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Metal Casting Technologies : September 2010
60 www.metals.rala.com.au Back to the much smoother assembly and, in some cases, it is difficult to see where the two halves join. Pasted cores could be blamed for some loose sand inclusions in castings. Gating practices vary The single gate or one lead-in gate is the type of gate the steel foundry should not use. It promotes scabs and erosions on the casting. It has been found that multiple gates produce a much better casting surface because of less sand erosion. Parting line gating Gating the casting at its parting line is the easiest to make, although it is less preferred for steel castings than for iron castings. Figure 1 shows how it looks. Whirl gating The whirl gate is not a good gate for steel casting. It has only one lead-in that promotes a hot spot and prevents temperature uniformity. Its whirling action actually promotes mold erosion. It is much better to use bottom gating, instead of a whirl gate to secure definite improvement in quality. Figure 2 shows how a whirl gate would look. Bottom gating Bottom gating has given the best results in pouring large castings. Bottom-gating here is meant going below the drag part of the casting with one, two or more leads. The action of the metal entering the casting does not erode over the surface as does a side-gate. This does require more molding time and sometimes an extra flask, but the compensation from a better surface quality makes the method very worthwhile. Figure 3 shows a type of bottom gate aside from the horn gate type. Ring gating For circular or ring castings, the ring gates are recommended. A ring running around the outside or inside of the casting is fed through two in-gates running from the sprue. There are then many in-gates as necessary from the ring that feed into the casting. The ring gate cross-section should be large enough to insure filling it up and run through the many in-gates to fill up the casting. Castings made in this way have been exceptionally clean. Figure 4 shows one type of ring gate; another type would be encircling the casting from the outside. Fig. 1. Two types of parting gates Fig. 2. A typical whirl gate Fig. 3. One type of bottom gate Fig. 4. One type of ring gate Drawings adapted from the AFS Gating and Risering Committee as sourced from Loper, Heine, and Rosenthal, The Principles of Metal Casting Fig. 5. A type of horn gate Fig. 6. A type of top gate The reverse horn gate would have the bigger diameter of the horn next to the casting to make velocity of entry slower The bottom of the top gate could be only slightly smaller in diameter than the hub in the case of the wheel casting.