EoW September 2012

Technical article

September 2012

www.read-eurowire.com

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

(illumination reduces

with d

) as well as for transversal

movement, where the object is leaving

the scan spot partially or completely

• 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

relative

measurement with high reproducibility

Normally

one

line

runs

different

conductor/insulation

diameters.

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.

▲

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

▲

Figure 6

Forced colour fault by putting blue masterbatch into the barrel feeding

▼

Figure 7

Raw data with stripe – well recognisable changes in a*- and b*-channel when the stripe is moving

through the scan field

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

distribution of

E (binning = 0.05)

sampling time [min]

E [AU]

Dual colour test (blue-green)

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

test length [AU]

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

Time [min]

E [L*a*b*]