The hazards and sources of stripes
Streaks are heterogeneous glassy inclusions in the main body of the glass. It is a common defect of glass inhomogeneity. It is different from the main body glass in terms of chemical composition and physical properties. Streaks can be understood as the interdiffusion of various components and the interpenetration and fusion of tissue structures through chemical reactions, physical changes, and physical and chemical reactions between several mixed glass liquids. Before being completely uniform, it is cooled to form glass, and there is an uneven structure inside. It has a different chemical composition, structural state and different physical and chemical properties from the main glass.
From the appearance of the product, the stripes are distributed in different parts of the product, some are inside the glass product, and some are on the surface of the glass product. The interface between the stripes and the main glass exhibits different shapes. Taking bottle glass as an example, most of the stripes are curved strips, twisted lines, and fibers. Most of the stripes start from the neck of the bottle, go through the shoulder, the body and the bottom of the bottle. Obvious stripes are thicker and wider, with a width of more than 10mm; inconspicuous stripes are thinner, like thin lines, with a width of less than 0.5mm. The more obvious stripes protrude on the glass surface and can be felt by hand. The inconspicuous stripes are faintly visible under the light of the naked eye, and some are even invisible to the naked eye. They can only be observed with the help of optical instruments. Stripes are called differently by many manufacturers, some are called "line", some are called "screw line", some are called "cat paw print" and so on. Streak is a kind of non-uniformity defect that is common in glass products.
From the point of view of the harmfulness of stripes, after the stripes appear, they first affect the appearance of the product and seriously affect the appearance quality of the product. It is a relatively obvious appearance defect, which greatly affects the pass rate of the product. According to the different standards required by different products for stripe defects, the degree of influence on the pass rate of the product is also different. For some high-end daily-use glass products, when streaks appear almost 100% affect the pass rate of the product.
When some stripes appear, characteristic stresses of different sizes are generated inside the product, usually called structural stress. This stress cannot be eliminated when the product is annealed and can cause the product to break by itself. The appearance of this stress has a great impact on the physical and chemical properties of the product, such as the mechanical strength, thermal stability and chemical stability of the glass. For some products with higher requirements, the reduction of these physical and chemical properties will greatly reduce the product’s Qualification rate, such as beer bottles that require relatively high compressive strength and impact strength.
The structural stress generated in the glass is related to the composition of the fringes. When the composition of the fringes differs from the composition of the main glass, the greater the difference in thermal expansion coefficient, the greater the characteristic stress.
The area where streaks are generated in the glass melting furnace
Through the analysis of the melting process in the production of glass containers, it is not difficult to find that the process of streaks should be in two areas. One area is the area in the melting pool before the molten glass passes through the flow hole of the glass melting furnace. This part is called Melting area; the other area is the area where the molten glass has entered the working pool (or the distribution channel) through the flow hole and the area in the supply channel. This area is called the cooling area.
2.1 Reasons for streaks in the melting area
(1) Streaks caused by poor glass melting;
(2) The streaks caused by the kiln trellis dripping into the glass molten pool without sufficient homogenization;
(3) After the refractory material is corroded, some of the components of the refractory material dissolve into the glass and produce stripes;
(4) Streaks caused by various stones melted into molten glass.
The analysis of the melting process of glass in the melting area shows that the glass melting process is first to put a batch composed of quartz sand, soda ash, limestone, cullet into the glass melting furnace. Under the action of high temperature, some raw materials are immediately The decomposition produces gas, and various solid phase reactions are carried out at the same time to form silicate body. Afterwards, part of the silicate body melted into a liquid phase, and then transformed into a melt with different components and different viscosities. This is the initial stage of molten glass.
The molten glass produced in the initial melting stage is rich in melts with different components and different viscosities. The melt is mixed with unmelted residual quartz sand particles and a large number of bubbles. Melts with different components and different viscosities are not easy to fuse with each other, and the boundary layer is relatively stable. Some of the glass bodies that are rich in silica or alumina have high viscosity and surface tension, and are mixed in the entire glass melt. If the glass body is originally spherical, it will remain unchanged without other external forces. , Will form an area rich in silica or alumina in the entire glass melt, which is the initial area of ??the stripes.
Through the above analysis of the glass melting process, it is not difficult to find the cause of the stripes in the melting area.
(1) In terms of batch materials, the quality of batch materials is a prerequisite to ensure the smooth elimination of streaks in the glass melting stage. When the following factors appear, they will cause streaks or even more serious. The first is the uneven mixing of batch materials, the stratification of batch materials during the conveying process, the excessively large particles of some raw materials in the batch materials, the unstable water content of quartz sand and other raw materials, and the unreasonable feeding method or matching The low water content of the material causes the flying of certain components. Secondly, the cullet occupies a large proportion in the batch, the selected cullet and the main glass composition are quite different, and the uniformity of the mixing between the cullet and the powder is poor. In addition, severe streaks will occur when the batch weighing system fails.
(2) In terms of the melting process, through the analysis of the melting process, it is not difficult to see the importance of the various process parameters of the batch material in the melting process. Correct and reasonable process parameters must be formulated and strictly controlled. First of all, the melting temperature, the pressure of the furnace during the melting period, the length of the flame during combustion, and the atmosphere of flame oxidation and reduction will all have an impact on the viscosity and surface tension of the clear glass liquid. All the factors that can affect the viscosity and surface tension of the liquid glass will affect the homogenization of the liquid glass and produce streaks.
In addition, the streaks caused by the change of the flow state of the molten glass in the molten pool, when the overall output of the furnace, the temperature of the furnace, the glass level, bubbling agitation and other processes undergo major changes, the overall flow of the molten glass will be greatly changed. The change will cause the molten glass at the bottom of the furnace or in the dead corner to participate in the flow of the liquid glass, which will cause streaks.
(3) The droplets and refractory materials are corroded, and the streaks caused by the melting of the stones. The streaks produced in these three aspects are all due to the high temperature of the glass melting furnace. Other heterogeneous materials that have nothing to do with the composition of the glass batch (such as masonry) The refractory material used in the furnace is burned, corroded, and melted to form a melt with a composition, viscosity, and surface tension that is seriously different from that of the main glass, and it is mixed and melted into the main glass to form stripes. When streaks are produced by the droplets, they are often accompanied by nodules.
2.2 Causes of streaks in the cooling area
According to the analysis of the structure of the fully separated and semi-separated cooling part, in the five stages of glass formation, when the glass liquid enters the working pool (distribution channel) through the flow hole, it is also the last stage of glass formation. ———Cooling stage. According to the different structure of the furnace, the flame separation form of the working part can be divided into semi-separated form and fully separated form. The semi-separated working part requires a relatively high molding temperature. The temperature of the molten glass in the working part is completely heated by the radiant heat of the flame space of the melting part. In addition, the working tank is generally semi-circular, and several feeding channels supplied by the melting furnace are directly discharged from different parts of the semi-circular working tank. The molten glass comes out of the liquid hole and flows into the forehearth immediately after entering the working tank. During this cooling process stage, streaks are rarely produced. This kind of kiln structure was widely used in China before the 1990s. In the production of corresponding wine and beverage bottles, stripes were rarely found in the working pool. Nowadays, in the production of electric lamp bubble shell products, the furnace structure of this working pool is still used, and stripes on the products are rarely seen.
After the 1990s, due to the introduction and upgrading of a large number of molding equipment, a double-drop row machine with stricter molding temperature requirements appeared, and advanced foreign furnace technology was also introduced. In the cooling stage of the molten glass, the working part structure of the distribution channel appears. This kind of working part flame is completely separated, and the temperature of the molten glass in the working pool is not affected by the temperature of the flame space in the melting part. At the same time, inside the flame space of the working part, the working part is divided into different parts according to the exit position of the feeding channel. Several flame spaces, each of which is equipped with a complete and advanced temperature control system. Before the molten glass enters each feeding channel, according to the molding temperature requirements of each feeding channel, the temperature is controlled at the required index through the temperature control system in the distribution channel. Similarly, each feeding channel is divided into several areas, and each area has a complete and advanced temperature control system to ensure the molding temperature of the glass liquid entering the material bowl. For such a perfect distribution material channel and cooling structure of the material supply channel, it is difficult to have streak defects. For example, many foreign-invested and joint-venture glass companies introduced in China after the 1990s have rarely found streaks due to their distribution channels and feed channels.
The above is the analysis of the structure of the working part of the furnace. On this basis, it is not difficult to analyze some conditions in the cooling process of the glass liquid in some of the domestic self-designed cooling areas (working pools in the form of distribution channels). Discover the cause of streaks during this stage.
(1) Streaks caused by temperature
The temperature of the molten glass in the distribution channel and the forehearth is generally in the range of 1270 to 1320°C before the molten glass exits the flow hole through the ascending channel and enters the distribution channel. The molten glass needs to be controlled by cooling and heat preservation in the distribution channel, and then enters the feeding channel. The temperature of the molten glass entering the feeding channel is generally in the range of 1210-1250°C. The molten glass in the feed channel must also be cooled, kept warm and heated, and then enter the material bowl. The temperature of the glass liquid in the material bowl is generally about 1100°C. In the material bowl, the control of the temperature control system is also needed to adjust to the drip temperature that can meet the product requirements.
In the normal production process, there is always a part of the molten glass remaining in the distribution channel and the supply channel when it flows. The temperature of this part of the glass is lower than the temperature of the just flowing in, or even much lower. . The middle part of the distribution channel and the feeding channel is flowing with the relatively high-temperature molten glass that has just flowed in, and the remaining low-temperature molten glass in the forging channel flows around. Therefore, the molten glass has to undergo a process of mixing, permeating, and homogenizing the high-temperature molten glass and the low-temperature molten glass while flowing through the distribution channel and the feeding channel. Because temperature has a great influence on the viscosity of molten glass, molten glass of different viscosities are always mixed together in the distribution channel and feed channel. If the viscosity difference is relatively large, streaks will occur. The greater the temperature difference, the more obvious the stripes will be. In severe cases, they will be thick and wide, and can be felt by hands.
Due to the different types of products, the molding drop temperature of various products is different, and the difference is relatively large. In order to ensure that the incoming material temperature can meet the dripping temperature of the product-it is neither very high nor very low. After simple temperature control and adjustment, the temperature required for molding can be reached and satisfied. The production plants are distributing the material channels and feeding materials. Various measures have been taken on the road, some of which have better results, and some methods have brought many other defects to the product although the molding temperature has been reached. For example, when the temperature of the incoming glass liquid is relatively high, some production plants cool down the temperature by blowing cold air or compressed air at certain positions of the forehearth. Although it barely meets the requirements of the molding temperature, the product appears more serious. Stripes.
There are many factors that affect the temperature of the molten glass in the distribution channel and the supply channel. The first is to be reasonable in the design of the initial distribution channel and the feeding channel. According to different types of products and the discharge volume of each distribution channel and supply channel, the reclaim ratio of each distribution channel and supply channel should be optimized to ensure that the temperature of the new glass liquid The temperature of the remaining molten glass is not much different. The second is that the length, depth, width and slope of the distribution channel and the supply channel should be designed reasonably to ensure that when the glass liquid flows in the distribution channel and the supply channel with the production flow, its overall flow is assisted by the heating system. The speed is basically the same, and the temperature of the upper, lower, left, and right glass liquids is basically the same without layering. In addition, the insulation system on the bottom and sides of the distribution channel and the feed channel should be reasonable in the initial design to prevent the residual glass in the three directions of the channel due to improper insulation and the main body glass. Furthermore, a complete temperature control system should be equipped on the distribution channel and the forehearth to ensure the required temperature of the molten glass in each stage of the distribution channel and the forehearth. At the same time, according to the actual production changes, the temperature control system should be reasonable. The adjustment to ensure that each part of the glass liquid is basically uniform.
(2) Streaks caused by other reasons
①There are some corners in the brick structure of the distribution channel type working pool, where some non-flowing glass frit is often condensed in this part, which is generally called "dead material". This part of the material is due to long-term condensation. The distribution and viscosity are different from the normal flowing glass liquid. Secondly, when the molten glass flows through the distribution channel and the forehearth for a long time, as time accumulates, some molten glass with relatively high viscosity is deposited on the bottom of the molten glass. When the output volume and the temperature of the molten glass change greatly in production At this time, the immovable and deposited materials in these parts will participate in the flow and form stripes. In addition, when some iron impurities are deposited on the corners of the distribution channel and the feed channel, yellow and brown stripes will be formed on the product.
②Under reducing conditions, the viscosity of glass liquid is two orders of magnitude higher than that under oxidizing conditions, and the surface tension is also greater than under oxidizing conditions. Substances with low surface tension tend to spread and form thin films on the surface of substances with high surface tension. Therefore, for the distribution channels and feed channels for burning gas and liquid fuel, when the surface of the glass liquid is in a reducing atmosphere, the surface tension will increase, and the surface glass will be immediately squeezed away and the lower material will be turned over. Two glass frits with different surface tension and viscosity are blended together to form stripes.
③In the distribution channel and supply channel, because the surface of the molten glass is in an open state, some volatile components evaporate, causing the composition of the molten glass on the surface to be different from the composition of the molten glass inside, resulting in a difference in the surface tension of the molten glass inside and outside. Make the upper and lower layers of glass liquid produce convection, and form stripes.
④In the distribution channel and supply channel, the temperature of the molten glass is low, and the molten glass does not erode the trough bricks much, but despite this, the molten glass still washes and erodes the trough bricks, and the eroded material has a higher viscosity. It is difficult to spread and adheres to the vicinity of the liquid line. When the liquid glass level fluctuates every time, it will drive the liquid glass rich in corrosives to rise or fall. When the temperature of the distribution channel and the forehearth and the discharge volume change greatly or the glass liquid level fluctuates greatly, This part of the erosion material participates in the flow and forms stripes.
