Ink Viscosity Control System Description and Operation

In recent blogs, we have discussed efflux cups, viscometers, and applications where ink viscosity control systems are employed to improve flexographic printing quality, ensure accurate viscosity measurement, and control costs.

Ink Viscosity Control

Simply stated, as ink viscosity rises due to the evaporation of solvent or another fluid type, the controller initiates a controlled injection of fluid into the ink to bring the viscosity back to the desired value. As simple as this appears, technology is required to ensure “simple control” can operate 24/7/365 without failure or downtime.

It is important to note that, by itself, a viscometer does not serve a useful purpose without an appliance to operate the device. Hence, the need for a system that operates the viscometer and controls the viscosity of the process fluid.

Viscosity System Components

An ink viscosity control system can include components such as a PC or PLC, a touchscreen HMI, a fluid injection valve, and installation materials including wires, cables, fluid lines, and other components integrated into a printing press or coating machine. Since INKSPEC uses PLCs in our controllers, we will focus on the benefits and characteristics of PLC controllers.

Following is the list and explanation of major components and their descriptions:

PLC – Programmable Logic Controller: The PLC is the brains of the system. All the control parameters for the viscosity system, such as viscometer power and operation, solvent addition, and alarms, are programmed via ladder or function block logic.

Advantages of a PLC in process control include:

• Security: PLCs are resistant to viruses and cyber-attacks.

• Versatile: PLCs are programmed to perform a wide range of tasks in an industrial environment.

• Expansion: Modular in design, PLCs accept expansion with more devices, for example, a temperature control module.

• Rugged and Reliable: Designed to withstand vibration, contamination, and electrical interference. PLCs have a longer operational life when compared to a PC.

• Costs: PLCs have lower maintenance costs and require less interaction for updates and repairs.

Touchscreen HMI (Human Machine Interface): The touchscreen HMI provides a graphical, intuitive, and user-friendly interface that allows operators to control, monitor, and maintain the fluid viscosity from a central location. The touchscreen allows the operator to set control parameters, save print jobs, and trend how well each printing or coating station operates over a given time.

Fluid Injection Valve: Either air- or electrically-actuated, the fluid injection valve adds the make-up material—solvent, amine water, or the like—to maintain the process fluid’s viscosity. This valve can be either manually operated or automatically controlled using the touchscreen.

Installation Materials: Usually supplied by INKSPEC as part of the scope of supply, these components are necessary to ensure system power, PLC and HMI communication, and injection valve operation.

For simplicity, we will use the INKSPEC control design to describe the system operation. Our system can display in cup seconds (Zahn or other efflux cups) or in centipoise (cPs).

System Operation

When initialized, the viscosity control process consists of the following cycles: Measurement, Analysis, Control, and Mixing.

• Measurement: As ink flows through the viscometer, the viscosity of the process fluid is continuously monitored while communicating the information with the controller. The values read by the viscosity sensors are displayed in the “Actual” box on the “Main Viscosity Screen.”

• Analysis: The controller compares the measured value (actual viscosity) with the desired preset value (setpoint) and applies corrective measures if/when necessary. The acceptable setpoint/actual viscosity variation is determined by the operator for each printing or coating job.

• Control: As ink viscosity rises due to evaporation of solvent or another fluid type, the controller initiates a controlled injection of fluid into the ink to bring the viscosity back to the desired value.

• Mixing Time: To ensure precise viscosity measurements and stable control, the controller has built-in presets with predetermined mixing time delays (also known as solvent delay), based on different pail sizes. These delays help eliminate wide swings in viscosity caused by excessive fluid injections during the control cycle.

For this exercise, we will use a typical flexographic print job as an example.

Snap Shot

The press operator or user should bring the ink to the press as close to the specified viscosity as possible. Doing so will minimize the setup time for the job. This is especially important when running a multi-station print job.

After powering up the system and allowing the ink to circulate, the operator can take a cup reading or match the color to the job requirements.

• When matching color: Once the color is within specifications, the value is input into the controller and the “Learn” mode is selected. The viscometer will automatically “read” the ink flowing through and, after a few seconds, will display a centipoise value on the Viscosity Main screen as a new desired value, identical to the actual reading. The controller will also set itself to the AUTO Control mode and start maintaining the viscosity at that new desired value.

• When using cup seconds or conversion: The operator or end user takes an ink sample using the Zahn (efflux) cup. For example, 22 Zahn Cup #2 seconds (it is usually best to take three cup readings and average the results).
  That 22-second value is input into the Main Viscosity Screen as the Actual Viscosity. Next, tap on the “Setpoint” text box, then input the desired viscosity value using the pop-up keypad. When you tap “OK,” the new Desired value is displayed on the Main Viscosity Screen.
  Once this step is completed, the system is operating in Automatic Control Mode. Once in automatic control, as viscosity rises, the controller automatically manages the injection to maintain ink viscosity at the desired setpoint value.

Conclusion 

For more information, please contact INKSPEC.  Sales@inkspec.com  +14504415005  www.inkspec.com