Thermoplastics extrusion die with ultrasonic thermo-pellicular effect

Transcription

1 Thermoplastics extrusion die with ultrasonic thermo-pellicular effect DANIEL STAN, IANCU SERBAN, LILIANA TULCAN, CRISTIAN TURC, AUREL TULCAN Materials and Manufacturing Engineering Department POLITEHNICA University of Timisoara, Faculty of Mechanical Engineering Piata Victoriei, nr. 2, Timisoara ROMANIA davostan@yahoo.fr, Abstract: - The ultrasonic thermo-pellicular effect is specific for the ultrasonic activation of the melted, under pressure, flowing polymers. Due to ultrasonic activation, the modifications of the properties at the interface between the melted material and the metallic die could postpone, the start of the stick-slip phenomenon, allowing thus the high speed processing, and an increase of the productivity and even of the product quality. The paper presents the particular design of an extrusion head, based on this effect, with incorporated ultrasonic transducer, the design principles and specific assumptions for dimensioning this. Key-Words: extrusion, thermo-pellicular effect, ultrasonic activation, industrial application, experimental 1 Introduction Until the end of the 90 s, ultrasonic activation was less involved in plastics forming technologies, and more involved in welding and transfer-injection [1], [2]. During the extrusion of thermoplastics, any modification of the flowing properties of the melt at the interface fluid-wall could delay the setting-up of the oscillating flow conditions improving the quality of extruded products for high productivity conditions. View of the interesting effects associated with the ultrasonic activation [1], [2] such an applications in plastics technology could modify the interfacial properties, thus increasing the rheological properties of the melted polymers. Conventional studies concerning the rheological behaviour of melted polymers during extrusion have dealt with the classical assumption (convenient boundary condition) [3]: -the flow is streamline (laminar), the velocity and the stress are continuous in flow section -mass and inertia forces are neglected, -at laminar flow there is no slip at the die walls due to the specific interfacial interaction solid-fluid, -in particular conditions, a stick-slip phenomenon is present, generating volume and surface faults, -a power law describes the viscosity, -hypothesis of null velocity (v = 0) of displacement at the wall of the flow section. Nevertheless, recent research, [4], [5], [6], confirms the hypothesis of the displacement through slide of the melt volume in the proximity of the extrusion die wall due to the particular structure of the melted material (i.e. interpenetrated macromolecules). This determines the viscous- elastic behaviour and the solidary movement of the material next to the wall, generating (for particular conditions such as temperature, high flow velocity, wall proprieties, the stick-slip effect) oscillating variations of the discharge and pressure at the emergence from the extrusion dies. Based on similarity hypotheses and results regarding the volume (thermal) effect and surface effect (Rayleigh or surface waves, reducing of the friction ratio on the lateral side of the horn), in , we attempted to extend the ultrasonic activation range experimenting on visco-elastic molten, running, pressurized polymers. HDPE, type A experimet LDPE, type A experim. PP, type A experiment Table 1 Axial Extruded Heat effect i displacem. q material [mm] of the horn temperat. T A = T A -T, a, [mm] [%] T, [ C] [ C] 0,5 0, ,95 3, ,40 5, ,76 7, ,13 9, ,5 0, ,0 3, ,5 6, ,5 11, ,5 0, ,0 3, ,5 6, ,5 11, ISSN: / ISSN: ISBN:

2 For studying the modifications of the flow properties of the molten polymers induced by the ultrasonic activation an extrusion angular rheometer, designed for laminar flow and adapted to the abovementioned conditions was used. Preliminary laboratory tests [5], and further focused factorial sets of experiments confirmed an important increase of the volumetric specific extrusion product, q, in the presence of ultrasonic activation, for HDPE, LDPE and especially for PP, cf. Table 1, at particular conditions of low pressure, p = 5 bar ( q = increase of volumetric extrusion product, i = flowing section breadth, Fig. 3. According to these experimental results, the ultrasonic activation of the extrusion dies could change the rheological properties of the melt at the interface with the transfer channel wall and could increase the extrusion velocity (and the productivity) at least by %, Table 1, in the proximity of the glassy state material temperature (Tv). These results are far beyond the increase in productivity, ( 20 %), given by other technical solution such as the coating of the interfacial surface with fluoro-tricloro-silane, [6]. This particular effect of the ultrasonic activation of the viscous-elastically media under pressure, was referred to as the thermo-pellicular effect and was characterized by the appearance of a fine layer of over fluid volume of processed material with a lubricant effect on the lateral side of the ultrasonic horn [5], [7], as results of : - a local heating of the polymer allowing the significant increase of the flow temperature and air velocity on the final part of the flowing trajectory. Near the top of the ultrasonic horn we noticed a local increase of the melt temperature up to C, at a maximum of 100 W ultrasonic output power; - a progressive friction decrease between the flowing polymer and the metal contact surfaces nearby the injection/extrusion zone and especially at the end part of the horn (sonotrode). The above-presented results opened an interesting research direction focused on the modification of the viscous-elastically fluids behaviour, under pressure and ultrasonic activation with notable results in the last few years [8], [9], [10], [11], [12], [13], demonstrating that, in particular for the presented conditions, the molten polymer actually slid on the activated wall of the die (no stick-slip effect). These conclusions are also confirmed by the research results from the European project ULTRA-MELT, [14]. 2 Industrial extrusion head with ultrasonic activation. According to the previous presented results and considerations, we proposed the thermoplastics extrusion tools design from the figure 2. Basically, the classical tool design was adapted to house a particular adjusted ultrasonic system. The dimensions of the ultrasonic transducer are given by the working frequency of the generator. This active device must be fitted into the extrusion head assembly. Detail D : i T A α a d0 D Fig. 2 Extrusion head with ultrasonic activation Experimental set A : a = variable, i = f (a) = variable, d 0 = 3 mm, constant, Experimental set B : d 0 =variable, i =f (d 0) = variable, a = 1 mm, constant, o Fig. 3 Experimental extrusion noozle ISSN: / ISSN: ISBN:

3 For a correct design of the horn, this requirement is fulfilled for a maximized conversion of the electrical into mechanical energy. Also, in an ultrasonic activation, the control of the amplitude offers the possibility of controlling also the amount of the effect and of the results. For different industrial applications, the sonic tool (the transducer), is submitted to a specific axial force, derived from the specific static pressure and the particular manufacturing conditions, [1]. The dimensioning of the horn is made with a relatively intricate mathematic algorithm with precisely working assumptions, [1], [2]. Any particular deviations from these could establish a substantially variation of the mechanical impedance of the ultrasonic system which itself comprises an elastic resonant oscillatory system exhibiting cyclic elastic deformations. So that, the real solution of the problem is much complicated, in principal, by the fact that the load on the tool is described by an unknown piecewisedefined function (for a half of the vibration period the output end of the tool could lose the contact with machined solid). More than that, a certain deviation from the theoretical model, improper sizing and poor results could be induced by the difference between the real physical properties of the employed materials and the declared nominal values of these. 3 The design of the ultrasonic horn The main functions of an ultrasonic system, Fig. 4, are the conversion of electric into mechanical energy and focusing this one in a limited application zone. In classical tools, the transducer settle the first mission generating mechanical vibrations and the solid metal wave guide, the horn, is designed for optimum transfer of the wave through eventually with increased amount of the amplitude. The strong unyielding contact between these two components is required. For the most common industrial applications, the front of the horn is compressed against the processed material with a static force while the other end of the ultrasonic device is free. It is assumed that each section of the rod executes axial harmonic vibrations at the resonance frequency at various amplitude along the axis of the horn. Theoretically, for the calculation of transversal consecutives sections of the exponential concentrator the most reliable relation was proposed by Eisner [2]. The system works in proper conditions at maximum amplitude if the frequency of the induce vibration is adapted to the mechanical impedance of the tool. Any deviations from the resonance frequency decrease the amplitude and the efficiency. As shown in Fig.2, the ultrasonic energy was injected into the pressurized molten polymer volume through the ultrasonic horn mounted as a central element centered to the nozzle. For the particular conditions from set A experiments, Fig.3, the Pascal pressure of the viscous-elastically fluid gave thermal and the surface effects (Rayleigh) on all the surfaces of the central wave guide (horn), including the frontal surface of this, and the processing conditions are similar to classical injection. For experimental set A, Fig.3, there is no mechanical axial load (on the front of the horn) and the experimental conditions could be assimilated to the extrusion process. Electric energy Mecanical energy Primary energy source Transducer UST Wave guide or horn! Processed material efficiency f f 0 = nominal resonance frequency self-stabilizer frequency system f 0 frequence Fig. 4 Efficiency of an ultrasonic system perturbations => f and low efficiency ISSN: / ISSN: ISBN:

4 Therefore, related to the design of the sonic tool, three different types of problems arise: - to establish the dimensions of components in order to increase efficiency, to increase the magnitude of the vibrations at the narrow end or the transformer or concentrator; - to minimize the heat and stress induced in the volume of transducer material because, however, the problem of heat dissipation is still present; - to maintain the functionality of the system narrow / next to the resonance frequency (self-stabilizer frequency system required). Several efforts have been made to include in an appliable algorithm these particular experimental conditions, giving unexpected deviations from the theoretical model. Based on the typical assumptions of Eisner, and on practical experience in industrial applications of ultrasonic activation, the specialized team from ISIM Timisoara has developed an algorithm and the associated soft for the design of ultrasonic concentrators. The main assumptions of this is derived from the Eisner theory and presume that the ultimate value of the stress induced in the concentrator is limited by permissible stress for the chosen material, in tand the system is capable of being operated in higher efficiency conditions. Basically, the classical tool design was adapted to house a peculiar adjusted ultrasonic system, Fig. 2, and the horn was designed and sized using the above mentioned software, accordingly to the previous presented considerations, Fig. 5. The algorithm is applicable even for particular shapes of the transducer as shown in Figure 4. The results were verified on a Branson Ultrasonic Analyser and the procedure was applied for other different industrial implementations. Fig. 5 Computer aided design of a non-conventional shape concentrator 4 Conclusions There was emphasized a particular effect of the ultrasonic activation namely the thermo-pellicular effect specific for the particularized activation conditions of the viscous elastically fluids under pressure and characterized by the appearance of a fine layer of over fluid volume of processed material with lubricant effect on the lateral side of the ultrasonic horn, [5], [7]. Further research [8], [10], [11], [12], [13], [14], confirmed that, for the particularized activation conditions of the viscous elastically fluids under pressure, the ultrasonic activation can change the properties of the contact interface, bust the transfer velocity in the proximity of the wall and offers an increased productivity in thermoplastics extrusion. ISSN: / ISSN: ISBN:

5 For right research conclusions and furthermore for efficient industrial applications, it is very important to know and understand the involved phenomenon s, to know the classical assumptions for those, to evaluate the real situation and to design the experiment for particular applications conditions An incorrect application of the theoretical principles, a misinterpretation of the classical assumptions or an uninspired adaptation of these at real working (or experimental) conditions, may generate faulty conclusions and failures in industrial implementation. In the design of the technical solutions for ultrasonic activation in processing melted polymers under pressure (injection, extrusion) could be considered the particular conditions and specific assumptions mentioned in this paper. The most frequent error sources are: wrong extrapolations from the classical assumptions, or wrong technical solutions for the experimental devices the overrating of the expected effects and benefits Acknowledgements Certain of the results presented in this paper are included in the first research report from the project PRECISUS MNT 7-021/2010 granted by the Romanian Ministry of Education and Research. References: [1] Kuttruff H., Ultrasonics-Fundamentals and Applications, Elsevier Appl.Sci.,Paris, 1991 [2] Rozenberg L., Physical Principles of Ultrasonic Technology, Vol. I&II, Plenum Press, N.York, 1973 [3] Agassant J.F., Avenas P., e.a., Polymer Processing Principles And Modeling, Hanser Publisher, 1996 [4] Benallal A., thèse de doctorat, L univ. de Pau et des Pays de L adour, (1991) [5] Stan D.V., thèse de doctorat, Univ. Montpellier 2/France, 1999 [6] El Kissi N., Piau J. M., Rhéologie Pour la Transformation des Polymères Fondus, In : Proceedings GDR 901, IMG Grenoble, 1991, p.171 [7] Stan D.V, Iclanzan T., Abadie M.J.M., L effet de Surface dans le Cas de L activation Ultrasonique des Milieux Visqueux sous Pression, Synthetic Polymer Journal, tome VII, 2000, p [8] Piau J.M., Piau M., Easier Flow of Viscoplastic Materials With Ultrasonic Longitudinal Wall Motion, J. of Non-Newtonian Fluid Mechanics, Vol.104, No.2-3, 2002, p.185 [9] Huilin L., Influence of Ultrasound on the Processing and Structure of Polypropylene During Extrusion, Polymer Engineering And Science, Vol. 7, No.1, 2002, (2002), p.87 [10] Guangshun C., Shaoyun G., Huilin L., Ultrasonic Improvement of Rheological Behaviour of Polystirene, J..Of Applied Polymer Science, Vol.84, No.13, 2002, p.2451 [11] Yurong C., Huilin L., Influence of Ultrasounds on the Processing and Structure of Polypropylene During Extrusion, Polymer Engineering & Science, Vol.42, No.7, 2002, p.1534 [12] Ghita E., Gillich G.R., Bordeasu I., e.a.- Aspect concerning polymers behaviour under tension, Materiale Plastice, Vol. 44, No.2, 2007, p. 158 [13] Postolache I. Fetacau C., Stan F., Nedelcu D., Study of the Polymer Flow Through Tubular Runner, Materiale Plastice, Vol.46, No.4, 2009, p. 458 [14] ***, ULTRA-MELT Europeean Project ISSN: / ISSN: ISBN: