Sand Castings : Manufacturing Process

Manufacturing Processes:

  • CO2 Sand Moulding (2kgs to 100 Kgs)
  • Shell Moulding (100 gms to 10 kgs)

CO2 Process:


The sand mixture used in this process is pure silica and sodium silicate. The molding mixture thus obtained is rammed around the pattern and gassed with carbon dioxide (CO2). The molding mixture gets hardened due to the formation of silica gel. After passing CO2, the pattern is removed and the mold cavity is ready for pouring.

 

Na20 + mSiO2 . xH20 + CO2 = Na2CO3 + mSiO2 . xH20

 

Properties of silicates vary according to the silica (SiO2)-soda (NaO) ratio of the sodium silicate base that is used to formulate the binder. Considerable care must be used when gassing with CO2 since overgassing and undergassing adversely affect the properties of the cured binding system. The amount of silicate binder used for cores and molds varies from 3% to 6% depending on the type of sand, grain fineness, and degree of sand contaminants.



Shell Molding Process:


Shell mold casting process is a technique in which liquid metal is poured into a cavity formed from a sand mixture. Shell molding process offers better surface finish, better dimensional tolerances, and higher throughput due to reduced cycle times and the fact that the process can be automated. These qualities of precision can be obtained in a wider range of alloys and with greater flexibility in design than die-casting and at a lower cost than investment casting. The process was developed and patented by Croning in Germany during World War II and is sometimes referred to as the Cronning shell process.

 

Like any other sand casting process, here too the first step in making shell molds is sand preparation. Sand of required quality & grade is sieved and then mixed thoroughly along with a thermoset plastic. When this sand mixture comes in contact with a heated match plate containing pattern and the required gating system, a thin shell of 3.5 to 5mm thick is formed on the surface of the match plate. The shell so formed is peeled-off from the match plate and then closed together by employing proper paste to form the mold cavity. Liquid metal when poured into this shell mold cavity, takes the shape of the casting, which is then fettled and processed as required.



 


This process can produce complex parts with good surface finish 1.25 µm to 3.75 µm (50 µin to 150 µin) rms, and dimensional tolerance of 0.5 %. Size limits of 30 g to 12 kg (1 oz to 25 lb). Minimum thicknesses can be as low as 1.5 mm (0.062 in) to 6.25 mm (0.25 in), depending on the material. Good surface finish and good size tolerance reduce the need for machining.

 

The equipment necessary for the production of shell molds may range from simple dump box rigging to highly mechanized and completely automatic blowing machinery. Production equipment is of two basic types, one in which the mold making steps of investment, curing, stripping, pattern cleaning and straying are incorporated in a single station machine and the other a multi-station unit where the mold making steps are arranged to take place simultaneously.

 

The process is good for producing parts requiring:

• Thin Sections
• Intricate Details
• Close Dimensional Tolerances
• Excellent Surface Finish
• High Volumes