Give some methods to improve the design of the casting that has porosity.how to avoid this problem?

Question

Casting design.

Answer 1

The casting porosity that I have encountered has come from shrinkage cavitation caused by the surface of the object cooling to the solid state while the interior is still liquid. As the interior cools further, contraction occurs, leaving holes. The last surface areas to cool will often show a depression as a clue to what is happening. To avoid or minimize this the casting metal should be poured at a temperature as near to the melting point as possible, the cooling rate should be as slow as possible so the interior drops in temperature as the surface temperature drops (ideally the whole thing "sets up" at the same time), and small quantities of casting material should be available to fill the depression at the fill hole as the final solidification occurs.

Answer 2

Decrease your cross sections - your casting design is too thick in spots. The voids form as the section cools; the material shrinks to the "outside" of the casting, leaving a "void" in the middle.

Also - check the placement of your sprues; you may be requiring the material to flow too far and it is solidifying on you before it fills the mold.