International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 4, July Aug 2016, pp.249 255, Article ID: IJMET_07_04_027 Available online at http://www.iaeme.com/ijmet/issues.asp?jtype=ijmet&vtype=7&itype=4 Journal Impact Factor (2016): 9.2286 (Calculated by GISI) www.jifactor.com ISSN Print: 0976-6340 and ISSN Online: 0976-6359 IAEME Publication MODELING THE FILLING PHASE OF INJECTION PROCESS OF TECHNICAL PARTS WITH THERMOPLASTICS COMPOSITES BASED ON HEMP FIBERS Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari Laboratory of Industrial Management and Energy and Technology of Plastics and Composites ENSEM Hassan II University Casablanca Morocco ABSTRACT The injected thermoplastics composites grow considerably in different industrial fields and provide increasing performance allowing manufacturers to consider innovative and competitive technical solutions. The extent of their physical and mechanical and aesthetic performance is constantly developed. However, the level of control of production technologies must be sufficiently high to better meet the specifications in terms of quality and performance and productivity, but also costs. In this paper, we present a numerical simulation of thermoplastic composites injection molding, in order to better predict the behavior of these materials during processing and propose operating conditions to give the best results and optimize the production. We studied the evolution of certain defects depending on implementation parameters: Temperature and pressure and speed of injection. The tested material is polypropylene reinforced at different rates (10 to 30%) of hemp fibers. Key words: Thermoplastic Composites, Injection Molding, Experimental, Short Hemp Fibers, Parts Quality, Processing Parameters. Cite this Article: Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari, Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics Composites Based on Hemp Fibers. International Journal of Mechanical Engineering and Technology, 7(4), 2016, pp. 249 255. http://www.iaeme.com/ijmet/issues.asp?jtype=ijmet&vtype=7&itype=4 1. INTRODUCTION Modeling of the filling the mold cavity during an injection cycle is difficult. On one hand, the geometry of the molded parts is often complex: multi-cavity mold, curved surfaces, three-dimensional parts. On the other hand, the filling thermorheology depends also on the polymer characteristics. It exists in the literature many studies on filling molds with simple geometry: rectangular cavity, cylinder, disc injected through the center Pearson, Kamal and Koenig [1-2-13] modeled the flow in disk geometry with pseudoplastic and thermo-dependent behavior law without considering the thickness solidified and neglecting the unsteady http://www.iaeme.com/ijmet/index.asp 249 editor@iaeme.com
Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari terms. Wu and al [4] have used the same geometry with consideration of the solidified thickness and thermal-mechanical coupling. Modeling flow in flat plate geometry was the subject of many studies. White then Titomanlio [5-6] proposes one-dimensional models. Van Wijngaarden and al [7] studied a two-dimensional model: The inlet flow rate is constant until a limit value of the pressure at the start of the plate. In addition, the calculation is carried out at constant pressure (and rate flow decreases with time). The retained behavior law is a thermodependent power law. This work consists in modeling the behavior of a thermoplastic composite based on short hemp fiber during the filling phase of the injection molding process. Rheological tests on the online rheometer at different temperatures allowed us to determine a thermo-dependent behavior law for each composite formulation in industrial conditions. This law will be used thereafter as input data for more accurate modeling of the filling phase. 2. EXPERIMENTAL PROCEDURE 2.1. Materials For industrial applications, the polypropylene homopolymer is most used. It offers the best mechanical and thermal characteristics. The characteristics of the polypropylene used are: density = 900kg/m 3, melt index I = 25g /10min, melting temperature T = 168 C, thermal conductivity at 25 C is λ = 0.3W/mK. In order to improve their properties to use them as reinforcements for composite materials, Hemp fibers have been treated with alkaline solution. The used fibers have a diameter of 250 μm and a length ranging from 2 to 2.5 mm. For better fiber matrix interfacial adhesion, we have added to the mixture 5% of polypropylene grafted maleic anhydride. The granules of polypropylene reinforced at various rates of hemp fibers were produced by the extrusion process. 2.2. Geometry of the mold cavity The mold cavity allows molding complex part for technical use in the automotive field. The finite element mesh components are triangles, which helped to limit the calculation time. The actual injection conditions (time and pressure) are considered in the simulation. Figure 1 The studied geometry 2.3. Simulation approach The originality of our simulation work is the introduction of a thermo-dependent rheological behavior law determined experimentally using rheological measurements on an online rheometer developed in our laboratory [8-9]. http://www.iaeme.com/ijmet/index.asp 250 editor@iaeme.com
Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics Composites Based on Hemp Fibers The determination of the thermo-dependent power law is made from measurements at three temperatures. Through the curves equations determined above, we deduce the coefficients of power law: η = A & γ exp( CT ) (1) B wr The plot log η= f(logγ& c ) determines by a linear regression the viscosity behavior law according to γ& c at each temperature. The representative power law of the rheological behavior of polypropylene reinforced with 25% of weight short hemp fibers in the studied temperature range can be written: η = 2063,81 & γ 0.62 wr exp( 0,013 T ) (2) This relationship will be used to model the composite flow in a mold cavity. Following the same approach, it was determined behavior laws for pure PP and PP with different rates of fiber. Figure 2 Simulation approach [10] http://www.iaeme.com/ijmet/index.asp 251 editor@iaeme.com
Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari 3. RESULTS AND DISCUSSION 3.1. Evolution of the pressure and temperature during the filling of the mold cavity We are interested in analyzing the evolution of the pressure during filling especially at the injection location. This pressure determines the complete filling of the mold cavity and the fiber orientation which entails the performance and appearance of the final part. Figure 3 Pressure evolution at the injection location during the filling phase During filling, the volume of the mold cavity is filled at a relatively low pressure. The pressure curve mainly depends on the material resistance to flow into the mold and in particular: the reinforcement content - the section of the channels and cavity - the distance traversed by the material in the mold- the viscosity of the molten material - the injection speed the material temperature - the mold temperature. Figure 4 shows a prediction of the temperature evolution in the mold cavity. Figure 4 Evolution of temperature in the mold cavity http://www.iaeme.com/ijmet/index.asp 252 editor@iaeme.com
Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics Composites Based on Hemp Fibers The evolution of the temperature in the mold governs the cooling time. This also depends on the part thickness and the material temperature and his thermal diffusivity. The cooling time must be determined for a temperature in the core of the part, giving no deformation at mold release and for a residual pressure value in the part causing no depression when the part is no longer in contact with mold walls. This phase is crucial to the quality of the manufactured part and productivity. 3.2. Effect of the injection location on the filling time We are interested to estimating the filling time of the mold cavity depending on the position of the injection location. Figure 5 Prediction of the part filling time at two different positions of the injection location. We note a difference of 0,18s, allowing a significant gain on the cycle time, especially if we think on an industrial scale on high rates. Furthermore, it remains to analyze the presence of defects on both configurations in particular weld lines. 3.3. Prediction of defects in the part Several types of defects can occur in the molded parts, in this section we will focus on the influence of the position of the injection location on the appearance of certain defects such as air traps and weld lines. Figure 6 Prediction of air traps occurrence in the part at different positions of the injection location http://www.iaeme.com/ijmet/index.asp 253 editor@iaeme.com
Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari After observation of Figure 6, it is observed the presence of many air traps in the second design, but generally this problem is encountered in the processing of composites with vegetable fibers. The fibers water content is one of the limiting parameters for hemp fiber, which has a direct influence on the molding: the viscosity of the material and the mechanical properties and surface appearance of molded parts. The presence of humidity in the material can cause poor adhesion fiber / matrix and contribute to the formation of internal bubbles in molded parts [11, 12]. Specific treatment for hemp fiber is required, (steaming the fibers before mixing). The appearance of a weld line is a major problem both aesthetically and mechanically in the design of injected part with thermoplastic composites. Figure 7 Prediction of weld lines appearance in the part at different positions of the injection location Figure 7 shows that the injection location has an influence on the appearance of weld lines in the room, in particular in areas with irregular geometry. The first design is still the best. Weld lines are formed when two melt fronts come into contact at low speed. Especially in areas of geometric variations: Change in thickness holes - the presence of insert [13-14]. These weld lines cause in addition to the drop in the mechanical properties of the parts, optical imperfections in particular when using high shine material [15]. 5. CONCLUSION We conducted a numerical simulation of filling of an industrial part for technical use in the automotive industry. Our simulations were based on a behavior law of the material experimentally determined. [16] We have shown the influence of the injection location on the part quality and on the presence of certain defects such as air traps and weld lines and finally on the filling time. Our results are in good agreement with experimental results. [17] Moreover, this work has allowed us, at first, to validate the part design, then, analyze the impact of the production cycle parameters on both the quality of the produced parts and productivity. REFERENCES [1] J. E. A Pearson. Mechanical principals of polymer melt processing. Centre for Advanced Engineering Study and Dept. of Chemical Engineering, Massachusetts Institute of Technology. London 1996 [2] M.R. Kamal, S. Kenig. Polym.eng.sci, Chap.13, pp.102. 1973 [3] M.R. Kamal, S. Kenig. polym. eng. sci 12, 4. 1982 [4] Wu C.F Huang, C.G Gogos; polym Eng sci 14, 223. 1974 [5] J.L. White, H.B Dee. Poly Eng.Sci 3, 14. 1974 http://www.iaeme.com/ijmet/index.asp 254 editor@iaeme.com
Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics Composites Based on Hemp Fibers [6] G. Titomanlio. Polym. Eng. Sci 22, 589. 1982 [7] Wijngaarden, J.F, Dijkman, P.Wesseling 11.175. 1982 [8] A. Haddout, G. Villoutreix ; Brevet N/REF 233 489D.14028DL. (1992). [9] M. Benhadou, A. Haddout, G. Villoutreix. Rheological behaviour of polymer composites PP/FG at high shear rate. Journal of reinforced plastics and composites, Vol 26 N13, 2007. [10] M. Benhadou, A. Haddout, A. Tazine, S. Ajana. The 10th congress of mechanical, Oujda Morocco from 19 to 22 April 2011. [11] A. Haddout, G. Villoutreix; Intern. J. polymer Processing XV p.291-296. 2000 [12] D.J Lee, M.W Kim, S.Y Kim, S.H Lee, J.R Youn. Korea-Australia Rheology Journal, 22(2), pp. 95 103, June. 2010 [13] Robert A. Mallo. Plastic Part Design for Injection Molding. Hanser Publishers. 1994 [14] J.G. Kovács, B. Sikló. Experimental validation of simulated weld line formation in injection moulded parts. Polymer Testing 29(7), pp. 910 914. 2010 [15] Shia-Chung Chen, Wen-Ren Jong, Jen-An Chang. Dynamic Mold Surface Temperature Control Using Induction Heating and Its Effects on the Surface Appearance of Weld Line. Journal of Applied Polymer Science, Volume 101, 1174 1180. 2006. [16] Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Houssam Ourchid, Study of thermorheological behavior of polypropylene composites reinforced with short hemp fibers during industrial injection molding process. International Journal of Mechanical Engineering and Technology, 7(4), 2016, pp. 29 37. [17] B. Benhadou, A. Haddout, M. Benhadou, H. Bakhtari, Study and optimization of the injection molding of composites based on short hemp fibers. International Journal of Mechanical Engineering and Technology, 7(4), 2016, pp. 38 47. http://www.iaeme.com/ijmet/index.asp 255 editor@iaeme.com