corrugated cartons used to belong to transporting package, but now tend to become consumer package in the tide of market economic development. Therefore, higher requirements on layout and printing of cartons are made.
Rubber plate printing technology had been applied by most carton manufacturers before 1995. Currently, flexographic printing is occupying an important position in the industry, thus how to improve flexographic printing quality is a big concern. In my opinion, printing quality mainly depends on color sequence, pre-press film output, flexographic plate-making and printing ink etc.
I Color Sequence
In flexographic printing, printing plates in different colors usually use solid plate and water-based ink that doesn’t have good transparency. Therefore, whether colors after being stacked can conform to requirement of the copy, and whether satisfying overprint effect can be achieved, shall be considered when arranging overprint sequence of color printing plates. Theses points are directly related with the quality of printing products.
Printing sequence is generally arranged according to copy design and plate-making selection. However, it may change upon the requirement of technology and product characteristics. Color sequence in normal flexographic plate printing is decided according to the following:
1. First print ink with strong covering capacity. Tint colors shall be printed first, and then follow deep colors.
2. Deep-color outline shall be printed first in case of multi-overprint colors and when deep-color outline does not serve as the overprint guide.
3. When it’s required to eliminate outline burr by covering the inward edge of image outline with the deepest color, the color shall be overprinted at last. However, sometimes the color also serves as overprint guide, while you may print a part of the deep color as the guide of other color overprint plate, and finally lap-print the deep-color outline.
4. The dominant hue of figures shall be the last to be overprinted.
5. For whole-color with dot and solid plate, in order to avoid dot filling-in and blurred solid plate, screen plate and solid plate shall apply plate-hanging printing respectively, so as to enable separate adjustment of printing pressure and adoption of different inks to ensure printing quality.
II Printing Plate
Films used to make printing plate shall make reference to the original plate and technology requirement.
1. Different printing color sequence of printed maters would affect the film and flexographic plate. For example: print tint color first, and then cover it with deep color. The center circle is black and excircle is yellow. If print yellow first, yellow is in a solid circle as big as the copy and black plate is in a circle with the same size of copy when outputting film. Use black to lap-print yellow and yellow would be covered by black. If print black first, the inner diameter of yellow ring shall be 1~2mm shorter than that of the copy, and black shows in a ring in the same size of that in the copy. That is to say, the black loop and yellow loop has 1~2mm overlapped part, which ensures accurate overprint of the two colors.
2. Dot figures are often involved in flexographic printing. With the development of plate-making technology and product quality, people tend to require more for beautiful figures, gorgeous and well-bedded color and luster, and make new demand on screen angle. Screen angle for dominant-color plate-making is usually 45 degree, as dot aligned in this angle looks extremely comfort and dynamic. Dot aligned in vertical angle looks sluggish and in 15 degree or 75 degree looks betwixt 45 degree and vertical angle. Therefore, it is recommended to adopt 45 degree, or 15 degree and 75 degree.
3. Most board production plants do resin-plate printing through rotary printing. Printing plate is pasted on rolling drum. Flexographic plate is usually 3.94mm thick. Printing figures would extent along circumference of the drum under the affection of thickness of printing plate and film of hanging plate, thus circle would change into oval. Therefore, paper-in direction shall be considered when outputting films. Calculate the shrinkage according to diameter of the drum and thickness of resin plate. Reduce its length along the direction of circumference to ensure that the printing figure would not deform. Consider shrinkage volume when conduct hanging-plate operation.
Ⅲ Printing Ink
Water-based ink is adopted by most corrugated carton manufacturers. The performance of water-based ink determines the printing quality of corrugated cartons. Ink dispersion degree and pigment content in ink are scaled with a terminology called coloring strength. Generally speaking, ink consistency shall be high when printing fine screen product, and be comparatively low when printing solid product. High speed, pressure and temperature require high consistency, per contra, requires low consistency.