EoW September 2012

Technical article

• The wire geometry is detected as a side view on a cylinder surface. This results in a colour variation from the cylinder centre view towards the cylinder border. This effect is also influenced by the surface roughness. As both conditions cannot be changed, the final colour value cannot be interpreted as an absolute measurement but as relative measurement with high reproducibility The device should be able to work with various geometries (over a certain range) without mechanical preparation or sensor recalibration. One more challenge is the measurement in a production of colour-coded wires (one or two stripes). As the final colour establishes after the cooling down of the polymer, sampling has to be done behind the cooling trough. Caused by redirecting wheels and the product itself (particularly stranded conductor), the wire can turn around the longitudinal axis in an irregular way. Therefore the sensor detects sometimes the main colour, sometimes the stripe colour, or both at the same time in the scan field. Figure 3 gives an impression of the sensor’s view on a two-coloured wire. With sophisticated mechanics the wire turning can be changed to be more regular and used for main and stripe colour detection with only one sensor. Normally one line runs different conductor/insulation diameters.

distribution of d E (binning = 0.05)

1.Col. Test (Yellow) 2011-04-28

d E [AU]

d E [AU]

sampling time [min]

▲ ▲ Figure 5 : Left side – ∆ E calculated from data in picture 4 (with setpoints 87.62/-66.04/39.10) Right side – Histogram of ∆ E with a binning of 0.05. Average ∆ E = 0.89

Yellow insulated cable with masterbatch fault from hopper (blue grain)

d E [L*a*b*]

Time [min]

▲ ▲ Figure 6 : Forced colour fault by putting blue masterbatch into the barrel feeding

Technical requirements and problems caused by wire geometry and processing Colour measurement on the base of CIE-Lab is today state-of-the-art in the paint industry or graphic art applications, with tolerance values of sometimes ∆ E < 1. Conditions for such exact measurements are plane objects, a scan spot with a diameter of some 5-10mm and a sampling time in the order of 100ms on a motionless object – but all these conditions are definitely not given at an extrusion line. That’s why an inline measurement has to consider the following points: • With a very short sampling time an averaging over a certain number of single shots eliminates local devia- tions. This is justifiable, as colour changes in extrusion have a relatively long transition time caused by mixing effects in the barrel

• Object movement (jitter) has to be minimised at the sensor position. This is important for the object-sensor distance d s (illumination reduces with d s 2 ) as well as for transversal movement, where the object is leaving the scan spot partially or completely

▼ ▼ Figure 7 : Raw data with stripe – well recognisable changes in a*- and b*-channel when the stripe is moving through the scan field

Dual colour test (blue-green)

raw signal L*, a*, b* [AU]

test length [AU]

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www.read-eurowire.com

September 2012