Experimental investigation of contact heat transfer coefficients in nonisothermal glass molding by infrared thermography

Abstract

Nonisothermal glass molding has recently become a promising technology solution for the cost‐efficient production of complex precision glass optical components. During the molding process, the glass temperature and its temperature distribution have crucial effects on the accuracy of molded optics. In nonisothermal molding, the glass temperature is greatly influenced by thermal contact conductance because there is a large temperature difference between the glass and mold parts. Though widely agreed to be varied during the molding process, the contact conductance was usually assumed as constant coefficients in most early works without sufficient experimental justifications. This paper presents an experiment approach to determine the thermal contact coefficient derived from transient temperature measurements by using infrared thermographic camera. The transient method demonstrates a beneficially short processing time and the adequate measurement at desirable molding temperature without glass sticking. Particularly, this method promises the avoidance of the overestimated contact coefficients derived from steady‐state approach due to the viscoelastic deformation of glass during the inevitably long period of holding force. Based on this method, the dependency of thermal contact conductance on mold surface roughness, contact pressure, and interfacial temperature ranging from slightly below‐to‐above glass transition temperature was investigated. The results reveal the dominance of interfacial temperature on the contact conductance while the linear pressure‐dependent conductance with an identical slope observed for all roughness and mold temperatures. The accurate determination of the contact heat transfer coefficients will eventually improve the predictions of the form accuracy, the optical properties, and possible defects such as chill ripples or glass breakage of molded lenses by the nonisothermal glass molding process.

Numerical and experimental determinations of contact heat transfer coefficients in nonisothermal glass molding

“…In contrast, the surface of the mold 3 has been undergone several molding cycles before the experimental investigation. Under this circumstance, there exist thin oxide layers on its surface, which has been illustrated by the scanning electron microscope investigation . The oxide layers contain not only higher hardness but also lower thermal conductivity compared to the newly machined surface material, and subsequently cause an increase in heat resistance.…”

“…In contrast, the surface presented in Figure C is initially machined by diamond turning and then undergone several glass molding cycles. It is observed that the roughness of mold 3 sharply decreases after the first molding cycles, mainly due to plastic deformation of surface asperities and wear phenomena such as oxidation, glass adhesion or scratch, and afterwards remains almost unchanged . Figure C provides the surface roughness measured after 20 molding cycles ( R a = 35 µm‐mold 3).…”

“…Consequently, the heat transfer occurring at the glass‐mold interface, ie thermal contact conductance, becomes the dominant mechanism influencing the thermal behavior of the contact pair . After being brought into contact with the cold mold, the glass temperature largely reduces, even up to hundreds of Kelvin . Such temperature drop of the glass at the contact surface certainly affects the form accuracy and the optical properties of molded glass components.…”

“…Several investigations for quantifying thermal contact conductance are performed, nevertheless, only at low contact pressure. Recently, a conjugate gradient method combined with infrared thermography has been introduced by the authors . By this method, transient contact heat transfer coefficients over a wide range of molding pressures, interfacial temperatures and surface finishes were experimentally determined.…”

“…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. Early studies indicated the influences of the glass temperature on the final shape of the molded lenses.…”