<prompt> Which is the most common site for suck-back porosity?Cusp tipGingival margin of the restoration when not well roundedOcclusoaxial line angle when not well roundedIncisal surface  <response>: Suck-back porosity often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded. The entering metal impinges onto the mold surface at this point and creates a higher localized mold temperature in this region, known as a hot spot. A hot spot may retain a localized pool of molten metal after other areas of the casting have solidified. This, in turn, creates a shrinkage void or suck-back porosity. This type of porosity defect can be eliminated by flaring the point of sprue attachment and reducing the mold-metal temperature differentialthat is, lowering the casting temperature by about 30C.Phillips 12th edition page 227 