Analysis of the Water Assisted Injection Technique by Using Ultrasonic

Transcription

1 Analysis of the Water Assisted Injection Technique by Using Ultrasonic Analysis of the Water Assisted Injection Technique by Using Ultrasonic Sebastian Becker and Ch. Hopmann IKV, Aachen, Germany Introduction Water-assisted injection technique (WAIT) is well established for the production of plastics parts with functional hollow sections. This technique is widely used to manufacture handholds and pipes for transportation of fluids in particular. The process sequence of WAIT is characterised by the injection of water into the mold part after the filling stage. Thus, the still liquid melt is displaced and a cavity is formed in the interior of the part. Since WAIT is primarily used for the production of technical parts, a comprehensive quality control and process monitoring is essential [2, 3]. The moulding process is much more complex compared to the injection moulding of compact parts. Furthermore, internal properties (e.g. the residual wall thickness, eccentricity) or internal part defects (e.g. water inclusions) play a decisive role in the compliance with quality requirements. A promising approach in this context is the utilisation of the ultrasonic measurement technology for the analysis of the hollow space formation and for the identification of internal part properties. Basics of Ultrasonic Analysis The science dealing with the study of sound is called acoustics. Acoustics enable the description of mechanical vibrations and their propagation in solid, liquid or gaseous substances. A subdomain of the acoustics is the ultrasound physics which deals with sound with a frequency range from 20 khz to 1 GHz [4]. becker_s@ikv.rwth-aachen.de Footnote: This paper was presented at the IKV Colloquium Smithers Rapra Technology, 2013 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1,

2 Sebastian Becker and Ch. Hopmann Figure 1. Propagation of longitudinal and transversal sound waves The model of an elastic body is shown in Figure 1. In this model, the body consists of individual mass particles that are bound to their positions by elastic forces. The propagation of sound in a medium occurs by external stimuli to these particles. Hereby the particles are deflected and swing around their rest position. Due to the mechanical coupling the adjacent particles are also stimulated to oscillate. This results in an advancing movement in the medium, manifested as a wave-form. When describing a wave, one can distinguish between longitudinal and transversal waves (Figure 1). Longitudinal waves are characterised by the transmission of normal stress. Thus, the direction of propagation of the sound wave follows the direction of oscillation of the atoms. On the other hand, transversal waves propagate by a transfer of shear stress. Therefore, the direction of propagation of the sound wave is normal to the plane of oscillation of the particles [3]. However, shear stress cannot be transferred in polymer melts, so the analysis of transversal waves is not an option. For this reason, only longitudinal waves are used in the following series of experiments. The propagation of acoustic waves in a medium is strongly influenced by the properties of the material and the present boundary conditions. In particular, acoustic properties of viscoelastic materials such as the sound velocity or the acoustic damping coefficient are strongly dependent on the pressure, temperature and the type of the medium. Figure 2 shows the longitudinal speed of sound as a function of temperature and pressure using the example of a polypropylene. 56 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1, 2013

3 Analysis of the Water Assisted Injection Technique by Using Ultrasonic Figure 2. Sound velocity as a function of pressure and temperature It can be seen that an increase of temperature causes a reduction of the sound propagation velocity. In addition to this, an increase of pressure results in an increase of the sound propagation velocity. Since polypropylene is a semicrystalline thermoplastic material crystallization can be detected in the range between 120 C and 140 C. This crystallization causes a rapid decrease of the sound propagation velocity [1]. Due to the injection of water new surfaces are formed in the moulded part which can be detected by means of ultrasonic analysis. The so-called pulse methods which can be divided into pulse-echo and pulse-transmission are suited for the ultrasonic analysis. In contrast to continuous methods, pulse methods use acoustic pulses that are sent out from a piezoelectric ultrasonic transducer in discrete time intervals into the part to be analysed. In the pulse-echo method which is used e.g. for the detection of cracks in metallic components, the sound wave strikes an acoustic boundary surface (crack), where it is partly reflected and then detected by the same ultrasonic transducer it was generated by. In the pulse-transmission method on the other hand, the part is passed through by the sound pulse which can be detected by a second ultrasonic transducer on the opposing side of the ultrasonic transducer. In the following research studies the water-assisted injection technique is to be investigated by means of ultrasonic measurement. Both methods, the pulse-echo and the pulse-transmission, are used for the measurements. In this context, the moulding process and the correlation of the ultrasonic measurements as well as the residual wall thicknesses are examined in particular. Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1,

4 Sebastian Becker and Ch. Hopmann Description of the Experimental Setup The trials were performed using an injection moulding machine HM 1600/100 Unilolog B4 made by Wittmann Battenfeld GmbH, Kottingbrunn, Austria. The fully hydraulic machine has a clamping force of 1600 kn and a screw diameter of 55 mm with a resulting maximum shot volume of 594 cm³. The water injection is carried out by a WAIT-system of the Power Module 15/210-2 PME type by fluidtec GmbH, Kappel-Grafenhausen, Germany. With this unit a volumetric flow rate of up to 30 l/min and a holding pressure of 210 bar can be achieved. During the moulding cycles the water injection is managed by a volume flow control. The WAIT-unit also provides the possibility to operate up to eight hydraulic core pullers used for the movements of the injector, the needle valve nozzle and the overflow barriers. Cavity Insert Pipe For the ultrasonic analysis of the WAIT-process a modular mould is used. The modular design offers the possibility to vary the geometry of the moulded parts to be examined by using different cavity inserts. In the following discussion a cavity insert shall be used which allows the manufacturing of a tubular geometry. Here, the outer diameter of this tubular part is 30 mm which is suitable for the use of gas-assisted injection molding (GAIM) and WAIT. The ultrasonic analysis of this geometry is intended to provide information on the residual wall thickness. In addition, the positioning of multiple ultrasonic transducers along the flow path allows the determination of the integral melt and fluid velocities online. The cavity insert contains several curves in addition to straight sections. These redirections allow the investigation of their influence on the residual wall thickness by means of ultrasonic measurements. Furthermore, the insert is connected to an overspill cavity in the injection mould that collects the displaced melt during the fluid injection phase (Figure 3). This overspill cavity can be closed by shut-off slide barriers, allowing the manufacturing of moulded parts both with the short shot and with the full shot method. In the short shot method, the cavity is initially partially filled with the injected thermoplastic material. The final shape is achieved when material is blown-up by water injection. In the full shot method, however, the cavity is initially completely filled with plastic melt. Prior to the subsequent water injection the shut-off slide is retracted, opening the inlet to the secondary cavity, where the displaced material from the core of the part is fitted in. The cavity insert allows both the use of the pulse-transmission method and the pulse-echo method. 58 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1, 2013

5 Analysis of the Water Assisted Injection Technique by Using Ultrasonic Figure 3. Assembly of the cavity insert and the overspill cavity Measurement Instrumentation The analysis of the WAIT-process is carried out using a data acquisition system assembled at the IKV which enables the analysis of the highly dynamic injection moulding process by a synchronous recording of all relevant data such as pressure, temperature or the ultrasonic signals [4]. The main components of this assembly are: ultrasonic sensors of the DS6 type by Karl Deutsch GmbH, Wuppertal, Germany pulse amplifiers of the 5800 PR type by Olympus GmbH, Mainz, Germany pt-sensors of the 6190CA0,8 type by Kistler Instrumente GmbH, Ostfildern, Switzerland industrial PC based on x86 architecture high speed analogue-digital converter card of the PCI 5122 type by National Instruments GmbH, München, Germany signal conditioning unit analogue-digital converter card of the PCI 6251 type by National Instruments GmbH, Munich, Germany The pulse amplifier enables the connection of a single ultrasonic transducer for pulse-echo mode or a pair of ultrasonic transducers for pulse-transmission mode. Further features of the pulse amplifier are a bandwidth of 1 khz to 35 MHz, analogue high-pass and low-pass filters, maximum pulse energy of 100 µj and a quartz stabilised pulse generator unit with pulse repetition rates between 80 Hz and 10 khz. The output of the pulse amplifier which corresponds to the amplified signals received by the receiver transducer Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1,

6 Sebastian Becker and Ch. Hopmann is routed to the high speed A/D converter card. Furthermore, this device is extended by an additional A/D converter card of the PCI 6251 type which allows the sampling of 16 channels. This A/D converter card is connected to the signal conditioning unit which allows the acquisition of 2 temperature signals (module for the thermocouple element of the K type), 2 pressure signals (module for piezoelectric pressure sensors) and 12 voltage signals ranging from -10 V to 10 V. These are used to acquire additional relevant signals from the machine, e.g. the fluid volume flow or the injection velocity which can be examined in the analysis later on. Analysis of the WAIT- Process Using Ultrasound Prior to the actual ultrasonic measurements, it is necessary to determine the optimal machine parameters and process variables to produce high quality parts. In addition to the information from the data sheets provided by the suppliers of the raw materials and the information derived from the developing of cavity pressure, the visual assessment of the moulded parts plays a particularly important role. The parts to be analyzed are produced using the full shot method. The manufacturing conditions used to manufacture optimal parts are summarized in Table 1. Table 1. Manufacturing conditions Production parameters Melt temperature 230 C Mould temperature 40 C Injection volume flow 270 cm 3 /s Water flow rate (WAIT) 150 cm 3 /s Water pressure (WAIT) 150 bar Fluid delay time (WAIT) 25 s Because of its easy processing a non-reinforced polypropylene (PP) type 505P produced by Sabic Germany GmbH & Co.KG, Düsseldorf is used in the following experiments. Analysis of the Process Course In the following injection moulding trials, the acquisition of the time of flight is used to describe and to interpret the moulding process of parts manufactured using WAIT technique. Here, the pulse-transmission method is used exclusively. 60 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1, 2013

7 Analysis of the Water Assisted Injection Technique by Using Ultrasonic The result of such an ultrasonic measurement is shown in Figure 4 and can be described as follows: Figure 4. Analysis of the process course by using ultrasound The data acquisition starts with the beginning of the machine signal close mould (phase 0). The actual injection of plastic melt begins at t = 5 s. At this moment there is still no plastics material between the ultrasonic sensors. Thus, ultrasonic pulses cannot be transferred yet due to the presence of an air gap. At t = 7 s the melt front reaches the position of the ultrasonic transducer so that ultrasonic pulses can be conducted for the first time. During the volumetric filling of the cavity (phase I) the cavity pressure increases at the measuring point. This leads to an increase in the sound velocity of the melt, thus reducing the time of flight. After the volumetric filling the packing phase (phase II) follows which is characterised by a strong increase in the cavity pressure. Furthermore, the moulded part starts to cool down, thus rapidly reducing the time of flight. After the packing phase and a brief holding pressure phase, the delay time starts (phase III). During this phase, no more new material is fed into the cavity so that the moulded part continues to cool down and the cavity pressure decreases gradually due to the volumetric shrinkage of the thermoplastic. At this point the mould is still containing a compact part with a wall thickness of 30 mm. In combination with the poor thermal conduction of the polypropylene, the heat condition in the component occurs rather slowly. As a result, the change in cavity pressure has more influence on the resulting sound velocity than the temperature within the cavity. Hence, the pressure drop results in a reduction in the sound velocity and thus in an increase in the time of flight of the ultrasonic wave motions between the two ultrasonic transducers. At the end of the delay time the water injection phase (phase IV) starts. At the beginning of this phase the inlet to the secondary cavity is opened by moving a shut-off slide. This leads to an expansion of Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1,

8 Sebastian Becker and Ch. Hopmann the pressurised plastic melt into the secondary cavity, quickly reducing the cavity pressure and thus increasing the time of flight at t = 20 s. The following run of the curve (phase V) shows that the time of flight decreases instantly at t = 22 s. This occurs because the water blister passes the measuring position of the ultrasonic transducers. The sound velocity is higher in water than in plastics, so the ultrasonic oscillations in the core area of the WAIT-parts are conducted faster than in compact ones. At t = 23.5 s the volumetric filling of the secondary cavity is completed. In the following water pressure holding phase (phase VI) the regulation of the WAIT unit adjusts the water volume flow so that a constant water pressure is maintained in the cavity. In this time period the part continues to cool down so that the resulting increase in the speed of sound causes a further reduction in the time of flight. At t = 30 s the water injection is stopped and the water is being pumped down. From now on a conduction of ultrasonic pulses is not possible anymore due to the formation of an air gap in the core of the moulded part so that the signal is interrupted. Variation of the Manufacturing Parameters In the following the influence of particular manufacturing parameters on the molding process in water-assisted injection technique is to be analysed by means of ultrasonic analysis. In these experiments the parameters melt temperature and water volume flow rate are varied due to their strong influence on the properties of the moulded parts. First, the influence of melt temperature on the results of the ultrasonic measurements will be examined. Here, both the water volume flow rate and the fluid injection delay time will be kept at constant values. The results for melt temperatures of 210 C and 250 C are shown in Figure 5. Obviously the time of flight tends to be higher when the melt temperature is high as well. This can be explained by the severe reduction in the sound velocity in polypropylene due to an increase in melt temperature. Furthermore another effect of an increase in melt temperature is the higher melt compression in the packing phase. This can be seen in the results of the ultrasonic measurements at t = 8.5 s. Here, at the beginning of the packing phase the time of flight in the high temperature melt decreases faster when compared to the low temperature melt. This indicates a higher pressure increase during compression and can be seen in the curve of the cavity pressure (Figure 6). The delay time ends at t = 25 s for both melt temperatures. The moment the inlet to the overspill cavity is opened, the melt from the main cavity can expand therein. The results of the ultrasonic measurements show that the time of flight increases instantly at this point. 62 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1, 2013

9 Analysis of the Water Assisted Injection Technique by Using Ultrasonic Figure 5. Influence of the melt temperature on the time of flight Figure 6. Comparison of different cavity pressures In addition to the melt temperature, the volume flow rate during the water injection plays a major role in the moulding process as well. Hence, the influence of the water volume flow rate on the results of an ultrasonic analysis shall be examined. Therefore, the melt temperature and the delay time of the fluid injection will be maintained at constant values. The results of such an ultrasonic analysis for volume flows of 100 cm³/s and 200 cm³/s are shown in Figure 7. First of all, the chart reveals that the individual curves of the times of flight do not differ from each other until the end of the fluid injection delay time. It is not before the water injection that the curves start to deviate. The results show that a higher volume flow rate leads to a faster water expansion within the moulded part. Thus, the injected water is detected earlier at the sensors position. Furthermore, the higher volume flow rate leads to an increase of Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1,

10 Sebastian Becker and Ch. Hopmann Figure 7. Influence of the water volume rate on the time of flight the cavity pressure, resulting in rather short times of flight in the time period between 15 and 18 s. Correlation Between Ultrasonic Measurements and the Residual Wall Thickness Because of the fact that the mechanical properties of parts made by WAIT depend on the internal part properties, an important quality characteristic is the residual wall thickness. It is not possible to determine the wall thickness online with commercially available state of the art sensors. In this context, the ultrasonic measurement technique constitutes an opportunity to determine these inner properties of the moulded part. Therefore, the theoretical calculation of distances by means of the acquisition of the times of flight shall be illustrated: The time t an ultrasonic pulse needs to cover a distance l depends both on the distance and on the sound velocity c in the medium (Equation (1)). t = l c (Eq. 1) Hence, the distance covered by a pulse can be calculated, provided the speed of sound in the medium is given. However, the temperature profile that is formed along the cross section of the moulded part in the course of the process cannot be detected by the temperature sensors. This fact has to be taken into account when analysing the WAIT-process. Thus, another approach to determine the residual wall thickness shall be used in the following 64 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1, 2013

11 Analysis of the Water Assisted Injection Technique by Using Ultrasonic examinations. Therefore, the tubular parts will be manufactured using the full shot technique and analysed by means of the pulse-echo method. Afterwards the residual wall thicknesses will be measured offline using a magnetic-inductive thickness gauge of the Magna-Mike 8000 type by Panametrics GmbH, Hofheim/ Taunus, Germany. To minimise the influence of the temperature-dependent speed of sound on the time of flight, it is indicated to take the measurements at a time when the temperature profile has been levelled due to temperature equalisation processes. Thus, only times of flight in a brief time period before the end of the fluid holding phase are considered in the interpretation of the analysis. A comparison of the measured times of flight and the residual wall thicknesses is depicted in Figure 8, where 5 different specimens of the same manufacturing conditions have been analysed. The comparison shows a qualitative match between the results of the ultrasonic analysis and the residual wall thickness. In particular, the reduced resulting wall thickness of specimen 4 is reflected in the time of flight of the corresponding ultrasonic signal. Therefore, these times of flight at the end of the process can also be used to determine the influence of the manufacturing conditions on the residual wall thickness. Figure 9 shows the dependence of the time of flight on the water volume flow rate employed using the pulsetransmissions method. The comparison shows that an increase in the volume flow reduces the times of flight. This effect occurs because a higher volume flow results in a larger displacement of material into the secondary cavity than a lower one. The result is a decline in the residual wall thickness and thus a reduction in the time of flight. Figure 8. Correlation of the time of flight with the residual wall thickness Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1,

12 Sebastian Becker and Ch. Hopmann Figure 9. Influence of the water volume flow rate on the time of flight at the process ending Conclusions and outlook The studies have shown that the water-assisted injection technique can be described by means of ultrasonic analysis. The results allow a division of the whole process into separate process phases. Process points like e.g. the volumetric filling, the packing phase or the opening of the secondary cavity inlet can be monitored online. Unlike the conventional process analysis using sensors like e.g. pressure or temperature sensors, the ultrasonic analysis allows the acquisition of additional information on the residual parameters of the moulded part. For instance, this method allows the detection of the arrival of water blister at the position of the ultrasonic transducers. Thus, qualitative statements on the spread velocity of the fluid during the injection can be made. Furthermore the use of ultrasonic measurements allows an online determination of internal part properties such as the residual wall thickness. The examination of the process by means of the pulse-transmission method shows the correlation between the times of flight recorded and the measured residual wall thicknesses. The acoustic properties of the whole component part are influenced by production parameters like e.g. the melt temperature or the fluid volume flow rate. Hence, deviations and variations of the individual production parameters can be detected by means of the ultrasonic technique. However, the fact that the acoustic properties of thermoplastics are temperature- and pressure-dependent must be taken into account. Therefore, it is necessary to determine these acoustic properties in order to evaluate the part properties quantitatively. An ultrasonic measuring cell is manufactured in order to measure the speed of sound and the acoustic damping under stationary conditions. These results shall be used to quantitatively determine 66 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1, 2013

13 Analysis of the Water Assisted Injection Technique by Using Ultrasonic the residual wall thickness online in the coming series of experiments by means of the pulse-echo method. Afterwards, the new insights will be transferred to the analysis of gas-assisted injection moulding technology. Acknowledgment The research project N of the Forschungsvereinigung Kunststoffverarbeitung was sponsored as part of the industrielle Gemeinschaftsforschung und -entwicklung (IGF) by the German Bundesministerium für Wirtschaft und Technologie (BMWi) due to an enactment of the German Bundestag through the AiF. We would like to extend our thanks to all organizations mentioned. Furthermore, our thanks go to all companies that have supported this work by the provision of plastics, machines and other resources. References 1. Hopmann Ch. and Becker S., Analysis of the Fluid Assisted Injection Technique by Using Ultrasonic Measurements. Proceeding of the Annual Technical Conference (ANTEC) of the Society of Plastics Engineers. Orlando (FL), USA, , Michaeli W., Grönlund O. and Gründler M., WIT unter Kontrolle. Kunststoff-Berater, 54 (2008) 1-2, pp Michaeli W., Qualitätssicherung bei der Herstellung von Kunststoffmedienleitungen mittels der innovativen Wasserinjektionstechnik. Institute for Plastics Processing, RWTH Aachen, final report of the BMBF research project No. 01 RI RI 05200, (2010). 4. Krautkrämer J. and Krautkrämer H., Werkstoffprüfung mit Ultraschall. Berlin Heidelberg New York: Springer Publishing House, Lingk O., Einsatz von Ultraschall zur Prozessanalyse beim Spritzgießen von Thermoplasten. RWTH Aachen, PhD thesis, 2010 ISBN: Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1,

14 Sebastian Becker and Ch. Hopmann 68 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 1, 2013