Slumping Research

The following is an abstract of an article investigating the behaviour of glass in slumping (or as they term it - sagging).  They are using optical glass which has a much higher viscosity at its slumping temperature than fusing glass, but the principles are transferable.  They indicate quick firing to the strain point (which is about 500C/930F) is possible, but that best results come from progressing slowly to the slump soak temperature.  

The article is as follows|: 

Numerical and Experimental Investigation of the Heating Process of Glass Thermal Slumping

• Author: Zhao Dachun, Liu Peng, He Lingping, Chen Bo

• Publish: Journal of the Optical Society of Korea Volume 20, Issue2, p314~320, 25 Apr 2016

https://oak.go.kr/central/journallist/journaldetail.do?article_seq=20778

The abstract of the article follows.

“Simulations performed for different heating rates resulted in different sag variation between glass and mold; higher heating rate caused larger sag variation. Therefore, to save time in manufacturing, the furnace should be heated to about 500℃ quickly, but then to the soaking temperature slowly. According to the simulation, the maximum sag variation decreased with higher soaking temperature. Considering the temperature-dependent viscosity of glass, the optimal soaking temperature was about 800℃.

Based on the optimized experimental conditions, glass samples of thickness 0.5 mm were formed in a furnace with a concave parabolic mold made of steel. Slumped glass was produced and tested. Comparison between surface profiles of the formed glass, theoretical data, and mold suggested that the quality of the slumped glass can be significantly improved by optimizing the shape of the mold. The RMS and PV sag deviations between formed glass and mold were 2.3 and 4.7 μm respectively, along the axial direction.

On the other hand, with a soaking temperature of 800℃ the formed glass was still not fully slumped at the edges, according to the radial deviation seen in experimental results, which meant that the glass was not making full contact with the mold. The experimental results agreed with the numerical predictions. Therefore, trimming the edges is necessary to achieve a better surface profile. Based on the simulated and measured results, improvements can be made in future research, and new mold materials or manufacturing processes should be adopted to improve the shape accuracy of the mold.”