In gravure printing for flexible plastic packaging, choosing a good doctor blade and using it correctly produces bright, saturated colors and reduces print defects. A doctor blade removes excess ink from the gravure cylinder surface, leaving ink only in the engraved cells. Areas without cells should be wiped clean to achieve the required print quality. Correct selection and use of a doctor blade can substantially improve print quality.
How to choose a good doctor blade
Gravure plates may be mechanically engraved or electronically engraved, and both have uneven surfaces. As the press runs at high speed, the blade moves back and forth with the machine. Without sufficient elasticity, it cannot quickly return to its original position and may leave streaks, blade particles and copper particles that significantly affect color. Blade materials should contain appropriate proportions of C, Cr, Mn and Si. A thickness of 0.1–0.18 mm is typically selected. An overly thick blade lacks elasticity and may leave residual ink and a hazy print, affecting print quality and subsequent color registration and causing significant variation in results before, during and after printing.
An overly thin blade can leave marks on the cylinder and rub against copper and iron particles in the ink, causing defects in large solid white areas. Blade hardness also affects quality. The blade is steel, while an electronically engraved cylinder has a chrome-plated copper surface. A hardness of 200–300 HV is generally used to remove excess ink while avoiding cylinder damage and blade marks. A rounded blade tip suits the cylinder's curved surface, helping wipe unprinted areas clean without trapping ink. A flat, sharp tip can damage the cylinder and impair ink removal and print quality.
Using doctor blades during printing
Inconsistent color density is a common issue for color printers, particularly in screened or tonal printing. Besides ink transfer, blade angle is an important factor. The angle between the blade and the cylinder directly affects quality. A small angle can produce a thicker, glossier ink layer but may not remove excess ink effectively, making images and text unclear. An angle of 35–45° is generally suitable.
The optimum angle also depends on cell depth: deeper cells require a larger blade angle, while shallower cells require a smaller angle. Press speed and the distance between the blade and the impression roller also require further adjustment.
For good printing results, the blade must remove excess ink cleanly so that ink in the cells transfers evenly and fully to the film. Screened ink areas are especially sensitive to doctor blade performance.