Finally, the mold is designed to minimize melt wastage and aid ejection of the ingot, as losing either melt or ingot increases manufacturing costs of finished products. A variety of designs exist for the mold, which may be selected to suit the physical properties of the liquid melt and the solidification process. Molds may exist in the top, horizontal or bottom-up pouring and may be fluted or flat walled. The fluted design increases heat transfer owing to a larger contact area. Molds may be either solid "massive" design, sand cast (e.g. for pig iron), or water-cooled shells, depending upon heat transfer requirements. Ingot molds are tapered to prevent the formation of cracks due to uneven cooling. A crack or void formation occurs as the liquid to solid transition has an associated volume change for a constant mass of material. The formation of these ingot defects may render the cast ingot useless and may need to be re-melted, recycled, or discarded. The physical structure of a crystalline material is largely determined by the method of cooling and precipitation of the molten metal. During the pouring process, metal in contact with the ingot walls rapidly cools and forms either a columnar structure or possibly a "chill zone" of equiaxed dendrites, depending upon the liquid being cooled and the cooling rate of the mold. For a top-poured ingot, as the liquid cools within the mold, differential volume effects cause the top of the liquid to recede leaving a curved surface at the mold top which may eventually be required to be machined from the ingot. The mold cooling effect creates an advancing solidification front, which has several associated zones, closer to the wall there is a solid zone that draws heat from the solidifying melt, for alloys there may exist a "mushy" zone, which is the result of solid-liquid equilibrium regions in the alloy's phase diagram, and a liquid region. The rate of front advancement controls the time that dendrites or nuclei have to form in the solidification region. The width of the mushy zone in an alloy may be controlled by tuning the heat transfer properties of the mold or adjusting the liquid melt alloy compositions. Continuous casting methods for ingot processing also exist, whereby a stationary front of solidification is formed by the continual take-off of cooled solid material, and the addition of a molten liquid to the casting process. Approximately