Why is the Hole in Aluminum Alloy die Casting should be Less Than 0.25mm?

We add a lot silicon into aluminum alloy for die casting. When aluminum alloy was condensed in the mold, silicon would float to the surface and form a thin layer of silicon film, which has high hardness and wear resistance. Some designers of OME (Original Equipment Manufacturer) use this feature to transform the hole surface into bearing surface. The thickness of silicon film is usually from 0.2 to 0.9mm. Too much processing would shorten the service life of bearing surface.