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Common Defects of Tempered Glass and Solution

means the condition of heat convection furnace

Defect phenomenon

Reason Analysis

Solutions

Glass appears “”

Concave shape

(Side warp)

1.Upper glass surface temperature is higher than lower surface when discharging.

2.Upper surface is cooled less than lower surface when tempering in chiller section.

1.Raise lower chamber temperature or reduce upper chamber temperature. 

Reduce heat equilibrium pressure. (not recommend to do.)

2 Raise air pressure of upper chiller or reduce air pressure of  lower chilleradjustment of air flow equilibrium

reduce frequency of top convection blower.

NOTEWhen adjusting, either  furnace temperature or air pressure of  chiller to be adjusted only.

Glass appears “”

Convex shape

(Center bending upward)

1. Upper surface temperature is lower than lower surface of glass when discharging.

2. Air pressure in upper chiller is higher than lower chiller when tempering in chiller section.

 

Raise upper chamber temperature or reduce the lower chamber temperature.

Raise heat equilibrium pressurenot recommend to do

2. Raise air pressure of lower chiller or reduce air pressure of upper chiller.adjust air flow equilibrium.

raise frequency of top convection blower.

NOTEWhen adjusting, either  furnace temperature or air pressure of  chiller to be adjusted only.

Double direction profile, glass appears concave shape or heaved shape

Middle temperature of glass is lower than edge temperature.

Change the setting of heating profile. Raise the middle temperature higher than edge temperature.

Using heat equilibrium to improve the temperature uniformity inside furnace.

Properly adjust the equilibrium of air flow.

Glass appears wave deformation  “”

        

Heating temperature too high or heating time too long.

Reduce heating temperature or heating time

But the  heating time is needed to properly lengthened under the following conditions:

1.edge grinding quality of glass is not perfect

2.there are holes, gaps, cutting edge on the glass sheet

Pockmark or roller mark on glass surface.

1.Excessive heating.

2. Ceramic roller is dirty.

1.Reduce furnace temperature or heating time

  2.Check and clean the ceramic roller

  3.Inject SO2 properly.

“white fog” at the centre of glass sheet or interference

Edge of glass warps inside furnace, it is caused by higher roller temperature, since the roller transfer more heat to lower glass surface while the radiant plate transfer less heat to upper glass surface. 

 

 

 

1.Adjust upper and lower temperature value setting inside furnace

2.Use SO2 gas with proper quantity

raise the frequency of convection blower on the top chamberto reduce the edge warping  time and secure the uniform reciprocation at primary heating time.

Reduce the furnace temperature

NoteSometimes this phenomenon happens when roller surface is rough or dirty, or not cleaned completely

These is crackle on glass surface;

Glass temperature is lower than normal when discharging

Raise temperature inside furnace or extend heating time.

Glass is damaged inside furnace;

1. These is some crackle on glass surface;

2. Edge grinding quality isn’t good;

3. When processing thick glass, the heating up speed is faster, resulting in big temperature difference between center part and glass surface

4.Secondary tempering for tempered or heat strengthen glass;

1. To guarantee the glass sheet and edge grinding quality;

2. Reduce the furnace temperature when processing thicker glass sheet;

reduce the frequency of convection blower of top chamber to avoid upper glass surface being heated faster;

Tempered or heat strengthen glass are not recommended for secondary tempering. 

Four corners of glass sheet are damaged inside furnace or both glass edges abraded.

Upper chamber

temperature is higher or the frequency of convection blower is higher cause to upper surface expand more and result in only glass corner and edge touching the roller.

1. Reduce the setting temperature of top chamber;

2. Reduce the  frequency of convection blower during preceding blowing stage;

Wave deformation of tempered glass at front and rear end;

1. The heating speed at front and rear end is faster;

2. The temperature at front and rear section is higher;

 1. Reduce furnace temperature, extend heating time, make glass heating uniform;

 2. Reduce setting frequency value of convection blower;

 3.Increase loading rate per batch;

 4.Reduce idling time;

5. Reduce the front and rear section temperature inside furnace;

Glass damaged in chiller section.

1. There is sand inclusion or crackle in glass sheet.

2. Bad grinding for glass hole.

3.Glass surface  temperature is too low or uneven

4. Quenching air pressure of thick glass is too high

5. Nozzles are blocked

1. Checking and guarantee the quality of glass sheet;

  2. Extend heating time;

 

3. Adjust air pressure and chiller opening

 

  4.Clean the blower room regularly;

  5.Clean the dirty inside chiller.

 

 

Scratch

1. Too more handling and collision;

1. Simplify technology and add material as liner between sheets;

2. Glass overlapping;

2. Put and fetch with single sheet or add paper between sheets;

3. Roller surface unclean;

3. Roller cleaning;

4. Roller level difference or running asynchronously;

4. Corresponding adjustments of the rollers;

Low impacting resistance

1.Heating temperature is low or temperature difference between inner and outer layer too great, heating insufficient, or stress is insufficient;

1. Properly Raise temperature of furnace or extend heating time;

2. Quenching intensity is too low, temperature gradient is not enough, and stress is not enough.

2. Raise air pressure or reduce the of nozzles height;

3. Rear part of glass enters chiller too late and temperature reducing too more due to roller speed too low;

3. Raise speed of conveyer to reduce temperature dropping;

Particles sizes too big or uneven.

1. Heating temperature is low, heating time is short or uneven temperature distribution inside furnace;

1. Raise furnace temperature or lengthen heating time, using heat equilibrium to make temperature uniform;

2. Quenching intensity is not enough.

2. Raise air pressure or reduce the nozzle height;

Spontaneous bursting

 ( Self – bursting )

There is inclusion inside glass sheet, such as refractories stone and NiS.

Carefully select and check glass sheet.

During application of glass, owing to that the glass contains NiS crystal which transforms from α to β phase cause to volume expansion and stress equilibrium broken, and resulting in glass bursting;

Use a heat soak oven (heat treatment with 300×1 hour) to induce spontaneous bursting.

 

 

Interference

1. Chiller position is too low.

1. Raise chiller height properly;

2. Quenching reciprocating time is too short.

2. Prolong reciprocating time;

Tempering iridescence

During float glass forming, SnO permeates into float glass surface in tin bath. When tempering, SnO is oxidized into SnO2 which causes volume expansion. Thus,glass surface appears micro-wrinkles due to being compressed, and  resulting in ray interference.

1. Choose glass sheet with high quality.

2. Control heating temperature at lower limit.

3.  Polish glass with fine powder.

 

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