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59

www.read-wca.com

Wire & Cable ASIA – January/February 2015

Foreign objects, foreign pellets or other organic contamination

(Optical)

System for pellet inspection and sorting inside

Installation of the system for pellet inspection and sorting

Temperature

measurement

CCV line

VCV line

System for

inspecting

and sorting

It is for that reason that the Chinese Standard for high

voltage cables, for example, demands the exclusion of

contamination from 75

μ

m in the processed materials.

Accordingly, it is necessary to inspect the material to 100

per cent before it enters the end product. Sample tests are

a first step but not sufficient to exclude all contamination

reliably.

Today, cable manufacturers use screens to catch

impurities in the XLPE compound before they get into

the cable. The screens are positioned directly in the melt

flow after the extruder, before the crosshead. However,

these screens can get clogged by scorches or excessive

amounts of contaminants after a certain run time. Then the

melt pressure in the extruder may increase significantly.

Finally, the production has to be stopped in order to

change the screens, which in turn means that later a joint

is required at that position.

Joints where the cables are welded together are always

critical, in particular when talking about power cables for

offshore applications.

That is why cable manufacturers aim at delivering large

cable lengths with only a minimum of joints as they contain

a potential risk for breakdowns. This can be achieved

by using highly pure raw material. As the integration of

screens reduces the productivity of the line, an approach

is to omit the screens.

This requires, however, reliable alternative methods to

detect and sort out contamination in the XLPE pellets at

an early stage. Scorches can be avoided by temperature

supervision and control of the melt flow by an ultrasonic-

based temperature measurement.

Green technology solution

for clean material

The new technology from Sikora inspects the material for

purity to 100 per cent. The system combines X-ray tech-

nology with an optical system. This allows the detection of

metallic and organic impurities of 50

μ

m in the pellet itself

and on its surface.

The combination of these two technologies is unrivalled

on the market. The specially developed X-ray technology

is able to inspect transparent and coloured (eg black)

pellets as well as semi-conductive XLPE material for

impurities.

Contamination that is detected by the X-ray or optical

technologies is identified by image processing soft-

ware, characterised as contamination and automatically

separated.

The basic principle of the X-ray technology is the different

attenuation of the material. XLPE mainly comprises two

carbon and four hydrogen atoms. The carbon atom has six

protons in its core while hydrogen has only one. A typical

contamination would be steel particles from the extruder

or granulator, which is mainly iron (FE).

Iron has 26 protons in its core. Because these 26 protons

have a much higher X-ray attenuation than the six protons

from the carbon it is possible to generate a contrast

between the two materials in the X-ray image.

Regarding the optical inspection, the illumination plays an

essential role.

By using a special diffuse light construction technique, the

smallest contamination is detected, among others, foreign

objects, foreign pellets or organic impurities. In order to

allow an industrial throughput up to 2,000kg/h, modern

camera technologies are used. They allow precise images

of the pellets to detect contamination down to a size of

50

μ

m.

In order to avoid contamination from the outside, the

feeding of the pellets is carried out via a hermetically

vibrating ramp.

This at first avoids contamination that might occur by

conveyor belts, and secondly an over-pressure avoids

contamination from dust of the ambient air.

1 Transport system

2 X-ray inspection

3 Optical inspection

4 Sorting unit

5 Bad stream

6 Good stream