Industry news / Types and causes of stripe defects in glass production

Types and causes of stripe defects in glass production

10-29-2024

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Glass stripe defect
 

Stripes are the manifestation of uneven chemical composition or temperature in glass products, and can also be understood as the transition stage before the melt or solution penetrates through chemical, physical, and structural aspects to achieve complete uniformity. Stripes may exist inside or on the surface of glass panels, without a clear boundary with the glass body, in the form of stripes or lines, mostly irregular in shape. In the production of flat glass, standing on the side of the running direction of the cold end glass strip at different incident angles can be observed, commonly known as "ribs". Specially thin stripes are also known as "streaks". In most cases, it is formed during the drawing process due to the presence of glass melt that has not yet been homogenized, has high viscosity, and high surface tension in the glass melt.

 

The pathway of stripe defects
 

one
 

Stripes formed due to uneven mixing or weighing errors of the ingredients, as well as irregular melting of the ingredients. Such as stripes rich in silica and lacking in silica, or stripes rich in alumina and lacking in alumina. Uneven mixing, layering of the mix, fluctuations in various raw material components, and excessive particle size of the mix can all cause stripes.

 

two
 

During the glass melting process, certain components evaporate from the surface of the glass melt, especially volatile substances (including alkali) that evaporate and decompose, causing inconsistency between the surface composition and the interior of the glass melt, resulting in uneven distribution of the glass melt. For example, as the content of silicon dioxide on the surface of the melt increases, stripes are produced. The gas partial pressure in the glass melting furnace plays a decisive role here.

 

three
 

The oxidation-reduction properties of glass undergo changes, such as an increase in oxidation strength, failure to decompose saltpeter, and the formation of stripes rich in alkali or calcium oxide when nitrate water appears on the surface of the melt. Or the reducibility becomes stronger. Glauber's salt has already decomposed in the early stage of glass melting. When the glass enters the clarification stage, no bubbles escape, which means there is a lack of gas stirring, resulting in poor homogenization and the production of rich silicon stripes or aluminum silicon stripes.

 

four
 

The stripes formed from the walls of the melting pool and other refractory masonry components are almost always caused by the formation of a reaction layer at the interface between the refractory wall and the glass melt, resulting in the formation of dissolved substances or mixtures.

 

five
 

The stripes caused by the upper part of the kiln mainly come from silica bricks, but also from zirconia bricks (such as zircon). Silicon bricks absorb alkali metal oxides and drip silicate melts rich in silicon dioxide; Bricks containing zirconium will seep out silicates containing smaller zirconia particles.

 

six
 

Crystallization and "stones" interact with the surrounding melt to form stripes. Both situations are related to the shift in solubility equilibrium. Crystallization is the separation of substances required for the formation of crystal nuclei and crystal growth from a melt. Stones "add some substances to the melt.

 

seven
 

There is a significant difference between the composition of broken glass and the glass composition introduced by the blending material. If the composition of the broken glass used is different from that of the glass material, the uniformity of the broken glass is poor, or due to the volatilization of alkali in the original blending material during melting, there is relatively less R2O in the broken glass. After melting, stripes may appear due to incomplete consistency with the surrounding glass composition.

 

eight
 

There are significant temperature and density gradients in the depth of the glass melt, which are common in the melting of milky white glass. Milk white glass generally contains fluorite as an opalescent agent, which has a particularly severe erosion effect on the refractory materials of the pool wall. The eroded refractory materials enter the glass liquid, adding refractory components such as alumina to the local glass liquid, resulting in the formation of stripes rich in alumina. In addition, milky white glass has poorer thermal conductivity than transparent glass liquid, and heat is not easily transferred to the bottom of the glass, which can cause difficulties in flow. If the glass components remain immobile for a long time, a large amount of them will evaporate, resulting in some components of the glass liquid being inconsistent with the components of the flowing glass liquid. It is particularly evident that the immovable glass liquid will experience severe loss of permeability. When the discharge volume increases, this part of the glass liquid begins to flow in a regular manner and forms stripes.

 

nine
 

Impurity coloring pollution. The coloring pollution of iron comes from the small amount of iron impurities contained in various mineral raw materials. If white flat glass appears colored, it is mainly due to the presence of a certain amount of colored metal ions in the glass matrix. The color of glass colored by iron is mainly determined by the equilibrium state between Fe2+and Fe3+, and the coloring strength is determined by the iron content. The valence state and related coloring of Fe ions are closely related to the furnace atmosphere (i.e. redox conditions). Figure 12-15 shows the brown stripes (color channels) caused by Fe. In addition, producing glass tubes with high PbO content and high viscosity can easily introduce a lot of stripes.

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