A flexographic press running at 300 meters per minute on a 12-hour shift evaporates approximately 1.5 to 3 liters of solvent per hour from each print station's ink tray. The ink viscosity rises as the solvent leaves. By hour three of the shift, the viscosity has climbed from the target 18 seconds (Zahn Cup #2) to 22 seconds — a 22% increase. The print result: higher dot gain in the 50% halftone areas, bridging in fine reverse text, and color strength variation across the web width. The operator adds solvent by hand, judging the right amount by how the ink drips from a draw-down bar. The correction is reactive, inconsistent between operators, and applied after enough scrap has been produced to notice the defect.
Automating solvent replenishment requires measuring the ink's condition in real time. Two fundamentally different measurement strategies compete for this function: concentration control (measuring the pigment-to-solvent ratio directly) and viscosity measurement (measuring the fluid's resistance to flow as a proxy for solvent content). This article compares the two approaches — what each measures, what each misses, and which printing applications benefit most from each method.
Viscosity Measurement: The Fast, Indirect Proxy
Viscosity is the ink's resistance to flow, measured in centipoise (cP) or seconds through a flow cup. In solvent-based flexographic and gravure inks, viscosity correlates inversely with solvent content: as solvent evaporates, viscosity rises. A viscometer in the ink circulation loop measures this rise and triggers a solvent add valve when viscosity exceeds the setpoint.
Commercial inline viscometers for printing fall into three categories:
- Falling-piston viscometers — a piston rises through the ink, then drops under gravity. The drop time correlates to viscosity. Simple, reliable, ±1% repeatability. Widely used on flexographic presses.
- Vibrating-element viscometers — a resonant element (tuning fork or rod) oscillates in the ink; the damping effect of the fluid changes the resonance frequency, which correlates to viscosity-density product. Fast response (<5 seconds), no moving parts, ±0.5% repeatability. Higher cost than falling-piston, preferred in gravure where tighter viscosity control is required for high-speed cylinder-to-web transfer.
- Rotational viscometers — a spindle rotates in the ink at a controlled shear rate. Measures viscosity at a known shear rate, important for non-Newtonian inks (most flexo and gravure inks are slightly thixotropic). Highest precision (±0.1% of reading) but higher maintenance due to bearings and seals in the ink path.
The weakness of viscosity measurement: it is a bulk fluid property, not a composition measurement. If the ink formulation drifts — the pigment-to-binder ratio changes because the press returns a different proportion of ink than it consumed, or because fresh ink added to the sump has a slightly different starting viscosity — the viscometer reports the same viscosity but the ink's print characteristics (color density, dot gain, adhesion) have changed. Viscosity control keeps the ink flowing consistently; it does not guarantee the ink composition is consistent.
Concentration Control: The Direct Composition Measurement
Concentration control measures the actual pigment or non-volatile content of the ink, typically by density (mass per unit volume). Since pigment and binder have higher density than the solvent carrier, an increase in density at constant temperature means the solvent fraction has decreased (pigment concentration has increased). A density meter — or a Coriolis flow meter like the E+H Promass I 100, which measures both density and temperature from the oscillation frequency of a single straight titanium tube — provides a direct concentration signal: density = mass of ink / volume of ink, and with known pigment and solvent densities, the pigment-to-solvent ratio can be calculated continuously.
The advantage of concentration control is that it measures what actually matters for print quality: the pigment concentration in the ink film transferred to the substrate. If the ink formulation drifts (different batch of base ink, different return-ink composition), a density-based concentration measurement detects the change. A viscosity measurement may not — because viscosity is a function of both concentration and the specific binder-solvent-pigment interaction, and two inks with the same viscosity can have different pigment concentrations if their base formulations differ.
The disadvantage: concentration control responds more slowly than viscosity measurement to rapid solvent evaporation. Density changes as solvent evaporates, but the density difference between typical flexo solvent (ethanol, ρ ≈ 0.79 g/cm³) and the ink base (ρ ≈ 1.05 to 1.20 g/cm³) is smaller than the viscosity difference — a 5% solvent loss produces a ~2% density change but a ~15 to 25% viscosity increase. The signal-to-noise ratio for early detection of solvent loss is higher with viscosity measurement for a given sensor cost.
Which Method for Which Application?
| Application | Recommended Method | Rationale |
|---|---|---|
| Narrow-web flexo (labels, tags) | Viscosity (falling-piston) | Short runs, frequent ink changes, ±1% viscosity control sufficient. Low cost per station is the priority — each print station needs its own measurement. |
| Wide-web flexo (flexible packaging) | Viscosity (vibrating-element) with periodic density check | Longer runs benefit from tighter control. Vibrating-element gives ±0.5% with fast response. Periodic density check (once per shift or at ink change) catches formulation drift. |
| Gravure (publication, décor) | Concentration (density) | Very long runs (100,000+ linear meters), high solvent evaporation rates, and tight color consistency requirements across the full run. Density-based control maintains pigment concentration — the parameter that directly determines optical density on the substrate. |
| UV flexo / UV inkjet | Neither continuous — temperature conditioning + periodic viscosity check | UV inks contain no evaporating solvent. Viscosity changes are driven by temperature, not solvent loss. Control the ink temperature to ±1°C and viscosity stays within specification without active solvent addition. |
When Both Methods Together Deliver the Best Result
In high-end gravure and wide-web flexo printing where the cost of off-spec output exceeds $500 per hour, combining inline viscosity measurement with periodic or continuous density measurement provides the most complete ink condition picture. The viscometer — fast, sensitive to solvent loss — triggers the solvent add. The density meter — slower but composition-specific — verifies that the ink formulation has not drifted and that the viscosity setpoint is still producing the correct pigment concentration. If the density trend deviates from the viscosity trend (viscosity at setpoint but density slowly rising), the operator is alerted that the ink formulation is changing — fresh ink addition or a sump dump-and-refill is indicated — before the print defect appears.
This dual-measurement approach, using instruments like the E+H Promass I 100 Coriolis meter with in-line viscosity measurement capability in a single instrument, provides the speed of viscosity control and the accuracy of concentration verification without doubling the sensor count in the ink loop.
Viscosity tells you how the ink flows. Concentration tells you what the ink is. For most flexographic printing applications, flow is the right parameter to control — it changes first, responds fastest, and the solvent-addition control loop based on viscosity is well understood and widely deployed. For gravure and other applications where color consistency across very long runs is the primary quality metric, concentration control directly targets the parameter that determines print density. And for the applications where the cost of being wrong is highest, both measurements on the same ink loop are cheaper than the cumulative cost of print defects that neither measurement alone would catch.
