Glaze pinholes and air bubbles are one of the main defects in ceramic production during the production of ceramic water cups. It reduces the transparency and glaze gloss of the product, affects the surface quality of the product, reduces the quality level of the product, increases the production cost, reduces the income of employees, and also affects the reputation and benefits of the enterprise. The causes of glaze pinholes and bubbles involve a wide range of factors, so how to reduce glaze pinholes and bubble defects as much as possible, improve product quality, and reduce production costs, the following is the solution.
1. The reasons for glaze pinholes and air bubbles in the raw materials for glaze of ceramic advertising cups are as follows:
1.1 There are too many organic substances and carbon contained in the raw materials. If these organic substances and carbon do not completely react in the oxidation decomposition stage due to reasons such as too fast temperature rise, low oxidation temperature, and insufficient oxidation atmosphere, but in the high temperature stage, the glaze has already melted When it reacts and releases gas, it may cause glaze pinholes or bubbles in the product. It reacts as follows:
C organic matter + O2→CO2↑ (350°C-450°C) C carbon + O2→CO2↑ (about 600°C or higher)
1.2 The raw material contains iron sulfide. Because iron sulfide is not magnetic, these impurities cannot be removed with iron absorbers and iron absorbers. The gas generated by the oxidation reaction of iron sulfide not only causes glazed pinholes or bubbles in the product, but also produces Fe2O3 It will also affect the appearance and color of the product, and Fe2O3 will be further decomposed or reduced at high temperature to release gas, which will easily cause glazed pinholes or bubbles in the product. It reacts as follows:
FeS2+O2→FeS+SO2↑ (350℃-450℃)
4FeS+7O2→2Fe2O3+4SO2↑ (500°C-800°C)
2Fe3O3→4FeO+O2↑ (1250°C-1370°C)
Fe2O3+CO→2FeO+CO2↑ (1000℃-1100℃)
1.3 Structural water contained in clay raw materials, because the removal temperature of structural water contained in clay raw materials is related to factors such as crystallization degree, mineral composition and heating rate, etc., such as improper operation such as too fast heating may also make the product Glaze pinholes or air bubbles are produced. The dehydration temperatures of several raw materials containing structural water commonly used in ceramic production are: kaolinite clay 400°C-600°C, montmorillonite clay 550°C-750°C, illite clay Clay-like clay is 550°C-650°C, pyrophyllite is 600°C-750°C, porcelain stone is 600°C-700°C, and talc is 800°C-900°C.
1.4 The raw materials contain carbonate, because the decomposition reaction of these carbonate minerals generally ends at about 1000°C, and at this time the glaze has been sintered or even melted, and the gas generated by the decomposition reaction is relatively difficult to get rid of, so it is also It is easier to cause glazed pinholes or bubbles in the product, and the reaction is as follows:
CaCO3→CaO+CO2↑ (600°C-1050°C) MgCO3→MgO+CO2↑ (400°C-900°C)
MgCO3·CaCO3→CaO+MgO+2CO2↑ (730°C-950°C)
4FeCO3→2Fe2O3+3CO2↑ (800°C-1000°C)
1.5 If the raw materials contain more sulfates and high-valent iron, it is very easy to cause glaze pinholes or bubbles on the product, because these sulfates and high-valent iron must be processed at a temperature higher than 1200°C in an oxidizing atmosphere. Decomposition, the reduction and decomposition reaction must be carried out at a temperature higher than 1080°C in a reducing atmosphere. At this time, the body already has a liquid phase, the glaze has begun to melt, and the gas generated by the reaction is difficult to get rid of, so It is very easy to cause glaze pinholes or air bubbles in the product. It reacts as follows:
MgSO4→MgO+SO3↑ (>600℃)
CaSO4→CaO+SO3↑ (1250℃-1370℃ violently)
Na2SO4→Na2O+SO3↑ (1200°C-1370°C)
2Fe2O3→4FeO+O2↑ (1250°C-1370°C)
Fe2O3+CO→2FeO+CO2↑ (1000℃-1100℃)
The temperature of the above reaction in reducing atmosphere is 1080℃-1100℃
The methods for overcoming glaze pinholes and air bubbles in the raw materials for glaze of ceramic water cups are as follows:
(1) The raw materials for mud glaze should try to choose raw materials that do not contain or contain sulfate and high-priced iron as much as possible, and also try to choose raw materials that do not contain or contain iron sulfide as much as possible, and should also try to choose those that contain organic matter and carbon. If there are few raw materials, the raw materials for mud should use as little or no raw materials containing carbonate as possible, and the raw materials for glaze should not be used too much.
(2) Wollastonite does not contain organic matter, adsorbed water and crystal water, so wollastonite hardly produces gas, so wollastonite is used instead of calcite or dolomite with mud, and wollastonite is used instead of calcite, dolomite and quartz with glaze At the same time, the amount of gas generated in the slip can be reduced, thereby reducing the pinholes or air bubbles in the glaze correspondingly.
2. The reasons for glaze pinholes and air bubbles in the firing operation of ceramic advertising cups are as follows:
2.1 The low-temperature flame is not clear and the combustion is insufficient, resulting in carbon deposits, which are burned at high temperatures with the increase of the excess air coefficient or at the end of the reduction and cooling periods, leaving pinholes or bubbles on the glaze.
2.2 The strong reduction atmosphere is too thick and the strong reduction time is too long, causing the blank glaze to absorb too much carbon and carbide, and these carbon and carbide are oxidized at the end of reduction or cooling period, thus leaving glaze needles holes or air bubbles.
2.3 In the stage of oxidation decomposition, if the temperature rises too fast, the oxidation temperature is too low, or the oxidation atmosphere is insufficient, the organic matter, carbon and iron sulfide are not completely oxidized and enter the reduction period, then these organic matter, carbon and iron sulfide are reduced. The final or cooling period may be oxidized to produce glaze pinholes or air bubbles.
2.4 In the oxidative decomposition stage, if the temperature rises too fast and other reasons, the carbonate is not completely decomposed and the crystal water of the clay raw material is not completely removed. The resulting gas cannot escape freely from the glaze, so glaze pinholes or bubbles appear.
2.5 For low-temperature fast-fired ceramics, although impurities such as organic matter, carbon, iron sulfide, and carbonate in the billet have been completely oxidized and decomposed before the glaze is melted, the gas produced is not released before the glaze is melted due to the rapid temperature rise. It can be completely discharged. After the glaze is melted, there is still a large amount of gas discharged. These gases break through the glaze surface and cause pinholes in the glaze surface. Bubbles are formed when they fail to rush out of the glaze surface. Because the temperature rises too fast, it is difficult for the glaze to flatten and the gas rushes out. The pits left by the glaze, even if the high temperature holding time is prolonged, will still leave glaze pinhole defects on the glaze.
2.6 The firing temperature is not enough, so that the glaze cannot be fully and evenly distributed, resulting in pinholes on the glaze surface.
2.7 Forced ventilation, excessive dust particles, and water vapor may cause pinholes or air bubbles in the glaze.
2.8 When the glaze is vitrified, the temperature rises too fast or the firing temperature is too high, the glaze will boil and cause glaze pinholes or air bubbles.
2.9 When the body is fired in a reducing atmosphere, attention should also be paid to the selection of two important temperature points, that is, the temperature point (atmosphere conversion temperature) from a strong oxidizing atmosphere to a strong reducing atmosphere and the temperature from a strong reducing atmosphere to a weak reducing atmosphere point. The atmosphere in the kiln and its strength are determined by the content of free oxygen and carbon monoxide in the flue gas. When the free oxygen concentration is 8%-10%, it is called a strong oxidizing atmosphere, and when the free oxygen concentration is 2%-5%, it is called Weak oxidizing atmosphere; when the concentration of free oxygen is less than 1%, and the concentration of carbon monoxide is 1%-7%, it is called a reducing atmosphere, and when the concentration of carbon monoxide is 2%-6%, it is called a strong reducing atmosphere, and the concentration of carbon monoxide is 1%-7%. 2% is called a weak reducing atmosphere. The conversion temperature should be properly selected according to the formula of the blank glaze. If the conversion temperature is too low or too high, that is, if the conversion temperature is too early or too late, it may cause pinholes or bubbles on the glaze surface of the product, as well as defects such as cloudy yellow and smoke.
2.10 The firing atmosphere also has a great influence on the pinholes of the glaze. The oxidizing atmosphere can promote the chemical reaction of the glaze melt and the combustion of organic matter and carbon, which is conducive to the rapid removal of gas before the glaze melts, thereby avoiding the formation of glaze Pinholes or air bubbles, while the reducing atmosphere inhibits the oxidation reaction in the glaze and the combustion of organic matter and carbon, causing the discharge of gas to be delayed until the glaze is melted (especially zirconium glaze), resulting in pinholes or air bubbles on the glaze surface .
The corresponding methods to overcome glaze pinholes or air bubbles in the firing of ceramic advertising cups are as follows:
(1) In the stage of oxidative decomposition, it is necessary to ensure sufficient oxidizing atmosphere, the temperature rise should not be too fast, and the oxidation temperature should not be too low. Before entering the strong reducing atmosphere, the product is generally kept at around 950°C-1050°C in a strong oxidizing atmosphere. Proper heat preservation for a period of time, in order to completely oxidize and decompose the impurities such as organic matter, carbon, iron sulfide and carbonate in the blank as much as possible before the vitrification of the glaze layer, and fully remove the crystal water.
(2) In the final stage of reduction, the heating rate should not be too fast, and the temperature difference should be reduced to promote the glaze to fully melt and spread evenly, but it must be prevented from overheating to cause the glaze surface to boil.
(3) The strong reducing atmosphere should not be too thick, and the end should not be too late, so as not to cause too much carbon deposition, and it is also conducive to the early burning of carbon deposition and other substances.
(4) The kiln operator should perform low-fire operation according to the type of product and the loading density of the kiln car. The loading density of kiln car cup products is higher than that of plates and bowls, the oxidation temperature can be appropriately higher, and the oxidation time can be extended; the oxidation temperature of grouting products can be compared with that of plastic products. The oxidation temperature should be about 25°C higher, and the oxidation time of grouting products should be longer than that of plastic products; the oxidation temperature of thick tire products should be 15°C higher than that of thin tire products, and the oxidation time should be longer than that of thin tire products. It can be appropriately longer, so that impurities such as organic matter, carbon, iron sulfide and carbonate in the product (products of each part of the kiln car) can be completely oxidized and decomposed as much as possible, and crystal water can be fully removed.
(6) When the green body is fired in a reducing atmosphere, the atmosphere transition temperature varies with the glaze formula of the blank, so it should be carefully selected. Generally, the atmosphere transition temperature should be determined at about 150°C before the initial melting temperature of the glaze layer, and the atmosphere transition temperature is too low. Or too early, because the organic matter, carbon, iron sulfide and carbonate are not completely oxidized and decomposed, it is easy to cause glaze pinholes or bubbles on the product, and defects such as smoke, etc., if the atmosphere conversion temperature is too high or too late, it is easy to The product will produce defects such as cloudy yellow; the temperature for changing the strong reducing atmosphere to a weak reducing atmosphere should also be properly selected according to the glaze formula. Generally, daily-use porcelain products are turned into a weak reducing atmosphere at around 1250°C. Defects such as yin and yellow, if the conversion is too late, the free carbon absorbed by the blank glaze will increase, which may cause glaze pinholes or bubbles on the product.
3. The reasons for glaze pinholes and air bubbles in the glaze formulation of ceramic advertising cups are as follows:
3.1 The glaze contains too much carbonate (especially limestone), the gas generated by the decomposition of carbonate at high temperature, and the CaO generated by the decomposition of carbonate can easily absorb free carbon and carbide during the firing process. Absorbed free carbon and carbides are burned off as the temperature increases, and the gases generated may cause glaze pinholes if they escape, or bubbles if they do not escape.
3.2 The high-temperature viscosity of the glaze is high (the high-temperature viscosity of the glaze is related to the particles, fluidity and chemical composition of the glaze), so that the gas discharged from the underglaze layer will initially appear small bubbles, and as the temperature continues to rise, Large bubbles are melted away, while small bubbles are left behind, and pinholes or bubbles will appear on the glaze surface; the high-temperature viscosity of the glaze is high, and the fluidity of the glaze is also poor, making it difficult for the pits to be formed when the gas escapes from the glaze surface. Be leveled to produce glaze pinholes.
3.3 The initial melting temperature of the glaze is too low, which prevents the atmosphere from penetrating into the green body, thereby delaying the progress of various redox reactions in the green body. When the glaze melt seals the green body, the gases generated by these reactions are easy to Make the product produce glaze pinholes or air bubbles; and the glaze material is prematurely vitrified, and the gas generated by oxidative decomposition is not easy to escape, and can only stay in the glaze layer to form bubbles. If these gases rush out with the increase of temperature The glaze surface will form glaze pinholes.
Post time: Dec-05-2022