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Glass Breaking During Tempering and Their Solutions (1)
Source : www.landglass.co    Release time:2013-10-25

When the glass sheet is tempered in the glass tempering furnace, there will be various of conditions happened sometimes, and glass breaking is a common phenomenon during the tempering process. Generally speaking, the possible breakages of glass during the tempering include the breaking during quenching, cracking during the bent tempering, breaking in the furnace during heating, and corner losing of plate glass. 


1. Glass breaking during the quenching

Glass breaking during the cooling mainly results from the following reasons: 

1) The cold working of glass sheet is improper, such as cutting, drilling and edge grinding, which causes uneven stress distribution after tempering and results in glass breaking. 


For different thickness of glass, the special shape cutting, drilling and notching are restricted by specific conditions about the size and position of hole and notch, and the distance between holes and notches (which are specified in the process manual of Landglass). If the cutting and drilling of glass sheet are unqualified, stress concentration area will be built in the tempering process, which results in uneven stress distribution inside the glass. 


Glass without edge grinding or finish grinding after cutting, drilling and notching are liable to the defects like nick, crack and scratch at the rough edges and corners. Subsequently the stress concentration area will be built, and the glass will break. Therefore, finish grinding is needed before tempering. Similarly, for special shaped glass and the glass with notches and holes, etc., edge grinding or chamfering shall be done after cold working, and the hole and edge-cut surface shall be smooth in case of stress concentration at the edges and corners. 


2) There are stones as impurities and nickel sulfide particles inside the glass. For the expansion coefficients of stone and glass are different, cracks exist around the stones; they are the weak points in the glass, or called as stress concentration area. The stress effect in glass will increase by times after tempering, which may lead to spontaneous breaking. 


3) Because of over low discharge temperature or incomplete heating of glass, it’s impossible to generate even internal stress during the cooling in chiller, and breaking occurs. 


Solutions:

1) Have cold working to glass strictly according to the process requirements, such as cutting, drilling, notching and finish grinding. Before heating, check the glass sheet again. 

2) Choose the high-quality glass sheet, and check whether there are stones inside the glass. 

3) Increase the temperature of heating furnace or increase the heating time so as to ensure sufficient heating of glass. 


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