Fluid absorption during forward roll coating of porous webs

Forward roll coating is a common process to deposit thin liquid films onto a continuous web. When the web is porous, some amount of the fluid is forced into the web in the nip. This removal of fluid, along with the deformation of the backing material, influences transfer in the nip as well as operational issues such as misting and coating defects. While much has been reported on forward roll coating for non-porous webs, little has been done when the web is porous.A laboratory roll coating device is used to characterize the pressure profile, the rubber deformation, and the film thickness as the fluid is in contact with a porous web. A pressure transducer is used to record the pressure profile in the nip. The film thickness on the steel roll surface and the gap between the rolls are measured with capacitance probes. Silicone oils, with three different viscosities, are used as test fluids. Three different papers are used in these tests. A model is proposed to describe the pressure profile, rubber deformation, and absorption in a forward roll coating device. The differential equations are solved to describe the nip behavior. A simplified model is also proposed, using an average nip pressure and Darcy's law, to predict penetration in the nip. The proposed models compare well with the experimental results and predict the dependence on viscosity, nip load, and paper permeability. The experimental results with low viscosity fluids show some speed dependence that is not captured by the model, but the high viscosity fluid behavior agrees well with model predictions.     

Estimation of roll press design parameters based on the assessment of a particular nip region

Granular solids are compacted in roll presses to obtain dust free products from fine powders. This mainly takes place through the increase of the bulk density due to the pressure applied by the press. The well known theory by Johanson [J.R. Johanson, A rolling theory for granular solids, Transactions of the ASME, Journal of Applied Mechanics, December 1965, 842-848.] was based on defining the nip angle which identifies a nip region where the material is trapped and pressed to higher bulk density. The nip region was introduced in terms of sets of parameters which needed a lot of experimental work to be obtained. In this study it is shown that these parameters are not needed to be determined. The output of the machine as described by the ratio of bulk density can give the design parameters of the roll press; mainly described by the roll dimensions and roll force. This is mainly obtained using one single test, where a particular nip region can be defined, which accounts implicitly for the parameters which represent the flow properties and the material-machine interaction parameters.     

Thermomechanical analysis and modeling of the extrusion coating process

During the extrusion coating process, a polymer film is extruded through a flat die, stretched in air, and then coated on a substrate (steel sheet in our case) in a laminator consisting of a chill roll and a flexible pressure roll. The nip, i.e. the area formed by the contact between the pressure and the chill rolls, constitutes the heart of the extrusion coating process. Indeed, in this region, some of the most critical properties, such as adhesion, barrier properties, optical properties, are achieved or lost. In this article, we first present an experimental investigation of the coating step, which enables to characterize the leading thermomechanical phenomena. It is shown that there is no polymer macroscopic flow in the nip, but a local flow within the asperities of the steel substrate surface. This microscopic flow, at the interface between the film and the substrate, is slowed by strong cooling conditions in the nip. Several models are then proposed, giving access to the temperature profile through polymer thickness and substrate, the pressure distribution in the nip as well as the behavior of the polymer melt in the nip at the interface with the substrate. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Roll compaction of a pharmaceutical excipient: Experimental validation of rolling theory for granular solids

Roll compaction is widely used in industry to produce free flowing agglomerates from a fine particulate feed. Two of the main advantages of this process are that it is dry and continuous. Despite being superficially a simple process, a quantitative understanding has proved difficult to develop because of the complex behaviour of particulate materials. Sub-optimal design and operation of the equipment can lead to unsatisfactory products. Johanson (1965, ASME, Journal of Applied Mechanics Series E, 32(4), 842-848) developed a theoretical model that enables the surface pressure, torque and separating force of the rolls to be predicted from the physical characteristics of the powder and the dimensions of the rolls. However, a detailed experimental validation of the theory has yet to be accomplished. The current paper describes such a study using a gravity fed instrumented roll press and a microcrystalline cellulose powder. The measured pressure profiles in the nip region of the roll press were comparable to the calculated values. The theory was also found to predict the effect of material properties on the nip angle and the peak pressure but it was unable to account for the influence of roll speed.     

A method to model web trajectory and release in forward roll coating

The trajectory of the web at the exit of a roll coating operation can influence the quality of the final coating. While methods to model the web trajectory have been given in the literature, these methods are limited in various ways. A method is proposed to describe the web trajectory and the pressure distribution in the fluid at the exit of a forward roll coater. The Reynolds lubrication equations for the fluid are coupled with the web by a force balance on web node points. The fluid pressure in the coating layer generates forces on the web. These forces deflect the web. Integration in time gives the web dynamics. The angle that the web is pulled from the nip and the tension are found to influence the pressure pulse in the divergent section of the nip to a large extent. Low tensions lead to a second pressure pulse followed by a sub-ambient or tack pressure. Pulling the web at various angles from the nip can cause the tack pressure to increase or decrease. Pressure pulses are predicted that are comparable to measurements by a laboratory device.     

Using intelligent software to predict the effects of formulation and processing parameters on roller compaction

Roll compaction is a dry, continuous granulation process, which is widely used in the pharmaceutical, chemical, metallurgical, mineral and agricultural industries to produce dust-free and free-flowing agglomerates. Intelligent software has been used to predict the relationships between tablet formulations, roll compaction process parameters and the roll compacted ribbon, from which granules for tablet manufacture can be produced. The software exploits the strengths of artificial neural networks, genetic algorithms and fuzzy logic to predict multivariate relationships from experimental data. Input data were generated from material characterisation studies and from investigations conducted on a 20 cm diameter laboratory-scale roll press with side plates, where process parameters such as roll speed (1-5 rpm), roll gap (0.5-1.4 mm) and compaction pressure (up to 230 MPa) could be manipulated. The relative significance of inputs on various outputs such as ribbon properties, nip angle and maximum roll compaction pressure was investigated using the commercially available artificial intelligence software FormRules (Intelligensys, Teeside, UK). The important inputs and required outputs were subsequently used in the model-development software INForm (Intelligensys, Teeside, UK) so that the conditions necessary to produce ribbons with specific desired properties could be predicted.     

A torque-based analysis of the reverse roll coating process

In a laboratory film coater operated at industrial speed, the torque exerted by the metering rod was used to investigate the hydrodynamics in the metering nip flow. CFD simulations were also performed taking into account the deformation of the roll cover to depict the elastohydrodynamics of the metering nip flow. From the experiments, it was found that the torque increases with transfer speed, load and viscosity. When runnability problems such as spitting were observed, the torque signal decreased. Torque measurements could also predict excessive friction between rod and roll, which may damage the elastic cover. The numerical simulations showed trends similar to the experiments. Regions of positive and negative torque values demonstrated how the fluid interacts with the rod, a phenomenon apparently related to the occurrence of secondary flows in the metering nip. Finally, the numerical results also showed that the minimum nip gap becomes constant at high loads.     

钢丝帘布压延线上辅机的设计与应用

介绍了钢丝帘布压延线上辅机锭子张力装置、电磁吸盘及整经压力辊的设计与应用情况。张力控制装置采用重锤式结构，重锤重量按公式Ｐ＝Ｆ１Ｄ２ｄｆ计算。电磁吸盘装置主要由机架、导辊、轴承、电磁吸盘和电控箱组成，左右两个靠近电磁吸盘的导辊调整的最佳高度是：在运行过程中大部分帘线与磁盘表面平行，只有个别张力值小于电磁吸力的帘线被磁盘吸附在其表面或处于若即若离的状态。整经压力辊装置主要由交叉板、整经辊、压力辊等组成。整经辊的沟槽深度等于帘线直径，压力辊的沟槽深度为帘线直径的一半。一般调整压力辊与３＃辊筒的间隙略小于所给的胶片厚度     

Two-dimensional elastic contact model for rubber covered rollers

Abstract

A model for the contact between a rubber covered roller and a rigid roller was developed using analytical functions. The model, being two-dimensional, connects the line load, geometric properties (roller radii, rubber thickness) and the shear modulus of the rubber to nip properties in the roller contact. The output from the model is the indentation of the rigid roller into the rubber, the nip maximum pressure, the nip width and the surface strain in the centre of the nip. Moreover, the shape of the pressure distribution is also given as an analytical expression. The basic assumption relies on the work of Parish (?58 and ?61), but a development of this work was performed, showing the influence of the rubber shear modulus and also how the surface strain in the nip can be described. The functional relationships are based on least squares fitting of analytical functions, depending on two master variables, to a large number of finite element analysis results.     

Roll coating of purely viscous liquids

Experimental measurements are reported for the flow of Newtonian liquids through the nip of two co- and counter-rotating rolls of various size and speed ratios. Results are presented in a dimensionless form for the total volumetric flux through the nip and for the distribution of this flux between the two rolls. Simple correlations have been developed and these should be useful in the design and analysis of roll coating equipment.Some preliminary data are also presented for a shear thinning inelastic non-Newtonian liquid. The forms of the correlating equations developed for the Newtonian systems are still applicable but the constants in the equations are slightly modified.     

新型锥形双螺杆混合元件三维流场分析

利用ANSYS有限元分析软件对带有混合块的新型锥形双螺杆的混合段流场进行了三维模拟分析，通过速度场求出流量，并求出剪切速率，剪切应力及回流量来衡量混合效果，找出了混合块几何条件（间隙，错列角）及操作条件（转速，压差等）对挤出机混合效果的影响。     

Influence of fillers and oil on mill processability of brominated isobutylene‐co‐paramethylstyrene and its blends with EPDM

Milling behavior of brominated isobutylene‐co‐paramethylstyrene (BIMS), and its blends with ethylene propylene diene rubber (EPDM), with special reference to the effect of addition of different fillers, filler loadings and processing oil, was investigated over a range of temperatures in a drop mill operation. The critical nip gap (CNG, at which a front to back roll transition (F‐B transition) occurs, was also measured. Addition of different fillers reduced the value of the critical nip gap and the mill band formation indices for BIMS as compared to the gum polymer, indicating a decrease in the tendency to go to the back roll. The systems containing reinforcing fillers showed a lower value of the critical nip gap, as compared to systems containing low or non‐reinforcing fillers. For different filled blends of BIMS and EPDM, the CNG decreased on increasing the EPDM content, indicating a decrease in the tendency for a F‐B transition. The CNG decreased on increasing the N330 carbon black‐loading, while it increased with the addition of procesing oil. The results were explained in terms of rubber to metal adhension and viscosity of the polymers.     

An engineering study of the film embossing process

A mathematical model of the dynamics and heat transfer of the film embossing process has been developed. The thermal analysis around the preheat roll is determined from an unsteady, two-dimensional heat conduction equation along with appropriate boundary conditions by neglecting the curvature of the preheat roll and choosing a Lagrangian reference frame. The heat transfer occurring between the preheat roll and the embossing rolls is based on a one-dimensional analysis, including both convective and radiative effects. The deformation occurring in the nip region is analyzed for two different situations. For the case where the surface features are small in comparison with the film thickness, a modified one-dimennsional calendering analysis is given, accounting for the irregular geometry of the embossing roll surface. For the case where the polymer does not make complete contact with the surface of the engraved channel, the local deformation is determined by means of a simple one-dimensional cavity filling model. The required pressure distribution is determined by means of a simple one-dimensional cavity filling model, The required pressure distribution is determined by means of a conventional calendering analysis. The analysis for the case of a Newtonian and power-law model is presented in detail. The model yields qualitatively correct results and is computationally simple.     

纵向拉伸机新进展

现代化的双向拉伸膜生产线速度已由８０年代的１５０ｍ／ｍｉｎ提高到３５０ｍ／ｍｉｎ，为适应这一要求，要求纵拉机能更精确的控制拉伸温度和拉伸倍数，本文对纵拉机预热、拉伸及热定型辊；工作辊的直径和排列；拉伸间隙；夹辊；张力辊；单点拉伸与多点拉伸技术；破膜检测装置和传动系统的技术进展作了简介     

同向啮合双螺杆挤出机捏合块流道三维流场分析

利用ANSYS有限元分析软件对啮合同向双螺杆挤出机的捏合块流场部分进行了三维模拟分析。通过速度场求出流量 ,并求出拉伸速率、剪切速率及剪切应力来衡量混合效果。得到了机筒表面和啮合区的压力分布 ,找出了捏合盘几何条件 (间隙、错列角 )及操作条件 (转速、压差等 )对挤出机混合效果的影响。     

A semi-empirical model of the forward roll coating flow of newtonian fluids

A theory of the forward roll coating flow of Newtonian fluids has been developed recently by Greener and Middleman [1] utilizing the usual lubrication approximations and some very simple physical notions regarding the region in which the total flux through the nip splits into two films. These basic notions restricted the validity of their model to the simple case of two rolls of equal rotational speeds and equal radii. Their analysis resulted in an expression for the dimensionless total flux through the nip λ as a weak function of a modified capillary number N.The aim of this paper is to extend the model presented by these workers [1] by introducing to it experimental evidence of the flux distribution [2] and thus generalizing its applicability to operations involving rolls of different diameters rotating at different speeds. The results of this extended model give the dimensionless total flux λ as a function of the modified capillary number N, the speed ratio r_s, and the radius ratio r_r. This function is a weak one over the range of N, r_s, and r_r, normally encountered in forward roll coating operations and is in good agreement with experimental data [2].     

Roll compaction of maize powder

This paper presents a study of a roll compaction process as a dry granulation method for typical food materials such as maize powder. This process is widely applied in industry as it can continuously produce large quantities of granular product at comparatively low cost. The objectives of this work were to predict the roll compaction performance from a simple measurement involving uniaxial die compaction using the classical Johanson model. This involved determination of the optimum operating conditions for the production of granules as evaluated by apparent density.In the current work, a smooth counter-rotating rolling mill with a roller diameter of 0.08 m and a roller width of 0.20 m was used. The operating conditions for the rolling mill are shown to be influenced by parameters such as the roll gap, the roll speed, the feed powder amount, and the friction ratio. Material properties such as the compressibility factor and the angle of wall friction were investigated using uniaxial die compaction. The angle of wall friction was analysed using both contact mechanical and continuum mechanical approaches.The results indicated that this simplified approach can be used to provide a quantitative prediction of the extent of the roll compaction performance, and can be used to design optimal roller geometries and operating conditions.     

Study of mixing efficiency in roll-mills

A fluid dynamics analysis package (FIDAP) based on the finite element method was used to simulate the 2-D, isothermal flow of a power law model fluid in the bank and nip area of a two roll-mill geometry, including the determination of the free surface. Dispersive mixing efficiency was analyzed in terms of the shear stresses generated and the elongational flow characteristics. The latter were quantified using a frame invariant flow strength parameter. We found that the converging region rather than the nip region provides better mixing flow characteristics. We also found that the presence of vortices in the bank region is not beneficial for dispersive mixing.