How to improve the roughness of iron castings?

We produce iron components mainly by lost foam casting and sand casting process. But roughness of iron castings by lost foam casting or by quartz sand casting are not good enough. For some iron castings, we choose to produce them by precoated sand casting process.The roughness can reach about Ra12.5 to Ra6.3.

Besides election of different casting process, there're some other factors that affect the roughness of 
iron castings. 

Here, let's talk about some factors.

1)Shape of sands used during casting process

The shape of sands affect the surface much. Widely use are round, square and triangle sands. Triangle sands are the worst. The gap between sands are big during sands mixing. If it's resin sand casting, it costs lots of resins.

2)Required material of castings

Some materials are not suitable for sand castings, such as some materials with very low manganese. Componets cast from such materials are with severe shrinkages and the surface is extremely rough.

3)Pouring system

If the pouring system is not designed reasonably, there will be shrinkage and insufficient cast defects. If the suppress slag system is not reasonable, the slags will flow into the casting cavities, which will cause the sand pore defects. This will affect the roughness.

4)Slagging

If the slags in melt iron waters are not remove enough or not filtrated clearly during pouring process, the slags will be poured into the cast cavity. This will caused the slag inclusion defects. This will affect the roughness.

5)Improper operation

Improper operation can also caused the casting defects and rough surface. Such improper operations include, sand not cleared enough during moulds assembling, sands not compacted during modelling, and wrong matching rate between sands. If the sands or the model is not strong enough, sand holes defects will be caused.

6)Percentage of P & S exceeded

If the P & S elements exceeded during material mixing, there will be cracks on the casting surface. This affect the surface roughness much.

According to experiece, adding some high temperature resistances can improve the surface roughness of precoated sand casting. Such high temperature resistances can achieve solid phase sintering, which increase the roughness or smooth level of iron castings.