TPT July 2016

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46 July 2016
T E CHNOLOG Y
Millimetre wave technology for measurement
during manufacturing of large plastic tubes
By Harald Sikora, director, Sikora Holding GmbH & Co KG
Introduction
During the manufacture of plastic
tubes with large diameters and wall
thicknesses, product quality and the
reduction of material costs have the
highest priority. Norms and standards
precisely define the minimum and
maximum diameter and wall thicknesses
of a tube dimension.
In addition, the determination of
sagging plays an important role. Due
to standards and growing demands
in tube extrusion, manufacturers use
measuring and control devices for
quality assurance in production lines.
Sikora, in cooperation with the
Fraunhofer Research Institute for
High-frequency Physics and Radar
Technology and the South German
Institute for Plastics, has developed a
new technology based on millimetre
wave technology for precise non-contact
online measurement of inner and outer
diameter, ovality, wall thicknesses and
sagging of large plastic tubes with a
diameter larger than 120mm.
The innovative concept of the measur-
ing system adapts the characteristics
of the extruded plastics and does not
require any calibration by the operator.
This new millimetre wave technology
allows for an increase of product quality
and ensures significant material and cost
savings during extrusion.
Dimension measurement of plastic
tubes during extrusion
Today, there are diverse technologies
used for quality assurance during the
productionof plastic tubes, suchas optical
methods, eg lasers for determination of
the diameter, or X-ray for the additional
measurement of the concentricity
and wall thicknesses. Conventional
technologies such as ultrasonic also
measure tube dimensions; however, they
often reach their functional limits.
An additional technology that is used
for quality control works with terahertz
impulse technique. This method uses
a fibre laser that generates terahertz
impulses directed at the measured
object. From the reflected echoes striking
the inner and outer boundary layers, the
wall thickness is determined.
Millimetre wave technology for the
measurement of large tubes
Precise measurement of large tubes
independent from environmental or
material influences can be assured by
the use of millimetre wave technology,
introduced in this article.
In recent years, enormous success
regarding measuring accuracy has been
achieved by researching metrological
applications with frequencies in the
millimetre wave range. Nevertheless,
the results could not yet be used for
the coating thickness measurement of
cylindrical products. The newly developed
millimetre wave technology creates the
prerequisite for reliable measurement
of the nominal size as well as the outer
diameter, ovality and wall thickness of all
kinds of extruded tubes.
Without any knowledge of the
properties of the extruded material and
its temperatures, the system measures
the outer contour as well as the wall
thicknesses simultaneously at several
positions of the circumference. Also,
individual layer thicknesses of multi-layer
tubes can be measured precisely. Thus,
the system represents a key technology
for future-orientated quality assurance in
the production of large tubes.
Measurement by millimetre wave
technology is based on the FMCW
runtime method. Several static or
rotating transceivers, arranged
around the circumference of a tube,
continuously send and receive
frequency modulated millimetre waves.
From the runtime difference, the
product dimensions are defined.
Boundary layers, as for example
each front and back site of a plastic
tube, reflect these radio waves. The
signals are detected and demodulated
by the receiver of the transceiver. The
signals contain information regarding
the distance between boundary layers
of different materials that means the
inner and outer diameter, ovality, wall
thicknesses and sagging.
After an algorithmic processing of
the received signals of each sensor,
the requested measuring results are
ready for visualisation and control of the
diverse tube dimensions in real time. A
connected processor system takes the
measured values and displays them
numerically and graphically. It also
includes comprehensive trending and
statistical information.
Millimetre wave technology for
optimisation of tube quality, as well
as time and cost savings
As product temperatures have no
influence on the measuring result when
using millimetre wave technology, the
system is installed for hot measurement
as well as at the cold end of the line
for final quality control. Directly after
the first cooling, the Centerwave 6000
provides precise information about
inner and outer diameter, ovality, wall
thickness and in particular sagging. The
millimetre wave technology selected
for the measurement covers the entire
range of plastics, such as PE, HDPE,
PP, PA6, etc, as well as PVC.
If we assume a line where tubes are
produced with an outer diameter of
400mm and a wall thickness of 27.5mm,
at a line speed of 0.5m/min, the machine
operator receives accurate measuring
results after around 10 to 30 minutes.
In contrast, the measurement of plastic
wall thicknesses with high temperatures
via ultrasonic technology represents
System for measuring diameter, ovality,
thicknesses and sagging of large tubes