Ceramic Mug Group, Ceramic mugs are all made by calcining in a kiln. So how are coffee mugs fired? What effect does the firing process have on ceramic mugs?
Ceramic mugs can actually be divided into three kinds in the firing process. One is a single firing. That is, from the blank clay to our commonly used ceramic coffee mugs, only through the kiln calcination once and molded. The second type is secondary firing. As the name suggests, ceramic mug blanks need to enter the kiln twice for calcination. into the kiln for calcination. The first time is unglazed firing, usually become "plain firing", the purpose is to increase the mechanical strength of the body, not easy to damage. Often larger or thinner bone china cups are often fired twice. After the first firing, the glaze is applied (for information on glazes, please refer to our factory's website). information, please refer to our factory's previous information), and then the second firing. Depending on the type of cup, the material and the glaze, the temperatures of the two firings are different. The third type is the triple firing, which requires three trips into the kiln. Often more complex ceramic pieces only need. The first time is plain firing, the second time is glaze firing, the third time is appliquéd or hand-painted and then fired. Then in the firing process, the kiln temperature, oxygen content, heating time and cooling time, what is the impact on the
ceramic coffee mug? The following details Introduction:
1. What is the oxidizing flame, reducing flame and neutral flame?
Flame in the kiln at different times have different properties. The nature of the flame can be roughly divided into three categories: oxidizing flame, reducing flame and neutral flame, different properties of the flame has different roles.
1). Oxidizing flame
Refers to the complete combustion of fuel when the flame, the flame must have a large amount of oxygen supply for complete combustion, when the kiln has sufficient oxygen, less CO. In order to make the billet in the moisture and all organic matter are evaporated and volatilized discharge, so that the billet to get the normal contraction, so in the kiln firing There must be an oxidizing flame stage in the process.
2). Reduction flame
Reduction flame is an incomplete combustion flame. At this time the kiln produces a lot of CO and hydrogen, no or very little free oxygen exists. Because the reducing flame can make the embryo body of high-value iron (Fe2O3) to be fully reduced to ferrous oxide (FeO), and become green, to eliminate porcelain Color yellowing phenomenon, so in the kiln firing process of daily-use porcelain, more reduction flame firing.
3). Neutral flame
Burning neutral flame produced in the kiln of carbon monoxide and hydrogen and into the kiln of air compounds almost equal, in a state of equilibrium. Its role is to make Yang ferrous no longer subject to oxidation, and finally make the body to achieve the purpose of complete glass. But control the neutral flame is very difficult However, it is very difficult to control the neutral flame, and it is often used to replace it with a weak reducing flame.
2. How to observe the flame color to determine the temperature and nature?
Ten years ago, Ceramic mug group is no testing equipment, engineers can rely on the naked eye to observe the flame color to determine the temperature and nature of the flame. This is because the color and concentration of light radiated by hot objects vary with the temperature. What we often see is the initial red flame with a temperature of about 600°C, dark red 700°C, cherry red 800°C, bright cherry red 1000°C, yellowish-white 1300°C, white 1400°C, and so on. This relies on experience to determine the temperature, can make up for each other with the detection equipment, has its irreplaceable.
The nature of the flame, in general, burning billet ceramics, 900 ℃ before the oxidizing flame, in order to prevent soot accumulation in the billet. 1000 ℃ or so is the reducing flame, so that the high value of iron into low value of iron, 1140 ℃ after the use of neutral flame. Five years ago, the plant installed testing equipment, for the The temperature control, oxygen, carbon monoxide detection is more accurate.
3. What is flat-burning insulation?
Flat firing insulation is to maintain a constant temperature for a certain period of time. The purpose is to keep the temperature up and down in the kiln at the same time, with no temperature difference.
1). Flat firing and holding during the dewatering period allows the water in the blank to evaporate safely, the quartz crystals to be transformed safely, and prevents the blank from cracking.
2). The purpose of the flat-firing heat preservation before glaze melting is to make the organic matter contained in the blank fully reacted under high temperature, and the waste gas produced is fully volatilized and cleaned up to avoid blistering of the cup.
3). In the glass stage of flat-firing insulation, so that the billet within the iron trioxide and silicate compounds to generate new substances - mullite, and feldspar and quartz synthesized feldspar glass.
4. What physical and chemical changes occur in the billet during kiln firing?
1). The physical changes that occur in the kiln firing process of the billet:
(1). Weight reduction: the result of evaporation of water, water of crystallization and organic matter in the raw material.
(2). Loss of plasticity.
(3). Change in color.
(4). Changes in voids, the evaporation of water and organic matter is accompanied by a gradual increase in the void ratio within the billet, to the point where glassing begins, the voids are again filled with solution, the particle cups are surrounded by liquid, and the voids are again gradually reduced, until the billet is sintered.
(5). Volume contraction: the volume of the billet is slightly swollen when the void increases, and gradually shrinks when the void decreases.
(6). Mechanical strength changes, the crystal lattice is destroyed, when the water of crystallization is evaporated, the crystals split, the mechanical strength of the billet decreases, to recrystallization, the mechanical strength becomes greater.
2). Chemical changes in the billet: changes such as oxidation and reduction, reduction occurs mostly before 250-900℃, decomposition occurs mostly before 500-1300℃.
(1). Decomposition of carbonate.
(2). Oxidation of carbon, sulfide and organic matter.
(3). The oxidative decomposition of carbonaceous and organic matter present in the billet begins above 600°C and continues to high temperatures, the gas produced at this stage must be completely discharged, otherwise it will cause blistering of the billet. C+O2=C02
(4). Strong reduction stage
This stage of the body of the blank of iron oxide and sulfate in the glaze layer before the closure of the body of the full reduction and decomposition.
(5). Ferrous oxide and silica to produce a glassy substance, promoting the sintering of the body.
5. firing kiln should pay attention to what problems?
1). Too rapid a change of temperature in the kiln will cause the ceramic mug to crack easily.
2). Insufficient oxidizing time and too fast kiln firing will lead to blistering of ceramic cups easily.
3). When the fire of the kiln is not even, it will easily lead to deformation of the body.
4). Uneven temperature in the kiln combined with over-firing will cause the ceramic mug to swell easily.
5). When the firing method and the nature of the flame are not suitable, the surface of the ceramic is easy to change color and produce stains.
6). When the cold kiln is too slow, the ceramic coffee mugs tend to be rough in section.
7). Burning reduction flame should not be too heavy, too long, otherwise the ceramic cup is easy to have soot stains.
6. The technical characteristics of rapid firing kiln
General tunnel kiln cross-section is large, ceramic products are densely stacked, the temperature difference between the various parts of the product is large, so it can not quickly and evenly heat and cool the ceramic products, resulting in a long firing cycle of more than ten hours.
Rapid firing of the kiln performance of the basic requirements are: the ability to uniformly heat and cool products, and can flexibly adjust the temperature and gas. In order to meet this basic requirement, rapid firing kilns have the following characteristics in terms of structure and materials:
1). Small cross-section
In order to achieve uniformity of temperature in the cross-section of the kiln, the cross-section tends to reduce the height and become flat. 1250-1300 ℃ fired ceramic mugs, the firing cycle requires only 60-90 minutes.
2). Low sulfur content
Rapid firing is best used in open flame direct heating, the fuel used requires no will, low sulfur content, such as natural gas, liquefied petroleum gas, light oil, etc.. The sulfur content of the fuel for open flame firing should be 0.2%.
3). High-speed nozzle
High-speed nozzle has three advantages, fast combustion speed, strong space heat, and uniform temperature distribution. High-speed nozzle is an important technology to reduce the temperature difference in the kiln and realize rapid firing.
4). High quality kiln construction materials
In order to make the kiln temperature can be adjusted flexibly according to the requirements of the temperature curve. The kiln construction materials should be lightweight heat-resistant materials and lightweight insulation materials with low thermal conductivity, small thermal fusion, and certain strength.
5). Reduction of kiln furniture
In rapid firing, the kiln should be minimized or not used. A single layer is used to place the product. Elimination of kiln furniture improves heat transfer efficiency and flexibility of the kiln.
6). Advanced Conveying Methods
The general tunnel kiln uses bulky kiln cars to convey the billet, the kiln car heat storage is large, easy to cause the ceramic products heating and cooling lag, not suitable for rapid firing. Conveying methods for rapid firing include walking beam, pushing plate, roller conveyor, etc.
