WCA January 2015

61 / 68

59
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 75μm 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 50μ 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
50μm.
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