|Sag Bending-External Manipulation is Indication of Failure|
|MIKA ERONEN - Safety Glass Experts International|
|Still today manufacturers of bus and special vehicle windscreens to OEM and ARG markets are using external manipulation, where a pressing tool is applied inside the bending chamber during the bending and final bending stages through a inlet in the bending furnace wall. With this pressing tool the glass is then typically forced to its final shape and maintained until below glass transition temperatures. To my opinion and experience the external manipulation is a indication of a failure in bending process design. This article will review the issues related to external manipulation and discuss the correct approach to bending process that enables to decrease and eliminate the external manipulation completely.
In external manipulation the bend glass surfaces are pressed with a mechanical hand extension, a pressing tool, that is controlled manually by the operator and pressure is applied to the areas of the glass that are not bending accordingly to the required shape. These areas are the inner regions of the glass pane where undesired reverse bending occurs and most commonly the corners of the glass. The pressing tool is typically a metal rod that is covered with insulative/protective materials from the tool end that is used to apply pressure. Althought manufacturers applying this method are constantly seeking for better protective materials for their pressing tools, it is evident that the protective materials to reduce pressing mark appearance, are not yet able to eliminate the marking issues alone as this measure still has a strong tendency to leave pressing marks in to the final product. These visible marks on the windscreens can be seen as black stripes, lines, scratches and impressions of different magnitude on the edge and inner regions on interior surface (S4) of the windshield. Naturally the application of pressing, its force, direction, shape of pressing tool, temperatures, location and float glass affect in the appearance and the covering, protective material alone is not to be blamed, but the other variables of this practise.
These marks are not decreasing the product robustness but is a non-desirable aesthetic defect. Although the pressing marks are typically located well outside the drivers main vision area A, many vehicle manufacturers are strict with any pressing imprints, marks, and scratches in all visible areas of the windshields. Polishing can be applied to remove the pressing marks and it can efficiently remove the marks to a certain degree, but polishing the glass surface will affect the glass surface quality easily causing further issues, not to mention the extra work to finish the product further increasing manufacturing costs.
Instead of applying external manipulation manufacturers should be encouraged to pursue improved bending process parameters and mold tooling design to eliminate the reverse bending and corner deformations during bending.
The full article explains the concerns and causes in application of external manipulation and provides insight to the principals how to bend and maintain the shape without external manipulation.
Before discussing this topic further and external manipulation any further it is important to highlight that this method is potentially dangerous to health and can cause injury or death when applied in a bending furnace that is not designed for such a method! Furthermore it can be against the local electrical safety legislations to enter manually with a hand extension to an open electrical circuit! Therefore we cannot and do not advise anyone to use this method in any form due to health hazards but also simply because acquiring and maintaining shape with this method has potential to create quality issues and process variation!
Full version of this technical article available at our MATERIALS section
©All graphics, photographs, and text appearing in this article belong to Safety Glass Experts International Oy Ltd. Redistribution or commercial use is prohibited without express written permission.
Last review: January, 2012
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