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126.96.36.199 Gating System Design
Successful casting once depended largely on an operator's skill. Today, the product engineer uses fundamental fluid-flow principles for gating design, as shown in gating for sand casting.
In order to attain better molten metal flow a gating ratio of 1:2:2 or 1:2:1.5 with a pouring basin, conical sprue, and sprue base well that are proportioned as in sand casting, is used for reducing heat loss and turbulence. Most molds are gated to fill from the bottom to the top (Figure 11.3). The metal flows from the sprue base well along the runner bottom, feeds the riser, and passes through a slot gate along the vertical side of the casting into the cavity. Note that a horizontal extension of the base runner receives the initial dross.
Table 11.1 Comparison of sand- and metal-mold casting processes.
Metals cast ferrous & non-ferrous non ferrous non ferrous
Figure 11.2 Design fundamentals for permanent-mold castings.
Table 11.2 Typical dimensional standards for permanent molds and cores.
Figure 11.3 A typical gating system for a permanent mold.
All castings must have directional solidification from thin remote sections toward the riser in order to achieve proper material feeding. The riser in turn is fed last with hot metal and has a surface-area-to-volume ratio such that it will freeze more slowly than the casting.
The concept of casting yield may be used to determine a reasonable estimate of the riser size for permanent molds. Yield refers to the ratio of the casting weight divided by the total weight of metal poured. Metals that exhibit greater shrinkage characteristics require larger risers to fill voids during solidification. Low-shrinkage alloys such as a eutectic alloy of aluminum with 12 percent silicon, require little feed metal for a sound casting. The figures given are once again approximations, because the weight of the gating system must be estimated and the surface-area-to-volume ratio is not considered.
Vents are used to discharge the gas in the gating system and mold cavity as fast as the metal enters the mold. Natural venting along sliding members and at the parting line is usually inadequate. Additional venting may be added as follows.
1. Cut slots as deep as 0.011 in. (.3 mm) and of suitable width across the parting seal surface.
2. Drill small clusters of holes 0.008 to 0.011 in. (.2 - .3 mm) in diameter in the mold wall at a point where venting is needed.
3. Drill holes and install slotted plugs (plug vent).
11.1.5 Mold Material
The mold material is chosen on the basis of three criteria: material cost, the expected total number of pours required, and the casting alloy. Most common and suitable for a permanent mold is high-quality pearlitic gray iron, inoculated at the ladle to achieve uniform grain size and highly dispersed fine graphite has the composition given in Table 11.3.
Table 11.3 Typical gray iron composition for permanent molds and cores.
Large castings or high pouring temperatures may require cores of alloy cast iron or H11 die steel. Undercuts or complex internal features can be formed with sand cores.