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Typically the sprue is shown connecting to the bottom center of the ring or the shank. This is a good location for producing a rubber mold because it allows the mold maker to create a smooth transition from the sprue to the shank. However, it may not be the best location when actually casting the piece.
The shank is usually the thinnest or narrowest section of the ring and when sprued to, can cause shrinkage porosity. Make sure that you sprue to the thickest or heaviest part of the ring, which in most cases is the top of the piece.
Plan the sprue connections to avoid sharp corners or directional shifts in the metal, both may cause bits of investment to break off as the metal flows into the flask.
BE CAREFUL, if you make the thickest section the point where the sprue meets the piece you will get porosity at the intersection. If the cross-section of the sprue is 5mm and the shank is 5mm, where the two meets are greater than 5mm (i.e. thin to thick to thin). When the sprue is cut off it is directly in the middle of the heaviest part, which is where porosity forms. Make sure there is a smooth transition from the sprue to piece and round or oval in cross-section.
We sell ready made wires to make the sprues for quick and easy spruing!