Gravity casting is one special casting process that pours melting metal into the permanent mould by gravity force through an inclinable casting machine as the picture indicated.
Casting may be classified as expendable mould and permanent mould two catalogues according to the mould material.
The casting process, such as sand casting, is the process in which expendable sand mould need to be made from a pattern for each casting.
While in permanent mould casting, the metal mould is used repeatedly for batch castings in its life cycle. The mould is so designed in such a way that casting can be removed easily from the mould.
In general, casting processes, such as sand casting or investment casting, may also be called as gravity casting as the melting metal is poured into mould by gravity force too.
However, only casting process that pours melting metal into the permanent metal mold can be called as gravity casting specifically.
Material and Applications
Materials that are cast in gravity casting are aluminum alloys, magnesium alloys, zinc alloys, copper alloy, and grey casting iron. However, the aluminum alloy is most popular and applicable because of its relatively lower melting temperature. Hot shock from melting temperature will shorten die life and makes process uneconomic.
Gravity casting is used in almost all industries and widely used in automotive, machinery components, gas and petroleum industry, etc. The classic application is the engine block, cylinder head, inlet or outlet manifold, nozzle handle, etc.
Mould and Cores Materials
The mold material is selected based on the consideration of metal melting temperature, the casting size, and the casting volume that is estimated to be made.
Grey iron is the most generally used die material, but alloy cast iron and alloy tool steel are also used for very large volumes or large parts. Graphite mold may be used for small volume production of aluminum and magnesium casting. The die life is short for higher melting temperature material, such as copper or grey cast iron.
In case making casting with hollow portions, cores are also used. The cores can be made out of metal or sand. When the sand core used, the process is called semi-permanent molding.
Tooling Design and Process Consideration
The gating and riser systems used are very similar to that of the sand casting. In fact, to get the proper gating arrangements, it may be desirable to experiment with various gating systems in sand casting and then finally arrive at the correct gating system for the metallic mould.
Mould cavity should be simple without any undesirable drafts or undercuts that interfere with the ejection of the solidified castings. Chills supported by heavy air blast may also be used to remove the excess heat. Alternatively, cooling channels may be provided at the necessary points to get proper temperature distribution.
In semi-permanent casting, the metallic core may not be complex with undercuts and similar. Also, the metallic core needs to be withdrawn immediately after solidification, otherwise, its extraction becomes difficult because of shrinkage. If metalcore is too complicated, the collapsible metal cores (multi-piece core) are sometimes used, but not frequently because of wide dimension variations. Hence the designer needs to give coarse tolerance for dimensions of complicated hollow features.
As mentioned, tooling material selection is up to the material and volume of casting to be cast. So, it is important to take a method to increase the tool life cycle for volume production. In doing so, the mould will be coated with refractory material, this coating will
- Prevent soldering of metal to the mould
- Minimize thermal shock to mould surface
- Control the rate and direction of the casting solidification
The coating normally is mixtures of sodium silicate, kaolin clay, soapstone and talc. The coatings are both insulating type and lubricating type, and inert to casting alloy. The coating may be applied by spraying or brushing. Coating shall be thick enough to cover any surface imperfections. The coating may be applied much thicker at the place that needs to be cooled slowly, the place such as sprue, runners, risers and thin sections.
Advantage and Limitation
- Mould fabricated from hot-working alloy tool steel, long life suitable for volume production
- Good surface finish and accurate dimension tolerance.
- Almost has no casting air bubble or void inside of casting, gravity casting strength is stronger than sand casting and suitable for further heat treating to enhance its strength and toughness
- Die cost relatively higher than that of sand casting
- Suitable to make small or medium size casting because of limitations on the capacity of die fabrication machine and casting machine
- Because of the thermal shock issue, only suitable to make lower melting point nonferrous metal. Aluminum alloy is the most common favorite
So far, a simple introduction to the gravity casting process has finished. The process selection relies on consolidation judgments based on material used, mechanical structure complexity, component size, and the most important design intent for field applications.
The theory is white and gray, and only practice trains people’s problem-solving skills and abilities. Sipaitech mfg has developed a wide range of gravity castings and has versatile machining equipment to meet your ultimate requirements.