Analysis of a die design and extrusion process of upvc profile: 3D modeling, simulation and experimental testing.

Transcription

1 C. Tomassini 1 and F. Angiolini 2 1 University of Rosario, Department of Plastics and Elastomers, Rosario, Argentina2 2 Alfavinil S.A., Buenos Aires, Argentina. Analysis of a die design and extrusion process of upvc profile: 3D modeling, simulation and experimental testing. CFD-simulation is performed for an existing die to produce upvc profiles by extrusion. Flow channel, free zone and die geometries are drawn in solid CAD 3-D. Experimental determinations are performed using an extruder with a calibrator experimentally designed and built specialty for this profile. The process is performed for two upvc compounds (mainly different thermal stabilizers) and behavior is compared between both material. Nonisothermal simulations are carried out taking into account the influence of shear rate and temperature on the viscosity of the PVC molten. Rheological testing are performed using an special rotational rheometer for PVC. Regarding of compare between simulations and testing results is defined to use the Ostwald law model for dependence of the viscosity with shear rate. Through the simulation is possible to determine the optimal distance where the deformations due to the die swelling of the profile molten is minimal, homogeneous and allow the profile to be calibrated correctly. Also, there are determined velocities distributions across each section of flow channel, shear rates, shear stresses, temperatures, pressures and die swelling, which gave us an indication of the final physical and mechanical properties of the profile in order to improve the design of the die design and process parameters. Finally, the simulated die swelling with the extruded one at 60 mm distance is compared for each material and it is possible to predict the behavior of the molten material at exit of die when the formulation of the compound is modified. 1. Introduction Thermoplastic profile extrusion is the production of continuous product by forcing plasticized materials through an appropriately shaped die having the desired final dimensions and geometry. In particular the use of PVC compounds have increasingly frequent use in the manufacture of frames for doors and windows and other products for the construction by its better properties (thermal and acoustic insulation, lighter, ecological, lower cost, etc.) respect to the conventional materials (steel and aluminum). Traditional methods of design depend heavily on the designer's experience and use the trial and error method, which may requires as much as 15 of these cycles for the development of a complex die head [1], raising costs and times development and construction. For instance, for a typical upvc window profile extrusion toolset (die and calibrator) the development costs are very high and trials may require equipment otherwise used for production, losing it time too The 3D modeling and simulation of the die design using computer simulation becomes a tool of great importance for industry, especially for small and medium-sized companies which do not use or have access to computer tools and trained personnel for the design of die and to optimize of the extrusion process. The PVC is a difficult material to be processed due to its poor thermal stability (begin to degrade at 140ºC) and also an increase of shear rate (screw speed) produces temperature rise by shearing and viscous friction, which worsens the situation. Since PVC is composed of different materials, must be correctly characterized the complex rheology [5] using an special rheometer. The objective of this work is to use computer 3D modeling and CFD(Computational Fluid Dynamics)[2] simulation software ANSYS Polyflow [6] to determinate the behavior of the flow of the molten material within die and compare the results with experimental runs. Also, it is important to determinate if the prediction improves the final quality of said profile (dimensions, geometry, avoiding or minimizing internal stresses, deformations, etc.); in addition to complementing this process with the optimum design of a calibrator modeled and specially made for it [3]. Through simulation also it is determined the optimum distance where the deformations of the cast profile at the head exit are minimal and allow to be calibrated to the required final dimensions within the recommended error [4]. The velocities distribution across each section of the flow channel[7], shear rates, shear stresses, temperature and pressure profiles are known, which define the physical and mechanical properties of the final. Finally, die swell at exit of head is measured, which determines if the molten profile can be calibrated correctly and the results obtained by simulation at 60 mm from die exit is compared for two PVC compounds and it was possible to predict the behavior of molten material for each one when the compound formulations is changed and improves quality.

2 C. Tomassini et al.: Analysis a die design and extrusion process of upvc profile: 3D modelling, simulation and experimental testing 2. Materials: The materials used in this study are commercial rigid PVC compounds, made by ALFAVINIL SA, code: RECOPP1-778 and RECOPF There are composed by different kinds of materials (resin, plasticizer, additives, mineral fillers, pigments, thermal stabilizers and others); which are showed in Table 1 and physical and thermal properties are showed in Table 2. Table 1: Chemical composition Material (PHR) RECOPP PVC resin index K Plasticizer (Epoxidized soybean oil) Thermal Stabilizer (Ca/Zn type 1) Thermal Stabilizer (Ca/Zn Pack type 2) RECOPF Mineral filler External lubricant Processing aid Pigment PHR: parts per 100 grams of resin. Table 2: Physical and thermal properties RECOPP1-778 RECOPF Density (δ) 1.65 Kg/m³ 1.62 Kg/m³ Thermal conductivity Mass Heat Capacity 0.12 W/(m ºK) 0.12 W/(m ºK) 1200 J/kg o C 1200 J/kg o C 3. Non-Isothermal conditions laws: It was considered heat transfer by conduction of the flow inside the die and convection out of it. PVC molten viscosity depend on shear rate and temperature, then it must be used the following laws respectively [4]: Ostwald de Waele (also called Power law): Ƞ: viscosity ɣ: shear rate. n: power law index K: consistency coefficient. Arrhenius: H(T) = exp α { [(1/T) - (1/Tα)]} α: ratio of activation energy to perfect gas constant. T: reference temperature (Kelvin degree) Tα: reference temperature for which H(T)= 1 4. Rheological tests: The values of rheological parameters "ŋ", "m" and "λ" were obtained by testing in the laboratory of PLAPIQUI (CONICET Bahía Blanca-Argentina). It was done in rotational rheometer (Rheometrics RDA II), which evaluated rheological behavior of upvc compound samples, which were supplied by ALFAVINIL S.A. (PVC compounds manufacturer). Frequency sweeps were performed at 190 C under N2atmosphere and obtained ŋ', ŋ" and finally ŋ* for each ɣ (frequency or shear rate). The results are showed in the follows table 3 and 4 for each material: Table 3: Shear Rate vs. Viscosities PVC RECOPP1-778 ɣ [1/s ] ŋ* [Pa.s ] , ŋ*: complex viscosity (viscoelastic system),which is square root of quadratic sum of viscosities ŋ' and ŋ" (ŋ': viscosity regarding to G' elasticity module and ŋ": viscosity according to G viscosity module) [8]. Table 4: Shear Rate vs. Viscosities PVC RECOPF ɣ [1/s ] ŋ* [Pa.s ] 0,

3 C. Tomassini et al.: Analysis a die design and extrusion process of upvc profile: 3D modeling, simulation and experimental testing , ,7 5. Rheological adjustment: Applying Ostwald law parameters ɣ and ŋ* are elaborated Chart 1 and 2 and obtained parameters K and n for each material: PVC RECOPP1-787: K= (1 - n) = -0.6 => n= 0.4 Chart 1: Rheological adjust PVC RECOPP1-778 Ƞ[Pa.s] y = 73702x -0,6 0, ɣ [Pa. seg] In solving for the free surface location, the position variables are also coupled to the temperature, velocity and pressure fields. To solve the coupled problem it must be defined two sub-tasks: one each for the fluid(sub-task 1) and the solid (sub-task 2). Each sub-task contains a particular model, domain definition, material properties and boundary conditions, including interface conditions with the other sub-task[6]. Sub-tasks are coupled because the global solution of the problem depends on the values of the solution variables at the intersection of the fluid and solid domains Geometric Model: The existing die and flow channel and exit zone large= 60 mm are drawn in solid 3D,(see below figure 1). This large was achieved in before simulations using the ANSYS Polyflow until to assure that the die swelling dimensions were inside of percent value recommended: 3-6 % for PVC rigid profiles with 3-4 mm of thickness, [4]. Since it must be simulated all the model, then, to reduce the computational run, it is draw a quarter of the model as is showed in the same Figure 1. Figure 1: Geometric models wire, solid view and a quarter of it in 3D solid (it is used in the simulation) PVC RECOPF K= (1 - n )= 0.544=> n= 0.46 Chart 2: Rheological adjust PVC RECOPF Ƞ(Pa.s) y = x -0,54 0, ɣ [Pa. s] Arrehnius law parameters are considered according to the temperature of molten polymers, [9], in our case for both materials this values are same: α= T= 190ºC 6. Simulation [6]: It is analyzed the coupled problem of non-isothermal flow of a fluid and heat conduction at surfaces-symmetric steel die. The melt enters the inflow domain at a fixed temperature= 190 C and at a given flow rate for each material. The problem involves flow heat transfer by conduction and convection, and heat generation by viscous dissipation. Energy, momentum and incompressibility equations are solved in the fluid domain. The energy equation for heat transport problems is solved in the solid (die) domain Mesh: It is imported a quarter of geometric model in solid 3D in the simulation software ANSYS Polyflow and are named sub-domains and boundaries (fluid, solid, free zone exit die, inflow, symmetries surfaces, etc.); since are showed in the follow Figure 1: Figure 1:Sub-domains and Boundaries definitions Then it is created the meshing (using automatic method), which divides each sub-domain in finite geometric parts: (triangular, tetrahedrons, hexahedrons, etc.)to apply the CFD (Computational Fluid Dynamic), see follow Figure 2: Figure 2: Meshing of geometric model quarter 3

4 C. Tomassini et al.: Analysis a die design and extrusion process of upvc profile: 3D modelling, simulation and experimental testing causes different cooling and contractions increasing the stresses into the melt (Figure 4). Figure 3: Velocities value along flow channel 6.3. Setup: It is defined enter parameters and boundaries conditions for fluid (flow channel inside and die exit) and solid (die) of each material in separate non isothermal simulations: Fluid: Material: Ostwald law parameters: K and n; Arrehnius parameters, density, inflow melt temperature, thermal conductivity, heat capacity per unit mass, etc. Boundaries conditions (for examples: inflow: mass flow, symmetries surfaces, outlaw: zero drop pressure, etc.). "When shearing occurs in a flow, the friction of the different fluid layers generates heat. When the fluid is highly viscous and/or the shear rate is high, the heating of the fluid caused by this phenomenon must be taken into account" [6], In the simulation software must be Select: Viscous + wall friction heating. Remeshing of free surface: it is done to obtain the dimensions and geometry of die swelling. Solid: Material data: alloy steel Cr-Ni-Mo, Thermal conductivity= 30 [W/m- C]. Thermal boundaries conditions: Thermal conductivity of die, heat convection transfer from melt to die walls, etc Solution: It is launched computational run; where ANSYS Polyflow solves complex ecuations applying CFD (Computational Fluids Dynamic): 6.5. Results: There are obtained the follow simulated parameter for each material: PVC RECOPP1-778: Velocities: The highest value of velocity(red color=> 2.85 m/s) is localized in the zone where flow channel change of geometry (transition zone); which is showed in the Figure 3. Distribution of velocities in 4 planes of across flow channel sections from inlet (0, 17, 35.5 and 74 mm): velocities vectors increase their values in the center of each section and it is also founded highest values(red color) in the transition zone. This unbalance generates shear stresses into the melt, which could provoke fails in service of the profile. Also, the melt arrives to different times, which Figure 4:Velocities vectors across flow channel sections: Temperatures: Highest temperature value(red color=> 491ºK= 218ºC) in the zone near to the internal wall of the die where it has lowest thickness(3 mm total and 1.5 mm considered half of thickness since is showed in Figure 5). Figure5: Temperatures profiles along flow channel Cut at 108 mm from the beginning of flow channel (inlet). This peak of highest is produced in the zone near(2.2 mm of total thickness of 3mm) since is showed in the Chart 4, where is the curve of temperature across section at 108 mm. The high velocity gradients near the die exit lead to an

5 C. Tomassini et al.: Analysis a die design and extrusion process of upvc profile: 3D modeling, simulation and experimental testing important viscousdissipation effect. The temperature of the polymer melt increases from the converging zone to the dielip. This temperature incrementmust be monitored to avoid melt degradation. The simulation helpsoptimize the geometry of the die, the flow section for the cooling fluid, and other conditions in orderto maximize the flow rate and the extrude speed. Chart 4:Temperature profile across die at 108 from intet In this point start the internal wall of the Die(to 3 mm to the other wall) - Dimensions in meters Figure 4: Pressures Shear stresses: Thehighest shear stresses(red color) is produced at transition zone (showed in the Figure 8)=> Maximum value= 1.055e+006 Pa= Bar, which increase the temperature and could produce degradation of the PVC molten. This zone of highest shear stresses is the same where the velocities are highest too (see Figure 4) Figure8: Shear Stresses It is 1 mm at internal die wall (die thickness= 3 mm since 0 x-axis) Pressures: Pressure is decreasing along channel melt and being cero at the die exit and the free zone. Pressure die inlet (red color) is aprox. 7.4 e+006 Pa = 74 Bar, (Figure 6); which is used to compare with sensor pressure. Figure 6: Pressure profile along flow channel Figure 9: Shear Stresses Shear Rates: It appeared a zone of highest shear rates(red color) = 188 1/s, which generates shear stresses and increase of temperature and could produce degradation of PVC molten (Figure 7) The shear rate value might be under 100 1/s(recommended) to avoid generates shear stresses, which could produce degradation of the melt. Figure 7: Pressure profile along flow channel Comparison Original (blue rectangle) and Profile to dieexit (Determination of Die Swelling at 60 mmof die exit) (Figure 10). Figure 10: Comparison Original and Profile to die exitat 60 mm 5

6 C. Tomassini et al.: Analysis a die design and extrusion process of upvc profile: 3D modelling, simulation and experimental testing Figure 12: Pressure Profile It is a half of profiles, then we have the follow dimensional and percent values of the Die Swell. Table 5: Shear Rate vs. Viscosities PVC RECOPF Origin Melt( Differences Percent( Profiles (mm) mm) (mm) %) Width Thickness PVC RECOPP20-787: Velocities: Figure 11: Velocities profile Temperatures: Figure 13: Temperature Profile - Along of Flow inside and exit of Die and Die It is observed a zone of highest temperature near of internal wall of die and also at exit of it. Max. Temperature Value(red)= 479,6 ºK = ºC. Figure 14: Die Swelling at 60 mm of die exit: Pressures: Maximum Velocity value(red)= 2.90 m/s Simulated pressure at inlet of molten material to the die is was 54.3 bar (red color zone - figure 12) Table 6: Comparison Original and Die swell profile. With Thickn ess Original Profile (mm) (half of total= 30 mm) Die Swell at 30 mm die exit (mm) Differ ences (%) (total) Die swelling was good at With=> 5.5 % (recommended values: 5 to 6 % for rigid PVC profiles of thickness= 2-3 mm) [4]. Die swelling was not good in the Thickness => 17.8 %, then it could have problem in the calibration process of

7 C. Tomassini et al.: Analysis a die design and extrusion process of upvc profile: 3D modeling, simulation and experimental testing profile. 7. Experimental Verification at C.T.P.E. (Technological Center of Plastics and Elastomers): It was used an extruder equipment Collins (made in Germany): screw diameter= 25 mm, compression relation= 2:1, experimental calibrator (designed and built specially for this profile), vacuum and cooling calibration and dragging units. Also, It was used an existing die for rectangular 30 x 3 mm PVC profile. Process parameters: Temperature inlet melt flow in die = 190 C Temperature profile (extruder): 149 C (feed zone), 171 C, 182 C (compression zone), and 190 C (die) Temperature water cooling: 20ºC Pressure (*): It was measure with pressure sensor in the melt at the extruder outlet or die flow inlet. There were made five experimental testing, where were modifying the flow (screw velocity) until to achieved the final dimension required at the exit of the unit calibratorvacuum-cooling and the parameters of each test were registered and showed in follow Table 7. Table 7: Process parameters Tests No. Velocity Pressure Output (RPM) (*) (Bar) (.kg h) (*) It was measured with a pressure sensor at inlet flow of die and his value was showed in the control system of the extruder. Also, the flow temperature in the die was measured with a temperature sensor at inlet flow of die. Figure 18: Pressure and temperature sensors Temperature sensor Note: Speed System (Drag)= 1.05 m/min. Pressure sensor It is increased screw velocity (RPM) until to reach values close to the final dimensions of the profile: Table 8: Dimension results of final profile Width (mm) Thickness (mm) Dimensions (mm) to 3.0 Percent (%) At last experimental verification (Nº 5) was possible to obtain values close to the required dimensions of the profile, but it wasn`t possible to continue due the material in the calibrator jammed, which caused not to work in continuous regime. Furthermore, it was possible to see the die swelling of the profile molten of the die exit as showed in the follow Figure 19 with the distance of 60 mm between the die exit and calibrator-vacuum system inlet. 60 mm Figure 19: Die swelling at die exit 8. Comparison rigid PVC ALVAVINIL RECOPP1-778 vs RECOPF at vacuum-calibrator system Table 9: Dimension results of final profile Profiles Original (mm) Die Swell RECOPP1-778 Die Swell RECOPF (mm) (%) (mm) (%) Width Thickness Experimental Verification at ALFAVINIL S.A. It was used a Rheomex single screw extruder of diameter=19 mm, length=25 L / D, compression ratio= 2: 1 and three heating zones. Besides, slit rheological matrix= 20mm wide x 2mm thick. This matrix has a mass temperature sensor and two melt pressure sensors up to 500 bar, separated by 50mm from each other. Between these two sensors it is assumed that the flow is perfectly developed and the difference in pressure is used for the calculation of viscosity. There are in a different Ca/Zn packages and in different proportions. In addition, there is no wax and the other less, and the flow modifiers are the most suitable for the development of primary PVC particles, which makes that flow of them less particulate and more viscous. This packages have between 8 and 12 individual components in their composition, so it is difficult to interpret or predict the behavior. It is produced profiles of thickness: 2 mm and 7 20

8 C. Tomassini et al.: Analysis a die design and extrusion process of upvc profile: 3D modelling, simulation and experimental testing mm in, which means that the increase over the width: 20 mm and measured on 3 different rpm(revolution per minute). It is summarized in the following Chart 5. Chart 5: Die swell measured on frozen slab at die exit The upvc compound RECOPP1-778 vsgave a die swelling of 1mm in excess of 20mm and is also not sensitive to the screw speed (rpm), while the RECOPF initially gave a higher swelling die (1.5) and also increases with the screw speed (rpm), showing greater elasticity in the melt. 11. Conclusions: 1. It was predicted using the simulation software ANSYS Polyflow that the die swell values(%) for compound RECOPP1-778and them are inside of the recommended values at the width and a little exceed for the thickness(values recommended are 5-6 % for rigid PVC profiles of 2-3 mm[4]. Also it was verified on the experimental tests. 2. It was found a zone of high shear stresses, which could produce degradation of molten PVC. 3. This highest shear stresses zone was located at the same highest velocity zone. 4. It was found unbalance of velocities across flow sections and zones of high shear rates; which produces shear stresses and different cooling due the flow arrived not at the same time; then, it could be solved modifying geometric model in the transition zone (more smooth) or using Inverse Method [6]; which defines best parallel zone. 5. It was possible predict with simulation software ANSYS Polyflow the behavior of die swell at die exit according to results experimental verifications at C.T.P.E. and ALFAVINIL S.A. of both compounds (same screw velocity: 62 rpm) when the compound formulation is changed. In our case, the increase of thermal stabilizer and add wax as a lubricant in the material formulation of RECOPF20-787, it produces an increase of die swelling at the thickness mainly. viscoelastic free surface flow with level set method", ANTEC, Society of Plastics Engineers Annual, [3] Srinivsar et al., Extrusion simulation and experimental validation to optimize precision die design, ANTEC, [4] Hofmann C. and Michaeli W., Extrusion Dies for Plastics, Hanser, 4th ed., 2016 [5] Vlachopoulos J. and Fattmann, The Role of Rheology in Polymer Extrusion, Department of Chemical Engineering, McMaster University, [6] ANSYS, Polyflow, Tutorial Guide, ANSYS, Inc. Release 18.0, Southpointe, USA, January [7] Nobrega et al., Flow Balancing in Extrusion Dies for Thermoplastic Profiles, International Polymer Processing, XVIII, 2003), Hanser Publishers, Munich. [8] Thermo Haake, Course-5 of Viscoelasticity, Viscometry and Reometry Seminar,Thermo Haake, 2003 [9] Laidler, K. J., The World of Physical Chemistry, Oxford University Press,1993 Acknowledgements The authors would like to thank to: ALFAVINIL S.A. (PVC compound manufacturer - Argentina), especially to Dr. Fernando Angiolini (Technical Manager) and Leonardo Tort(Technical Customer Support). Eng. Carlos Olivares G, Academic/Startup account executive, southern cone. ESSS - ANSYS. Magister Eng. Monica Bollatti, Vicedirector of Higher Polytechnic Institute(I.P.S.), U.N.R. T.U.P.E. Gustavo Pierson, Professor of Processed 2 (Extrusion process) and Consultant at C.T.P.E. T.N.Q. Hugo Peleteiro and T.U.P.E. Sergio Cuello, Authorities of Plastic and Elastomers Dept, I.P.S. U.N.R. (National University of Rosario) Date September, 2018 References [1] Szarvasy et al., Computer Aided Optimization of Profile Extrusion Dies, International Polymer Processing, XV, 2000, Hanser Publishers, Munich [2] Zhao, W., Finite Element Simulation of 3D unsteady