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Theoretical Investigation of Heat Transfer in Glass Forming
Journal of The American Ceramic Society
Abstract
A theoretical study to investigate internal heat transfer in glass undergoing cooling between glass and mold, as well as plunger, during and after pressing, is described. A thermal model has been formulated to simulate the cooling. The heat-transfer analysis accounts for the spectral nature of radiation in glass, the dependence of the thermophysical properties of glass on temperature, and the contact heat transfer between and after pressing, as well as subsequent cooling. Heat exchange between glass and mold by contact conduction across a very small gap and that by thermal radiation are considered separately. Numerical solutions have been obtained for typical conditions simulating symmetric and nonsymmetric cooling, and the results obtained are presented and discussed. During the dwell time, thermal-contact conduction between glass and mold is the dominant mechanism for heat extraction from glass. Results show that radiation from the surface of the glass plays a relatively small role in the heat extraction from the glass, but that radiation from the interior of the glass is much more significant.
... For example, Wilson et al. [8] studied the heat transfer across tool-workpiece interface by using an FEM code, DEFORM TM -2D. Viskanata and Lim [9] proposed a physical model for internal heat transfer in glass and heat exchange across glass-mold interface in one dimension. Yi and Jain [10] applied DEFORM TM -2D to the simulation of aspherical glass lens molding. ...
... During the first pressing in step (2), the changes of the position of the lower mold and the pressing load were recorded and the data were processed using Eqs. (9) and (10), and the viscosity of the glass preform was obtained. The measured viscosity of glass is plotted against temperature in Fig. 6. ...
Glass molding is as an effective approach to produce precision micro optical elements with complex shapes at high production efficiency. Since glass is deformed at a high temperature where the mechanical and optical properties depend strongly on temperature, modeling the heat transfer and high-temperature deformation behavior of glass is an important issue. In this paper, a two-step pressing process is proposed according to the non-linear thermal expansion characteristics of glass. Heat transfer phenomenon was modeled by considering the temperature dependence of specific heat and thermal conductivity of glass. Viscosity of glass near the softening point was measured by uniaxially pressing cylindrical glass preforms between a pair of flat molds using an ultraprecision glass molding machine. Based on the numerical models and experimentally measured glass property, thermo-mechanical finite element method simulation of temperature rise during heating and material flow during pressing was carried out. The minimum heating time and pressing load changes were successfully predicted.
... conductance, becomes the dominant mechanism influencing the thermal behavior of the contact pair. 2 After being brought into contact with the cold mold, the glass temperature largely reduces, even up to hundreds of Kelvin. 3 Such temperature drop of the glass at the contact surface certainly affects the form accuracy and the optical properties of molded glass components. ...
Heat transfer at the interfacial contact is a dominant factor in the thermal behavior of glass during nonisothermal glass molding process. Recent research is developing reliable numerical approaches to quantify contact heat transfer coefficients. In most previous studies, however, both theoretical and numerical models of thermal contact conductance in glass molding attempted to investigate this factor by either omitting surface topography or simplifying the nature of contact surfaces. In fact, the determination of the contact heat transfer coefficient demands a detailed characterization of the contact interface including the surface topography and the thermomechanical behavior of the contact pair. This paper introduces a numerical approach to quantify the contact heat transfer by means of a microscale simulation at the glass‐mold interface. The simulation successfully incorporates modeling of the thermomechanical behaviors and the three‐dimensional topographies from actual surface measurements of the contact pair. The presented numerical model enables the derivation of contact heat transfer coefficients from various contact pressures and surface finishes. Numerical predictions of these coefficients are validated by transient contact heat transfer experiments using infrared thermography to verify the model.
... As a result, the strong dependence of contact heat transfer on boundarynear temperatures under transient conditions has a particularly significant relevance for highly dynamic systems. Such systems include, for example, metal cutting [Augspurger, 2018;Deppermann, 2017] or glass molding processes [Viskanta and Lim, 2001;Moreau et al., 2003]. ...
The development of methods for the description of thermal behavior is a significant challenge for numerous technical applications. Particularly relevant is the accurate prediction of thermal boundary conditions, which influence the thermal system and thereby act as the cause of temperatures and heat flows. However, the precise determination of thermal boundary conditions is a demanding procedure as no direct measurement is possible. Instead, such boundary conditions are determined by measuring the temperature field resulting from them. This constitutes an inverse problem, which reacts sensitively to measurement errors and, thus, requires elaborate measurement and evaluation procedures. The present thesis describes a measurement method, which obtains temperatures by means of infrared thermography and allows the determination of thermal boundary conditions by comparing measurements with predictions from a numerical model of the observed thermal system. The methodology is described for contact interfaces between machine components. Thermal boundary conditions at contact spots are a result of microscopic surface roughness, which leads to a reduction of the actual surface area in contact, thereby causing thermal resistance. In addition, a numerical tool is presented, which allows prediction of contact heat transfer coefficients without the need of an experiment. The mechanical deformation of surfaces under contact pressure is simulated, using half-space description of topographically measured surface profiles. Both methods are investigated for various contact pressures, sample materials and for different interstitial fluids within the contact zone. A comprehensive description of possible sources for errors and an evaluation of the quality of results is given for both methodologies. Furthermore, the experimental methodology is evaluated through an exemplary technical application, for which heat flow into a cutting tool is determined during milling.
... Hydrocarbon polymers shrink during pyrolysis, and upon out gassing form carbon with an amorphous, glass-like structure, which by additional heat treatment can be changed to a more graphite-like structure [33][34][35][36][37]. Other special polymers, where some carbon atoms are replaced by silicon atoms, the so-called polycarbosilanes, yield amorphous silicon carbide of more or less stoichiometric composition. ...
There is the rapid increase in the application of ceramic materials in the industry. One area that is currently receiving significant attention from the scientific community is the different types of application of Ceramic Material Composite in the industry as well as in the Defence applications. The motivation to develop CMCs was to overcome the problems associated with the conventional technical ceramics like alumina, silicon carbide, aluminium nitride, silicon nitride or zirconia-they fracture easily under mechanical or thermo-mechanical loads because of cracks initiated by small defects or scratches. The crack resistance is-like in glass-very low. To increase the crack resistance or fracture toughness, particles (so-called mono crystalline whiskers or platelets) were embedded into the matrix. Ceramic Composite armour materials are currently being investigated by researchers to take the advantage of potential for improved performance and reduced by weight. Within the defence industry , advanced ceramics are at the heart of modern armour system due to their comparatively low weight and high performance during ballistic-scale impacts.
... Hydrocarbon polymers shrink during pyrolysis, and upon out gassing form carbon with an amorphous, glass-like structure, which by additional heat treatment can be changed to a more graphite-like structure [33][34][35][36][37]. Other special polymers, where some carbon atoms are replaced by silicon atoms, the so-called polycarbosilanes, yield amorphous silicon carbide of more or less stoichiometric composition. ...
There is the rapid increase in the application of ceramic materials in the industry. One area that is currently receiving significant attention from the scientific community is the different types of application of Ceramic Material Composite in the industry as well as in the Defence applications. The motivation to develop CMCs was to overcome the problems associated with the conventional technical ceramics like alumina, silicon carbide, aluminium nitride, silicon nitride or zirconia-they fracture easily under mechanical or thermo-mechanical loads because of cracks initiated by small defects or scratches. The crack resistance is-like in glass-very low. To increase the crack resistance or fracture toughness, particles (so-called mono crystalline whiskers or platelets) were embedded into the matrix. Ceramic Composite armour materials are currently being investigated by researchers to take the advantage of potential for improved performance and reduced by weight. Within the defence industry , advanced ceramics are at the heart of modern armour system due to their comparatively low weight and high performance during ballistic-scale impacts.
... Numbers of researchers have been devoted to investigating the heat transfer in the process. Viskanta and Lim 5 conducted a theoretical study to investigate internal heat transfer in glass undergoing cooling between glass and mold, and a thermal model had been formulated to simulate the cooling process. Hohne et al. 6 set up a laboratory testing unit to investigate the influence of radiation emitted by different glass compositions and the influence of different mold materials. ...
In a precision glass molding process, glass preform is compressed at a high temperature well above its transition temperature and the temperature distribution inside plays an important role in determining the quality of final products. In this research, a 2D axisymmetric numerical heat transfer model, integrated with an innovative PID control subroutine, was employed for simulating the temperature control process and obtaining transient temperature distribution in the glass preform. Feasibility of this method was validated by experiments with good agreements. Finally, influence of air gap between glass preform and upper mold as well as temperature control mode were investigated. The results showed that temperature difference of the molds had a severer influence on the temperature distribution of glass preform with the decrease of air gap. In addition, a two-point control strategy was demonstrated as a valid method to improve the temperature uniformity in the glass preform.
... Both sides of the glass have contact with the metal, i.e. the interior of the glass with plunger and at the same time, exterior with mould. Due to high temperature at interfaces, the conduction and radiation both occur during heat exchange between these interfaces ( Kleer and Doell, 1997;Viskanta and Lim, 2001) which accelerates the formation of oxide scale. Material Science, 5 No. 3 October-December 2017 41 However, oxidation effects are minimized by introducing nitrogen in press chamber ( Weck et al., 2003). ...
This paper reviews all coating techniques, including Chemical Vapour Deposition (CVD), Physical Vapour Deposition
(PVD), Magnetron Sputtering, and Ion Beam Assisted Deposition used for the coating on the working surface of dies used
in glass manufacturing industry. Coatings used with these techniques were single layer coatings of nitrides, borides, and
oxides, multilayer coatings of nitride or carbides and noble metal like Platinum-Iridium (Pt-Ir) coatings. During glass
manufacturing, dies are subjected to high temperature ranges from 900 °C to 1150 °C. The characteristics of coatings
like wettability, wear and corrosion resistance against thermal cycling at high temperature are important factors for die
manufacturing materials. Problems related to these characteristics have been elaborated in detail which reflects the
behaviour of coatings particularly used for dies, and their effects have been studied. Oxide scale formation on the
working surface and coatings due to high temperature corrosion is also a burning issue and it has been explained in this
review paper. Among the various coatings, Pt-Ir coatings are the best in terms of performance. However, these noble
metals are costlier than other materials which have been used for coatings. Further the different coatings for high
temperature corrosion applications have been taken into consideration to know the effective results associated with
their use and the materials that have provided the protection to the working surfaces at high temperatures. It has been
found that the coating process, its parameters and selection of material directly affect the surface properties at high
temperatures and it is proposed that the use of new coating techniques, such as microwave cladding and thermal spray
coating with a number of variants in their processes and feedstocks can effectively modify the surface properties of die
materials.
... For example, Wilson, Schmid, and Liu (2004) studied the heat transfer across a tool-workpiece interface by using an FEM code, DEFORMTM-2D. Viskanta and Lim (2001) proposed a physical model for internal heat transfer in glass and heat exchange across glass-mold interface in one dimension. Yi and Jain (2005) applied DEFORMTM-2D to the simulation of aspherical glass lens molding. ...
Microstructures are increasingly applied in optical systems, precision measurement, arms navigation, biomedical fields, and so on. Microstructures include microgrooves, micropatterns, microprisms, and microlenses. This chapter provides an overview of the application of microstructures and glass molding processes. It highlights commonly used materials for glass molding processes; the materials to fabricate the mold are also introduced. The latest examples of glass molding simulations are discussed, including viscoelastic constitutive modeling, the microstructure molding process, and a simulation coupling heat transfer and viscous deformation. Molding quality control and molding defects in a glass molding process are also introduced in this chapter.
This paper reviews all coating techniques including Chemical Vapor Deposition(CVD), Physical Vapor Deposition(PVD), Magnetron Sputtering and Ion Beam Assisted Deposition used for the coating on the working surface of glass manufacturing components . Coatings used with these techniques were single layer coatings of nitrides, borides, oxides such as TiBC, TiBCN, TiN,BN,TiAIN, multilayer coatings of nitride or carbides based, Diamondlike carbon coatings and Noble metal like Platinum-Iridium(Pt-Ir) coatings .During glass manufacturing, tools are subjected to high temperature ranges from 900°C to 1150°. The characteristics of coatings like wettability, wear, corrosion and resistance to thermal cycling at high temperature are required for glass manufacturing materials. These coatings did not attain all the required characteristics for tool materials. Pt-Ir coatings are best in terms of performance, however it is costlier. It is proposed that the use of new coating techniques such as claddings and thermal spray coatings with a number of variants in their processes and feedstock’s (micro and nano- powders) can be used for effectively modifying the surface properties of glass mould materials.
An integrated cooling simulation method for the temperature field of the panel during the forming process was proposed. The simulation system for the optimization of processing parameters and in-mold cooling system structure design was developed. A finite-difference method based local one-dimensional transient analysis in the thickness direction is adopted for the panel part. A three-dimensional, boundary element method is used for the numerical implementation of the heat transfer analysis in the mold region. The part and mold analysis are coupled so as to match the temperature and heat flux on the glass-mold interface. This paper presented an integrated and coupled numerical model for analyzing the panel cooling process. The numerical results are in good agreement with the experimental ones.
The required accuracy for the final dimensions of the molded lenses in wafer-based precision glass molding as well as the need for elimination of costly experimental trial and error calls for numerical simulations. This study deals with 3D thermo-mechanical modeling of the wafer-based precision glass lens molding process. First, a comprehensive 3D thermo-mechanical model of glass is implemented into a FORTRAN user subroutine (UMAT) in the FE program ABAQUS, and the developed FE model is validated with both a well-known sandwich seal test and experimental results of precision molding of several glass rings. Afterward, 3D thermo-mechanical modeling of the wafer-based glass lens manufacturing is performed to suggest a proper molding program (i.e., the proper set of process parameters including preset force-time and temperature-time histories) for molding a wafer to a desired dimension and quality. Moreover, the effect of some important process parameters such as cooling rate and pressing temperature on the final size and residual stress inside the wafer is evaluated. Finally, it is noted that the suggested molding program minimizes the costly empirical efforts and raises the process efficiency.
Pressing is one of the most versatile and important manufacturing processes capable of mass-producing complicated glass parts in net shape with excellent dimensional tolerance. Numerical simulation of glass pressing has been an active research area for many years, as part quality and yield requirements become more stringent. This paper reviews the research and development in numerical simulation of glass pressing. It organizes prior studies into three categories, namely, glass flow, heat transfer, and residual stresses. Based on the research progress to date, this paper suggests that the coupled and integrated simulation considering material, mould, and process, the use of empirical models and soft computing techniques to complement the numerical simulation, and small-scale experiments are the three most important areas for further advancement.
In the precision glass moulding process, the heat transfer and the resulting transient temperature distributions of the molten glass are of great importance because they significantly affect the productivity as well as the thermally induced residual stresses in the final product. Thermal modelling of the heating system in the glass moulding process considering detailed heating mechanisms therefore plays an important part in optimizing the heating system and the subsequent pressing stage in the lens manufacturing process.The current paper deals with three-dimensional transient thermal modelling of the multi-stage heating system in a wafer based glass moulding process. In order to investigate the importance of the radiation from the interior and surface of the glass, a simple finite volume code is developed to model one dimensional radiation–conduction heat transfer in the glass wafer for an extreme case with very high temperature difference considering temperature dependant thermal conductivity and heat capacity. Afterwards, by using three-dimensional FEM modelling along with a predefined experimental test, the equivalent glass–mould interface contact resistance is determined for two different pressures. Finally, the three-dimensional modelling of the multi-stage heating system in the wafer based glass moulding process is simulated with the FEM software ABAQUS for a particular industrial application for mobile phone camera lenses to obtain the temperature distribution in the glass wafer. In the numerical modelling, the interface boundary conditions for each heating stage are changed according to the determining heat transfer mechanism(s). Numerical results are compared with experimental data to show the validity of the numerical modelling. The obtained results show that the right thermal modelling is highly dependent on the proper choice of thermal boundary conditions in different stages according to the real physical phenomena behind the process.
Numerical simulation of glass pressing has been an active research area for many years. This paper reviews the research and development in numerical simulation of glass pressing. It organises prior studies into three categories, namely, glass flow, heat transfer, and residual stresses. Based on the research progress to date, this paper suggests that the coupled and integrated simulation considering material, mould and process, the use of empirical models and soft computing techniques to complement the numerical simulation, and small scale experiments are the three most important areas for further advancement.
Compression molding is an effective high volume and net-shape fabrication method for aspherical lenses and precision glass optical components in general. Geometrical deviation (or curve change as often referred to in industry) incurred during heating, molding, and cooling processes is a critically important manufacturing quality parameter. In the compression glass molding process, there are many factors that could lead to curve change in final products, such as thermal expansion, stress and structural relaxation, and inhomogeneous temperature distribution inside the molding machine. In this research, an integrated numerical simulation scheme was developed to predict curve change in molded glass aspherical lenses. The geometrical deviation in the final lens shape was analyzed using both an experimental approach and a numerical simulation with a finite element method program. Specifically, numerical simulation was compared with experimental results to validate the proposed manufacturing approach. The measurements showed that the difference between numerical simulation and experimental results was less than 2 mu m. Based on the comparison, the mold curve was revised using numerical simulation in order to produce more accurate lens shapes. The glass lenses molded using the compensated molds showed a much better agreement with the design value than the lenses molded without compensation. It has been demonstrated in this research that numerical simulation can be used to predict the final geometrical shape of compression molded precision glass components. This research provided an opportunity for optical manufacturers to achieve a lower production cost and a shorter cycle time.
In the pressing process of picture tube panel, the thermal and mechanical loads cause the mold blocks to deform. This deformation results in large deviations on the display surface of the panel. A thermoelastic model is presented in this paper to predict the deformation of the mold during pressing, which suggests the amounts that the mold cavity should be machined by. The model is based on the steady mold temperature field and thermoelastic boundary element method. Quadratic subparametric elements are used in the thermoelastic analysis, and a semi-analytical scheme coupled with element subdivision is employed for the integration. The predicted deformation has been side verified by comparing the dimensional accuracy of the panels produced by the molds considering a uniform shrinkage and considering the predicted mold deformation.
Purpose
This paper aims to develop an integrated cooling simulation for the temperature history of the panel during the forming process.
Design/methodology/approach
A local one‐dimensional transient analysis in the thickness direction is adopted for the panel part, which employs finite‐difference method. And a three‐dimensional, boundary element method is used for the numerical implementation of the heat transfer analysis in the mold region, which is considered as three‐dimensional conduction. The Renormalization‐Group turbulence model is applied for the jet impinging cooling. The part and mold analyses are coupled so as to match the temperature and heat flux on the glass‐mold interface.
Findings
The paper provides mathematical model and numerical strategy adapted to the problem, with experimental verification that shows a good agreement.
Practical implications
Cooling design in the forming operation of picture tube panel is of great importance because it significantly affects the part quality associated with residual stresses and productivity. The developed simulation package is a tool in the optimization of processing parameters and in‐mold cooling system structure design.
Originality/value
This paper presents a realistic, integrated, and coupled numerical model for analyzing the panel cooling process that retains important aspects of the problem. The paper could be very valuable to the researchers in this field as a benchmark for their analyses.
A laboratory experiment is described which was designed to investigate the thermal contact between glass and mold during pressing. The variation of the glass-mold heat transfer coefficient has been used as a measure of the thermal contact in relation to other variables such as the initial mold temperature and contact pressure. A rapid initial decrease in the heat transfer coefficient was observed over the range of pressing pressures investigated (20-400 kPa). It was also found that sticking of the glass to the heat resistant steel mold occurred above an initial mold temperature of 605 degree C.
Two phenomena which influence the energy transfer by thermal radiation between two solid dielectrics separated by a vacuum gap are described. For typical dielectric systems, it is shown that these coupled phenomena, which are termed wave interference and radiation tunneling, influence the radiative transfer to the extent that the classical Stefan-Boltzmann radiation law is inadequate to describe the net energy transfer. Since both phenomena are significant only when the distance of separation between the surfaces of the solids is of the same order of magnitude as the wavelength of the radiation, their effect is greatest when the spacing is small. However, the actual magnitude of the difference between the present treatment and the classical approach is a complicated function of the surface spacing, the refractive indexes of the solids, and the temperatures of the solids, and can be as large as an order of magnitude.
Experimental measurement of the dynamic internal temperature distribution in soda-lime glass plates using thermocouples fused in the glass has been carried out. Experimentally measured temperatures are compared to predictions obtained from the solution of the transient energy equation where the internal radiative transfer has been accounted for using rigorous radiative transfer theory. A discussion of the experimental method, the process used to fuse the thermocouples in the test plates and the rigorous formulation of the energy equation, for semitransparent materials is given. Predicted and measured instantaneous surface temperatures are compared and very good agreement is obtained. It is also concluded that the empriical equation for the phonon conductivity used in the analysis underpredicts the conductivity. A 20% increase in phonon conductivity is shown to significantly improve the agreement between measured and predicted centerline to surface temperature differences.
Heat transfer by combined conduction and radiation has been investigated in a one-dimensional glass layer. The layer is semitransparent to radiation and the dependence of the absorption coefficient on wavelength is accounted for. The discrete ordinates method and the diffusion approximation are used to analyze radiative transfer. The differencing scheme for the discrete ordinates method has been investigated to obtain the constant total heat flux distribution across the glass layer as required by the energy conservation equation, and the best uniformity is obtained with the diamond scheme. The results predicted by the discrete ordinates method are in good agreement with those based on the exact (integral) equation formulation of radiative transfer. The diffusion approximation greatly underpredicts the temperature and heat flux distributions in the glass layer when the thickness or the opacity of the layer is small. The predictions of the diffusion approximation are only reasonable for thick glass layers. This approximation should be used with extreme caution to obtain quantitatively accurate results.
An attempt is made on the basis of electromagnetic theory to predict spacing effects on the radiative transfer between two closely-spaced, semi-infinite metals separated by a nonconducting dielectric. Electromagnetic energy transmission factors are derived for a general, three-medium system and their simplification for the metal-dielectric-metal system is shown. The net energy flux is calculated with these transmission factors and the estimated intensities in the metals. Numerical results are given which exhibit the effects of metal spacing, metal type, temperature level, and type of dielectric. Variations in the heat flux of several orders of magnitude are shown to exist in a system whereas the Stefan-Boltzmann relation indicates a constant flux, independent of spacing.
Effects of Thermal Conditions on Generation of Sink-Marks of a Press-Formed Glass Product in Process " ; pp. 69–75 in Enhanced and Multiphase Heat Transfer—A Festschrift for A
- K Tatsukoshi
- Y Satoh
- Y Kurosaki
- I E Satoh
- Bergles
K. Tatsukoshi, Y. Satoh, Y. Kurosaki, and I. Satoh, " Effects of Thermal Conditions on Generation of Sink-Marks of a Press-Formed Glass Product in Process " ; pp. 69–75 in Enhanced and Multiphase Heat Transfer—A Festschrift for A. E. Bergles. Edited by R. M. Manglik and A. D. Kraus. Begell House, New York,