Forming of extrusion for clay brick

After the selected brick making raw materials treated and prepared, they enter the shaping workshop. China sintering brick molding method is mostly plastic extrusion. Plastic extrusion could be classified into three types: plastic extrusion molding, semi hard plastic extrusion and hard plastic extrusion, which are distinguished according to the different molding moisture content. When the moisture content of wet billet is over 16 %( dry basis, same as below), it is plastic extrusion. When the moisture water content is 14-16%,it is semi hard plastic extrusion. When the moisture water content is 12-14%, it is hard plastic extrusion.

Green brick shaping includes: feeding, mixing, adding water and wheel-rolling equipment after entering shaping workshop and before entering extruder; after the extruder, forming the qualified mud bar and green bricks. Shaping is to achieve the shape and structure of product. Therefore, shaping is the basis. That is to say, the product external shape and structure are formed by shaping process, which is the definition of ‘molding is the basic of brick making processes. Because the quality of the green bricks after shaping is directly related to the final product appearance quality, these wet green bricks need to be dehydrated and dried out from the shaping workshop.

In the sintering brick making processes, there are two ways of drying: natural drying and manual drying. Natural drying is to stack the wet green bricks on ridges of natural drying place, adopt manual inverted flower rack, and dry the wet green bricks into dried bricks with natural air. Manual drying means adopt manual dry chamber to realize the dry process,which could be divided into three types: large section tunnel drying chamber, small section tunnel drying chamber and room drying chamber. Whichever of these three drying types is adopted, the green bricks are stack on kiln cars by manual or by machine, and then the kiln cars with stacked wet bricks are moved into drying chamber for drying. The heat medium of the drying chamber generally comes from the waste heat of the sintering kiln or the hot blast stove. No matter which drying type or which manual drying type is adopted, it must be followed that the green bricks do not deform nor crack during the drying process. In case any problem in the drying, the appearance quality of the product would not be guaranteed, the scrap rate would be higher, the output capacity would decrease, the cost of finished product would increase, and the economic benefit of the factory would naturally be bad. Therefore, the green brick drying is often called ‘guarantee’, which emphasizes the importance of wet green bricks drying in brick making process.

After drying, when the residual moisture content is less than 6%, the green bricks will enter the sintering kiln. Rotary kiln and tunnel kiln are widely adopted for bricks sintering. In Rotary kiln, the green bricks are stacked in the kiln path by manual and the fire runs in the kiln for the baking. In tunnel kiln, bricks are stacked onto the kiln cars by manual or by machinery. The kiln cars with stacked bricks enter from the kiln head and exit from the kiln tail, and during the process, the green bricks on the kiln cars are roasted into sintered bricks. The fire in the tunnel kiln doesn’t move, while the kiln cars with bricks move in the kiln. No matter adopting rotary kiln or tunnel kiln, two points must be followed: 1. mustn’t burn the bricks not long enough, occur raw fire bricks   2. Mustn’t burn the bricks too much, occur over-burnt bricks. Therefore, it is often said that firing is the key of brick making processes, illustrating the importance of firing.

The new standards have higher technical index requirements to the hole rate, and the hole size requirement of rectangular hole improves the difficulty of brick shaping, which is relatively high requirement for vacuum extruder. The mud is evenly stirred by the spiral reamer of mixing machine, and fully homogenized in the conveying process, which avoids the uneven shrinkage of the adobe caused by uneven mixing and prevents the adobe cracks during drying and firing process. The mixed mud is vacuumed to remove the air in the holes, which further improves the mud’s plasticity. When producing rectangular perforated bricks, the extrusion pressure and vacuum degree of the extruder shall be appropriately increased. The shaping moisture shall be generally controlled to be 1-3% lower, normally no more than 18%. Since the moisture content is lower, to achieve the extruding strength, it is necessary to improve the extruding pressure and vacuum degree. The selection of extruder should be above model 50, with extrusion pressure no lower than 3.0MPa, and extrusion pressure more than -0.08Mpa. Only in this way, the green bricks strength and quality could be ensured, which is the basis of stacking bricks.  Otherwise, the increase of holes rate will cause the bottom green bricks strength decrease, thus causing the bottom bricks deform and collapse, and further affecting the stack effect and firing effect.

In the production of rectangular perforated bricks, reasonably core tooling is also very vital. The core tooling plays the role of perforation and adjusting the mud move speed at each part, which has a direct impact on whether qualified bricks could be extruded. By adjusting resistance of each related parts like the tool holder, core rod, core head, and etc. balance the mud’s extrusion speed as much as possible.  According to the shrinkage of raw materials, select the appropriate mouth size and brick thickness. In addition, the appearance quality of green bricks could be improved by reducing the size of cutting steel wire. One thing to note is the core tooling’s head and rod is rectangular, with different shape and different stress direction. So in case not fixed firmly, the core head could easily twist and have displacement caused by the deviation of stress, which will affect the shape of the extruded blank , failed meet standard and causing unqualified products. So when installing the core head, measures have to be found to fix it firmly.

Post time: Oct-25-2021