STYLE/SHRINKAGE POROSITY

The most common type of porosity is shrinkage porosity. This porosity is found on large surface area pieces, on parts with shanks that increase in thickness and parts with many angles. The porosity is usually seen in the shoulder area of rings or approximately 1/2-2/3 of the distance away from the gate on a single gated part. This porosity will be universal in nature, occurring with all types of metals and casting equipment. The shrinkage takes place due to a non uniform solidification of the part. The look of this porosity is similar to the look of the tree used for casting - rough, pitted and possibly dendritic (if oxides are trapped in the casting). Dendrites appear as a dark etched surface pattern (crisscross). The solution to this problem is to make a proper gate, that uniform filling and solidification of the flask cavity takes place. In rings this involves adding assist gates to the shoulder. In parts such as bracelets this involves a minimum of three gates, one at each end and one in the center. When casting these difficult parts, it is important that metal and flask temperatures are not too hot, as this will allow for a greater amount of shrinkage to take place. A rapid solidification is the aim.

NOTES: When using a single gate always place it on the heaviest section. Use a round gate whenever possible.