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Some Knowledge about Tempering Machine---Reasons and Solutions for Breakage of Tempered Glass(2)
Source :    Release time:2013-06-21

III. Technical Quality of Tempering    
 a. excessive tempering of glass 

To evaluate the tempering degree, we shall focus on the broken fragments of tempered glass. The smaller the fragments are, the larger the pressure stress of the glass surface. Spontaneous breakage will be easier to occur if the glass has been excessively tempered by glass tempering furnace, therefore, in order to meeting the national standard, too high pressure stress shall be avoided.

To avoid excessive tempering, glass heating temperature and tempering in the tempering furnace body pressure shall be controlled effectively. Excess heating temperature and tempering pressure can cause excess internal tensile stress and surface pressure stress of the glass, which will increase the probability of spontaneous breakage. Hence, in practical production process, a set of accurate technological parameters which is suitable for your own glass tempering furnace should be worked out according to the thickness and color of the glass, and the adjustment of the technological parameters during tempering process should be strictly controlled.

b. The size of the glass reaches or exceeds the max loading capacity of the glass tempering furnace 

If the size of the glass almost reaches or exceeds the max loading capacity of the glass tempering furnace, the spontaneous breakage probability of glass with this specification will be obviously higher than that of glass with other specifications. The reasons are as follows: The larger specification of glass shall cause non-uniform heating of the glass, and there will be temperature difference among different areas of the glass surface, which will result in stress non-uniformity after tempering; when conducting air tempering of the glass, the blowing non-uniformity will cause an uneven distribution of stress.

To solve the spontaneous breakage problem caused by uneven stress distribution when the glass specification is too large, on the one hand, bigger size glass tempering furnace should be chose when producing glass with large specification; on the other hand, in respect of the large specification glass which nearly reach the production capacity of the glass tempering furnace, the difficulty lies in the uniform heating and uniform blowing to each part of the glass. Technological control shall focus on these two points. Firstly, for uniform heating control, observe the temperature display of each part in the furnace, regulating the temperature in each area to the same level, and turning on the heat-balance system, then drop the heating temperature and extend the heating time appropriately. After exiting one batch of glass from the furnace, don't start to produce the second batch immediately. You'd better let the furnace be empty for a period until the furnace temperature comes to its original level; secondly, for the control of the uniform tempering, check if the blowing nozzles are completely ventilated, especially those in the corner areas of the chiller. Furthermore, regulate the air pressure of the bottom chiller to the same level; check whether the distances between each area of the chiller and the glass are consistent. When conducting the air tempering to the glass, try best to enlarge the oscillating distance and extend the tempering and cooling time.

IV. The Glass Sheet Has Nickel Sulfide 

Mixture of the glass sheet with nickel sulfide can result in spontaneous breakage. The reason is the dilatation coefficient of the nickel sulfide and that of glass are very different; crystal phase transition of the nickel sulfide is resulted from the changes of temperature. When the glass is with high temperature, nickel sulfide shall expands slowly; when the temperature is dropping slowly, the difference of the dilatation coefficient of the nickeliferous vitreous body will cause external tension extrusion of nickel sulfide, and the force of external tension extrusion of the nickel sulfide is the main reason resulting in glass breakage.

Because the float glass' temperature is uniformly dropping when annealing, the outward extruding force of the nickel sulfide from the center of glass is not enough to cause the glass broken in general conditions. However, in the process of glass tempering, the glass reaches a temperature of 680-720 centigrade, then it enters into the chiller and is tempered rapidly, which will cause the expansion of the edge fissure of the nickel sulfide. When the fissure is expanded to a situation that the pressure stress cannot balance, it will break. Some will be broken in the process of glass tempering, others will have spontaneous breakage hidden danger in the process of using, that is to say, when the glass' pressure stress can't balance the tension of the nickel sulfide in the process of using,  glass breakage will occurs.

To avoid the glass spontaneous breakage resulting from nickel sulfide, we shall focus on two aspects in technological treatment. For one thing, the float glass producers could adopt effective on-line nickel sulfide identification and detection technology to avoid the nickel sulfide existence in the glass as far as possible; for another thing, conduct homogenizing treatment to the glass before it leaves the factory. Putting the separated and fixed glasses in the homogenizing furnace vertically or horizontally, then heating them to (290±10) centigrade and keeping the temperature for 2 hours, then cooling them, so that the glasses that has spontaneous breakage hidden danger and with nickel sulfide can be detonated.  (to be continued)

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