28 / 36
28
Extrusion International 1/2017
MEASUREMENT
the eccentricity of the products. Thereby, in addition to the
quality control andprocess optimization, the savingof plastic
material and cost reduction play an essential role. Conven-
tionally available technologies are, for example, based on
ultrasound technology. This method is suitable for the basic
measurement of the wall thickness of single layer products
but reaches its limits due to its function and dependence on
material properties, the plastics temperature and the cou-
plingmedium. A precisemeasurement of all product param-
eter without the dependence on environmental or material
influences is nowadays ensured by the X-ray technology.
Ultrasound technology
The ultrasound technology is only partly suitable for on-
line quality control of hoses and tubes. For example, the
ultrasound is not able to penetrate the aluminum layer
that is used as a vapor barrier in composite pipes and is
therefore not applicable for this application. Measuring
rubber hoses, the ultrasound signals are largely absorbed
by the porosity and absorption of the rubber so that they
are also not reliably measurable. Furthermore, multi-layer
rubber hoses contain typical fabric reinforcements, which
divert the ultrasound echo and make a measurement im-
possible. The ultrasound measurement is usually realized
in a water bath as the water is used as a coupling medium
for the transmission of the sound. A precise temperature
compensation is necessary as the propagation speed of the
ultrasound, which is used for the calculation of the eccen-
tricity, depends on temperature and material. This tech-
nology requires a calibration. Further, an estimation of the
wall thickness is typically only possible by combining the ul-
trasonic technique with an additional gravimetric system.
X-ray technology
The X-ray technology is based on an imaging principle.
The X-ray technology does not require an adaptation to
Pic. 4: X-ray measuring system installed
in a hose extrusion line
Pic. 5: The measuring system based on
millimeter waves technology for the
determination of the diameter, wall
thickness and the sagging based on the
time difference analysis of reflected signals