Spool valve and what should be specifications of the spool valve. Typically hydraulic valves are made of cast iron. Sand cores are used inside the cast iron hydraulic valve to create cavity for internal rooms and oil drain.
In this phase casting and sand core production technology is the most important point. If the sand core is not in high quality the cavity which needs to be created will not be created so directional control valve will not work properly. On the other hand there can be other problems due to cast iron. These types of casting problems are mainly seen on Far Eastern oriented directional control valves.