Heat transfer and viscous polymer melting capacity correlation in self-controlled torsion induced extrusion

https://doi.org/10.1016/j.icheatmasstransfer.2021.105424Get rights and content

Abstract

A novel design concept of torsion screw configuration has been proposed to improve polymer melting ability. The heat transfer and melt flow behavior of a two-phase viscous polymer, induced by torsion elements in a screw compression section, were analyzed with the computational fluid dynamics simulation. It has been found that screws with added torsion elements have larger Nusselt number and liquid mass fraction, and shorter melting zone compared to that of a conventional screw. The heat transfer and melting capacity improve with the increasing number of torsion elements and their preferred arrangement in the screw. A simplified simulation model of torsion and standard elements has been developed to predict the heat transfer and melt flow mechanism, and the simulation results are in consistent with the field synergy theory. The existence of torsional flow in the region of torsion channels increases the perturbation of the fluid, improves interaction between temperature gradient and velocity vectors, resulting in improved mass and heat transfer and rapid melting of polymers. Furthermore, the paper proposed a qualitative melt flow model of polymers in the channel of torsion element.

Introduction

In both extrusion and injection molding, a limited heat transfer from barrel wall to screw channel of a molten polymer is a prime focus of manufacturing quality products [[1], [2], [3], [4], [5]]. A nonuniform heat distribution in the polymer melt between the barrel wall and the screw root often leads to poor mechanical, optical, thermal and other functional properties. It is largely attributed to the development of unwanted sink marks, lusterless, crack, warpage, residual stress in the molded and extruded products [6]. With the increasing demand of high-end precision products, precise control of heat and mass transfer in the polymer melt becomes important. In order to ensure uniform melt viscosity, a more controlled heat transfer from barrel wall to polymer melt as well as an efficient thermal management of the polymer melt in a dynamic flow condition are needed.

Due to the high viscous dissipation of polymer melts and their low thermal conductivity, the temperature distribution in the viscous polymer phase can be broad [7]. The later process subsequently results in unwanted heat loss and uneven melt viscosity. In past two decades, researchers have made a significant progress on the thermal management of polymer melt to advance the engineering science of heat flow in a viscous fluid. For example, Bu et al. [8] investigated the propagation of thermal fluctuations along the screw and its effect on melt homogeneity at the end of the injection molding screw. Vera-Sorroche et al. [9] quantitatively analyzed the thermal homogeneity of the melt in the extruder using a non-intrusive infrared temperature sensor mounted on the barrel and measured the melt temperature distribution in the screw homogenizing zone. Likewise, Abeykoon et al. [6,10,11] measured melt thermal distributions at the extrusion die by adopting a thermocouple mesh. They also concluded that the screw configuration is one of the most important factors for achieving improved melt temperature distribution. Jinescu et al. [12] found that temperature variation along the axial direction of screw allows effective positioning of intense mixing zones, resulting in enhanced thermal homogeneity and good product quality. Also, Jinescu [13] introduced a concept of thermal homogenization efficiency for flow optimization by accounting for the heat generated by internal friction of viscous polymer.

It is well known that the polymer is melted by the combined effects of thermal conduction from the heater outside of the barrel and shear-induced heat development in viscous polymer melt (viscous dissipation) during the plasticization process. In addition, shear induced heat generation depends largely on the thermal and rheology of the polymer [8,14]. Therefore, the heat and mass transfer processes have a significant effect on melt homogeneity of polymer. Monchatre et al. [15] investigated the temperature and frictional heat development (viscous dissipation) in a reciprocating single-screw extruder. Results indicated that a high Nusselt number is probably related to the disturbing flow (induced by the pin elements) and the pulsating flow (induced by the axial movement of the screw) to facilitate active convection and favor thermal exchange. Kuzyaev [16] developed a theoretical model to optimize the thermal and mass transfer processes in the working channel of extruder by taking into account the changes in the geometrical and technological characteristics. Karkri et al. [7] presented the influence of different flow rates and heat conditions on Nusselt number in the steady laminar convective heat transfer through an extrusion die, and discussed the viscous dissipation of polymer on heat transfer. Although most of those previous works enhanced the understanding of polymer flow behavior in melt process viscous heat transfer and in depth analysis of the radial heat flow in a highly viscous fluid and their effect on the ultimate polymer properties remains to be addressed.

While studying the heat transfer behavior of Newtonian fluid in heat exchanger, Guo et al. [17,18] developed a field synergy theory to guide the structural design of heat-exchange equipment. According to their study, the decreasing included angle (in the range of 0–90°) of velocity and temperature gradient vectors could improve Nusselt number. This novel theory of heat transfer enhancement has been further validated, applied and developed by many researchers. In later studies applications are focused on heat exchange equipment where mostly turbulent flow with Newtonian fluid (Reynolds number, Re>2000) are considered [[19], [20], [21], [22], [23], [24]]. In the laminar flow domains (Re<2000, generally), Liu et al. [25] discussed the synergy effect among multiple physical parameters in laminar flow field by mathematical modeling and quantified the synergy effect by simulations. Du et al. [26] simulated the effect of novel sinusoidal ribs on convective heat transfer of water (400 < Re < 1800). Cheng et al. [27] simulated the flow patterns and heat transfer of water inside a twisted oval tube (50 < Re 〈2000). Tong et al. [28] proposed a novel bellows by adopting sinusoidal function and investigated the effect of different sine camber on the flow properties and heat transfer (500 < Re 〈1000). Wu et al. [29] simulated the heat transfer performance in the fin surface fitted with vortex generators (800 < Re < 2000). In yet another study, Kuo et al. [30,31] simulated the fluid flow and heat transfer behaviors for comparing different design of gas flow channel to improve the property of polymer electrolyte membrane (PEM) fuel cells (Re ≈ 200). Most of those cited works indicated that the field synergy principle could well describe and explain the heat transfer phenomena for laminar flow. Furthermore, Li et al. [32,33] simulated the heat transfer and mass flow behaviors of water in microchannel with trapezoidal and triangular sections. Their results indicated that when Re is lower than 100, the synergistic effect of velocity and temperature gradient is more significant than in the case where Re is greater than 100. We can infer that the interaction of velocity and temperature gradient could have stronger influence on heat transfer enhancement in a low Re condition. In the non-Newtonian fluids domains, Chen et al. [34] simulated the heat transfer behavior of power-law fluids (Re < 1) in a rectangular impeller agitator. Their results indicated that an impeller with high aspect ratio give rise to heat transfer than the low aspect ratio. They explained that the latter would induce lesser degree of synergistic effect than the former, i.e. the temperature gradient field is mainly vertical to the velocity field.

Most detailed reviews of field synergy principle are mainly focused on the Newtonian Fluid and to the best of our knowledge, no relevant literature has been found in plasticization process with complex non-Newtonian fluids such as polymers, where Reynolds numbers of polymer melt flow is very small (Re < <1). Recently, we developed a novel torsion element (TOE), and analyzed the heat transfer behaviors of single-phase polymer melt in those screw channels metering section by adapting field synergy principle and compared them with standard screw (STD) [35,36]. Results indicated the presence of the torsional flow in working channel of torsion elements induce strong distributive mixing and favored thermal exchange to obtain a high Nusselt number, which in turn, promotes the synergy between velocity and thermal flow fields.

In order to overcome the challenges of melt heterogeneity and melting efficiency during the plasticization of polymers, we further explore the heat transfer and melt processing behaviors of solid-liquid polymer phase in compression section of screw channels by proposing insertion of novel torsion elements (TOEs) in line with the field synergy theory. We are devoted to quantify the positive influence of the TOEs on the melting and heat transfer of polymer from solid to liquid and to verify the synergistic interaction of velocity and temperature gradient fields in a viscous polymer when Reynolds numbers is much less than one.

Section snippets

Physical model

The geometric structures of the proposed torsion element (TOE) is illustrated in Fig. 1. The TOE is evenly separated into several torsion channels by torsion flights, and between every two adjacent torsion flights, there are two surfaces that are twisted by 90°along the axial direction. When polymer flows over the torsion channel, it is expected to undergo a torsional rotation (tumbling) under the forces generated from viscous friction with barrel wall and with the steering between two 90°

Mass transfer property

To understand the effects of TOEs on the mass transfer behavior of polymer fluid in the screw system, the velocity field and flow characteristics were investigated. The streamlines and transverse velocity vectors for simplified TOE and STD are shown in Fig. 5. From the streamlines in Fig. 5a, it can be clearly seen that a spiral flow appears in the simplified TOE channel, while parallel laminar flow was observed in the simplified STD channel where radial mass transfer does not occur in Fig. 5b.

Conclusions

The effect of polymer two-phase flow with new torsional screws compared with a conventional screw was explored in an extrusion process to examine their heat transfer and melting performances. Results confirm that the heat transfer and melting properties of a viscous polymer has been improved by incorporating TOEs in the screw, which is in agreement with the field synergy and transient melting theory. In addition, heat transfer and melting mechanisms are affected by the structures of TOEs and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant number 51576012). Support from China Scholarship Council is also gratefully acknowledged for Ranran Jian's joint PhD grant.

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