Gravity Die Casting refer to molten alloy pour into mould via gravity.Widely,it include sand casting,permanent mold casting,lost wax casting,lost foam casting etc.Precisely,gravity die casting normally refer to permanent mold casting.
Gravity die casting normally require a high temperature resistance steel mould.The casting method include alloy melting,degassing,tilt pouring,ventilation,cooling,open mould,shot blasting,heat treatment,machining.
Aluminum alloy used for gravity die casting normally contain relatively high amount of silicon,which can allow molten alloy flow into mould smoothly.Magnesium is necessary for alloy.It can improve the strength after heat treatment.A356 is quite common aluminum alloy used for gravity die casting.It is kind of Al-Si-Mg alloy and very suitable for casting.Fluidity and shrinkage and hot crack resistance are excellent.The strength can be improved very much by heat treatment.For example,the production of car wheel rim use A356 mostly.A357 is common aluminum alloy for gravity die casting.It contain more magnesium than A356.To guarantee the final strength,composition of magnesium need to be controlled strictly. Volatilization of molten magnesium is main issue which affect final strength of castings.The common solution is to add magnesium ingot during smelting.
The reason why choose gravity die casting is for following reason
- Casting can be heat treated.The strength of casting after heat treatment is more than high pressure die casting parts.
- The air pore inside casting can be controlled properly.
- By positioning sprue gate properly,the shrinkage porosity can be limited maximumly.
- The mould structure is much simpler than die casting mould.The cost is much lower than die casting mould.Therefore,customer do not need to put big investment at initial stage of production.
- Easy to make mould modification.The maintenance cost is quite low.
- Life span of mould is longer than die casting.
There are some disadvantage for gravity die casting.It will be done without any pressure applied,so it is not suitable for thin wall parts casting.The minimum wall thickness should be 4mm to be gravity die casted.The forming time is slower than high pressure die casting which only need few of seconds.Therefore,the average unit parts cost will be higher than die casting.The surface roughness is higher than die casting,but it can be solved by shot blasting.For some aesthetic finishing,better to do fine polishing over the surface.For some complex inner structure,die casting sometimes can not be used for production but gravity die casting can do it via sand core.
All in all,none of casting methods can be suitable for any kind of products.Better to choose proper casting method according to its design.