Channel: Safety glass
Safety Glass Experts: elimination of process variation from gravity bending

2011-03-01 | Safety glass

Uncontrolled process variation has an effect on manufacturing processes, reducing repeatability, and causing fluctuation in quality and productivity. Additionally, uncontrolled process variation greatly increases the waste (scrap) rate. The influence of individual process variation varies from minor to significant. However, all existent variations contribute to the magnitude of effect and can create a negative synergy that potentially leads to greatly fluctuating end results and waste pieces. There are several sources of process variations within the gravity bending process, and sources must be defined, measured, analyzed, improved and controlled - ideally to the point of elimination. These actions have an essential role in improving gravity bending process repeatability and reduction of waste. This article reviews the common sources of process variations within gravity bending systems and discuss the required actions to both control and eliminate these variations.

Safety Glass Experts: elimination of process variation from gravity bending
Process variations are undesired changes in production processes that can affect final product quality characteristics. These effects can be seen as fluctuations in: edge size, edge shape, geometry (sagging), wing radius, optics, and residual stresses. In order to control and eliminate any process variation, all key indicators must be identified, monitored, measured and adjusted accordingly. Key process variable #1 - temperature The most important key process variable in gravity bending processes is, naturally, temperature. The glass is sagged to its final shape with heat-treatment using infrared (IR) heating elements. The windshield is annealed and cooled at a controlled rate using a combination of natural airflow and forced cooling. In the gravity bending process, temperatures are controlled by pyrometers and thermocouples. The pyrometer measures the glass surface temperature and controls automation. Thermocouples are used to control empty chamber temperatures, both as a safety function and as a process controlling device in such case where the glass surface cannot be measured due to printing, loading location or shape of the glass. Thermocouples are very precise instruments.
However, in bending furnace technology, the applications cannot be considered as reliable response variables, because they are merely an indication of ambient temperature information in individual locations of the furnace affected by various conditions making the reading at...


Article taken from Glass-Technology International bi-monthly magazine (6 issues per year)
Year: 2011 number: 2
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