A press operator checks the ink at 22 cup seconds, while the inline controller shows a different number. Resetting the controller until both displays agree is usually the wrong response. A flow cup and an automatic viscometer apply different measurement principles, and printing inks may react differently to each. The useful target is the ink condition that produces the approved colour and transfer on press. Cup and sensor readings should both be tied to that condition, with the method and temperature recorded.
Define what the cup reading actually means
A cup result is not complete when the job sheet says only '22 seconds'. Record the cup type and number, the ink temperature, the sampling point and the point at which timing stops. A different orifice, cup volume or endpoint produces a different result. ISO 2431 specifies flow cups with defined dimensions and includes checks for wear, while ASTM D4212 covers dip-type cups used for viscosity control.
The cup must be clean, undamaged and filled in the same way each time. Dried ink at the orifice, a bent edge or a change in timing technique shifts the result. ASTM describes dip cups as robust tools for plant control, but it also states that adequate controls are needed before results are treated as comparable. Keep one verified reference cup and check pressroom cups against it at a planned interval.
Expect the sensor and cup to show different values
An efflux cup measures the time a fixed volume takes to drain through an orifice under gravity. Inline instruments can use vibration, torsion or another sensing method while ink is circulating. InkSpec notes that these methods apply different shear conditions to the fluid. A flexographic ink that is shear-thinning or thixotropic may therefore give unlike readings even when both instruments are working correctly.
ASTM D4212 makes the same limitation clear: dip cups are intended for Newtonian and near-Newtonian liquids, and controlled-shear methods are preferred when a material is non-Newtonian. Do not create a universal conversion table from one cup number to one sensor number for every ink. Build the correlation separately for the actual ink series, reducer system and operating temperature.
Establish the reference at an approved print condition
Bring the ink close to the job specification, circulate it through the normal press circuit and allow mixing and temperature to stabilise. Confirm the anilox, plate, substrate, speed and dryer or curing condition. Then adjust colour using the plant's approved procedure. The reference should be taken only after the print result is accepted, because a convenient cup value is not proof that colour strength, trapping and drying are correct.
At that point, take several cup readings with the same trained operator and average them if that is the plant procedure. Record the ink temperature and capture the inline sensor value at the same time. InkSpec's control guidance describes either learning the sensor value when colour is correct or entering a cup-based target through the controller. In both cases, the approved press result remains the anchor.
Control the sensor setpoint, not the cup number
Once the approved ink condition is stored, let the inline system control around its own validated setpoint. Do not repeatedly change the setpoint because a spot cup test differs by one or two seconds. First check whether the sample temperature, cup, operator, mixing and sampling location match the original method. If they do not, the comparison is weak.
InkSpec systems measure the circulating ink, compare the actual value with the setpoint and add controlled amounts of solvent, water or other make-up fluid when required. The controller also uses a mixing delay so the added fluid can disperse before another correction. That delay matters. Consecutive manual additions made before the circuit has mixed can take the ink below the approved condition.
Keep temperature beside every viscosity value
Viscosity changes with temperature, so a cup reading from cool ink at start-up should not be compared directly with a sensor value after the press and ink circuit have warmed. Record temperature with the cup result, sensor setpoint and approved colour measurement. InkSpec's current sensor guidance states that its system measures viscosity and temperature together for process control.
If a job regularly moves outside its control band as the shift progresses, trend temperature before changing the viscosity target. A cooling fault, warm solvent return, high pump energy or a changed circulation path can produce a viscosity symptom. Correct the temperature or circulation cause before treating every change as solvent loss.
Use the cup as an independent pressroom check
Automatic control does not make the cup useless. A verified cup can support ink preparation, confirm an incoming batch, investigate a sensor alarm and provide a simple comparison when a job moves between presses. Its value comes from a controlled method, not from pretending it is identical to the inline measurement.
Set a response rule for disagreement. Check the cup and orifice, ink temperature, agitation, foam, contamination, sampling point, sensor cleanliness, circulation flow and the time allowed after an addition. Escalate the job if the print result and sensor trend have changed together, or if the approved condition cannot be restored without repeated manual correction.
Save enough data for the repeat job
The job record should identify the ink series and batch, reducer or make-up fluid, cup type, average cup time, temperature, sensor setpoint, control band, mixing delay, press speed and approved colour measurement. Add the anilox identification and substrate because changes in transfer can be mistaken for a viscosity problem.
For an application review, send Royal Coat the press and ink-circulation arrangement, number of stations, ink chemistry, present cup method, operating temperature range and the print defect or colour drift being investigated. Royal Coat represents InkSpec viscosity, temperature and ink-management systems and can direct the details to the appropriate technical team.
Questions from production and purchasing teams
What to ask about ink viscosity measurement
Should an inline viscometer show the same number as a Zahn cup?
Not necessarily. The methods apply different flow and shear conditions. Correlate both readings at the same approved ink condition and temperature instead of forcing numerical agreement.
How many cup readings should be taken when setting a job?
Follow the plant procedure and instrument guidance. Multiple repeat readings by the same trained operator can expose an inconsistent cup method before a value is stored.
Why must temperature be recorded with cup seconds?
Ink viscosity changes with temperature. A cup time without the ink temperature cannot be compared reliably with the original job condition or an inline reading.
Can one conversion table be used for every ink?
No. Different ink chemistries can respond differently to the shear and flow conditions created by each measuring method. Establish the relationship for the actual ink series and process.
What should be checked when the cup and sensor suddenly disagree?
Check temperature, cup condition, timing method, mixing, foam, sampling point, sensor cleanliness, circulation and the delay after the last fluid addition before changing the setpoint.