The inline determination of process and product parameters is of great help for the evaluation and optimization of new procedures. Therefore, an ultrasound process tomography system has been developed, which enables the imaging of the local filler distribution in plastic melts.
The objects investigated were extruded rods made of polypropylene (PP) with radial filler gradients. During extrusion, sound velocity and attenuation of the plastic melt were determined and processed via a modified reconstruction algorithm according to Radon transform into 2-D sectional images. Despite the challenges of higher attenuation and impedance mismatch of 60 mm filled PP melt compared to water, the resulting images are of good quality. An important factor for the image quality after tomographic reconstruction is the opening angle of the used ultrasound transducers. Furthermore, a simulation environment was developed in Matlab, which serves as a testing platform for the measurement system.
Ultrasound (US) waves are mechanical, acoustical – both shear as well as
compressive – waves in the frequency range from 20 kHz up to several GHz,
depending on the elastic and dissipative properties of the medium. There are
two basic properties which are used for measurements. First, the speed of
sound
In transmission arrangement (i.e., transmitter and receiver are arranged opposite to each other) the pulse is attenuated when passing through the melt and the propagation time depends on the polymer. This allows conclusions about the viscous and elastic properties of the plastic melt. Especially signals with frequencies between 100 kHz and 10 MHz can also propagate effectively in highly filled compounds (Alig et al., 2000) – but only as long as the size of the filler particles (scatterers) is much smaller than the wavelength. Previous studies have shown that the volume fraction of different filler materials in different base polymers can be determined by variations in sound velocity (Alig et al., 2000, 2005). However, this depends on pressure and temperature. In the same manner, the distribution of particles also has an influence on the attenuation of the US signal. Therefore, the filler size distribution and agglomerate formation can be determined as well (Alig et al., 2006; Schober et al., 2014; Wöckel et al., 2016).
Since piezoelectric materials are temperature sensitive they need to be
protected from high temperatures. For this reason testing in hot media needs
specially designed US test probes protected by appropriate measures (Putz et
al., 2014). High temperature environments require delay lines which allow a
thermal decoupling between the temperature-sensitive US transducers with
their adhesive joints and the hot melt. Such delay lines are made of
materials which possess good sound transmission but low heat conductivity
properties, such as glass, ceramics, special steel alloys, or
high-performance plastics (Deutsch et al., 1997). When sound waves propagate
through interfaces between different materials the reflection coefficient
When extruding semi-finished plastic products, the subsequent component properties are intentionally modified through incorporating additives, for example, to fulfill special requirements on mechanical properties and resistance against light or chemicals. In this case, the functional additives are typically homogeneously distributed across the entire extruded profile. An intentional distribution of aggregates in the component can either be accomplished through co-extrusion in the form of sharply defined layer systems with concentration jumps on the boundary layers (Zhu et al., 2006) or through discontinuous procedures (Wu et al., 2004; Sun et al., 2008; Krumova et al., 2001). A continuous procedure for the extrusion of rods made of polypropylene (PP) with radial gradients of fillers is currently being developed (Hirt et al., 2015). As an optimal adjustment of the gradient (no local displacements, no agglomeration of the additives) is decisive for the component properties of the final semi-finished product and allows unambiguous conclusions about the quality of the production process, the continuous monitoring of the extrusion process is of advantage. Typically, the examination is offline and takes place in the lab after production either visually by sample or microscope or with the use of X-ray computed tomography (CT). However, offline methods entail a long dead time in the area of hours between sample-taking (including cooling of the rod) and the presence of a measurement result, which makes the process development and optimization very time-consuming. Furthermore, very small differences in density (e.g., as in the case of polymer blends) cannot be resolved by X-ray CT.
For an inline determination and real-time process control an USPT system has been developed for an inline imaging of the melt during the extrusion of rods. In contrast to X-ray CT, US requires no safeguards and can be integrated into the process directly.
Comparable approaches already exist in the medical sector for mammographies (Schwarzenberg, 2009), in the plastic sector, for example, to dissolve the temperature distribution in a melt channel (Praher et al., 2014; Hopmann et al., 2016), or for non-destructive component testing (Haach and Ramirez, 2016). An US tomography-based application for spatially resolved determination of the filler distribution in plastic melts has not been reported so far.
The USPT system was designed to the geometry of rods and consists of a sensor
ring with an inner diameter of 60 mm, allowing an easy adoption on the melt
channel right before the exit of the shaping nozzle (see Fig. 1). The used
basic polymer was Sabic 505P, a PP with a density of 905 kg m
View on the delay lines of the US transducers inside of the USPT adapter (top) and USPT adapter (wrapped in yellow polyimide tape) mounted between gradient tool and shaping nozzle of the used extrusion line (bottom).
The speed of sound in media depends on temperature. If the material
parameters are known, the speed of sound can then be derived from (Hochrein
and Alig, 2011)
Estimation of the reflection coefficients
The attenuation coefficient of sound in water can be calculated using the
k-Wave toolbox (Treeby and Cox, 2010) in Matlab. For a temperature of
22
Due to the higher impedance mismatch and higher signal attenuation of the plastic melt, high transmission powers of the US transducers are required to pass through a diameter of 60 mm. For a good tomography imaging of the melt channel, each location point has to be scanned from as many directions as possible (Kak and Slaney, 2001). This can be accomplished either with a large number of transducers or with a wide radiation angle per transducer. With the given geometry, a large number of transducers would be synonymous with smaller apertures and therefore also larger opening angles. However, the maximum sound power per transducer decreases with its aperture due to the smaller contact surface to the melt.
The USPT was equipped with 40 equiangular transducers with plain apertures of
2 mm
In contrast to typical tomographic methods, the projection angle is not varied by a mechanical rotation of transmitter and detector but electronically. The transducers in the different angular positions are successively set to transmitting mode while the other 39 transducers are in receiving mode. If the attenuation of the investigated medium (e.g., plastic melt) is high, up to four transducers can be bundled and used as a transmitter array. Through phase-delayed controlling of several transducers a focusing effect can be achieved and even a swivel of the sound field is possible.
The US transducers were driven with a rectangular windowed sinusoidal signal
with a center frequency of
System electronics of the USPT system (Inoson PCM 12343).
All delay lines for all US transducers in the system were produced manually, so geometric differences may occur. The length of the delay line in combination with its temperature dependence directly affects the measured sound velocity. In addition, the US transducers can exhibit different transmitting/receiving properties depending on the adherence of the piezo crystals to the delay line. Taking these factors into consideration, the system has to be calibrated accordingly. Distilled water was used as calibration medium since the speed of sound and the attenuation are well-known for defined temperatures.
For an absolute determination of the speed of sound within the USPT, the propagation time offset caused by the delay line of each transmitter and receiver pair (TRP) is necessary. With the measured distances between each TRP this time offset can be calculated for a given temperature. When using the system at higher temperatures it has to be adapted.
The calibration of the sound attenuation for this system is a bit tricky,
since the transducers cannot directly be compared to one another by an
opposing measurement setup. Therefore, a custom-made omnidirectional US
transducer made by Inoson GmbH (see Fig. 3) was used in the center of the
USPT system. The transducer has a diameter of 10.3 mm and was connected to a
channel of the PCM. In transmission mode the transducer was driven with
2.12 MHz and a transmit voltage of 47
Custom-made omnidirectional US transducer made by Inoson GmbH.
When using fillers, scattering effects at the particles has to be considered.
Here the wavelength
To receive a sectional image from the individual projections, a classic back projection algorithm for a non-diffracting case according to Radon (Kak and Slaney, 2001) was adopted.
Only direct paths between transducers are considered, and the line integral
for each TRP was calculated by
With the inverse Fourier transform,
Beside the measurement control and analyzation software, an additional simulation environment was developed in Matlab. Here, images, in which the different sound velocities and attenuation factors are depicted by different pixel values, are converted into matrices. The pixel values are integrated along the respective pixel line between a TRP. For the simulation the opening angle can be varied, which determines the number of opposing receivers used for reconstruction. By this, defective receivers can be simulated. In order to generate virtual data for the empty areas between the individual (and for defective) transducers the values were linearly interpolated. The reconstruction was done by creating 2-D sectional images with lines between TRPs of the integrated pixel value and subsequently summing them up. Afterwards a weighting function was applied, which took into account how often lines passed through a certain pixel.
The simulation enables the verification of real results, and the simulation algorithm can be improved on the basis of real measurement data. Thus, the suitability of the used USPT system can be determined beforehand for specific conditions (defective transducers, opening angle) and material systems. Moreover, the use of different corrective features, such as interpolation or filtering of the sinogram in the Fourier space (filtered back projections), can be examined.
The measuring procedure with real media is performed analogous to the
simulation. Therefore, the same algorithm with some extensions concerning
signal acquisition and data pre-processing is used. The measurement data are
acquired with the PCM. The recorded A-scan signals (see Fig. 4) are bandpass
filtered (finite impulse response) at the center frequency
Bandpass-filtered A-scan of an US signal (53
While only one transducer emits, every other transducer receives a signal.
This allows the use of a high number of receivers for the reconstruction,
resulting in a bigger opening angle. Initially, it has to be checked which
receivers can be used for transmission. This can be done by comparing the
maximum normalized signal amplitudes for every receiver while one transmitter
is working. Figure 5 shows the comparison of the maximum normalized
amplitudes of the receivers when transmitter 1 emitted for water and
PP at different settings. Receiver 21 opposes transmitter 1. Based
on several experiments, an opening angle of 60
Comparison of maximum normalized signal amplitude for every receiver
when transducer 1 emitted for water (53
The propagation time together with the known distance of the TRP allows the calculation of the sound velocity along that particular path. The intensity of the pulse train is used to determine the attenuation. However, a measured reference pulse train is required to do so. This can either be a back wall echo (see Fig. 4) or a transmission pulse from a reference measurement. Optionally, by interpolation between measured values of adjacent receivers, virtual values can be generated for areas without US transducers or for defective transducers. Subsequently, the sinogram is created and a sectional image can be reconstructed depicting the distribution of the sound velocity or the attenuation within the USPT system.
The effect of different opening angles of the transducers as well as the use of an interpolation on the reconstruction is exemplarily demonstrated on the basis of simulation images of agglomerates in Fig. 6 and a gradient of filler concentration offset from the symmetry axis in Fig. 7. The “original” pictures were created as grayscale images with maximum pixel values (255) for the agglomerates and the upper value of the gradients (the lowest was 0). The algorithm itself uses another color mapping. It can be perceived that the tradeoff made during the setup of the measurement system between number and aperture of the transducers leads to tomographic artifacts, such as ring artifacts, shadows, or blurring. These effects are removed through appropriate filtering and adjustments, improving the quality of the image.
Simulation of a reconstruction of spatially distributed agglomerates for different opening angle and optional interpolation (int).
The imaging resolution is limited by the width of the US transducers to 2 mm. Due to the limited opening angles of the transducers the effective area for a meaningful reconstruction is restricted. The larger the opening angle the larger the area with good reconstruction results. Also the use of interpolation seemed helpful. Both effects are also demonstrated in Figs. 6 and 7.
Simulation of a reconstruction of a de-centered gradient for different opening angle and optional interpolation (int).
The time for a single measurement with the typical settings mentioned in
Sect. 2.1 is 0.2 s per transducer. For a full tomographic measurement using
all 40 transducers, the whole measurement time sums up to 8 s. Within the
extrusion process, the polymer melt continuously flows during the measurement
with a melt flow velocity of
Nevertheless, homogeneity in the flow direction is essential to avoid motion
artifacts in the reconstruction. As expected, measurements on PP melts showed
more signal loss (impedance mismatch, attenuation) as well as more noise than
in preliminary tests with water. As can be seen in Fig. 5, a higher maximum
transmit voltage and a higher gain setting is necessary to achieve comparable
signal amplitudes in PP to those in water. In this case, the pulse detection
is distinctly more challenging. It is possible to penetrate 60 mm of filled
PP melt, and a transmission pulse can still be detected clearly. But its
length cannot be determined easily anymore because of noise and clutter.
Accordingly, the first back wall echo is no longer detectable. Figure 8 shows
an example of an A-scan of two opposing transducers measuring a PP melt
filled with TiO
Bandpass-filtered A-scan of an US signal (102
Moreover, the fact that only signals up to an opening angle of 60
Graded filled rods (PP
Reconstruction of the experimentally determined sound velocity of
pure (top) as well as filled PP melts with radial filler gradient. Due to
the opening angle of 60
Unfortunately, the gradient cannot (yet) be determined as accurately as it actually can be created during the manufacturing process. Furthermore, an inhomogeneous temperature distribution cannot be excluded, even though the examination of pure PP melt exhibits a certain degree of homogeneity. The reconstruction of the attenuation also proves pretty difficult in the case of PP because the pulse trains cannot be defined clearly and scattering effects have a strong influence as well (see Fig. 8). Moreover, diffraction effects may occur when filler agglomerates are present in the direct path between a TRP.
We have shown that the examination of the local filler distribution is
possible with the USPT system. The geometry of the examined graded rods
required a tradeoff between the number of transducers, transducer size,
opening angle as well as achievable transmission power. As a result, the USPT
system used here merely provides adequate image quality of the middle part in
the reconstruction due to the limitation of the opening angle of maximum
60
Filled PP melts could be penetrated with only one US transducer despite the 60 mm melt channel. However, due to impedance mismatch and increased signal attenuation, enhanced signal amplification is required, which, however, also amplifies the noise. Therefore, a larger number of measurement repetitions are necessary to receive an adequate SNR.
The artifacts (rings, shadows) caused by the limited opening angles can only be removed partially through filtration and interpolation. Alternatively, an iterative reconstruction procedure could be used, which, however, requires more computing power. Advanced computing methods like algorithm parallelization and the use of multicore processors or graphic processing units can overcome this obstacle and, furthermore, even speed up the entire measurement.
The use of matched filter detection was discarded due to the 1600 TRP, which would have to be treated individually because of different manufacturing and different angles to each other. For the further development it surely offers the possibility to enhance the pulse detection. The creation of values between the receivers by linear interpolation proved to be a worthy tool, but it nevertheless needs to be verified in a specific setup with a dummy model. The evaluation of a reconstruction uncertainty is rather difficult at the moment. This is also a topic for upcoming experiments. Wave characteristics, diffraction and scattering will be taken into account for the next generation of the algorithm. Also an imaging for attenuation is planned. It also has to be investigated whether the filler concentration can be locally resolved. Finally, a reference measurement method needs to be developed which helps to validate the USPT system by being able to measure at the same system (or dummies) with a well-defined concentration distribution.
In this paper we have shown that it is principally possible to locally detect the spatially resolved distribution of fillers in plastic melts using an USPT system. Considering all influences either by factorization or corrections within the reconstruction algorithm, it is possible to depict the filler distribution in addition to the local temperature distribution. Composing the individual sectional images a full 3-D image of the rod can be generated subsequently. Thus, US tomography has great potential to open up further applications for inline process control of plastic melts. But there is still a well-filled agenda of things to do in further research work.
The supplement contains the original image files used for the simulation as well as the raw data (Matlab files) of the measurements on water, PP melt and filled PP melt, which were used for this work.
The research project VIII/3-3852/45/5 was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology. We would like to thank them for the financial support. Edited by: J. Czarske Reviewed by: four anonymous referees