Ink Viscosity and Its Role in Printing
- 商品名： Ink Viscosity and Its Role in Printing
View Details
Overview of Ink Viscosity
Viscosity is one of the most important parameters describing the rheological behavior of printing inks. Maintaining an appropriate viscosity is essential for proper ink transport and uniform laydown. It directly influences print quality—affecting ink film uniformity, image sharpness, and gloss—and is closely related to other key ink properties such as tack, flow, yield value, thixotropy, and stringing. As a result, viscosity is a critical aspect of printability.
Because printing methods and printed products vary widely, viscosity requirements differ accordingly.
Offset inks typically require high viscosities of 10–80 Pa·s.
Gravure and flexographic inks, by contrast, operate with much lower viscosities:
- Plastic gravure inks: 0.02–0.2 Pa·s
- Publication gravure inks: 0.01–0.1 Pa·s
- Solvent-based flexographic inks: 0.09–0.15 Pa·s
- Water-based flexographic inks: 0.15–0.3 Pa·s
Due to their low viscosity, gravure and flexo inks are often measured with Zahn cups during press operation.
Effects of Printing Conditions on Ink Viscosity
Press Speed
Higher printing speeds require lower ink viscosity.
For instance:
- Sheetfed offset at ~6,000 impressions/hour: 20–50 Pa·s
- Web offset at ~20,000 impressions/hour: 10–30 Pa·s, sometimes lower than 10 Pa·s
Lower viscosity ensures the ink can transfer and separate cleanly at high speeds without misting or uneven laydown.
Paper Properties
Paper structure and surface strength also influence viscosity selection:
- Porous, weak-surface papers → require lower-viscosity inks
- Dense, high-strength papers → can tolerate higher viscosity
Newsprint, for example, typically uses lower-viscosity inks than standard offset papers.
Image Type
Different image structures place different demands on viscosity:
- Linework and fine-screen halftones → require higher viscosity for edge definition
- Solid areas → print better with lower viscosity for uniform coverage
Wet-Trapping and Four-Color Printing
In wet-on-wet trapping:
- The overprint ink should have higher viscosity than the underlying ink to maintain sharpness and ensure good interlayer adhesion.
In four-color lithographic wet trapping:
- Overprint inks typically require lower viscosity than the first-down ink to avoid picking and ensure stable trapping.
- Viscosity is generally decreased progressively from the first to the last unit.
Printing Problems Caused by Improper Viscosity
When Viscosity Is Too High
Excessive viscosity leads to several issues:
-
Ink misting: Strong stringing causes filaments to snap between rollers and form airborne droplets, especially at high speeds.
-
Picking and delamination: Ink tack can exceed the surface strength of the paper, causing fiber lift or ply separation.
-
Poor ink transfer: Because transfer efficiency decreases as viscosity increases, high viscosity leads to uneven ink distribution, weak color, and exposed image foundations.
-
Thick ink films and slow drying: This increases the likelihood of set-off and blocking, and in offset printing may cause sheets to wrap around the rollers.
When Viscosity Is Too Low
Low viscosity also causes defects:
-
Spreading on paper: Ink spreads excessively, enlarging halftone dots and reducing sharpness.
-
Weak adhesion and low gloss: The dried ink film bonds poorly and lacks luster.
-
Pigment piling: Low-viscosity ink may not carry larger pigment particles effectively, leading to buildup on rollers, blankets, or plates.
-
Emulsification in offset: Thin, free-flowing inks are more prone to emulsifying with fountain solution, resulting in scumming and contamination.
Adjusting Ink Viscosity
To match ink viscosity with specific printing conditions, adjustments must be made based on printing method, press speed, paper characteristics, and ambient temperature.
-
No. 6 reducer (No. 6 ink oil):
Used to increase ink fluidity, improve transfer, and reduce ink strength, viscosity, and drying rate. -
Diluent (toner reducer):
Added when ink color is too strong. It lightens color strength with minimal impact on viscosity.
Proper viscosity control ensures stable press performance, consistent ink transfer, and high-quality printed results.
Related Products
AIが支える信頼の試験品質!
お問い合わせ
3F, Bldg A, No.257 Junye Rd, Huangpu Dist, Guangzhou, P.R.China
+86-20-89851362