Precision casting is a method in which the mold is created by covering the demolition models with ceramic slurry. The model (usually made of wax or plastic) is removed from the mold chamber by burning or melting.
Property :
In sand molding methods, wooden or metal models are used to embed the shape of the piece inside the mold material. In such methods, the models can be used again, but the template is used only once. In the exact method, both the model and the template are used only once. In the exact method, both the model and the template are used only once.
Benefits and limitations
A: The most important advantages of accurate casting method are: Mass production of parts with complex shapes that can not be produced by other casting methods is possible by this process. Mold materials as well as the high technique of this process make it possible to repeat the production of parts with dimensional accuracy and smooth surface. This method is used to produce all metals and cast alloys. It is also possible to produce parts from several different alloys. By this process, it is possible to produce parts with minimal need for machining and all-round operation. Therefore, the restriction on the use of all-machined alloys is eliminated. In this method, it is possible to produce parts, especially metallurgical, better. Adaptability for melting and casting parts in a vacuum. The separation line of the parts is removed and as a result, the defects that are caused by the separation line are eliminated.
B: The most important limitations of the precision casting method are: The size and weight of the parts produced by this method are limited and generally parts with a weight of less than 5 kg are produced. The cost of equipment and tools in this method is higher than other methods.
Types of precision casting methods:
In this process, there are two distinct methods in preparing the mold, which are the shell method and the solid method. In general, these two methods do not differ in model preparation, but differ in the type of molds. Ceramic shell molding process Ceramic shell precision casting: For the production of cast parts of simple carbon steel, alloy steels, stainless steels, heat resistance and other alloys with high melting point, this method is used. These processes show that they are:
A: Preparation of models: Wax or plastic models are prepared by special workshops.
B: Assembly of models: After preparing wax or plastic models, usually some of them (this number depends on the shape and size) are assembled in a cluster around a path. The methods are more common and include:
Method 1: The joint is dipped in molten wax and then glued to the designated place.
Method 2: This method, known as wax welding, is such that the melted joints are joined together.
Method 3: The third method is to use special adhesives that the joint is glued together with special adhesives wax or plastic. The method of connecting plastic models is similar to wax models.
A: The cluster model and its appendages are immersed in two ceramic tubs. As a result, one layer covers the two ceramic gills on the model
D: At this stage, the cluster model is exposed to the rain flow of refractory sand particles. A thin layer is formed on its surface.
E: The ceramic shells created in the previous step are completely dried to become hard and firm. Steps (c) (d) (e) are repeated several times. The number of times this repetition depends on the required mold shell thickness. Usually, in the early stages, slurries made from soft powders are used, and gradually, two grouts as well as coarser refractory sand particles can be used. The smoothness of the surface of the cast depends on the particles of the two primary dusts as well as the primary refractory sand.
G: Wax or plastic modules are removed from the mold chamber by melting or burning, this operation is called waxing. In waxing operations, care should be taken that the expansion of the wax does not cause stress and cracking in the mold.
H: Melt loading operation is performed in the produced molds.
I. After the melt freezes, the ceramic shell is broken.
ی. In the last step, the parts are separated from the gate.
Refractory materials in precise shell process:
Due to its low thermal expansion, silica is widely used as a refractory in the precision shell method. This refractory material is used for casting ferrous alloys and cobalt alloys. The following is perhaps the most commonly used refractory in the shell process. This material creates the best quality on the surfaces of the part and is stable at high temperatures and is resistant to corrosion by the melt. Aluminum is less used due to its low resistance to heat shock. However, in some cases it is used because of its resistance to high temperatures (up to about 1760 ° C).
Adhesives: نس Refractory materials adhere to each other by adhesives. These adhesives are usually chemical: ethyl silicate, sodium silicate and colloidal silica. Ethyl silicates give a very good finished surface. Colloidal silica also produces an excellent finished surface.
Other components: In addition to the above materials, a suitable composition includes other materials, each of which is used for a specific purpose.
These materials are as follows: Viscosity control materials – Wetting materials to control the fluidity of the two gums and the ability to moisten the model – Anti-foaming materials to remove air bubbles – Gelatinous materials to control the drying and reduction of cracks The process of preparing solid molds in the cast Accurate Gray: Figure along the schematic of the steps