Studies on wire coating extrusion. I. The rheology of wire coating extrusion

Wire coating extrusion was studied, both experimentally and theoretically, using a pressure-type die. For the experimental study, a wire coating apparatus of laboratory scale was constructed, consisting of a pay-off device, extruder, cross-head and pressure-type die, cooling trough, and take-up device. The materials used were low- and high-density polyethylenes and thermoplastic rubber. The following measurements were taken during the experiments: (1) the axial pressure profiles in the die, (2) melt flow rate, and (3) take-up speed. The measurements were then used to determine the effect of the rheological properties of the polymers on the performance of the wire coating operation. It was found that a reduction in axial pressure gradient and a reduction in the recoverable elastic strain of a molten polymer at the die exit can be realized as the speed of the wire is increased. For the theoretical study, using a power-law model, the equations of motion were solved numerically to predict the volumetric flow rate as functions of the pressure gradient in the die and the rheological properties of the polymer being extruded. Solution of the system equations permitted us to predict the velocity profile and shear stress distributions of a molten polymer inside a pressure-type wire coating die.     

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.     

The influence of resin characteristics on the high speed melt spinning of isotactic polypropylene. II. On-line studies of diameter,birefringence, and temperature profiles

An investigation was carried out of the high speed melt spinning of three polypropylene resins with melt flow indices of 12, 35, and 300. On-line measurements were made of diameter, birefringence, and temperature as a function of distance from the spinneret for a range of spinning conditions for each polymer. A plateau (decrease of cooling rate) in the temperature profile was associated with the occurrence of crystallization in the spinline. The position of this plateau correlated with a rapid rise in the birefringence profile and a rapid decrease in the rate of drawdown in the diameter profile. The temperature and birefringence profiles were used to determine the temperature and position on the spinline at which the onset of crystallization occurred. It was found that the position and temperature of crystallization onset varied considerably with changes in take-up velocity, extrusion temperature, and resin melt index (weight average molecular weight). The crystallization onset occurred nearer the spinneret and at higher temperatures with (1) an increase of take-up velocity, (2) a decrease of extrusion temperature, or (3) a decrease of resin melt flow index. An analysis was carried out to estimate the rate of stress development with distance along the spinline; the results were also used to estimate the stress at the onset of crystallization for each spinning condition. It was concluded that the observed behavior could be attributed to the role of spinline stress in producing molecular orientation and consequent increase of crystallization rate.     

隔热条异型材挤出模具设计

通过对隔热条异型材截面形状及原材料挤出成型性能的分析,设计了异型材挤出口模形状、流线形流道及挤出模具结构。经实践证明,该模具口模形状及尺寸设计正确,流线形流道设计合理,能生产出满足尺寸精度要求的隔热条异型材。     

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     

Temperature profiles for polymer melts in tube flow. Part II. Conduction and shear heating corrections

A temperature probe system to measure radial temperature profile of polymer flowing in a rod die and a method to systematically correct the conduction and the frictional shear effects were developed. Experimental data obtained on a 1-1/2-inch extruder using a blow molding compound show that both conduction and frictional shear heating effects are significant in melt temperature measurement and that the radial temperature profiles of the melt in the rod die are influenced by the RPM of the screw and the die-wall temperature. The reliability of the temperature data obtained was compared with the solution obtained from the equations of motion and energy. A good agreement between the predicted versus experimental temperature profile exists. For this polymer system, the relationship between local Nusselt number and the velocity parameter could be adequately described with the theory of Van LeeuWen.     

Fluid flow stability analysis of multilayer fiber drawing

We here investigate drawing of multi-layered Newtonian and non-Newtonian fluid fibers, drawn under isothermal and non-isothermal conditions. We first develop one-dimensional equations governing mass, momentum, and energy balances and solve them numerically to obtain steady state draw root shape, velocity, and temperature profiles. These solutions are then used to perform linear stability analysis. For the case of isothermal draw, the system displays an oscillatory instability when the draw ratio (ratio of cross-sectional areas of fiber at the entrance and exit of the drawing) is higher than a critical draw ratio (highest stable draw ratio) of about 20.21. Investigation of stability behavior under non-isothermal draw conditions is performed by considering radiative heating and convective cooling. Employing only radiative heating enhances the critical draw ratio, and simultaneous heating and convective cooling increase the critical draw ratio even further. For the case of simultaneous heating and cooling, with increasing convective cooling strength, the critical draw ratio first increases, reaches a maximum, and then gradually decreases. However, with only convective cooling, the critical draw ratio decreases with an increase in convective cooling strength. We also find that the stabilizing effect of a non-isothermal operation can be enhanced by considering fluids with higher viscosity sensitivity to temperature, increasing the maximum temperature, and for sharper attenuation of the fiber cross-sectional area with length. For the case of isothermal drawing of non-Newtonian fluid fibers, the system has a higher critical draw ratio for shear thickening fluids (power-law exponent, n>1). In contrast, the use of a shear thinning fluid (n<1) reduces the critical draw ratio. Consideration of a non-isothermal operation of non-Newtonian fluid fibers reveals that the critical draw ratio is primarily determined by the non-Newtonian behavior rather than the non-isothermal drawing.     

Heat transfer to molten polymers flowing through tubes

A theoretical and experimental study of heat transfer to polymer melts flowing through circular tubes is presented. The mathematical model provides for shear heating and expansion cooling effects, and also heat of reaction during flow for various wall boundary conditions. Experimental results, obtained using low density polyethylene, show reproducible temperature and velocity profiles. The measured inlet melt temperature profile and the axial wall temperature profile provide the boundary conditions for the calculations. The experimental data confirm the predictions of the magnitude of the shear heating and expansion cooling effects during tube flow.     

A dual slit in‐line die for measuring the flow properties of S‐PVC formulations

We have developed an instrumented dual slit die mounted on a twin‐screw extruder. This device allows us to distinguish the predominant flow pattern and calculate the shear viscosity, Cogswell elongational viscosity, and a Mooney wall‐slip velocity. The melt‐down process is also monitored by measuring the screw torque together with temperatures and pressures along the screw barrel. So far, we have seen that many pipe and profile formulations have a predominant plug or slip‐dominated flow behavior in the die, while others can be more sticky. Generally, the sticky highly viscous formulations will be more affected by shear heating effects when exposed to high rates during processing. We also give a detailed discussion, with examples, of how data from the device are to be analyzed and how the correct flow boundary condition is to be identified.     

美式截面PVC塑料异型材挤出模具的设计

以典型的美式截面型材为例,从塑料异型材组装、使用及成型的角度,对挤出模具口模部分的各段流道进行了分析,对定型模中水气路排布、尺寸放量与口模的匹配程度、物料特别是对内筋在定型模中冷却定型规律进行了总结,对定型水箱中定型块等作用进行了分析,探讨了如何完善塑料异型材模具的设计问题。     

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.     

模具设计中如何提高塑料异型材的表面质量

通过对挤出模具口模部分预分流段、分流段、汇流段、压缩段、成型段等各段流道的分析,和对定型模中水气路排布、尺寸放量与口模的匹配程度、物料特别是对内筋在定型模中冷却定型规律的综合考虑,明确挤出模具中各主要部分对塑料异型材表面质量的影响,探讨如何在模具设计中平衡取舍,通过模具的优化设计可切实提高塑料异型材的表面质量。     

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     

Dynamics of air drawing in the melt blowing of nonwovens from isotactic polypropylene by computer modeling

The dynamics of stationary air drawing in the melt blowing of nonwovens were determined on the basis of a single‐filament model in a thin‐filament approximation that accounts for polymer viscoelasticity, heat of viscous friction in the polymer bulk, and surface energy. Predetermined distributions of the air velocity and temperature along the melt blowing axis were assumed. Axial profiles of the polymer velocity, temperature, elongation rate, filament diameter, tensile stress, and extrapressure were computed for the melt blowing of isotactic polypropylene. The effects of the air‐jet velocity, die‐to‐collector distance, and polymer molecular weight are discussed. We predicted that the filament attenuation and velocity at the collector located in the air‐drawing zone would increase with increasing die‐to‐collector distance. The air‐drawing zone was shorter for higher air velocities and lower molecular weights. No online crystallization was predicted before the achievement of the collector, and melt bonding of the filament in the web should have occurred during cooling on the collector, accompanied by spherulitic crystallization. Significant online extrapressure in the filament was predicted in the case of supersonic air jets as resulting from polymer viscoelasticity, which could have led to longitudinal splitting of the polymer into subfilaments. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Application of the SIMS Method in Studies of Cr Segregation in Cr-Doped CoO: II, Depth Profiles

Surface segregation-induced depth profiles of Cr were determined for Cr-doped CoO single crystals equilibrated at different temperatures (1373–1673 K) and oxygen partial pressures (10²–10⁵ Pa). It was shown that the shape of the depth profiles depends on both conditions of annealing and subsequent cooling procedure. It has been observed that during slow cooling of specimens from the condition of segregation equilibrium at elevated temperatures to the temperature of the experiment (room temperature), there is substantial change of the concentration profile involving desegregation of Cr (Cr diffusion from the surface to the bulk) resulting in its depletion. It has also been shown that the rate of the desegregation process (the process which results in a decrease of surface concentration) is slower than the lattice transport kinetics of Cr in CoO. It is concluded that the desegregation is rate-controlled by the decomposition of a spinel-type bidimensional surface structure which is formed at elevated temperatures as a result of Cr segregation.     

On the neck-like deformation in high-speed spun polyamides

The neck-like deformation process occurring in high-speed melt spinning of polyamide 66 and polyamide 6 filaments was investigated at take-up speeds of 4200 to 5500 m/min by on-line laser light scattering, thermographic contrast compensation, and wide-angle X-ray scattering (WAXS) measurements. New information about the onset of crystallization along the spinline was obtained by measuring simultaneously diameter and temperature profiles in the neighborhood of the neck. Crystallization rates, as a function of take-up speed, are estimated for both polyamides. Based on the present experimental results of diameter profiles, temperature profiles, and WAXS patterns, a picture of the physical mechanism responsible for the neck-like deformation of high-speed melt spun polyamides is proposed. © 1993 John Wiley & Sons, Inc.     

Shear amplification of orientation in Nylon 11/clay nanocomposites during melt casting and resulting electrical properties

In this study, the effect of organically modified clay on the orientation enhancement in Nylon 11 in melt casting was investigated. Nylon 11 was mixed with 1 and 3 wt% Cloisite 20A using twin screw extrusion and they were cast into films with varying take-up speeds. The addition of clay in Nylon 11 helped increase orientation levels substantially in melt cast films, both as a function of clay concentration as well as take-up speeds. This was primarily due to shear amplification effect caused by the movement of adjacent clay nanoparticles due to the shear flow gradient within the die. At low clay concentrations, the sub-T_m stretchability, and electrical breakdown strength improve as the presence of clay reduces inter/intrachain hydrogen bonding. At higher clay concentrations, both orientation and electrical breakdown levels decrease. The latter is primarily caused by increased percolation path of charge carriers. Nevertheless, clay nanoplatelets were very effective in their role as melt processing aids, as they enhance orientation levels of Nylon 11 thin films by shear amplification effect where they increase local chain orientation of chains trapped between clay platelets while their orientation relaxation is suppressed.     

涤纶有光扁平FDY生产中毛丝形成的原因及对策

分析了喷丝板的设计和工艺参数如干燥条件、纺丝温度、冷却条件、上油方式、热辊温度和拉伸倍数等对涤纶有光扁平FDY生产的影响。指出为减少生产中毛丝的形成,应采用较缓和的预结晶条件,适当提高纺丝温度,缓和冷却条件,选择合理的热辊拉伸倍数和温度。     

Extrudate swell during the melt spinning of fibers—influence of rheological properties and take-up force

A theoretical and experimental study has been carried out on extrudate swell B, especially the influence of rheological properties and applied take-up force on the emerging melt. The problem is analyzed in terms of (1) dimensional analysis, (2) force–momentum balances, (3) partially constrained elastic recovery. Analyses in terms of force–momentum balances are only able to give extrudate swell B in the asymptote of high Reynolds numbers. For low Reynolds numbers, they simply relate the take-up force to the pressure field in the spinneret. Increasing the take-up force predicts a decrease in the exit pressure. The partially constrained elastic recovery theory yields an expression for B as a decreasing function of applied take-up force. Specifically, this is where B(0) is the extrudate swell in the absence of applied forces, λ_eff is the effective relaxation time, μ is viscosity (both evaluated at the capillary wall), and D is the spinneret capillary diameter. An experimental study of extrudate swell of several rheologically characterized melts (high density polyethylene, low-density polyethylene, polypropylene, polystyrene) has been carried out at 180°C by four different methods (frozen, annealed in hot silicone oil, photographed emerging into air, photographed emerging through 180°C silicone oil) in the absence of applied take-up forces. Extrudate swell for a melt emerging from dies with differing diameters correlates with capillary-wall shear rate. A comparison of extrudate swell with normal stress–shear stress ratio shows the best agreement for frozen extrudates and photographs of melts emerging into air. The data is compared to the Tanner theory of extrudate swell. B has been determined during melt spinning and shown to be a function of take-up force for both a high-density polyethylene and polypropylene melt. B decreases rapidly with applied take-up stresses. The results are compared to the predictions of the partially constrained elastic recovery theory.     

Stress measurements in high-shear granulators using calibrated “test” particles: application to scale-up

Granulation is a process by which fine powders are agglomerated into larger particles using a liquid binder. In high-shear granulation the powder–binder mix experiences intense agitation inside a mixing vessel as binder is dispersed and granules form and strengthen under the influence of shear and compacting forces in the device.

It is an implicit assumption that in a “high shear” mixer, large forces are transmitted to the powder and this results in a short and efficient granulation process. Owing to these desirable characteristics, high-shear granulation was adopted by several industries including pharmaceuticals and detergents where the process is used almost exclusively. In the work reported here, we attempt to measure shear forces in a moving powder inside a mixer-granulator. The method is based on previous numerical simulations [Powder Technology 110 (2000) 59] and experiments [Journal of Fluid Mechanism 347 (1997) 347] where we showed that at equilibrium between stresses in the mixer and the yield strength of the particles, granules attain a characteristic elongated shape. The measuring method adopted is indirect in the sense that pellets with well-defined mechanical properties were used to interrogate forces inside the granulating vessel at the point where they attain their characteristic elongated shape. We subsequently used the condition of equal shear forces in the device as a scale-up criterion so as to preserve the magnitude of stresses at both scales and thereby to expose forming granules to similar forces in both the small- and large-scale machines.

We found that shear forces in a “High-Shear” mixer-granulator with a vertical axis (Fielder) are actually not always high. The mixer has the potential to produce high shear forces but these forces are transmitted to the powder mass only if the powder is sufficiently cohesive or becomes cohesive due to binder addition. Shear forces in the granulator are strongly wet-mass-dependent and they increase rapidly as soon as a “granulation limit” is achieved, i.e., at the point where granules start to form in the shearing powder mass. We found that granulators with geometrically similar bowls can be scaled to generate comparable shear forces by decreasing the impeller rotational speed of the large machine by the factor (D/d)ⁿ, where D and d are the impeller diameters of the large and small machine, respectively, and n is a scaling index that depends on impeller geometry but not on wet mass properties. For the equipment studied in this work, the coefficient n was obtained as 0.80<n<0.85. We also propose an improved granulation process in which dry powders are pre-wetted before introduction into the main granulating device. This scheme has the potential to produce larger shear forces during wetting and binder introduction and thereby improve homogeneity and consequently final granule properties.