Abeykoon, C.

Brown, E. C.

Coates, P. D.

Deng, J.

Harkin-Jones, E.

Karnachi, N.

Kelly, A. L.

Li, K.

Price, M.

Vera-Sorroche, J.

The University of Manchester - Institutional Repository

The University of Manchester ResearchThe Effect of Molecular Weight on Thermal Efficiency forSingle Screw Extrusion of HDPEDocument VersionAccepted author manuscriptLink to publication record in Manchester Research ExplorerCitation for published version (APA):Vera-Sorroche, J., Abeykoon, C., Kelly, A. L., Brown, E. C., Coates, P. D., Deng, J., Li, K., Karnachi, N., Harkin-Jones, E., & Price, M. (2013). The Effect of Molecular Weight on Thermal Efficiency for Single Screw Extrusion ofHDPE. In host publicationPublished in:host publicationCiting this paperPlease note that where the full-text provided on Manchester Research Explorer is the Author Accepted Manuscriptor Proof version this may differ from the final Published version. If citing, it is advised that you check and use thepublisher's definitive version.General rightsCopyright and moral rights for the publications made accessible in the Research Explorer are retained by theauthors and/or other copyright owners and it is a condition of accessing publications that users recognise andabide by the legal requirements associated with these rights.Takedown policyIf you believe that this document breaches copyright please refer to the University of Manchester’s TakedownProcedures [http://man.ac.uk/04Y6Bo] or contact uml.scholarlycommunications@manchester.ac.uk providingrelevant details, so we can investigate your claim.Download date:30. Jun. 2022Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany) THE EFFECT OF MOLECULAR WEIGHT ON THERMAL EFFICIENCY FOR SINGLE SCREW EXTRUSION OF HDPE   J. Vera-Sorroche1*, C.Abeykoon1, A.L.Kelly1, E.C.Brown1, P.D.Coates1, J.Deng2, K.Li2, N.Karnachi3, E.Harkin-Jones3 and M.Price3  1IRC in Polymer Engineering, School of Engineering, Design & Technology,University of Bradford BD7 1DP,UK-      J.VeraSorroche@student.bradford.ac.uk; Y.Abeykoon@bradford.ac.uk; A.L.Kelly@bradford.ac.uk; E.Brown@bradford.ac.uk; p.d.coates@bradford.ac.uk 2School of Electronics, Electrical Engineering and Computer Science, Queen's University, Belfast, BT9 5AH, UK-jdeng01@qub.ac.uk; k.li@qub.ac.uk 3School of Mechanical & Aerospace Engineering, Queen’s University Belfast, BT9 5AH, UK- n.karnachi@qub.ac.uk; E.Harkinjones@qub.ac.uk; m.price@qub.ac.uk     Abstract Polymer extrusion is an energy intensive process whereby the simultaneous action of viscous shear and thermal conduction are used to convert solid polymer to a melt which can be formed into shape. To optimise efficiency, a homogeneous melt is required with minimum consumption of process energy. In this work, a highly instrumented single screw extruder has been used to study the effect of molecular weight on the thermal efficiency of the extrusion process. Three different molecular weight grades of high density polyethylene (HDPE) were extruded at a range of extrusion conditions.  Three geometries of extruder screws were used at several set temperatures and screw rotation speeds. The extruder was equipped with real-time quantification of energy consumption; thermal dynamics of the process were examined using novel thermocouple grid sensors at the entrance to the die in conjunction with an infra-red sensor located in the extruder barrel.  Results showed that molecular weight of the polymer had a significant effect on process energy consumption and thermal homogeneity of the melt.  Highest specific energy consumption and poorest homogeneity was observed for the highest molecular weight grade.  Extruder screw geometry, set extrusion temperature and screw rotation speed were also found to have a direct effect on energy consumption and melt consistency.  In particular, energy consumption was lower using a barrier flighted screw compared to single flighted screws at the same set conditions. These results highlight the complex nature of extrusion thermal dynamics and suggest that rheological properties of the polymer can significantly influence thermal efficiency of the process  Keywords: Polymer extrusion, melt temperature, energy, polymer rheology     Introduction   Polymer processing and new technologies of polymer product applications have greatly increased over the last few decades due to their many attractive uses in diverse industrial sectors. In general, in polymer processing extruders, such as single screw extrusion, the raw polymer material is fed into the machine through the hopper and conveyed along the screw by the application of external heat, internal heat generation, and the application of pressure which is required to push the molten material into the die and form the final product. Polymers exhibit viscoelastic behavior and thus to understand the science of polymer processing a fundamental understanding of the rheological properties of polymer melts is required [1]. Polymeric melts show properties of elastic and viscous behavior and as a result, weight average molecular weight (Mw) and its distribution (Mw/Mn) will determine the polymer processability during the extrusion process, in addition to the extrusion temperature, pressure, shear, thermal history, drag flow deformation, extrusion die and screw designs [2].   The main objective of this work was the study of the rheological processing behavior of HDPE for single screw extrusion using real-time measurement techniques. Novel thermocouple grid sensors enabled characterisation of the thermal dynamics of the extrusion process which in combination with real-time energy consumption gathered evidence relating to thermal extrusion performance [3, 4].  The role of extruder processing conditions, three extruder screw geometries and set extrusion temperatures, and the effect of Mw and (Mw/Mn) on the measured temperatures and energy were quantified, Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany) highlighting  the potential energy savings from careful selection of the extruder processing conditions, screw geometry and polymer type.  Experimental  The experimental work was carried out on a 63.5 mm single screw extruder (Davis standard BC60). Three different molecular weight grades of high density polyethylene (HDPE) were used throughout the studies. The first material was a high density copolymer grade (Rigidex HD5050 EA, Ineos) with a narrow molecular weight distribution and a quoted melt flow index (MFI) of 4.0 g/10 min, 2.16kg load. The second resin was a medium molecular weight polyethylene (Rigidex HD6007 S, Ineos) and (MFI) of 0.6g/10 min, 2.16 kg load. The third material was a high molecular weight copolymer grade (Rigidex HM5411EA, Ineos)  supplied for medium and large blow moulding applications and (MFI) of 10g/10 min, 21.6 kg load.   The molecular weight characteristics of the three polymers are presented in table 1.  Table1: General characteristics of HDPEs   Mn Mw Mw/Mn HD5050EA 22592 91727 4.1 HD6007S 20500 119000 5.8 HM5411EA 17100 256000 15  The rheological behaviour for all materials was studied using a Rosand RH10 off-line capillary rheometer over an extrusion shear rate range of 10 – 1000s-1. A comparison of shear viscosity is shown in figure 1 to examine the effect of temperature-molecular weight dependence.  Fig.1. Shear viscosity of the HDPEs Viscosity decreased with increasing temperature and decreasing molecular weight as could be expected. Three extruder screws, typically used in the polymer industry, were used throughout the experiments with a length to diameter ratio of 24:1. These polyolefin screw designs were selected to reflect the important role of geometry on the melting performance in single screw extrusion. Schematic representations of the screw designs are shown in figure 2.  a) 3:1 compression ratio, with gradual compression and % free volume (FV) = 31.64  (b) 3:1 compression ratio, with rapid compression and % free volume (FV) = 26.99  (c) 2.5:1 compression ratio, barrier flighted with spiral Maddock mixer and % free volume     (FV) =    36.05  Figure 2: Extruder screw geometries used  Additionally three set temperatures were chosen with profiles of 130°C-155°C -165°C -180°C, 140°C -170°C -185°C -200°C and 150°C -185°C -205°C -220°C in barrel and die temperatures of 180°C, 200°C and 220°C. In- process monitoring techniques were carried out to assess the extrusion process by assembling an instrumented die adaptor (internal diameter 38mm) at the entrance to a 6mm diameter rod die (figure 3).     Fig. 3. Instrumented extruder die showing location of thermocouple grid at die entrance.  Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany) In the adaptor, a wall thermocouple (3 mm diameter J-Type) and an insulated J-Type thermocouple of 0.5 mm diameter protruding 1.0 mm (set in an insulating ceramic surround) were fitted. This enabled measurement of two melt temperatures values, one being measured as wall melt temperature and one being measured some distance close to the die wall. Additionally, a thermocouple mesh was also placed prior to the extruder die which in conjunction with the thermocouples described above generated detailed information concerning melt temperature profiles and temperature variation.  This allowed 2D profiles of real-time melt temperature flowing through the die to be measured, and enabled characterization of the thermal dynamics of the extrusion process [5.6].   A melt pressure transducer was fitted in this adaptor to monitor melt pressure and examine the pressure variation and its relationship with melt temperature providing valuable information on fluctuations cause by melting instabilities.   Real- time quantification of energy consumption was monitored using a 3-phase unbalanced loads energy meter (Hyoki 3169) connected to the 3-phase power supply to the extruder. This measured total energy consumption of the extrusion process, including consumption by the motor, heaters and cooling fans. As a result, a better understanding of the links between thermal processing conditions, melt pressure and energy consumption can be established.  Results and discussion  Thermocouples meshes or grids were developed in an attempt to provide an accurate temperature measurement of the flow with a fast response time to study dynamic melt temperatures at different screw speeds, set extruder temperatures, using three different screws. Dynamic melt temperatures of the three polymers measured for tapered compression screw at a set extruder temperature of 200°C and screw rotation speed of 90 rpm over a short- time of period (0-30sec) are compared in figure 4. It can be observed that the range of temperature variation at each junction significantly increased with increasing the (Mw) of HDPEs. The largest ranges of fluctuations were measured between radial positions of 8.8 and 14.7 mm from the centre of the flow. For the highest molecular weight grade of high density polyethylene (HM5411EA), a difference of temperature of 65°C was observed at 8.8 mm from the centre of the flow. However, melt temperature variations were smaller with barrier flighted screw for HM5411EA at the same conditions, 200°C and screw speed of 90 rpm (figure 5). It has been verified that for the geometries tested here, the barrier flighted screw performed significantly better than the single flighted screws, achieving much better temperature homogeneity across the melt flow  highlighting the improved thermal extrusion performance for this particular screw geometry.   Fig 4a: HD5050EA, tapered compression screw, 200°C, 90rpm    Fig 4b: HD6007XA, tapered compression screw, 200°C, 90rpm       Fig 4c: HM5411EA, tapered compression screw, 200°C, 90rpm  Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)   Fig 5: HM5411EA, barrier flighted screw, 200°C, 90rpm  The effect of shear viscosity on extrusion process energy demand is shown in figure 6.This data clearly demonstrates the dependence of the shear viscosity on both temperature and molecular weight, and reflects the important role of melt viscosity in the extrusion melting performance. Figure 6 shows that, decreasing the molecular weight of HDPEs, similar viscosity consistencies were achieved for both single flighted screws which resulted in similar demands of energy consumed during the process. Moreover, energy consumption decreased decreasing the set temperature.      Fig 6: Effect of shear viscosity on process energy demand (darker lines represent higher set temperatures) The processability and stability of the extrusion process and their relation to energy consumption improved decreasing the weight  average molecular weight and its distribution of HDPEs. It was clear the effect of the barrier flight on shear viscosity and energy consumption compared to single flighted screws. Barrier flighted screw consumed less energy than both non-barrier screws at all conditions, highlighting the improved melting performance, corresponding to the consistent melt temperatures seen in figure 5. Interestingly, although higher shear rates and consequently higher shear viscosities were achieved, lower energy consumption was required.    Conclusions  The effect of the extruder processing conditions on the thermal extrusion performance was studied using three barrel set temperatures and screw geometries at different extruder screw speeds. In addition a comparison between three different molecular weight grades of high density polyethylene (HDPE) was made to investigate the rheological processing behaviour of HDPE for single screw extrusion by monitoring real-time melt temperature measurements. The results reflected high levels of variation in radial melt temperatures across the die flow path, dependent on screw geometry, screw rotation speed and polymer type. Single flighted screws exhibited poorer temperature homogeneity and larger fluctuations. These fluctuations increased with increasing the molecular weight grade of polyethylene. The energy consumption per unit mass (J/g) was significantly higher for the highest viscosity grade of polyethylene, and was highly dependent upon screw geometry. The role of shear viscosity on the extrusion process was quantified reflecting improvements in processability and stability for lower molecular weight grade of polyethylene highlighting the careful selection of the extruder processing conditions and screw geometry.    Acknowledgements  This work was funded through an inter-disciplinary research programme (EP/G059330/1) by the Engineering and Physical Sciences Research Council of the UK. The technical assistance of Steve Brook, Ken Howell and John Wyborn at the University of Bradford are gratefully acknowledged.   References  1. J.F. Agassant; J.P. Villemaire; Makromol. Chem., Macromol. Symp. 1998, 22, 39-57.  2. R.K. Krishnaswamy; D.C. Rohlfing Polym. Eng. Sci. 2004, 44 (12) 2266-2273 3. A. L. Kelly; E. C. Brown; P. D. Coates. 2004 Polym. Eng. Sci. 2006, 46 (12) 1706-1714 4. J. Vera-Sorroche; A. Kelly; E. Brown; P. Coates; N. Karnachi; E. Harkin-Jones; K. Li; J. Deng Applied Thermal Engineering, 2012, in press 5. E. C. Brown; A. L. Kelly; P. D. Coates Rev.Sci. Instrum. 2004 75 (11) 4742-748 6. A.K. Wood, UK Patent 1996 GB2,291,197         

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The Effect of Molecular Weight on Thermal Efficiency for Single Screw Extrusion of HDPE

The Effect of Molecular Weight on Thermal Efficiency for Single Screw Extrusion of HDPE

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