Producing accurate color on paper can be difficult. If the color drifts at any point in the production of laminated, testliner, security, colored, or white papers, with or without optical brightening agents (OBA), the entire roll is wasted and must be reworked. This is resource-intensive, time-consuming, and costly. To keep color and brightness in harmony, paper manufacturers must constantly monitor color on fast-running paper machines.
Many production variables can cause color shifts throughout a run. Variability in raw materials, recycled paper, fillers, opacity, thickness, additives, and dye concentration can all impact the final color. Environmental factors like a dirty environment or product temperature can also impact the final color.
To ensure color quality, many paper manufacturers pull a sample to measure in the lab, but by the time a color issue is identified, most of the roll is already wasted. Manufacturers must then stop production to adjust or switch colors on the reel. This slows production, wastes materials, and consumes more energy in an already energy intensive process. In addition, dyes, pigments, and OBAs are expensive. It’s essential that mills get color right the first time to maximize efficiency and keep costs down.
Inline color measurement systems and closed-loop color control systems help address these challenges and allow operators to achieve the right color quickly and easily by setting machine parameters. The inline system adjusts for dyestuffs and OBAs. This saves time at start-up and for every shade and grammage change.
WHAT IS AN INLINE COLOR MEASUREMENT AND CONTROL SYSTEM?
Inline color measurement solutions manage color while a product is being made. An inline system monitors and alerts operators as soon as color begins to drift out of specification so corrections can be made before the product is wasted. Here’s how it works.
An inline system includes a non-contact spectrophotometer and a custom frame to position the device at the right distance over the paper production line. The frame moves the system away during a sheet break, lowers the arm to feed up the paper, and returns to position when it’s time to run. The spectrophotometer connects with quality control software to monitor the actual color and store the data throughout the run; it also adjusts colorants at the dyeing station at the beginning of the process.
Even the smallest color deviations are immediately displayed on screen to maintain narrow tolerances. Color corrections are carried out automatically—even when the actual color is far away from the target—using a mathematical algorithm to calculate all necessary dye adjustments in one step. This takes the guesswork out of the process.
To make a shade change, the operator chooses the standard color from the database and the inline system will automatically adjust the dye pumps and start the run. The quality control (QC) software can also report up to eight color trends in L*a*b*, Delta E, whiteness, and opacity so operators can understand color deviation and implement process improvements on the spot.
Inline devices are built to operate in harsh production environments so ambient light, vibrations, web speed, and web flutter do not influence measurement results. Systems are designed to swivel off in case of a paper break to facilitate the threading of the new web. As soon as the paper is stable again, the measuring device automatically swings into measuring position and starts measuring. An inline system can also be set up with two instruments to simultaneously monitor and control both sides of a paper.
WHEN TO MEASURE PAPER COLOR
The most common place to measure color is at the end of the machine shortly before reel up because it offers excellent correlation with the finished product and results from the laboratory. However, an inline system can also measure the liquid pulp stage for an early warning system, and laminated paper after the press section for better correlation to the paper laminated on the wood. There are different reasons for measuring color at different stages. The illustration on page 23 indicates where in the process each stage occurs.
1. Inline color measurement in the liquid paper stock: Measuring color inline in the liquid stage provides an early warning system even before the pulp is on the machine. Although the measured color will not correspond to that of the finished paper, detecting a color change in the pulp enables operators to make an adjustment before the pulp is on the machine or even before production begins. Using an inline system to monitor color at the liquid stage can compare batches; define the mixing of two material flows, such as waste paper with “clean” material; determine the influence of adding scrap; and detect optical brighteners.
2. Inline color measurement before reel up: Measuring color before rewinding is ideal for testliner, colored tissue, fine colored or white paper, cardboard, décor paper, security paper, and thin printing paper. Measuring the finished paper before winding offers a very good correlation to the laboratory values, where the paper is measured in the stack. Since the inline measurement is done only on a single layer, it is necessary to also measure opacity for non-opaque papers and recalculate to an infinite layer to correlate with the lab.
Measuring color before rewind is also ideal for closed-loop color control because color fluctuations in raw materials will end up as shade changes at the end of the machine. An inline system reliably detects even the smallest deviations, calculates the necessary countermeasures for all dyes in one step, and sends the calculated actions to the dyeing station. Thus, the color at the end of the machine will have only minor variations even when the raw materials change.
3. Inline color measurement between press and dryer: Measuring color inline between the press and dryer is ideal for pressed and laminated décor paper. The color of the pressed paper must match exactly, but the refractive index of the resin or varnish will change the color impression of the paper.
For laminated paper, an inline system can measure the wet paper before drying with a good correlation to the finished product because the water in the paper has a similar refractive index to that of the pressed paper. If necessary, a re-dyed strip can be produced and measured without having to make another lamination, further shortening transition times.
BENEFITS OF USING INLINE
As paper mills embrace digital transformation, inline color measurement systems provide mill operators with real-time color information to avoid costly line errors and rework. Today’s advanced inline and closed-loop solutions—such as X-Rite’s ColorXRA 45 and ESWin Closed Loop Color Control software—lead to more stable and consistent color workflows while reducing start-up times and cutting shade change times by up to 50 percent, saving on dye usage, and minimizing waste and rework. This also helps save energy and water resources to meet sustainability initiatives.
On average, paper mills see a return on investment (ROI) in six months with inline systems. This helps to improve a mill’s bottom line while keeping color and brightness in harmony from laboratory measurement to pulp, wet sheet, and before reel up.
Michael Saeger is general manager, inline measurement solutions, for X-Rite. He leads a team of specialists that provide customized inline color measurement solutions for the paper, plastics, textile, and automotive industries. He has more than 25 years of experience in R&D and color measurement applications and control and production machines.