Investment casting 101
This is a polystyrol sample method, the mold material is dry sand, without adhesives. The sample is buried in the sand mass free of adhesive but formed by vacuum aspiration. Pouring metal cavity into the mold, the liquid metal will gradually replace the porous sample and form the casting.
Investment casting characteristic
Unlike other binder casting technologies, combustion sample casting (also called gasification) does not use a binder to bond the sand. Patterns and branches are made of polystyrene resin or copolymers. The castings have complex shapes, each piece will be fabricated individually. Then, they will be glued together and glued to the main branch (main branch can be a sprue, the branch can be a slag filter groove, ditch). The sample after being glued to the branch will be painted (covered) with insulation. After the coating dries, the sample is placed in the mold case and covered with sand. The sand is tightened by vibration. When the mold is finished, the mold box is vacuumed and waiting for pouring.
Process
The combustion modeling technology is summarized as follows:
+ Fabrication of sample details by polystyrol.
+ Leave to air naturally to resize the sample (about 5 to 7 days).
+ Merge the details together into a complete sample (for large and complex details).
+ Sampling beam and filling system.
+ Paint the sample with the paint used exclusively for the combustion samples.
+ Dry or air-dry the paint until it is completely dry.
+ Insert sample and filling system into mold box (can be vacuum).
+ Pour liquid metal into the mold. Where the metal goes, the polystyrol sample burns there until the entire sample volume has been completely replaced by the metal. During molding, gas formed by the burnt specimen passes through the mold wall due to the pressure difference caused by vacuum aspiration.
After complete solidification, remove the sample beam from the mold box. If the mold is made under vacuum, after the vacuum is stopped, the grains of sand will come out of the casting.
Advantages
+ Investment casting creates a relatively beautiful surface quality, but the product does not have high accuracy.
+ This technology is also easy to automate to improve productivity, the creation of foam patterns is relatively simple. It is important that the mass-produced foam samples are based on a molded case so that the products are virtually devoid of error. Investment casting is quite independent on human labor
+ Easy to mechanize and automate, minimize hard labor, and reduce dust emissions causing environmental pollution.
Disadvantages
+ High cost.
Big initial cost, especially with big details.
+ Samples may be damaged or distorted due to their low durability.
+ When casting cast iron or steel, due to the high temperature, the sponge is decomposed very quickly and creates a lot of gas. a vacuum is inserted to absorb the other air. The high temperature causes the porous particles to be decomposed into a bitumen-like substance, which can sometimes escape through the coating. it promotes the formation of a form of carbon called “lustrous carbon”, which is responsible for the wrinkle defects and indentations above the surface of the casting. With alloys with a high carbon content, such as ductile iron, it is easier to develop defects.
Application of Investment casting
For the manufacture of all engine blocks, cylinder heads, crankshafts, carrier differentials and transmission boxes, …
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