If hot glass comes into contact with cold forming tools, three different kinds of typical hot forming defects usually limit the process window in glass forming processes. The first defect is sticking of the glass at the forming tools (chemical reactions between glass and forming material occurs), if the temperature is above a certain value. The second type of defects are hot forming cracks and the last forming defect is the appearance of chill ripples, which occur if the temperature of the forming tools is too low. Chill ripples are also known as press ripples or flow ripples. Sometimes these ripples can be observed on hollow ware products, such as pressed wine glasses, in the form of concentric waves on the surface of the foot.
Figure 1: Temperature distribution and chill ripples formation during pressing a glass gob, computed with CFD simulation software NOGRID points
NOGRID points computes the real glass flow and temperature distribution during the complete forming step and is also able to predict the appearance of chill ripples. Comparisons with various experimental data have shown the high precision of the simulation of this intricate flow process. The short computation time allows our customers to incorporate the simulation into the design process to find an optimal design, thus saving time and effort in the design phase and increasing product quality.