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The ROC-Color- Control system analyses pellets sampled from the production line

materials testing | Colour measurement “The repeatability is very good, if we reposition the

sample and take the measurement again, the standard deviation is close to zero,” he says. Dietrich adds that ROC introduced the ColorControl

system as a means for adjusting colour by having it linked to a pigment dosing system. “But we found out that some customers are not using it so much for the original purpose, but more as a means of monitoring the complete compounding process. If ColorControl picks up a deviation in colour, material can be diverted to a separate silo for testing, thus avoiding contamina- tion of the main production.” ROC-ColorControl can also be used for reducing

start-up times. Rather than stopping the line each time a sample is taken to make a plaque for testing – a procedure that can take two or three hours – with samples being taken every three minutes the start-up time is reduced to no more than half an hour. Dietrich says that ROC-ColorControl can pick up

faults in a compounding line much more quickly. He cites the example of an installation in which a raw material feeder was changing speed when switching between gravimetric mode and volumetric mode. He says it picked up small peaks that were indicative of a fault before it became a serious problem.

Detecting variations On a separate occasion, the ROC-ColorControl picked up variations that were being caused by a malfunction- ing heater band that was causing the extruder to continuously speed up and slow down slightly, before material properties moved outside alarm parameters. “Everything that happens shows up in the colour of

the compound,” says Dietrich. “But if you have to make a plaque before you can do your test, you may be destroying the evidence. If you have a problem with colour dispersion, for example, you will see it in the

granules, but the process of injection moulding will hide it. So the average quality may be OK, but the customer should not have to hope that injection moulding will iron out the variations; they want to be sure that the colour will be right every time. ” The latest development from ROC is called ImageIn- spect, which the company is now beginning to add to existing installations. “ImageInspect allows you to watch the images taken from the pellets and examine them while the production is running,” Dietrich says. “The images of the most recent pictures from all supervised production lines can be brought up on a touch screen computer. The user can select the line and image they want to examine closely if, for example, the colour goes out of spec or they see some irregularities in the pellet shape or consistency. They can zoom in on the image, watch the pellets masked or unmasked, change the contrast and pan around. This is a powerful tool since the user does not have to walk to the production line itself, all the relevant information is right there on the screen.” The ROC-ColorControl system is not cheap (Dietrich prefers not to cite prices), but Dietrich claims that it will

This diagram supplied by Germany-based in-line measure- ment specialist ROC Rapid Optical Control shows colour measurement results in dL*, da* and db* relative to the target colour. Measurements show that the colour fluctuates for 30 minutes at the start of production until the colour reaches specification. 20 minutes after start-up, the colour becomes lighter (dL increases), redder (da) and more yellow (db). 30 minutes later, the effect is repeated. The operator checked the refill cycles of all feeders and

found that the blip was caused by a raw material feeder increasing its rpm while switching to volumetric mode for refilling. This was not sufficient to create an alarm on the feeder, but the ROC-ColorControl did provide a warning to the operator, since colour drifted beyond tolerance limits.


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