Flow Of Molten Polymers Research Articles

A generalized method aiming at predicting the polymer melt flow field in the metering zone of large-scale single-screw extruders

Single-screw extruders (SSE) are commonly used in a wide variety of applications, ranging from polymer-extrusion to pellet additive manufacturing (PAM). Existing mathematical models focus on Newtonian and power-law rheologies to model melt flow in the last screw vanes. However, molten polymers usually follow more complex rheological patterns, and a generalized extrusion model is still lacking. Therefore, a semi-analytical model aiming at describing the flow of molten polymers in SSE is presented, to encompass a wide range of non-Newtonian fluids, including generalized non-Newtonian fluids (GNF). The aim is to evaluate the molten polymer flow field under the minimum set of dimensionless parameters. The effect of dimensionless extrusion temperature, flow rate, channel width, and height on the flow field has been investigated. A full factorial plane has been chosen, and it was found that the impact of dimensionless flow rate is the most prominent. The results were initially compared to numerical computations, revealing a strong agreement between the simulations and the proposed GNF method. However, significant deviations emerged when employing the traditional power-law model. This is particularly true at high values of flow rate and extrusion temperature: the mean error on overall flow speed is reduced from 12.91% (traditional power-law method) to 1.04% (proposed GNF method), while keeping a reasonable computational time (time reduction: 96.70%, if compared to fully numerical solutions). Then, the predicted pressure drop in the metering section was benchmarked against established literature data for industrial-scale extruders, to show the model’s accuracy and reliability. The relative errors of the traditional model range between 34.33 and 62%. The proposed method reduces this gap (errors ranging between 5.34% and 10.97%). The low computational time and high accuracy of the GNF method will pave the way for its integration in more complex mathematical models of large-scale additive manufacturing processes.

Dynamic optimal control of flow front position in injection molding process: A control parameterization-based method

High dynamic and precise control of process variables within permissible limits is the key to high product quality in the injection molding industry. Among these variables, the injection flow rate of molten polymer is particularly significant. In this paper, an effective optimal open-loop and state feedback controller design method is designed to realize the optimal tracking control of the melt polymer flow front position flow of melt polymer fluid during injection molding. To this end, the dynamic model of the filling process is firstly established, taking into account the nonlinearity of hydraulic control system and the effect of molten polymer flow on the dynamics of the injection ram, and then the dynamic optimal tracking control problem of flow front position of molten polymer in injection molding machine is proposed. Then, an open-loop controller and a multi-level feedback control law are well designed, wherein the control signal and the control feedback kernels are parameterized by control parameterization. The controller design problems are transformed into sequential optimal parameter decision problems. The explicit expressions of the gradient information of the objective function and constraints on the parameters of the decision variables are solved by using the state sensitivity equation analysis method, and the gradient information is combined with the nonlinear programming algorithm to solve the optimization problems efficiently. Finally, the viability and effectiveness of the proposed design method are verified by numerical simulations.

Numerical and experimental studies on design of multi-lumen tube extruded with drawing and air flow

This study represents the design and optimization procedure of a multi-lumen tube used for endoscopic retrograde cholangiopancreatography (ERCP) catheter. The geometry of tip and die for the multi-lumen tube is designed based on numerical analysis of molten polymer and air flow. In particular, this numerical study consider the interaction between the molten polymer and the air flow in the lumen which is the main process variable of extrusion, and analyze the heat transfer and pressure distribution through the air flow for the first time. In addition, to reflect the reality of the multi-lumen extrusion process, the velocity is controlled at the end of the tube ejected to the free surface in consideration of the extrusion process, which is drawn by the puller downstream of the extruder. The shear rate and temperature dependent viscosity of PEBAX 7233 SA01 MED is measured for non-isothermal numerical simulations. The extrusion process for multi-lumen tubes with drawing is simulated using ANSYS Polyflow. The profile of extrudate is numerically predicted with the initial design of the tip and die with the manufacturing parameter is optimized. The predicted profile of extrudate is compared to the target profile with dimensional requirements. The design of the tip and die is modified based on numerical results following the design procedure. To suppress the die swell phenomena, the flow area of molten polymer is increased, and the velocity difference is reduced. Finally, the multi-lumen tube with the optimized tip and die is experimentally extruded and compared to the target profile of the extrudate. It has a good agreement with the target profile and satisfies the dimensional requirements. This design procedure and numerical analysis help achieve the desired target profile of extrudate.

Flow Disturbance Characterization of Highly Filled Thermoset Injection Molding Compounds behind an Obstacle and in a Spiral Flow Part.

In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. The position, length, and angle of weld lines are dependent on the gate location, injection speed, injection pressure, mold temperature, and, especially, the direction and degree of the polymer melt velocity in the mold-filling process. However, the wall surface velocity of the thermoset melt in the mold-filling process is not zero, which is not found for thermoplastic injection molding. The main reason leading to this difference is the slip phenomenon in the filling phase between the thermoset melt and the wall surface, which is directly affected by the filler content. In this study, commercial thermoset phenolic injection molding compounds with different amounts of filler were employed to investigate not only the mechanism of weld line formation and development behind an obstacle in the injection molding process but also the flow disturbance of the thermoset melt in the spiral flow part. In addition, the effect of the wall slip phenomenon on the flow disturbance characterization and the mechanism of weld lines of selected thermoset materials was carefully considered in this research. Furthermore, the generated material data sheet with the optimal developed reactive viscosity and curing kinetics model was imported into a commercial injection molding tool to predict the weld line formation as well as the mold-filling behavior of selected thermoset injection molding compounds, such as the flow length, cavity pressure profile, temperature distribution, and viscosity variation. The results obtained in this paper provide important academic knowledge about the flow disturbance behavior as well as its influence on the mechanism of weld line formation in the process of thermoset injection molding. Furthermore, the simulated results were compared with the experimental results, which helps provide an overview of the ability of computer simulation in the field of the reactive injection molding process.

Statistical continuum damage healing mechanics (SCDHM)

Damage healing mechanics has been under tremendous development during the past decade in which many constitutive models for different classes of materials have been proposed in the literature. However, to bridge the gap between the physics of healing and the theoretical framework, the statistical nature of the damage healing mechanics must be considered. The statistical nature of the crack nucleation and propagation processes is well-known, and the healing mechanisms follow similar probabilistic processes. In general, the healing process consists of two steps: (I) sealing, in which the free crack surfaces are brought in contact with the help of external forces, and then, (ii) healing, which may include a variety of techniques including the flow of molten polymers or liquid healing agents into the crack free surface areas and solidifying. Both sealing and healing processes can be considered within statistical mechanics to include multiple uncertainties involved in the healing process. This work proposes a framework to integrate statistical mechanics into the continuum of damage healing mechanics. The proposed statistical damage healing mechanics may help designers and scientists to perform more realistic simulations for components under complex damage and healing cycles. The key outputs of the proposed statistical framework are (I) healing method choice, and (II) healing parameter optimization.

Experimental and Theoretical Study of Polymer Melting in a Co-rotating Twin Screw Extruder

Abstract The flow of molten polymers in corotating twin screw extruders has been largely studied, but little attention has been paid until now to the melting process. In the present work, we develop an experimental study of melting in twin screw extruders, including pressure and temperature measurements, dead-stop experiments and sampling, observation of cross sections and quantification of solid fraction. The influences of process parameters (screw speed and feed rate), screw profile, pellets size and extruder size are also investigated. An original model of the melting process is proposed, deduced from these observations. This model allows one to calculate the evolution of mean shear rate, pressure gradient, dissipated energy, pellet radius and both solid and liquid temperatures along the screws, for a sequence of right- and left-handed screw elements and blocks of kneading discs. The results of the model are in good agreement with the experiments.

Modeling of Twin Screw Extrusion of Polymeric Materials.

An issue of modeling of twin-screw extrusion of polymeric materials is reviewed. The paper is written in honor of Prof. James L. White who was a pioneer in studying this issue. A global approach to process modeling is presented which includes solid polymer transport, polymer plasticating, and the flow of molten polymer. The methodology of CFD modeling of twin-screw extrusion is presented as well as the examples of this modeling which show the details of the process. Optimization and scaling of twin-screw extrusion are also covered. And finally, the future prospects of developments and research of twin screw extrusion is discussed.

Influence of PEO and mechanical keying on the strength of AA 5052 alloy/polypropylene friction stir spot welded joints

A study has been carried out of the effect of plasma electrolytic oxidation (PEO) on the strength of AA 5052 alloy/polypropylene joints prepared using friction stir spot welding (FSSW). The joint strengths were determined using lap-shear tests and failure modes were investigated using scanning electron microscopy. Comparisons were made between control joints prepared with the alloy in the as-rolled condition or the as-rolled condition with a mechanical key and with PEO-treated alloy, with or without a mechanical key. Mechanical keying alone, provided by infiltration of polymer into holes of either 3 or 4.5 mm diameter drilled in the alloy, yielded enhancements of the joint strength by a factor of 1.8 and 3.8, respectively, compared with the as-rolled alloy. In contrast, PEO pre-treatment provided a much greater improvement in strength, by a factor of 21.3, with no significant influence of the presence of a mechanical key. The fracture of PEO-treated joints involved a mixture of de-bonding at the polymer/coating interface, cohesive failure within the coating and ductile failure of the polymer. The last resulted from formation of gas bubbles within the polypropylene due to thermal degradation of the polymer during FSSW. In contrast, in the absence of PEO, the fracture path of the as-rolled and also the as-rolled and mechanically keyed joints passed along the alloy/polypropylene interface and through gas bubbles within the polymer. The enhanced strength of the PEO-treated joints resulted from the strong polypropylene/coating bond resulting from flow of molten polymer into the coating porosity.

Visualization of particles arrangement during filling stage of polyamide 6 – metal insert injection molding

An isotactic polypropylene filled with talc and mica particles was used for visualization of molten polymer flow lines during filling of mold cavity with metal insert. The talc particles orientation in polypropylene matrix was followed by scanning electron microscopy. For the quantification of platelet particles orientation in molten polymer flow localized near metal wall, the Herman's function was applied. An existence of regions with well oriented, randomly organized talc particles depending on the shape and cross‐section of the channels, was found and reported. The domain with the most disorganized platelet particles was detected in the region where polymer streamed around the sharp corner of the metal insert. Such a kind of disarranged region could be the weakest part of the whole injection molded polymeric element. POLYM. ENG. SCI., 59:E271–E278, 2019. © 2019 Society of Plastics Engineers

Numerical Analysis of the Physical Properties of the Flow of Molten Polymers in Grooved Profiles

There is a wide field of research on the physical properties of non-isothermal pseudoplastic fluid flow. These researches help to avoid processing failures in the fields of thermoplastics. The present work presents the effects of flow inside cavities with groove profiles. For the simulations, the Ansys fluent software was used, considering steady-state flow without incompressible fluid slip. The government equations used in the simulations were mass conservation, momentum conservation and energy conservation. In order to discretize the equations, the finite volume method was used. Similar processes are seen in the thermoplastic injection industry which justifies the intention to identify the aspects of the process which hinders thermal exchange and favors the formation of undesirable physical properties in the injected parts. The implemented viscosity model was validated with the literature and consequently allowing the application of a complete analysis of the physical properties in the flow of molten polymers.

Flow visualisation of additive alignment inside a polymer matrix depending on hydraulic diameter of the mould channel

The alignment of fibre-type additives inside the polymer composites has a considerable effect on the properties of materials. In manufacturing processes such as injection moulding, the angles of additives are determined when a mixture of molten polymer and additive flows through the channels inside a mould and is solidified, which in turn influences the properties of the materials. This study conducted a flow visualisation experiment that simulated the manufacturing process of polymer matrix composites. When a mixture solution of short carbon fibre and polydimethylsiloxane liquid flowed through transparent mould channels, the additives showed a variation in angle, which was then analysed using image processing calculation. The angle of the additives was traced according to the hydraulic diameters of channels. The trend obtained in this way was compared with the gradient of the velocity field of the flow.

유동가시화를 통한 섬유형 첨가제 함유 액상 폴리머의 금형 내 유동 모사

유동가시화를 통한 섬유형 첨가제 함유 액상 폴리머의 금형 내 유동 모사

Thin-wall injection molding of polystyrene parts with coated and uncoated cavities

Low-friction mold surface coatings can be used to promote filling of thin-wall parts through reduction of the melt flow resistance by causing wall slip at the polymer-mold interface. This work investigates the effects of different mold coatings (DLC, CrN and CrTiNbN) on the flow resistance of molten polystyrene in thin-wall injection molding. The design of the mold allowed high-speed visualization of the molten polymer flow during the filling phase and measurement of the velocity profile across the cavity thickness. The evaluation of the speed profiles allowed the characterization of the wall-slip phenomenon, indicating the absence of conventional ‘fountain-flow’ filling mechanism. The results indicate that a DLC deposited on a chrome substrate can significantly reduce the flow resistance of polystyrene, by increasing the slip velocity of polymer melt in contact with the mold surface. Moreover, the contact angle of molten polystyrene over the considered coatings was found to be inversely proportional to the melt flow resistance, indicating the importance of the adhesion at the polymer-mold interface.

Laser-bulge based ultrasonic bonding method for fabricating multilayer thermoplastic microfluidic devices

Ultrasonic bonding is a commonly-used method for fabrication of thermoplastic microfluidic devices. However, due to the existence of the energy director (a convex structure to concentrate the ultrasonic energy), it is difficult to control its molten polymer flow, which may result in a small gap between the bonding interface or microchannel clogging. In this paper, we present an approach to address these issues. Firstly, the microchannels were patterned onto the PMMA sheets using hot embossing with the wire electrical discharge machined molds. Then, a small bulge, which was formed at the edge of the laser-ablated groove (LG), was generated around the microchannel using a CO2 laser ablation system. By using the bulge to concentrate the ultrasonic energy, there was no need for fabricating the complicated and customized energy director. When the bulge was melted, it was able to flow into the LG which overcame the ‘gap’ and ‘clogging’ problems. Here, two types of two-layer microfluidic devices and a five-layer micromixer were fabricated to validate its performance. Our results showed that these thermoplastic microdevices can be successfully bonded by using this method. The liquid leakage was not observed in both the capillary-driven flowing test and the pressure-driven mixing experiments. It is a potential method for bonding the thermoplastic microfluidic devices.

Using CO2-laser bugle for ultrasonic bonding of thermoplastic microfluidic devices

A novel method for ultrasonic bonding of thermoplastic microfluidic devices was demonstrated. When the CO2 laser beam scanned around the microchannel of the microdevice, a small bulge, which was called CO2 laser bulge (LB), was formed in the rim of the laser-ablated groove (LG). After applying the ultrasonic energy the LB was melted and the bonding process was achieved. In this method, the molten LB flowed into the LG, which addressed the issues of controlling the molten polymer flow between the bonding interfaces. Moreover, there was no need for fabricating the complicated and customized energy director, which will make it simple and flexible. The relationship between the laser parameters and the LB height was investigated. The ultrasonic bonding parameters were optimized. The influence of the LB height on the bonding strength was studied. The results showed that the thermoplastic microdevice can be successfully bonded by using this method. The channel loss was 0.42±0.25μm in depth and 0.32±0.02μm in width. The liquid leakage was not observed in the experiment. The bonding strength can be 1.92±0.31MPa. Hence, it can be a potential method for the bonding of microfluidic devices.

Friction stir lap joining of aluminium alloy to polypropylene sheets

Joining feasibility of aluminium alloy to polypropylene sheets via friction stir lap joining was examined. Effects of heat-input on microstructure and mechanical behaviour of the joints were investigated. A covering plate was used to confine flow of molten polymer. The results showed a distinctive interaction layer at polymer/aluminium interface, consisted mainly of C, O and Al. Shear strength of the joints decreased by enhancement of the heat-input due to increase in the thickness of the interaction layer as well as the gap width between this layer and both aluminium and polymer matrices. Maximum shear-tensile strength of 5.1 MPa (∼20% of polymer shear strength) was obtained, which was higher than or comparable to that of the joints produced by other processes.

Multiphase Simulation of Rubber and Air in the Cavity of Mold

In the polymer shaping process that uses molds, the quality of the molded products is determined not only by the flow of the (molten) polymer but also by the air venting in the cavity. Inadequate air venting in the cavity can cause defects in the product, such as voids, short shot, or black streaks. As it is critical to consider the location and size of the vents for proper venting of the air in the cavity, a method that predicts the flow of air and material is required. The venting of air by the flow of rubber inside the cavity was simulated by using a multi-phase computational fluid dynamics method. Through computer simulation, the interface of rubber and air over time was predicted. Then, the velocity and pressure distribution of the venting air were observed. Our research proposes a fundamental method for analyzing the multi-phase flow of polymer materials and air inside the cavity of a mold.

High through-plane thermal conduction of graphene nanoflake filled polymer composites melt-processed in an L-shape kinked tube.

Design of materials to be heat-conductive in a preferred direction is a crucial issue for efficient heat dissipation in systems using stacked devices. Here, we demonstrate a facile route to fabricate polymer composites with directional thermal conduction. Our method is based on control of the orientation of fillers with anisotropic heat conduction. Melt-compression of solution-cast poly(vinylidene fluoride) (PVDF) and graphene nanoflake (GNF) films in an L-shape kinked tube yielded a lightweight polymer composite with the surface normal of GNF preferentially aligned perpendicular to the melt-flow direction, giving rise to a directional thermal conductivity of approximately 10 W/mK at 25 vol % with an anisotropic thermal conduction ratio greater than six. The high directional thermal conduction was attributed to the two-dimensional planar shape of GNFs readily adaptable to the molten polymer flow, compared with highly entangled carbon nanotubes and three-dimensional graphite fillers. Furthermore, our composite with its density of approximately 1.5 g/cm(3) was mechanically stable, and its thermal performance was successfully preserved above 100 °C even after multiple heating and cooling cycles. The results indicate that the methodology using an L-shape kinked tube is a new way to achieve polymer composites with highly anisotropic thermal conduction.

Injection-rolling nozzle flow channel design and optimisation in polymer continuous injection direct rolling processes

The injection-rolling nozzle is one of the most important components in polymer’s continuous injection direct rolling process. The basic theory for molten polymer mould channel design and the advantage of extrusion channels lead us to design a new channel for the injection-rolling nozzle. Simulation results show whether the channel structure design is feasible, and will allow the molten polymer flow to be even, yet also optimized for uniform velocity. The finite element method is used to optimize several key structural parameters for nozzle, which are then used as optimization object parameters. Compared with the previous results, molten flow velocity is more uniform after optimization. The velocity difference of 0·3 mm s−1 is slower at the outlet compared with 60 mm s−1, and the total length of the nozzle has decreased. Finally, experiments results show that the channel with optimization has better flow speed uniformity.

Numerical and mathematical modeling of the injection for incompressible fluids through rigid cylindrical duct: application of melted polymers PPH

Our melted polymer is modeled by incompressible non-Newtonian fluids. The mathematical formulation leads to very complex equations. These complexities can arise, on the one hand, from the law of behavior of the fluid of interest (Arrhenius model) and the nature of the flow and on the other hand the nature of the wall considered. We are interested in this study to establish a numerical code to study the flow of molten polymers in rigid cylindrical ducts. Using an iterative numerical finite volume method, we determine the axial profile of the velocity and the temperature.