sand casting and other expendable casting

sand casting and other expendable casting process processor

1. Lecture No.3 Sand & Other Expandable Casting Process By Ass. Lect. Ali H. Almaily [email protected]

2. 3.1 Sand casting Also known as sand-mold casting, consists of pouring molten metal into a sand mold, allowing the metal to solidify, and then breaking up the mold to remove the casting. The casting must then be cleaned and inspected , and heat treatment is sometimes required to improve metallurgical properties . Figure 3.1: Steps in the production sequence in sand casting. The steps include not only the casting operation but also pattern making and mold making.

3. 3.1.2 Patterns Patterns define the external shape of the cast part . Sand casting requires a pattern—a full-sized model of the part, enlarged to account for shrinkage and machining on the total quantity of castings to be made. 3.1.2.a- Pattern materials Wood Patterns : is a common pattern material because it is easily shaped. Its disadvantages are that it tends to warp, and it is allowances in the final casting. Metal patterns: are more expensive to make, but they last much longer. Plastics Patterns: represent a compromise between wood and metal.

4. 3.1.2.b- Types of Patterns Solid Pattern: The simplest is made of one piece-same geometry as the casting, adjusted in size for shrinkage and machining. Although it is the easiest pattern to fabricate, it is not the easiest to use in making the sand mold . Solid patterns are generally limited to very low production quantities. Split patterns: consist of two pieces, dividing the part along a plane coinciding with the parting line of the mold. Split patterns are appropriate for complex part geometries and moderate production quantities. match-plate patterns : the two pieces of the split pattern are attached to opposite sides of a wood or metal plate. Holes in the plate allow the top and bottom (cope and drag) sections of the mold to be aligned accurately. Cope-and-drag patterns : are similar to match-plate patterns except that split pattern halves are attached to separate plates, so that the cope and drag sections of the mold can be fabricated independently, instead of using the same tooling for both.

5. Figure 3.2 Types of patterns used in sand casting: (a) solid pattern, (b) split pattern, (c) match-plate pattern, and (d) cope-and-drag pattern.

6. 3.1.3 Foundry sands: Silica (SiO2) or silica mixed with other minerals, features of the sand include: Good refractory properties - capacity to endure high temperatures. Small grain size yields better surface finish on the cast part. Large grain size is more permeable, allowing gases to escape during Pouring. Irregular grain shapes strengthen molds due to interlocking, compared to round grains . Disadvantage: interlocking tends to reduce permeability. 4.4.1 Binders Used with Foundry Sands Sand is held together by a mixture of water and bonding clay Typical mix: 90% sand, 3% water, and 7% clay. Other bonding agents also used in sand molds: Organic resins (e g , phenolic resins) Inorganic binders (e g , sodium silicate and phosphate) Additives are sometimes combined with the mixture to increase Strength and/or permeability .

7. Several indicators are used to determine the quality of the sand mold : 1. Strength: the mold’s ability to maintain its shape and resist erosion caused by the flow of molten metal; it depends on grain shape, adhesive qualities of the binder, and other factors; 2. Permeability: capacity of the mold to allow hot air and gases from the casting operation to pass through the voids in the sand; 3. Thermal Stability: ability of the sand at the surface of the mold cavity to resist cracking and buckling upon contact with the molten metal; 4. Collapsibility: ability of the mold to give way and allow the casting to shrink without cracking the casting; it also refers to the ability to remove the sand from the casting during cleaning; and 5. Reusability: can the sand from the broken mold be reused to make other molds.

8. 4.4.2 Sand molds are often classified as: Green sand molds: Are made of a mixture of sand, clay, and water, the word green referring to the fact that the mold contains moisture at the time of pouring. Green-sand molds possess sufficient strength for most applications, good collapsibility, good permeability, good reusability, and are the least expensive of the molds. Moisture is the mean problems in the green-sand which can cause defects in some castings. 2. A dry-sand mold: is made using organic binders rather than clay, and the mold is baked in a large oven at temperatures ranging from (200 to 320 o C ). Oven baking strengthens the mold and hardens the cavity surface. A dry sand mold provides better dimensional control in the cast product, compared to green-sand molding. However, dry-sand molding is more expensive, and production rate is reduced because of drying time. Applications are generally limited to medium and large castings in low to medium production rates.

9. 3.Cement bonded sand molds: A mixture of silica sand containing 8-12% cement and 4-6% water is used. When making the mold, the cement-bonded sand mixture must be allowed to harden first, before the pattern is withdrawn. The mold obtained is then allowed to cure for about 3-5 days. Large castings with intricate shapes, accurate dimensions and smooth surfaces are usually produced by this method. The only shortcoming being the long time required for the molding process.

10. 3.2 Shell Molding process include: A . Shell molding casting : is an expendable mold casting process in which the mold is a thin shell (typically 9mm) made of sand held together by a thermosetting resin binder., the process is described and illustrated in Figure 3.3 . The shell mold casting process consists of the following steps: 1. a match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin; 2. box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; 3. box is repositioned so that loose, uncured particles drop away; 4. sand shell is heated in oven for several minutes to complete curing; shell mold is stripped from the pattern; two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished. The finished casting with sprue removed is shown in (7 ).

11. Figure 3.3: step of shell casting.

12. 3.2.1 Advantages to the shell-molding process: (1) The surface of the shell mold cavity is smoother than a conventional green-sand mold, and this smoothness permits easier flow of molten metal during pouring and (2) Good dimensional accuracy with better surface finish on the final casting .The good finish and accuracy often precludes the need for further machining. (3) Collapsibility of the mold is generally sufficient to avoid tearing and cracking of the casting. 3.2.2 Disadvantages of shell molding include: a more expensive metal pattern makes shell molding difficult to justify for small quantities of parts.

13. Lost-wax casting: For investment (sometimes called lost-wax) casting, the pattern is made from a wax or plastic that has a low melting temperature. Around the pattern is poured a fluid slurry, which sets up to form a solid mold or investment; plaster is usually used. The mold is then heated, such that the pattern melts and is burned out, leaving behind a mold cavity having the desired shape. This technique is employed when high dimensional accuracy, reproduction of fine detail, and an excellent finish are required—for example, in jewelry and dental crowns and inlays. Also, blades for gas turbines and jet engine impellers are investment cast. The Steps in investment casting are: Wax patterns are produced; Several patterns are attached to a sprue to form a pattern tree; The pattern tree is coated with a thin layer of refractory material; The full mold is formed by covering the coated tree with sufficient refractory material to make it rigid.

14. (5) The mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity; (6)The mold is preheated to a high temperature, which ensures that all contaminants are eliminated from the mold; it also permits the liquid metal to flow more easily into the detailed cavity; the molten metal is poured; it solidifies; and (7)The mold is broken away from the finished casting. Parts are separated from the sprue. Figure 3.4: Steps in investment casting.

15. 3.3 Continuous Casting Continuous casting is widely applied in aluminum and copper production, but its most noteworthy application is in steelmaking. The continuous casting process, also called strand casting, is illustrated in Figure 3.5 . Molten steel is poured from a ladle into a temporary container called a tundish , which dispenses the metal to one or more continuous casting molds. The steel begins to solidify at the outer regions as it travels down through the water-cooled mold. Water sprays accelerate the cooling process. While still hot and plastic, the metal is bent from vertical to horizontal orientation. It is then cut into sections or fed continuously into a rolling mill in which it is formed into plate or sheet stock or other cross sections.

16. Figure 3.5: Continuous casting; steel is poured into tundish and distributed to a water-cooled continuous casting mold; it solidifies as it travels down through the mold. The slab thickness is exaggerated for clarity.