Flow balancing in die design of wood flour/HDPE composite extrusion profiles with consideration of rheological effect

This article presents an experimental study on the flow balance of an extrusion die for various wood flour (WF)/high‐density polyethylene (HDPE) compositions. Flow balancing, in the design of a thermoplastic extrusion die, is dependent on the material rheological properties so that a change in the material, in some cases, may result in a total redesign of the die. To investigate the importance of this particular effect, the flow balance of an extrusion die, with a U‐shaped profile having uneven wall thicknesses, was undertaken. The main feature of the die was an adjustable restrictor implemented for fine balancing similar to that employed in the slit dies. The rheological influence of wood plastic composites (WPCs) on the flow balance of the die via loading various WF contents, 40, 60, and 70% by weight, was experimentally investigated; flow balancing was performed via varying the height of the restrictor bar. Interestingly, the results showed that, for a high WF content (above 60%), the issue of flow balancing for an uneven wall thickness profile is much less complicated because of the plug flow behavior of the composite. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

An innovative method of die design and evaluation of flow balance for thermoplastics extrusion profiles

In this article, a computational and experimental method for flow balancing of a U‐shaped die profile with nonuniform thicknesses is presented. The approach was to implement a flow restricting mechanism along the melt flow path. A parametric study based on the restrictor dimensions was carried out to attain a preliminary optimal design. Simulations were performed using Fluent software to analyze the flow velocity at the die exit. Experimental study was then carried out at various restrictor positions for the purpose of attaining a desirable flow balance. The velocity at various segments of the die exit was measured utilizing an innovative procedure by implementing the “separating blades.” Experimental findings were compared with those of simulations which showed an acceptable agreement. The results suggest that a flexible die can be designed to achieve a flow balance under various processing conditions. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Residence time distribution of polymer melt in the T-die

Theoretical or rheological calculations for crosshead die geometry were thought not worthwhile until recently, and restrictor, or choker, bars were often excessively relied upon for film uniformity. Thus, the residence time distribution of a polymer melt was infrequently calculated especially in the T-die, because it was assumed to be very wide in T-dies. This report provides a general equation expressing the residence time distribution of polymer melts in T-dies, and indicates how to take an optimum combination of the flow-path dimensions in order to obtain both a high flow uniformity and a comparatively narrow residence time distribution across the die width. Such a T-die designed by the above considerations will produce a shorter heat history and improved physical properties of the sheet or film produced.     

Chute Performance and Design for Rapid Flow Conditions

Many industrial chute applications are characterised by rapid flow conditions in which the bulk solid stream thickness or depth is less than the chute width. Under these conditions, it is possible to describe the stream flow by means of a lumped parameter model which takes into account the frictional drag around the chute boundaries as well as making allowance for inter‐particle friction. Equations of motion to describe the chute flow are presented and their application to the determination of chute profiles to achieve optimum flow is illustrated. By means of design examples, the problems associated with the feeding of bulk solids onto belt conveyors and conveyor transfers are discussed. Criteria for the selection of the most appropriate chute geometry to minimise chute wear and belt wear at the feed point are presented. The determination of optimum chute profiles to achieve specified performance criteria is outlined.     

Autothermal Reforming of Methanol in an Isothermal Reactor – Concept and Realisation

Based on a new isothermal approach a reactor design for the autothermal reforming of methanol is developed. The new reactor concept addresses the special requirements of mobile applications, such as compact design and good dynamic behaviour. With the catalyst carried in porous disks, an even temperature profile combined with a constant flow distribution can be achieved. The disks provide a copper‐matrix, that fixes the small catalyst‐particles, transports heat and provides an open structure for mass‐transport. Therefore heat and mass transfer are not limiting, and the reactor is operated at optimum temperature, resulting in high reaction rates with low concentrations of by‐products like carbon monoxide or methane. There are no changing temperature profiles and negligible product‐fluctuations as a response to load steps of more than one order of magnitude. This enables the easy and precise control of the reaction. For start‐up the reactor is heated by catalytic oxidation of methanol. Starting from room‐temperature, a temperature of 300 °C is reached within less than a minute.     

Velocity measurements on a polypropylene melt during extrusion through a flat coat‐hanger die

An experimental investigation of various flow regimes observed during the extrusion of a polypropylene melt through a flat coat‐hanger die by laser‐Doppler velocimetry (LDV) is presented. LDV measurements of the velocity profiles across the gap of the die at various locations along the die reveal three different extrusion regimes. At small wall shear stresses, the velocity profiles can be fitted by symmetrical curves with the velocities becoming zero at the die walls. These profiles are not uniformly distributed along the die. An increase of the wall shear stress reveals a second flow regime characterized by a uniform distribution of the velocity profiles along the die. As the wall shear stress is increased even further, a third flow regime characterized by wall slip on the glass windows is observed. This flow regime is systematically characterized by measurements of the slip velocities at various temperatures and throughputs. The maximum velocities along the die are taken to assess the uniformity of flow which decisively influences the thickness of the extruded film. By measuring velocity profiles, at different throughput, and temperatures, the conditions for constant velocities along the die were determined. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Simulation of particle migration in free‐surface flows

The migration of particles in free surface flows using the diffusive flux model was investigated. As the free‐surface flows, a planar jet flow and a slot coating flow were chosen. The suspension was assumed to be a Newtonian fluid with a particle concentration dependent viscosity. The governing equations were solved numerically by the finite volume method, and the free‐surface problem was handled by the volume of the fraction model. The result shows that even though the velocity profile is fully developed and becomes flat, the particle distribution never reaches the uniform distribution for both of the cases. It is also shown that the die swell of the free jet is reduced compared to the Newtonian fluid and when the particle loading is 0.5, die contraction is observed. The change in die swell characteristics is purely due to particle migration since the suspension model does not show normal stress differences. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Compensating for die swell in the design of profile dies

Because of the effects of die swell, the final shape of an extrudate is often substantially different from that of the exit opening of the die. As a result, the design of profile dies producing complex shapes often involves more than just “balancing” the die but also compensating for the effects of die swell. Typically, a successful design of such dies is achieved only through much “cut and try,” However, with the use of a fully three‐dimensional finite element flow algorithm along with quick mesh generating capabilities, the usual cut and try involved in the design of many profile dies can be greatly reduced, if not eliminated. This paper demonstrates how the effects of die swell can be compensated for in the design of profile dies. For profiles with one plane of symmetry, this includes compensating for the sideways translation of the extrudate as well as the change in shape that the extrudate experiences. Completely asymmetric profiles undergo a “twisting” downstream of the die. This twisting, which appears not to have been reported in the literature (at least for isothermal extrusion), is also accounted for here, along with the change in shape that the extrudate undergoes. The translation or twisting of profiles downstream of a die is often attributed to non‐Newtonian or non‐isothermal effects. Only isothermal Newtonian examples are considered here. These results clearly show that asymmetry of the profile will result in a translation and twisting of the extrudate even in the isothermal Newtonian case.     

塑料型材成型模拟技术

为了满足塑料异型材挤出的需要，根据熔料的流动状态，选择了最佳的口模结构。同时，介绍了计算机模拟技术在塑料异型材挤出中的应用。     

Computer-aided geometric design of the preform dies for flat film extrusion through 3-D finite element flow analysis

A rational computer-aided design procedure is presented for determining the optimum flow channel geometry of a flat film die, which yields a minimum pressure drop and produces a uniform transverse flow rate distribution. The three-dimensional die surface is generated by analytic expressions that represent a dumbbell-like contour. The die surface may be specified by several geometric parameters. The length of the transition zone turns out to be the controlling parameter. Because of the complicated geometric boundary, it is not possible to optimize the flow channel geometry explicitly; instead, a computer trial procedure is employed. The numerical computation is based on an isothermal three-dimensional flow model, which assumes power law behavior for the polymer melts. The calculated results indicate that for a particular polymer and a particular aspect ratio of slit, there may exist an optimum transition length from which the flow channel geometry of a flat film die may be uniquely defined.     

Optimization of blow molded part performance through process simulation

In this work, a gradient‐based numerical optimization scheme is proposed to determine the optimal process operating conditions to produce a blow molded part by with a given performance. Finite element simulations are used to relate the part performance to the processing conditions. A performance optimization is first performed to find the minimum part thickness distribution that minimizes the part weight while satisfying mechanical performance constraints such as maximum part deflection or maximum stress for an applied load. Then a process optimization finds the optimal operating conditions, e.g. the die gap opening profile, that minimize the part weight while respecting the minimum thickness distribution dictated by the performance optimization. The results show that the optimization scheme minimizes the part weight with minimal constraint violation. The addition of a constraint associated with process stability is proposed.     

Noncontact measurement of parison thickness profiles affected by swell and sag in continuous extrusion blow molding

Optimization of final part thickness distributions is crucial in the extrusion blow molding process in order to minimize resin usage. Prediction of part thickness distributions from basic process and material parameters would be ideal. However, attempts to do so have been unsuccessful, largely because of the inability to predict parison thickness profiles. One must therefore resort to measurement of the parison thickness profile and estimation of the final part thickness distribution by computational methods. This paper describes a new technique for the noncontact estimation of parison thickness profiles in continuous extrusion blow molding. The method accounts for sag and requires no previous knowledge of rheological data. It can be employed on-line for the purposes of process monitoring and control. The approach is based on the measurement of the parison length evolution with time during extrusion, the parison diameter profile, the flow rate, and the melt temperature gradient along the length of the parison. These parameters are utilized in conjunction with a theoretical approach that describes the extrusion of a parison under the effects of swell, sag, and extrusion into ambient conditions. Results are presented for three resins of various molecular weight distributions. The degree of sag is minimal at the top and bottom of the parison, and reaches a maximum near the center of the parison. Results are also presented to demonstrate the versatility of the method under other process conditions, such as varying flow rate, die temperature, and die gap.     

Flow behavior of polypropylenes with different molecular structures in a coat‐hanger die

An experimental investigation of the flow behavior of three polypropylene melts with different molecular structures during extrusion through a coat‐hanger die is presented. Two linear and one long‐chain branched material, rheologically characterized in shear and elongation, were investigated. Using laser–Doppler velocimeter measurements of the velocity profiles across the gap height were performed at five various locations along the die. The uniformity of the velocity distribution along the die has been assessed using the maximum velocities v₀ of the corresponding velocity profiles across the gap. The velocity distribution along the die changes with throughput and temperature. Regarding the rheological properties, it was found that the power‐law index of the viscosity as a function of shear rate has a decisive influence on the uniformity of flow but that the pronounced strain hardening in elongation typical of the long‐chain branched polypropylene is not reflected by the velocity distribution along the die. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

The effect of wall slip on the performance of fiat extrusion dies

Flat extrusion dies are commonly used in a wide variety of film. Sheet and coating applications. Although flal dies can be designed to produce an exit flow distribution that is very uniform across most of the width, there will usually be a region along each side where it drops gradually to zero. This often requires trimming the edges of the film or sheet downstream in order to meet product specifications. It is commonly believed that treating the land area of the die with coatings that promote a small amount of wall slip will reduce the size of this edge effect and therefore improve die performance. This analysis shows that slip over the entire land region of the die will adversely affect die performance. Better performance is possible but only if the sides of the land are treated.     

Model for a two‐cavity coating die with pressure and temperature deformation

Coating dies uniformly distribute liquid for application as a film to moving substrates using one or two cavities spanning the coating width and adjoining precision narrow slots of much higher resistance to flow. If the slots are deformed by the pressure of the liquid or by temperature gradients in the die bars, degradation in flow distribution can result. Consequently, dies are designed to be sufficiently stiff and are thermally jacketed to keep slot deformations within fabrication tolerances. To aid in design and operation, a model of low computational load is developed in which the flow and deformation analyses are directly coupled. Die deformation is modeled using classical beam theory taking account of the varying thickness of the bars due to cavity geometry. Two‐dimensional finite element analysis of die deformation gives marginally higher slot deformation. Three‐dimensional finite element analysis agrees with the two‐dimensional analysis except near the center of the die where the symmetry boundary condition reduces deformation. The effects of die geometry on deformation and flow distribution are illustrated. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

The effect of screw geometry on melt temperature profile in single screw extrusion

Experimental observations of melt temperature profiles and melting performance of extruder screws are reported. A novel temperature sensor consisting of a grid of thermocouple junctions was used to take multiple temperature readings in real time across melt flow in a single screw extruder. Melt pressure in the die and power consumption were also monitored. Three extruder screws at a range of screw speeds were examined for a commercial grade of low density polyethylene. Results showed melt temperature fields at low throughputs to be relatively independent of screw geometry with a flat‐shaped temperature profile dominated by conduction. At high throughputs, melting performance and measured temperature fields were highly dependent upon screw geometry. A barrier‐flighted screw with Maddock mixer achieved significantly better melting than single flighted screws. Low temperature “shoulder” regions were observed in the temperature profiles of single‐flighted screws at high throughput, due to late melting of the solid bed. Stability of the melt flow was also dependent upon screw geometry and the barrier‐flighted screw achieving flow with lower variation in melt pressure and temperature. Dimensionless numbers were used to analyze the relative importance of conduction, convection, and viscous shear to the state of the melt at a range of extrusion conditions. Polym. Eng. Sci. 46:1706–1714, 2006. © 2006 Society of Plastics Engineers     

A parallel coextrusion technique for simultaneous measurements of radial die swell and velocity profiles of a polymer melt in a capillary rheometer

This article proposes a new experimental technique to simultaneously measure radial die swell and velocity profiles of polystyrene melt flowing in the capillary die of a constant shear rate rheometer. The proposed technique was based on parallel coextrusion of colored melt‐layers into uncolored melt‐stream from the barrel into and out of the capillary die. The size (thickness) ratio of the generated melt layers flowing in and out of the die was monitored to produce the extrudate swell ratio for any given radial position across the die diameter. The radial velocity profiles of the melt were measured by introducing relatively light and small particles into the melt layers, and the times taken for the particles to travel for a given distance were measured. The proposed experimental technique was found to be both very simple and useful for the simultaneous and accurate measurement of radial die swell and velocity profiles of highly viscous fluids in an extrusion process. The variations in radial die swell profiles were explained in terms of changes in melt velocity, shear rate, and residence time at radial positions across the die. The radial die swell and velocity profiles for PS melt determined experimentally in this work were accurate to 92.2% and 90.8%, respectively. The overall die swell ratio of the melt ranged from 1.25 to 1.38. The overall die swell ratio was found to increase with increasing piston speed (shear rate). The radial extrudate swell profiles could not be reasoned by the shear rate change, but were closely linked with the development of the velocity profiles of the melt in the die. The die swell ratio was high at the center (～1.9) and low (～0.9) near the die wall. The die swell ratio at the center of the die reduced slightly as the piston speed was increased. Polym. Eng. Sci. 44:1960–1969, 2004. © 2004 Society of Plastics Engineers.     

生物滤池反冲洗过程中生物量和生物活性的分析

用生物膜过滤柱对污水进行深度处理的中试研究表明,滤柱挂膜期间低强度的反冲洗,使局部积累的生物在滤柱全层得到重新分布,生物活性得到增强,挂膜时间缩短30%.生物量和生物活性同滤层高度有良好的正向相关性,从而推导出q(x)为单位体积滤料的生物平均需氧量,利用该式可得出生物膜过滤柱最佳反冲洗气强度.突破传统普通快滤池用膨胀率来确定反冲洗气强度的做法,采用生物量和生物活性来确定反冲洗气强度对于生物膜过滤柱来说更符合其本身的生物特性,从而更具有实用价值.     

Effect of process variables on melt velocity profiles in extrusion process using single screw plastics extruder

Abstract

Single screw extruders are used to generate a continuous flow of molten polymer in many industrial polymer processes. The melt velocity profile as extruded is important in determining the properties of the final product and influences process related phenomena such as die swell and the onset of sharkskin. The factors that influence the velocity profile would be expected to be the melt temperature (this affecting the viscosity of the melt), the screw and die geometry, and the output rate from the extruder. In the present work a thermocouple mesh sensor coupled with a cooled stainless tube has been used to determine velocity profiles in melts exiting from the screw of a single screw extruder. The results show that the technique can be used successfully to determine velocity profiles in the extrusion process.

It was found that the main influence on the magnitude of the melt velocity was the extruder screw speed. Melt temperature, and hence melt viscosity, were found to have little effect on the velocity profiles measured. The flow in the centre of the duct was retarded slightly owing to the flow across the screw tip and no rotational component of flow was observed. The velocity profiles measured seemed to be reasonably stable, only small changes being observed in the velocity profiles as the melt flowed along a duct of uniform cross-section, although these changes were limited in nature. Die diameter and length had a limited effect on the velocity profiles generated, although the die entry angle did have a significant effect on the shape of the velocity profile at higher screw speeds.     

Radial pressure profiles in a cold‐flow gas‐solid vortex reactor

A unique normalized radial pressure profile characterizes the bed of a gas‐solid vortex reactor over a range of particle densities and sizes, solid capacities, and gas flow rates: 950–1240 kg/m³, 1–2 mm, 2 kg to maximum solids capacity, and 0.4–0.8 Nm³/s (corresponding to gas injection velocities of 55–110 m/s), respectively. The combined momentum conservation equations of both gas and solid phases predict this pressure profile when accounting for the corresponding measured particle velocities. The pressure profiles for a given type of particles and a given solids loading but for different gas injection velocities merge into a single curve when normalizing the pressures with the pressure value downstream of the bed. The normalized—with respect to the overall pressure drop—pressure profiles for different gas injection velocities in particle‐free flow merge in a unique profile. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 4114–4125, 2015