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| By Ulrich Lange, Schott Glas, Mainz, Germany
Optical fiber preforms without (left) and with (right)Chill ripples, which are also known as press ripples or flow ripples, sometimes occur on the glass surface under certain conditions in many hot-forming processes, such as pressing and casting. These ripples can be observed on many low-quality hollow ware products, such as pressed wine glasses, in the form of concentric waves on the surface of the foot. The name "chill ripples" reflects the fact that this phenomenon occurs if the temperature of the tool used in the process is too low. Indeed, any low temperature wall that comes into contact with the glass can cause ripples to form. The obvious countermeasure of using a higher tool temperature is very limited, since the glass tends to stick to the tools if their temperature is too high. The window between the critical temperature for the onset of ripples and the sticking temperature can be very small. Hence, for high quality products either the process conditions must be controlled very carefully, or expensive post-processing must be performed. Using POLYFLOW, two case studies were performed to study the formation of chill ripples. A model of a casting process predicted the formation of ripples on the glass surface if a low initial mold temperature was assumed. In agreement with experiments, no ripples appeared for higher initial mold temperatures. A close inspection of the temperature and flow field corroborates the conjecture that chill ripples are a consequence of the strong dependence of glass viscosity on temperature, and that thermal expansion effects are not the dominant cause. More precisely, the model calculations show that the ripples are caused by an "arching flow"; once the glass comes into contact with the mold, the vicinity of the contact point is cooled rapidly and thus immobilized. Less viscous glass flows around the immobilized region and eventually touches the mold in a new contact point.
The evolution of a chill ripple by an "arching flow" in a casting process; temperature contours on the glass and mold are shown
The formation of chill ripples in a pressing process with temperature contours on the glass and tool (y-direction stretched in pictures on the right hand side) In an examination of a pressing process, the quantitative agreement between the model predictions and published experimental data1 for the critical tool temperature is good if the initial glass temperature is high, but not as good if the initial glass temperature is very low. More importantly, these simulations revealed many interesting details of the evolution mechanism of chill ripples. An attempt to observe this evolution in an experiment would be very expensive, because of the high temperatures and the optical inaccessibility of the process. Reference: 1 Kluge W.D., PhD Thesis, University of Freiberg, 1988. |
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