Investigation of glass thickness effect on thermal slumping by experimental and numerical methods

Thermal slumping process is a high volume and low cost manufacturing method for fabricating aspherical and freeform glass mirrors with large dimensions. It has been aggressively tested for X-ray mirrors for space-based telescopes and for solar panels as well. In case a concave mold is used, the relationship between the upper surface contour of a slumped mirror and that of the mold becomes non-linear. On the other hand, the final surface contour of the slumped glass is a very important parameter which directly affects the optical performance of the mirror. In this research, experiments of thermal slumping glass plates on a parabolic concave mold were performed to study the thickness effect on the slumping process and the final surface contour of the upper surface of the glass plate. In addition, numerical simulation was conducted to ensure the internal stresses were removed at selected cooling rate and predict the corresponding surface contour. A comparison between simulation and experiments showed that the finite element method (FEM) simulation is adequate for predicting the surface contour if the glass was fully slumped. It was also discovered that under certain process conditions, thinner glass plates may not be fully slumped therefore proper remedies to the manufacturing process are necessary.