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materials testing | Colour measurement


The ColVisTec probe is


mounted in the extruder die plate via a standard


sensor thread


system can operate simultaneously on two lines. ColVisTec has been offering the system in its current


form to the plastics industry in Europe for just over four years. American customers are serviced by Equitech International, which offers the same German-made system. The two companies now concentrate on applications


that require direct contact of the sensor with the medium and where in most cases the medium is at an elevated temperature. Currently, ColVisTec has a sensor that Johnsen says is “comfortably capable” of withstanding continuous temperatures up to around 320-350°C, but the goal is 500°C. The sensor is also resistant to viscous and abrasive media, and withstands operating pressures up to 200 bar.


Closed-loop control


Johnsen says some systems already in operation at compounders are running with closed-loop control on a limited number of elements on the line. On a com- pounding line with a single feeder, this is relatively straightforward, but most lines have more than one


feeder, of course. “We are actively working on multiple- parameter closed-loop systems,” says Johnsen. “There is a desire in the industry for a system that will regulate a line with multiple feed ports, but that is a hard goal to reach, particularly when you talk about colour.


The ColVisTec system picks up regular variations in colour caused by dosing pump pulsations on a PVC fl ooring line


ColVisTec has several new developments under way that enable immediate translation of raw data into usable information. One package, relating to residence times, has recently been released commercially. A compounding extruder mixes and transports, but distribution is far from symmetric, and some material fractions stay in the barrel much longer than others. This obviously can have an adverse effect on material properties, especially when temperature sensitive materials are involved. “It would be useful to have a simple and rugged way to characterize this behaviour for a particular setting for the extruder by sending materials with tracers through the process, and have a fast visualization of how properties are changing,


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