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Ultrasonic investigations of the thermoplastics injection moulding process
Polymer Testing 24 (2005) 205–209 www.elsevier.com/locate/polytest
Test Methods
Ultrasonic investigations of the thermoplastics injection moulding process W. Michaeli, C. Starke* Institut fu¨r Kunststoffverarbeitung (IKV), Aachen, Germany Received 8 July 2004; accepted 19 August 2004
Abstract The ultrasonic measuring technique constitutes an interesting possibility for process analysts to gain a deeper understanding of a particular process. During initial tests for thermoplastic compact injection moulding it was found that the ultrasonic signal can sometimes supply information for much longer than, for example, pressure signals. It was also found that a relationship exists between the signal and the shrinkage process of the moulded part. The point when the signal ceases coincides with the time the moulded parts become detached from the cavity wall during the cooling phase. The measuring method reflects all variations in the process conditions very sensitively indeed. q 2004 Elsevier Ltd. All rights reserved. Keywords : Ultrasonic investigation; Injection moulding; Thermoplastic material; Process characterization; Longitudinal waves; Ultrasonic runtime; Relative amplitude
1. Present use of ultrasonic techniques in the polymer processing The formal connection between mechanical and acoustic properties of plastics opens a broad and attractive field of applications for the ultrasonic measuring technique, before, during and after the processing. Elastic waves in distortable media are called acoustic waves. The most important wave forms are the longitudinal and the transverse waves, whereby the latter are exclusively transmitted in solids, due to the lack of propagating shear forces in liquids. The different propagation mechanisms of both wave forms also result in different propagation speeds, depending on the respective material properties [1]. Additionally, the relationship from longitudinal to transverse speed exclusively depends on Poisson’s ratio. Therefore, it is possible to calculate the modulus of elasticity, the shear * Corresponding author. Tel.: C49 241 809 3832; fax: C49 241 809 2262. E-mail address: [email protected] (C. Starke). 0142-9418/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.08.009
modulus and the Poisson’s ratio of a medium depending on temperature, pressure and cooling speed just with the help of the sound speeds. The acoustic measurement of elastic properties of solids is state of the art. For example, the ultrasonic reflection procedure provides the real and imaginary portions of the elasticity and shear moduli including the transverse contraction ratio within a single pressure-free measurement [2–4], so that very good characterisation of polymer materials is in principal possible [2,5–7]. Typical operational areas of the ultrasonic measuring technique in plastics processing concentrate basically on extrusion [3,8–11] and the processing of thermosets [12–14]. In addition, it is also possible for injection moulding [15,16] or compression moulding. Systematic investigations of plastic melts, by means of ultrasound, had already begun at the IKV, Aachen, Germany in the nineteen seventies with investigations of the extrusion process [17]. At present, the applications of the ultrasonic measuring technique in the field of extrusion extend among others [4,5,9,18] to proving changes of morphology [3], residence times [10], mixing qualities [11] or melt inhomogeneities [19]. The applications
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W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
Table 1 Pros and cons of the ultrasonic measuring technique in plastics processing Pros
Cons
No marks on the parts surface
High investment
Integral signals, i.e. information of superficial and interior part properties
Ultrasonic parameters are not easily imaginable like temperature and pressure
Signals of part and process properties
Interpretation of the ultrasonic signals requires a lot of process understanding
More parameters for the quality control and documentation in the course of DIN ISO 9000/9001
Transport mechanism of ultrasound as functions of T, p, dT/ dt and f are not widely explored
of the ultrasonic measuring technique in the field of injection moulding are still limited in spite of some advantages in relation to conventional injection moulding measuring techniques (Table 1). However, this technology is already used in first attempts in on-line process control [16] or with gas assisted injection moulding [20]. Within gas assisted injection moulding, the necessary ultrasonic sensors for the investigation can be integrated into the mould in a similar manner to pressure sensors. Here, they can also be attached behind the cavity wall, in order to avoid surface markings. Among other things the progress of the gas bubble can be observed and the remaining wall thickness can be determined (Fig. 1). Finally, break-through of the gas bubble through the melt front (Fig. 2) or foaming (Fig. 3) can be detected [20].
2. Experimental setup and execution The investigations were accomplished with three amorphous materials (ABS from the company Schulman, Kerpen, Germany; PMMA from the company Roehm, Darmstadt, Germany; PC from the company Bayer,
Fig. 1. Schematic ultrasonic runtime graphs, effect of pressure.
Fig. 2. Schematic ultrasonic runtime graphs, gas break-through.
Leverkusen, Germany), two crystalline materials (PP from the company DSM, Heerlen, Netherlands; PA6 from the company Schulman, Kerpen, Germany) and a crystalline, short glass fibre-reinforced material (PA66GF30 from the company Rhodia, Freiburg, Germany). A simple plate shaped part with band gate and fish tail manifold was applied (Fig. 4). Pressure and temperature sensors, which were positioned close to the gate and far from the gate, served for the correlation of the measured pressures and temperatures with the ultrasonic data taken up at the centre of the part. The ultrasonic signals were taken with longitudinal wave sensors using the transmission technique (i.e. the ultrasonic signal is sent by the transmitter and goes through the plastic volume exactly once before it is detected by the receiver) with a rated frequency of 4 MHz. In the first part of the investigations for each material the end of the holding pressure time was determined by iteratively increasing the latter in combination with weighing of the moulded parts. The time when no further weight increase of the parts was recognized (gate seal-off time) and after time was considered as the end of the holding pressure time. With the help of a cooling formula [21] the corresponding cooling period was calculated. The holding pressure was selected in such a way that the moulded part detaches from the cavity wall at the position of the ultrasonic sensors at the computed end of the cooling period. Finally, the measured data attained for each material were compared with one another and evaluated in relation to the final part properties.
Fig. 3. Schematic ultrasonic runtime graphs, foam generation.
W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
Fig. 4. Moulded part of the injection moulding studies.
The second part of the investigations consisted of the determination of the influence of certain manufacturing parameters on the measured ultrasonic data. Therefore, the cavity wall temperature and injection speed were varied in two stages as well as the holding pressure in three stages. Also, the measured ultrasonic data of each material were finally compared with one another and evaluated in relation to the final part properties.
207
Fig. 6. Ultrasonic runtime and amplitude (PC, Bayer, Leverkusen, Germany).
It was determined in the course of the first part of the investigations that the ultrasonic signal can supply information about the process for longer than, for example, pressure signals (Fig. 5). In addition, a relation exists between the signal and the shrinking process of the moulded part. The time of the ‘breaking off’ of the signal corresponds to the detachment of the moulded part of the cavity wall (breaking off is not physically correct; the signal is rather strongly absorbed starting from an air gap of more than 1 mm, so that it is no longer detectable with the transmission technique). In the conventional process the signals of pressure sensors approach the zero point asymptotically, so that only a time interval can be outlined vaguely for the detachment of the part from the cavity wall, in contrast to a well defined point in time with ultrasonic signals. As the ultrasonic speed has a pronounced dependence on the temperature and less pressure dependent mechanical
properties, the ultrasonic speed gives an approximate idea of the course of the mean temperature of the scanned volume of the moulded part. Figs. 5–7 present the ultrasonic runtime curves, which behave inversely proportional to the ultrasonic speed, so that these qualitatively represent the fading of the mean temperature of the part from the time when the melt is passing the sensor position up to the time of detachment from the cavity wall. A previous detailed and also pressure dependent calibration is naturally necessary for the accurate computation of this temperature. The course of the ultrasonic amplitude, measured during the injection moulding cycle, is multi-faceted. A simple distinction of amorphous and crystalline polymers is possible, due to the material specific, characteristic course of the ultrasonic amplitude. Investigations on polypropylene (PP) or polycarbonate (PC) showed strongly different courses of the curve (compare Figs. 6 and 7), which are due to the different solidification processes of the materials. The process of the ultrasonic amplitude curve of amorphous polycarbonate (Fig. 6) begins with an initial value unequal to zero and a linear rise from the time when the melt front passes the ultrasonic sensor up to a local maximum at the end of the injection phase tIE. The beginning of the holding pressure phase leads to a break in the course of the ultrasonic amplitude, whereupon the amplitude drops to a local minimum at the end of the holding pressure phase tHPE, with which, at least in the past investigations, the gate
Fig. 5. Cavity pressure and ultrasonic runtime (PA66GF30, Rhodia, Freiburg, Germany).
Fig. 7. Ultrasonic runtime and amplitude (PP, DSM, Heerlen, Netherlands).
3. Results of the investigations
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W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
Fig. 8. Ultrasonic velocity and amplitude in dependence of the mould wall temperature wW (PA 6, Schulman, Kerpen, Germany).
Fig. 10. Ultrasonic velocity and amplitude in dependence of the holding pressure pN (PA 6, Schulman, Kerpen, Germany).
seal-off time of the moulded part could be detected simultaneously. A second rise of the ultrasonic amplitude curve follows due to the increasing solidification of the material because of the cooling in the mould, which finally leads into a plateau of constant amplitude as a sign of the almost finished solidification. These observations were made with all examined, unfilled amorphous materials and seem suitable for characterisation of the material. Initially, the course of the ultrasonic amplitude curve of crystalline materials behaves similarly, but differs substantially starting from the beginning of the holding pressure phase, as can be shown by the example of crystalline polypropylene (PP) (Fig. 7). Instead of going through a local minimum at the end of the holding pressure phase, the ultrasonic amplitude falls constantly up to the detachment of the moulded part from the cavity wall. Another phenomenon seems to work against a second rise of the ultrasonic amplitude due to solidification. The present assumption takes the crystallization of the material into account, with which the dispersion of the ultrasound at the developing spherulites leads to the decay of the received ultrasonic amplitude (Fig. 7). The results obtained in the second part of the investigations are exemplarily shown on the basis of the crystalline material polyamid 6 (PA6). With constant settings of the other processing parameters, a significant change of the ultrasonic speed can be observed by the variation of the cavity wall temperature (Fig. 8). The ultrasonic speed increases more in a colder mould, which
accordingly indicates a higher cooling rate and solidification of the moulded part. The course of the ultrasonic amplitude is probably also characterised (see assumption of the first part of the investigations) by a change of crystallization kinetics. Apparently, at first the crystallization in the cold mould runs faster, whereby the ultrasound is more strongly dispersed at the larger number of developing spherulites. Consequently, at first the ultrasonic amplitude sinks faster with lower cavity wall temperatures. In addition, this crystallization seems to finish faster, so that the effect of the rise of the ultrasonic amplitude outweighs the dispersion, due to the cooling and the increasing solidification of the material. In the range selected for these investigations, a variation of the injection speed did not exhibit substantial change of the ultrasonic speed or amplitude (Fig. 9). Only the gradient of the ultrasonic amplitude, within the linear running range of the injection phase, is steeper for higher injection speeds. The shift of the detachment of the moulded part from the cavity wall results from the different times for the cavity filling. The holding pressure represents the only manufacturing parameter examined which has a direct influence on the change of the detachment time of the moulded part from the cavity wall (Fig. 10). However, the different ultrasonic speeds of the comparable time intervals do not change significantly. Small differences probably result because of better heat transport conditions from higher contact pressure of the moulded part to the cavity wall with rising holding pressure. This can be seen by higher ultrasonic speeds with increasing holding pressure. The assumption is supported by the increase of the ultrasonic amplitude with rising holding pressure, since the transmission quality is also dependent on the dominant contact pressure.
4. Conclusions
Fig. 9. Ultrasonic velocity and amplitude in dependence of the injection speed vEin (PA 6, Schulman, Kerpen, Germany).
The results presented give an idea of the possible potential of the ultrasonic measuring technique for process control and management of the injection moulding process of thermoplastics. However, the complete interpretation of
W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
the observations is not possible due to missing knowledge of the ultrasonic speed and/or absorption, which depend on the material behaviour in an injection moulding specific wide range of temperatures, pressures and cooling speeds. Further research projects have to concentrate on the ultrasonic measuring technique before definitive relationships between the ultrasonic condition curves and the final part properties can be found. These are necessary for the development of controlling concepts based on this promising technique.
Acknowledgements The investigations set out in this report received financial support from the Federal Ministry of Economics and Labor (BMWA) by the AiF e.V. (No. N04294/03), to whom we extend our thanks.
References [1] J. Krautkra¨mer, Werkstoffpru¨fung mit Ultraschall, Springer, Berlin, 1998. [2] I. Alig, D. Lellinger, Ultrasonic methods for characterizing polymeric materials, Chemical Innovation 30 (2) (2000) 12–18. [3] I. Alig, D. Lellinger, Nachweis von Morphologiea¨nderungen im Extruder, DKI, Darmstadt, 2001. [4] S. Tadjbakhsch, Entwicklung eines Ultraschallreflexionsverfahrens zur Bestimmung der viskoelastischen Eigenschaften von amorphen und teilkristallinen Polymerfilmen, TU, Darmstadt, 1998. [5] R. Gendron, M.M. Dumoulin, J. Tatibouet, L. Piche, A. Hamel, Measuring on-line polymer properties using an ultrasonic technique, in: ANTEC 1993, SPE, New Orleans, USA, 1993, pp. 2256–2261. [6] R. Gendron, J. Tatibouet, J. Guevremont, M.M. Dumoulin, L. Piche, Ultrasonic behavior of polymer blends, in: ANTEC 1994, SPE, San Francisco, USA, 1994, pp. 2452–2456. [7] T. Pfeiffer, M. Benz, Qualita¨tssicherung fu¨r Verbundkunststoffe durch Ultraschallru¨ckstellung, GAK 53 (11) (2000) 764–768.
209
[8] I. Alig, D. Lellinger, Nachweis von Schmelzeinhomogenita¨ten im Extruder mit Hilfe von Ultraschall, DKI, Darmstadt, 2001. [9] I. Alig, D. Lellinger, R. Lamour, J. Ramthan, InlineMonitoring mit Ultraschall, Kunststoffe 90 (5) (2000). [10] R. Gendron, L.E. Daigneault, J. Tatibouet, M.M. Dumoulin, Resident time distribution in extruders determined by in-line ultrasonic measurements, in: ANTEC 1994, SPE, San Francisco, USA, 1994, p. 167. [11] J. Tatibouet, M.A. Huneault, In-line ultrasonic monitoring of filler dispersion during extrusion, Internals of Polymer Processing 12 (1) (2002) 49–52. [12] W. Michaeli, J. To¨pker, Verarbeitung von Phenolharzen im resin transfer moulding. Abschlussbericht, Institut fu¨r Kunststoffverarbeitung, RWTH Aachen, Aachen, 2000 (Report No.: 11693 N). [13] W. Michaeli, J. To¨pker. Process control of Resin Transfer Molding (RTM) by ultrasonics, in: 46th International SAMPE Symposium 2001, California, USA, 2001, pp. 300–309. [14] W. Michaeli, J. To¨pker, Einsatz der Ultraschallmesstechnik beim RTM-Verfahren, in: 4. Internationalen AVK-TV Tagung fu¨r faserversta¨rkte Kunststoffe und Formmassen 2001, Baden Baden, 2001. [15] C.L. Thomas, A.A. Tseng, A.J. Bur, J.L. Rose, Closed loop control of injection molding hold time, Advances in Polymer Technology 15 (2) (1993) 151–163. [16] H. Wang, B. Cao, C.K. Jen, K.T. Nguyen, M. Viens, On-line ultrasonic monitoring of the injection moulding process, Polymer Engineering and Science 37 (2) (1997) 363–376. ¨ berwachen von Kunststoffsch[17] M. Petrik, Kontinuierliches U melzen im Extrusionsprozess durch Ultraschallmessungen (Dissertation), Rheinisch-Westfa¨lische Technische Hochschule, Aachen, 1975. [18] I. Alig, D. Lellinger, Frequency dependence of ultrasonic velocity and attenuation in two-phase composite systems with spherical scatterers, Journal of Applied Physics 72 (1992) 5565. [19] I. Alig, D. Lellinger, Ultraschallwandler fu¨r Langzeitanwendung, DKI, Darmstadt, 2001. [20] W. Michaeli, A. Brunswick, On-line Prozessu¨berwachung bei der Gasinjektionstechnik, in: 20stes Kunststofftechnisches Kolloquium 2000, IKV Aachen, Aachen, 2000, pp. 7–10. [21] G. Menges, Werkstoffkunde der Kunststoffe, Carl Hanser Verlag, Mu¨nchen, Wien, 1990.
Test Methods
Ultrasonic investigations of the thermoplastics injection moulding process W. Michaeli, C. Starke* Institut fu¨r Kunststoffverarbeitung (IKV), Aachen, Germany Received 8 July 2004; accepted 19 August 2004
Abstract The ultrasonic measuring technique constitutes an interesting possibility for process analysts to gain a deeper understanding of a particular process. During initial tests for thermoplastic compact injection moulding it was found that the ultrasonic signal can sometimes supply information for much longer than, for example, pressure signals. It was also found that a relationship exists between the signal and the shrinkage process of the moulded part. The point when the signal ceases coincides with the time the moulded parts become detached from the cavity wall during the cooling phase. The measuring method reflects all variations in the process conditions very sensitively indeed. q 2004 Elsevier Ltd. All rights reserved. Keywords : Ultrasonic investigation; Injection moulding; Thermoplastic material; Process characterization; Longitudinal waves; Ultrasonic runtime; Relative amplitude
1. Present use of ultrasonic techniques in the polymer processing The formal connection between mechanical and acoustic properties of plastics opens a broad and attractive field of applications for the ultrasonic measuring technique, before, during and after the processing. Elastic waves in distortable media are called acoustic waves. The most important wave forms are the longitudinal and the transverse waves, whereby the latter are exclusively transmitted in solids, due to the lack of propagating shear forces in liquids. The different propagation mechanisms of both wave forms also result in different propagation speeds, depending on the respective material properties [1]. Additionally, the relationship from longitudinal to transverse speed exclusively depends on Poisson’s ratio. Therefore, it is possible to calculate the modulus of elasticity, the shear * Corresponding author. Tel.: C49 241 809 3832; fax: C49 241 809 2262. E-mail address: [email protected] (C. Starke). 0142-9418/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.08.009
modulus and the Poisson’s ratio of a medium depending on temperature, pressure and cooling speed just with the help of the sound speeds. The acoustic measurement of elastic properties of solids is state of the art. For example, the ultrasonic reflection procedure provides the real and imaginary portions of the elasticity and shear moduli including the transverse contraction ratio within a single pressure-free measurement [2–4], so that very good characterisation of polymer materials is in principal possible [2,5–7]. Typical operational areas of the ultrasonic measuring technique in plastics processing concentrate basically on extrusion [3,8–11] and the processing of thermosets [12–14]. In addition, it is also possible for injection moulding [15,16] or compression moulding. Systematic investigations of plastic melts, by means of ultrasound, had already begun at the IKV, Aachen, Germany in the nineteen seventies with investigations of the extrusion process [17]. At present, the applications of the ultrasonic measuring technique in the field of extrusion extend among others [4,5,9,18] to proving changes of morphology [3], residence times [10], mixing qualities [11] or melt inhomogeneities [19]. The applications
206
W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
Table 1 Pros and cons of the ultrasonic measuring technique in plastics processing Pros
Cons
No marks on the parts surface
High investment
Integral signals, i.e. information of superficial and interior part properties
Ultrasonic parameters are not easily imaginable like temperature and pressure
Signals of part and process properties
Interpretation of the ultrasonic signals requires a lot of process understanding
More parameters for the quality control and documentation in the course of DIN ISO 9000/9001
Transport mechanism of ultrasound as functions of T, p, dT/ dt and f are not widely explored
of the ultrasonic measuring technique in the field of injection moulding are still limited in spite of some advantages in relation to conventional injection moulding measuring techniques (Table 1). However, this technology is already used in first attempts in on-line process control [16] or with gas assisted injection moulding [20]. Within gas assisted injection moulding, the necessary ultrasonic sensors for the investigation can be integrated into the mould in a similar manner to pressure sensors. Here, they can also be attached behind the cavity wall, in order to avoid surface markings. Among other things the progress of the gas bubble can be observed and the remaining wall thickness can be determined (Fig. 1). Finally, break-through of the gas bubble through the melt front (Fig. 2) or foaming (Fig. 3) can be detected [20].
2. Experimental setup and execution The investigations were accomplished with three amorphous materials (ABS from the company Schulman, Kerpen, Germany; PMMA from the company Roehm, Darmstadt, Germany; PC from the company Bayer,
Fig. 1. Schematic ultrasonic runtime graphs, effect of pressure.
Fig. 2. Schematic ultrasonic runtime graphs, gas break-through.
Leverkusen, Germany), two crystalline materials (PP from the company DSM, Heerlen, Netherlands; PA6 from the company Schulman, Kerpen, Germany) and a crystalline, short glass fibre-reinforced material (PA66GF30 from the company Rhodia, Freiburg, Germany). A simple plate shaped part with band gate and fish tail manifold was applied (Fig. 4). Pressure and temperature sensors, which were positioned close to the gate and far from the gate, served for the correlation of the measured pressures and temperatures with the ultrasonic data taken up at the centre of the part. The ultrasonic signals were taken with longitudinal wave sensors using the transmission technique (i.e. the ultrasonic signal is sent by the transmitter and goes through the plastic volume exactly once before it is detected by the receiver) with a rated frequency of 4 MHz. In the first part of the investigations for each material the end of the holding pressure time was determined by iteratively increasing the latter in combination with weighing of the moulded parts. The time when no further weight increase of the parts was recognized (gate seal-off time) and after time was considered as the end of the holding pressure time. With the help of a cooling formula [21] the corresponding cooling period was calculated. The holding pressure was selected in such a way that the moulded part detaches from the cavity wall at the position of the ultrasonic sensors at the computed end of the cooling period. Finally, the measured data attained for each material were compared with one another and evaluated in relation to the final part properties.
Fig. 3. Schematic ultrasonic runtime graphs, foam generation.
W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
Fig. 4. Moulded part of the injection moulding studies.
The second part of the investigations consisted of the determination of the influence of certain manufacturing parameters on the measured ultrasonic data. Therefore, the cavity wall temperature and injection speed were varied in two stages as well as the holding pressure in three stages. Also, the measured ultrasonic data of each material were finally compared with one another and evaluated in relation to the final part properties.
207
Fig. 6. Ultrasonic runtime and amplitude (PC, Bayer, Leverkusen, Germany).
It was determined in the course of the first part of the investigations that the ultrasonic signal can supply information about the process for longer than, for example, pressure signals (Fig. 5). In addition, a relation exists between the signal and the shrinking process of the moulded part. The time of the ‘breaking off’ of the signal corresponds to the detachment of the moulded part of the cavity wall (breaking off is not physically correct; the signal is rather strongly absorbed starting from an air gap of more than 1 mm, so that it is no longer detectable with the transmission technique). In the conventional process the signals of pressure sensors approach the zero point asymptotically, so that only a time interval can be outlined vaguely for the detachment of the part from the cavity wall, in contrast to a well defined point in time with ultrasonic signals. As the ultrasonic speed has a pronounced dependence on the temperature and less pressure dependent mechanical
properties, the ultrasonic speed gives an approximate idea of the course of the mean temperature of the scanned volume of the moulded part. Figs. 5–7 present the ultrasonic runtime curves, which behave inversely proportional to the ultrasonic speed, so that these qualitatively represent the fading of the mean temperature of the part from the time when the melt is passing the sensor position up to the time of detachment from the cavity wall. A previous detailed and also pressure dependent calibration is naturally necessary for the accurate computation of this temperature. The course of the ultrasonic amplitude, measured during the injection moulding cycle, is multi-faceted. A simple distinction of amorphous and crystalline polymers is possible, due to the material specific, characteristic course of the ultrasonic amplitude. Investigations on polypropylene (PP) or polycarbonate (PC) showed strongly different courses of the curve (compare Figs. 6 and 7), which are due to the different solidification processes of the materials. The process of the ultrasonic amplitude curve of amorphous polycarbonate (Fig. 6) begins with an initial value unequal to zero and a linear rise from the time when the melt front passes the ultrasonic sensor up to a local maximum at the end of the injection phase tIE. The beginning of the holding pressure phase leads to a break in the course of the ultrasonic amplitude, whereupon the amplitude drops to a local minimum at the end of the holding pressure phase tHPE, with which, at least in the past investigations, the gate
Fig. 5. Cavity pressure and ultrasonic runtime (PA66GF30, Rhodia, Freiburg, Germany).
Fig. 7. Ultrasonic runtime and amplitude (PP, DSM, Heerlen, Netherlands).
3. Results of the investigations
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W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
Fig. 8. Ultrasonic velocity and amplitude in dependence of the mould wall temperature wW (PA 6, Schulman, Kerpen, Germany).
Fig. 10. Ultrasonic velocity and amplitude in dependence of the holding pressure pN (PA 6, Schulman, Kerpen, Germany).
seal-off time of the moulded part could be detected simultaneously. A second rise of the ultrasonic amplitude curve follows due to the increasing solidification of the material because of the cooling in the mould, which finally leads into a plateau of constant amplitude as a sign of the almost finished solidification. These observations were made with all examined, unfilled amorphous materials and seem suitable for characterisation of the material. Initially, the course of the ultrasonic amplitude curve of crystalline materials behaves similarly, but differs substantially starting from the beginning of the holding pressure phase, as can be shown by the example of crystalline polypropylene (PP) (Fig. 7). Instead of going through a local minimum at the end of the holding pressure phase, the ultrasonic amplitude falls constantly up to the detachment of the moulded part from the cavity wall. Another phenomenon seems to work against a second rise of the ultrasonic amplitude due to solidification. The present assumption takes the crystallization of the material into account, with which the dispersion of the ultrasound at the developing spherulites leads to the decay of the received ultrasonic amplitude (Fig. 7). The results obtained in the second part of the investigations are exemplarily shown on the basis of the crystalline material polyamid 6 (PA6). With constant settings of the other processing parameters, a significant change of the ultrasonic speed can be observed by the variation of the cavity wall temperature (Fig. 8). The ultrasonic speed increases more in a colder mould, which
accordingly indicates a higher cooling rate and solidification of the moulded part. The course of the ultrasonic amplitude is probably also characterised (see assumption of the first part of the investigations) by a change of crystallization kinetics. Apparently, at first the crystallization in the cold mould runs faster, whereby the ultrasound is more strongly dispersed at the larger number of developing spherulites. Consequently, at first the ultrasonic amplitude sinks faster with lower cavity wall temperatures. In addition, this crystallization seems to finish faster, so that the effect of the rise of the ultrasonic amplitude outweighs the dispersion, due to the cooling and the increasing solidification of the material. In the range selected for these investigations, a variation of the injection speed did not exhibit substantial change of the ultrasonic speed or amplitude (Fig. 9). Only the gradient of the ultrasonic amplitude, within the linear running range of the injection phase, is steeper for higher injection speeds. The shift of the detachment of the moulded part from the cavity wall results from the different times for the cavity filling. The holding pressure represents the only manufacturing parameter examined which has a direct influence on the change of the detachment time of the moulded part from the cavity wall (Fig. 10). However, the different ultrasonic speeds of the comparable time intervals do not change significantly. Small differences probably result because of better heat transport conditions from higher contact pressure of the moulded part to the cavity wall with rising holding pressure. This can be seen by higher ultrasonic speeds with increasing holding pressure. The assumption is supported by the increase of the ultrasonic amplitude with rising holding pressure, since the transmission quality is also dependent on the dominant contact pressure.
4. Conclusions
Fig. 9. Ultrasonic velocity and amplitude in dependence of the injection speed vEin (PA 6, Schulman, Kerpen, Germany).
The results presented give an idea of the possible potential of the ultrasonic measuring technique for process control and management of the injection moulding process of thermoplastics. However, the complete interpretation of
W. Michaeli, C. Starke / Polymer Testing 24 (2005) 205–209
the observations is not possible due to missing knowledge of the ultrasonic speed and/or absorption, which depend on the material behaviour in an injection moulding specific wide range of temperatures, pressures and cooling speeds. Further research projects have to concentrate on the ultrasonic measuring technique before definitive relationships between the ultrasonic condition curves and the final part properties can be found. These are necessary for the development of controlling concepts based on this promising technique.
Acknowledgements The investigations set out in this report received financial support from the Federal Ministry of Economics and Labor (BMWA) by the AiF e.V. (No. N04294/03), to whom we extend our thanks.
References [1] J. Krautkra¨mer, Werkstoffpru¨fung mit Ultraschall, Springer, Berlin, 1998. [2] I. Alig, D. Lellinger, Ultrasonic methods for characterizing polymeric materials, Chemical Innovation 30 (2) (2000) 12–18. [3] I. Alig, D. Lellinger, Nachweis von Morphologiea¨nderungen im Extruder, DKI, Darmstadt, 2001. [4] S. Tadjbakhsch, Entwicklung eines Ultraschallreflexionsverfahrens zur Bestimmung der viskoelastischen Eigenschaften von amorphen und teilkristallinen Polymerfilmen, TU, Darmstadt, 1998. [5] R. Gendron, M.M. Dumoulin, J. Tatibouet, L. Piche, A. Hamel, Measuring on-line polymer properties using an ultrasonic technique, in: ANTEC 1993, SPE, New Orleans, USA, 1993, pp. 2256–2261. [6] R. Gendron, J. Tatibouet, J. Guevremont, M.M. Dumoulin, L. Piche, Ultrasonic behavior of polymer blends, in: ANTEC 1994, SPE, San Francisco, USA, 1994, pp. 2452–2456. [7] T. Pfeiffer, M. Benz, Qualita¨tssicherung fu¨r Verbundkunststoffe durch Ultraschallru¨ckstellung, GAK 53 (11) (2000) 764–768.
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