Effective riser use.

A riser is a reservoir of molten metal that is used to feed a casting, which shrinks during solidification from molten to solid metal. Risers are an important part of a casting's gating system, which is the path molten metal takes to enter the mold cavity. The molten metal goes through several stages of volumetric contraction as it cools in the mold, or what foundrymen call "shrinkage."

Because the metal is poured into the mold at a temperature above its melting point, there is shrinkage as the molten metal cools to the temperature where solidification begins. As the metal cools through the solidification range of the alloy, the casting solidities and more shrinkage occurs. Finally, as the solid, hot casting cools to room temperature, a third and final shrinkage occurs. The total volume of shrinkage depends primarily on the alloy and the temperature at which that alloy is poured.

Compensating for Shrinkage

The pattern of a part to be cast is made slightly larger in dimension than the desired part to compensate for shrinkage. This shrinkage is commonly called "patternmaker's shrink."

All metals shrink during cooling and solidification, but a riser is not always necessary to produce a sound casting. If the casting volume is small, the other gating components may provide adequate feed metal to compensate for shrinkage. The required feed metal volume depends on the alloy being poured. During the solidification of gray or ductile iron, pensates for some of the shrinkage, and smaller feed metal volumes are generally required for these alloys. As a result, gray and ductile iron castings require fewer and smaller risers than other alloys.

Risers work because the shape and size of the riser keeps the metal in the riser liquid longer than the casting section it is feeding.

Casting Solidification

Solidification in castings progresses via two mechanisms: progressive solidification and directional solidification. In progressive solidification, solidification proceeds from the outside surface of the casting toward the casting center, as shown in Fig. 1a. The solid metal fronts approach from opposite sides of the casting, limiting the flow of feed metal deep into the casting's interior, which often can lead to centerline shrinkage in the casting. For this reason, risers are said to have a limited feeding distance. Knowing the feeding distance for an alloy helps determine the number of risers needed to produce a sound casting.

[FIGURE 1a OMITTED]

In directional solidification (Fig. 1b), solidification progresses from the end of the casting toward the end where the gate or riser is attached. In this manner, a molten metal channel is maintained to provide feeding to all sections of the casting. Theoretically, an infinite feeding distance would exist.

[FIGURE 1b OMITTED]

In practice, both progressive and directional solidification are taking place at the same time (Fig. 1c). The casting design often promotes one mechanism over the other, however. Casting designers should employ casting designs that promote directional solidification whenever possible since this provides sounder castings, improves yields and reduces casting costs. Figure 2 illustrates this point. A wedge shape is a perfect example of how the casting design can affect the solidification model. Even by using the progressive solidification model, the casting easily is fed through its entire length, since a liquid channel is maintained throughout the casting during solidification.

[FIGURE 1c-2 OMITTED]

"Padding" a casting (Fig. 3), or adding extra metal to provide a feed path into the casting, can be used to reduce shrinkage defects. Padding provides the wedge shape that helps promote directional solidification. Unfortunately, the padding must be removed by machining after the casting is produced.

[FIGURE 3 OMITTED]

Computer modeling of casting solidification is widely used in today's foundries. The study of solidifying metal fronts provided by these programs can accurately predict shrinkage defects in the castings. This valuable tool allows foundrymen and designers to compare the soundness of castings of various designs.

Riser Location

The risers illustrated in Figs. 1-3 are side risers (they are attached to the side of the casting). When the feed path is restricted between a side riser and the casting section being fed, a top riser (Fig. 4) often is used. Top risers usually require a larger contact between the casting and the riser, which must be subsequently removed from the casting. Also, since the riser is located at the top of the casting, the coldest metal is in the riser. This is the reverse of what is preferred. By working with your foundry supplier and computer solidification software, you may find ways to eliminate the use of a top riser.

[FIGURE 4 OMITTED]