper workshop conditions—such as controlling temperature between 18–20°C and relative humidity below 65%—can be beneficial. Additionally, replacing low-quality inks with better alternatives can significantly reduce the risk of sticking.
In some cases, the plastic film itself may be unsuitable for printing. Films made from non-packaging-grade resins or those lacking sufficient slip agents can contribute to adhesion problems. Replacing these films with more suitable materials is often necessary.
Static electricity can also cause issues, as it leads to film-to-film adhesion. Adding antistatic agents to the film or the ink can help reduce this effect.
Color registration problems are another common challenge in gravure printing. Issues such as misalignment in the longitudinal or lateral directions can occur due to improper tension control, incorrect plate cylinder diameters, or misadjusted photoelectric sensors. Regular checks and adjustments of the printing equipment are essential to maintain accurate registration.
Poor printing fastness can result from inadequate corona treatment of polyolefin films, poor ink compatibility, or unstable workshop conditions. Ensuring proper surface tension and adjusting the solvent mixture can help improve adhesion and durability.
Pattern size shifts during printing are often caused by improper shrinkage margins or inconsistent film tension. Adjusting the plate design and optimizing tension settings can minimize these issues.
Printed material reproducibility problems, such as overly dark or light highlights, can be addressed by modifying the ink formulation, adjusting the doctor blade angle, or changing the printing plate depth.
Other challenges include dry ink layers, uneven gloss, rough edges, orange peel effects, scratches, and color biting. Each of these issues has specific causes and solutions, ranging from adjusting ink viscosity and drying conditions to improving machine maintenance and handling practices.
Overall, successful gravure printing requires attention to detail, consistent monitoring of environmental factors, and regular maintenance of both the printing equipment and materials used. By addressing these factors proactively, printers can significantly reduce defects and improve overall print quality.
Foshan Gruwill Hardware Products Co., Ltd. , https://www.zsgruwill.com