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Analysis of fluid-assisted injection techniques by use of ultrasonic measurements.

Gas-assisted injection molding (GAIM) and water-assisted injection molding (WAIM) are well-established processes for producing parts with (functional) hollow space. The quality of these parts greatly depends on internal part properties, such as residual wall thickness. In this context, the ultrasonic measurement technique is suitable both for describing the process and for detecting internal characteristics. This article deals with the online use of ultrasonic measurements to visualize hollow space formation in gas-assisted and water-assisted injection processes and to determine the residual wall thickness of the molded parts.

The hollow parts produced by GAIM and WAIM are frequently tubes or plate-like parts with reinforcing rips; they feature a high degree of function integration. The general course of both processes can be described as follows: First, the cavity of the mold is partly or completely filled with the polymer. Then, gas or water is injected by special injectors. The fluid displaces the melt-liquid core and shapes a hollow space inside the part. The resultant internal properties such as the residual wall thickness are very important for the quality of the part, because mechanical properties and the flow cross section largely depend on this wall thickness. Compared with compact injection molding, however, the forming process is significantly more complex. Hollow space formation can be divided into five ranges (Figure 1).

Within the range of the flow front of the polymer (IV), a fountain flow is present. Here the melt particles of the center are transferred to the edge of the mold and cooled down immediately. The zone between flow front and fluid bubble (III) is a parallel, laminar, one-component flow. In this area, a frozen outer layer is shaped. The formation of the residual wall thickness is significantly affected in the range of the spreading fluid bubble (II). The residual wall thickness consists of the frozen outer layer and the melt-liquid layer, which has not yet been displaced by the fluid. Finally, a one-component flow can be found in Range I.

Because GAIM and WAIM are used primarily to produce technical parts, comprehensive quality assurance and process control are essential. (2,3) In addition to classical sensor technology for pressure and temperature measurement (pT measurement), mass-flow sensors and infrared thermography are applied for quality control. However, pT measurements don't allow the detection of inner properties, and infrared thermography can only be used inline. Therefore, suitable systems for online quality control of important quality indicators--such as the residual wall thickness or for nondestructive detection of part flaws like blowholes, foaming, or water entrapment--are missing. Thus ultrasonic measuring technology seems to be an appropriate method for nondestructive and permeating testing for online quality control.

Basics of Ultrasonic Measurements

Acoustical testing methods analyze the propagation of mechanical oscillations in solid, liquid, or gaseous materials. An established model for acoustical measurements describes a medium as an elastic body consisting of individual mass particles. These mass particles are bound to their positions by flexible forces. Sound propagation in a medium is initiated by an outside excitation of these particles. Subsequently, the particles are deflected and swing around their rest position. Because of the mechanical coupling, the nearby mass particles also begin to oscillate. The outcome of this is a progressive wave movement inside the medium.

The propagation of sound waves within a medium is essentially influenced by its mechanical material properties. Equation 1 shows the dependency of the longitudinal sound-propagation velocity [c.sub.l] of an ideal elastic material on the Young's modulus E, the density [rho], and Poisson's ratio v. (4)

[c.sub.l] = [square root of (E(1 - v)/ [rho](1 + v) x (1 - 2v))] (1)

However, the dependency of viscoelastic materials like plastics is significantly more complex. The mechanical properties of viscoelastic materials can be described by using the complex modulus. As an example, the real part of the complex shear modulus G' as a function of the density p, the transversal sound-propagation velocity [c.sub.t], the damping coefficient [alpha], and the wave length [lambda] is shown in Equation 2. (5)

G' = [rho][c.sup.2.sub.t][1 - [([alpha][lambda]/2[pi]).sup.2]]/ [[1 + [([alpha][lambda]/2[pi]).sup.2]].sup.2] (2)

Especially, acoustic properties such as sound-propagation velocity or damping of polymers depend on the state of the medium. Figure 2 exemplifies the dependency of the longitudinal sound-propagation velocity on the pressure and temperature of a polypropylene.

It can be seen that an increase in temperature causes a reduction in sound-propagation velocity. In addition, an increase in pressure results in an increase in sound-propagation velocity. The analyzed polypropylene is a semicrystalline thermoplastic material. Therefore a crystallization temperature can be found between 120[degrees]C and 140[degrees]C. This crystallization causes a rapid decrease of the sound-propagation velocity.

Additionally, other characteristics of sound waves are important for the analysis of GAIM and WAIM. Ultrasonic waves are partly reflected and partly transmitted at interfaces (Figure 3).

The ratios of the sound pressures of the reflected wave [p.sub.r], the transmitted wave [p.sub.t], and the incident wave [p.sub.e] are shown in Equations 3 and 4.

R = [p.sub.r]/[p.sub.e] (3)

T = [p.sub.t]/[p.sub.e] (4)

The reflection factor R and the transmission factor T can be calculated by using the acoustic radiation impedance of the interface-forming materials [Z.sub.1] and [Z.sub.2] (Equations 5 and 6).

R = [Z.sub.2] - [Z.sub.1]/[Z.sub.2] + [Z.sub.1] (5)

T = 2 x [Z.sub.2]/[Z.sub.2] + [Z.sub.1] (6)

The detection of the reflected waves allows the determination of the residual wall thickness, which is an important quality feature of a molded part made by the fluid-assisted injection technique. Therefore the theoretical computation of a distance l using the ultrasonic time of flight t is shown in Equation 7.

t = l/c (7)

Hence the computation of the residual wall thickness is possible if the sound-propagation velocity of the medium is known.

Experimental

For this project, a cavity insert was developed allowing the application of the ultrasonic measurement technique to analyze gas-assisted and water-assisted injection molding (Figure 4).

A pipe element was designed as being representative of the majority of parts produced by fluid injection technology. The external diameter of the pipe was 30 mm. By the positioning of several ultrasonic transducers along the flow path, the arrival of the fluid bubble at these measurement points can be detected. Therefore, determining the melt and fluid propagation speed is possible. In addition to straight elements, the cavity insert had different directional changes. Hence the dependency of the residual wall thickness on the deviation radius could be analyzed. Inside the mold, the cavity insert was connected with the overspill cavity in which the melt was displaced during fluid injection. The process-describing data were recorded by combining ultrasound measurements with conventional measuring methods such as cavity pressure or temperature measurements. A measurement data-acquisition system was compiled at the Institute for Plastics Processing (IKV) at RWTH Aachen University, Germany. This system consisted of the following components:

* Ultrasonic transducer DS6, Karl Deutsch GmbH, Wuppertal, Germany

* Pulse generator 5800 PR, Olympus GmbH, Mainz, Germany

* pT-sensor 6190CA0,8, Kistler Instrumente GmbH, Ostfildern, Germany

* Industrial-PC based on x86 architecture

* High-speed-A/D-transformer card PCI 5122, National Instruments GmbH, Munich, Germany

* A/D-transformer card PCI 6251 National Instruments GmbH, Munich, Germany

These experiments used a fully hydraulic injection molding machine (HM 1600/100 Unilog B4, Wittmann Battenfeld GmbH, Kottingbrunn, Austria); this machine provides a maximum damping force of 1600 kN and a screw diameter of 55 mm. The maximum shot volume is 594 [mm.sup.3]. Water injection was implemented using an external WAIM device (Power Module 15/210-2, PME fluidtec GmbH, Kappel-Grafenhausen, Germany); this device allows a maximum water volume flow rate of 30 l/min and a maximum water holding pressure of 210 bar. For gas injection, an Airmould[R] GAIM device was used (Wittmann Battenfeld GmbH); this machine provides a maximum gas pressure of 300 bar. The resin used for the experiments was polypropylene 505P, provided by Sabic Deutschland GmbH & Co.KG, Dusseldorf, Germany.

Process analysis by use of ultrasonic measurements can be accomplished by the "pulse echo" technique or the "pulse transmission" technique (Figure 5).

The pulse echo technique uses only a single ultrasonic transducer, which serves as a transmitter and a receiver simultaneously. Initially, the sound waves are generated by this transducer and pass through the medium. Every time the waves encounter an interface, a partial or complete reflection occurs. Subsequently this echo can be measured by the transducer. The pulse transmission technique, by contrast, uses two different ultrasonic transducers. The first transducer generates the sound waves that pass through the medium and all interfaces. This acoustic pulse can be measured by the second transducer. In this context, note that an interface between polymer/gas and mold/gas leads to a total reflection of sound waves; therefore the pulse transmission technique can be used to analyze WAIM only.

In the following experiments, both ultrasonic measurement techniques are examined. First, the water-injection technique is analyzed by using the pulse transmission technique. The results allow a description of hollow space formation during the process. Second, the pulse echo technique is used to analyze internal properties of GAIM parts. The manufacturing conditions are shown in Table 1.

The fluid delay time [t.sub.v] describes the length of time between the end of the holding pressure phase and the beginning of fluid injection.

Results and Discussion

The ultrasonic analysis of WAIM using the pulse transmission technique is shown in Figure 6.

Data acquisition starts with the machine signal "mold closing" (phase 0). Melt injection begins at t = 5 s (phase I). At that moment there is no polymer between the ultrasonic transducers. Because of the great differences in the acoustic radiation impedances of mold and air, the sound waves are totally reflected at the mold/air interface. Hence, no signal can be detected by the second ultrasonic transducer.

At t = 7 s, the melt arrives at the position of the transducers and the first ultrasonic wave is measured. During the volumetric filling, the cavity pressure increases. According to Figure 2, this leads to an increase of the sound-propagation velocity of the polymer. Therefore, the time of flight between both ultrasonic transducers decreases. After volumetric falling, the packing phase occurs (phase II). The packing phase can be characterized by a great increase of the cavity pressure. This leads to an onward decrease of the time of flight.

After the packing phase and a short holding pressure phase, the fluid delay time begins (phase III). During this time, no more material is conveyed into the mold. As a result, the melt cools down and the cavity pressure decreases because of the volume shrinkage of the polymer. Because of this large wall diameter (30 mm) and the low heat conductivity of the polypropylene, the temperature compensation processes are comparatively slow. Hence the changing of the cavity pressure is more important than the temperature relating to the sound-propagation velocity. The decrease of the cavity pressure leads to a decrease in the sound-propagation velocity and therefore to an increase in the time of flight.

After the fluid delay time, the core puller is activated and the water-injection phase begins (phase IV). Now the pressurized melt flows directly into the opened overspill cavity. This leads to a severe decrease of cavity pressure, and the time of flight increases abruptly at t = 20 s. At t = 22 s, the water bubble reaches the position of the ultrasonic transducers (phase V). Owing to the higher sound-propagation velocity of water, ultrasonic waves move faster between the two transducers. Hence the time of flight decreases. At t = 23.5 s, the overspill cavity is completely filled and the water holding pressure phase begins (phase VI). In this period of time, the WAIM device provides a constant water pressure inside the mold. The molded part cools down continuously so that the increase of the sound-propagation velocity results in a decrease of the time of flight.

After t = 30 s, water injection is ended, and the water is removed from the part. Because of the air gap that occurs between the mold and the part, no further ultrasonic waves can be transmitted.

After this general description of the process, the influences of individual manufacturing parameters on hollow space formation are examined in the following experiments. Figure 7 shows the results of ultrasonic measurements for melt temperatures of 210[degrees]C and 250[degrees]C.

As expected, an increase in the melt temperature causes a decrease in sound-propagation velocity. Further, it can be shown that the arrival of the melt at the position of the transducers also depends on the temperature. The 250[degrees]C melt reaches the transducers at t = 7 s. However, the colder melt arrives at the position at t = 7.5 s. This is because the viscosity of the warmer melt is lower than the viscosity of the colder melt. Hence the mold can be filled faster by using higher melt temperatures. Further, it can be seen that the water bubble propagates faster in the warmer melt. The water flow front is detected at t = 28 s, and the overspill cavity is completely filled at t = 29 s. By using a melt temperature of 210[degrees]C, the gas bubble is detected at t = 28.5 s and the overspill cavity is filled at t = 31 s.

Internal properties are very important for the quality of a molded part made by WAIM or GAIM. In this regard, the pulse echo technique is likely useful for online determination of the residual wall thickness. However, the acoustic properties of thermoplastic materials are highly dependent on pressure and temperature. Furthermore, a temperature profile is shaped across the part diameter during the process. Hence, the acoustic properties of the residual wall are not exactly known. Therefore, another approach was used in the following: Parts were manufactured by use of the gas-injection technique. Subsequently, the residual wall thickness at the position of the ultrasonic transducer was measured by a magnetic-inductive thickness measuring device (Magna-Mike 8000, Panametrics GmbH, Hofheim, Germany). Accordingly, the wall thickness was correlated with the time of flight at the end of the process (Figure 8).

It can be seen that an increase of the wall thickness leads to an increase of the time of flight. The results show that the residual wall thickness can be qualitatively well displayed by means of ultrasonic analysis. Especially, the decreased wall thickness of specimen 4 can be detected using the pulse echo technique.

Conclusions

These experiments show that the ultrasonic measurement technique is suitable for describing hollow space formation during GAIM and WAIM. Process phases such as packing phases or fluid delay time can easily be determined. Furthermore, manufacturing conditions such as the melt temperature significantly influence the acoustic properties of a molded part. Hence, variations of these production conditions can be detected by ultrasonic measurements. Compared with conventional measuring techniques such as cavity pressure, ultrasonic measurements allow the determination of internal properties. Residual wall thickness is an important quality characteristic. The correlation of the time of flight with the wall thickness shows good agreement. Therefore, ultrasonic measurements are suitable for quality control of molded parts made by GAIM or WAIM.

However, the acoustic properties of thermoplastics are highly dependent on pressure and temperature. For this reason they must be initially determined in a measuring cell under stationary conditions. With the aid of these data, the next step will be to improve accuracy in calculating residual wall thickness, with the aim of monitoring the process online.

Acknowledgment

Research project 16285 N of the Forschungsvereinigung Kunststoffverarbeitung was sponsored as part of the "industrielle Gemeinschaftsforschung und -entwicklung (IGF)" by the German Bundesministerium fur Wirtschaft und Technologie (BMWi) due to an enactment of the German Bundestag through the AiF.

Editor's note: "In ultrasonic flow meter measurement, time of flight (TOF) is used to measure speed of signal propagation upstream and downstream of flow of a [medium], in order to estimate total flow velocity. This measurement is made in a collinear direction with the flow."--From Wikipedia

References

(1.) A. Lanvers, Analyse und Simulation des Kunststoff-Formbildungsprozesses bei der Gasinjektionstechnik, RWTH Aachen, PhD thesis (1993).

(2.) W. Michaeli, O. Gronlund, and M. Grundler, "WIT unter Kontrolle," Kunststoff-Berater, 54, 1-2, pp. 62-65 (2008).

(3.) W. Michaeli, Qualitatssicherung 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 05196--01 RI 05200 (2010).

(4.) J. Krautkramer and H. Krautkramer, Werkstoffprufung mit Ultraschall, Berlin, Heidelberg, New York: Springer Publishing House (1986).

(5.) I. Perepechko, Acoustic Methods of Investigating Polymers, Moscow: Mir Publishers (1975).

(6.) O. Lingk, Einsatz von Ultraschall zur Prozessanalyse beim Spritzgiessen von Thermoplasten, RWTH Aachen, PhD thesis (2010).

Note: The authors presented a version of this paper at ANTEC[R] 2012.

Prof. Dr.-Ing. Ch. Hopmann, Prof. Dr.-Ing. E.h.W. Michaeli, and Dipl.-Ing. S. Becker

Institute of Plastics Processing at RWTH Aachen University (IKV), Germany

Table 1. Manufacturing Conditions.

                                         WAIM   GAIM

melt temperature [T.sub.m], [degrees]C   230    230
mold temperature [T.sub.m], [degrees]C    40     40
  fluid volume rate V, [cm.sup.3]/s      150     --
        fluid pressure p, bar            150    100
     fluid delay time [t.sub.v] s         10     7
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Author:Hopmann, Ch.; Michaeli, E.h.W.; Becker, S.
Publication:Plastics Engineering
Geographic Code:4EUGE
Date:Jun 1, 2012
Words:2895
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