As the old saying goes, "Three parts printing, seven parts plate making." This highlights the critical importance of the plate-making process in ensuring high-quality printing results. In gravure printing, the preparation and maintenance of the printing cylinder play a vital role. Understanding how to choose and inspect the gravure cylinder is essential for maintaining consistent print quality. This article will provide an in-depth look at the key requirements for selecting and processing gravure cylinders, including electroplating, grinding, and engraving.
**Basic Requirements for the Printing Plate**
1. **Material Selection and Processing of Gravure Cylinder**
Choosing the right material is crucial when manufacturing a gravure cylinder. There are two main types: open hollow cylinders and closed solid ones. The dimensions of the cylinder depend on the size of the printing machine and the printed material. It’s best to use seamless steel tubes, as they offer a smooth inner surface without any seams, which ensures uniformity and durability.
The manufacturing process includes rough machining, semi-finishing, and finishing. During rough machining, the wall thickness is adjusted to ensure even weight distribution, which is important for balanced rotation during printing. Semi-finishing focuses on aligning the shaft with the cylinder, while finishing involves precise shaping of the outer surface to meet design specifications. Proper processing directly affects the cylinder's lifespan, electroplating performance, and overall print quality. All dimensions must meet strict quality standards to avoid issues later on.
2. **Plate Electroplating**
Electroplating is a key step in preparing the gravure cylinder. Before copper plating, the surface must be thoroughly cleaned using manual or electrolytic methods to remove oils and contaminants. Next, the cylinder is pickled to remove rust and oxide layers. A thin layer of nickel is then applied to improve adhesion before copper plating. The copper layer serves as the base for engraving, where the image is etched by an electronic engraver. The quality of this copper layer significantly impacts the final engraving result, so it must be done carefully and precisely.
3. **Barrel Grinding**
After electroplating, the copper surface must be ground to achieve a high level of smoothness, typically reaching a finish of â–³8 to â–³10. Proper grinding ensures that the final plate has a uniform surface, preventing defects like spots, water marks, or scratches. If the copper layer is not properly finished, it can lead to poor ink transfer and inconsistent print quality. Especially for large areas, attention must be paid to surface finish to avoid visual imperfections on the final product.
4. **Electro-Engraving Quality**
The quality of the electro-engraving process is essential for accurate and consistent printing. Industry standards define the size and depth of the engraved cells, but these can vary depending on factors such as the engraving needle’s condition, the angle of the engraving head, and the copper thickness. For example, with 70 lines/cm and a 30-degree angle, the cell width might range from 34–36 microns in highlights and 175–178 microns in shadows. Any missing walls or incomplete cells can lead to weak lines, blurred edges, or blistered screens, all of which affect print quality.
5. **Chrome Layer Quality**
To increase the durability of the gravure cylinder, a chromium layer is applied after engraving. Chromium has a much higher hardness (HV800–1000) compared to copper (HV90–180), making it more resistant to wear. However, if the chrome layer peels off during printing, it can damage the cylinder and cause waste. Therefore, it's important to inspect the cylinder carefully for any signs of chipping or peeling. If found, the cylinder should be repaired immediately and not used until it is fully restored.
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