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materials testing | Colour measurement The key expression is “process monitoring,” says

ColVisTec CEO Jan Johnsen. In a continuous process like compounding, process parameters have to be stable to guarantee a consistent result, he notes. “Any change in these parameters we would identify as change in the spectrum and as a result in the colour values.” “Users can monitor the stability of the extrusion

process and the feeder system. As soon as this changes from where you want it to be, it shows up in the colour spectrum, and usually you can have a good indication of where that change was coming from. “For some of our customers, colour is the primary

The ROC system examines every pixel, with the chosen ones marked pink, allowing the operator to identify any false measurements

concern, but for others, it is not so important. What is most important is that the concentrations of the various additives in the compound remain constant and the product is homogeneous. So it depends on the applica- tion. In some cases it is more important to measure in the UV range rather than in the visible range.

Broader measurement “So it’s a much broader type of measurement than in other colour control systems. And it’s continuous process monitoring, rather than the more traditional procedure of periodic sampling from the line for analysis in the lab, with the hope that in the meantime nothing changes on the line. We think that that approach is reaching the limits of what it can do,” says Johnsen. “People who are looking beyond the next two or three

ROC’s new ImageInspect allows users to examine images of the pellets while production is running

pay for itself inside 18 months. ColVisTec claims to be unique in offering in-line

continuous colour measurement of the melt, with the possibility of closed-loop control over certain process parameters. Its system is based around a “Refl ection Polymer Melt Probe” with a sapphire window that fi ts into the die plate of the extruder via a standard sensor thread. Illumination of the molten material through the

sapphire window is achieved by optical glass fi bres around the circumference of the window that convey light from a Xenon fl ash lamp. The refl ection from the illuminated surface is detected by one centre-positioned fi bre. The light is then interpreted by the spectropho- tometer. Colour values such as L*, a*, b* C*, h are calculated from this spectral curve, and displayed as trend charts. The sampling rate is freely adjustable up to three readings per second. The ColVisTec in-line system monitors the whole

UV-visible spectrum between 220 and 820 nm. It directly detects any alteration in the compound that manifests itself as a change in that part of the spectrum.


years have a lot of detailed questions about the homoge- neity of materials, certifi cation of products, lack of contaminants in downstream processes, 100% control, that sort of thing. ‘What happens during the night shift?’ is a typical question they are asking. And to provide the answer, you need to be able to extract and store complete data covering the process,” he says. The inline system can also help in minimizing

changeover times when switching from one batch to another, since it indicates when the new batch is completely free of contaminants from the previous one. Rather than relying on experience from previous changeovers, users know when the new batch is clean, Johnsen says. “You no longer have to have an unneces- sarily long changeover period, just to be on the safe side.” “We see a lot of problems with feeding systems,” Johnsen adds, “things you may not see if you are only taking samples at long intervals. You will only pick up things like oscillations if you take measurements every few seconds.” According to Johnsen, a major polymer and com-

pound producer is currently bringing the system on-line at one of its production facilities, hooking up multiple lines at a plant in the Netherlands. Each ColVisTec

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