Design Consideration For Casting

For design consideration of casting..

1. Design Consideration For Casting Presented By – Mohit Joon 130130103062 B.Tech (ME) , 7 th Sem

2. Introduction Successful casting practice requires the proper control of a large number of variables: characteristics of the metals (or alloys) casts, method of casting, and various process parameters. The flow of the molten metal in the mold cavities, the gating systems, the rate of cooling, and the gases evolved all influence the quality of a casting.

3. CASTING Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting , which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together. Example - epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.

4. Design Considerations in Casting Design the part so that the shape is cast easily. Select a casting process and material suitable for the part, size, mechanical properties, etc. Locate the parting line of the mold in the part. Locate and design the gates to allow uniform feeding of the mold cavity with molten metal. Select an appropriate runner geometry for the system. Locate mold features such as sprue, screens and risers, as appropriate. Make sure proper controls and good practices are in place.

5. Design Considerations in Casting - Design of cast parts Corners, angles and section thickness : avoid using sharp corners and angles (act as stress raisers) and may cause cracking and tearing during solidification. Use fillets with radii ranging from 3 to 25 mm

6. Design Considerations in Casting - Design of cast parts Figure 12.1 Suggested design modifications to avoid defects in castings. Note that sharp corners are avoided to reduce stress concentrations.

7. Design Considerations in Casting - Design of cast parts Sections changes in castings should be blended smoothly into each other. Location of the largest circle that can be inscribed in a particular region is critical so far as shrinkage cavities are concerned (a & b). Because the cooling rate in regions with large circles is lower, they are called hot spots . These regions can develop shrinkage cavities and porosity (c & d). Cavities at hot spots can be eliminated by using small cores (e). It is important to maintain (as much as possible) uniform cross sections and wall thicknesses throughout the casting to avoid or minimize shrinkage cavities. Metal chills in the mold can eliminate or minimize hot spots.

8. Design Considerations in Casting - Design of cast parts Figure 12.2 Examples of designs showing the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.

9. Design Considerations in Casting - Design of cast parts Figure - Examples of design modifications to avoid shrinkage cavities in castings. Figure - The use of metal padding (chills) to increase the rate of cooling in thick regions in a casting to avoid shrinkage cavities

10. Design Considerations in Casting - Design of cast parts Flat areas : large flat areas (plain surfaces) should be avoided, since they may warp during cooling because of temperature gradients, or they develop poor surface finish because of uneven flow of metal during pouring. To resolve this one can break up flat surfaces with staggered ribs. Shrinkage: pattern dimensions also should allow for shrinkage of the metal during solidification and cooling. Allowances for shrinkage, known as patternmaker’s shrinkage allowances, usually range from about 10 to 20 mm/m .

11. Design Considerations in Casting - Design of cast parts Draft : a small draft (taper) typically is provided in sand mold pattern to enable removal of the pattern without damaging the mold. Drafts generally range from 5 to 15 mm/m. Depending on the quality of the pattern, draft angles usually range from 0.5 o to 2 o . Dimensional tolerances : tolerances should be as wide as possible, within the limits of good part performance; otherwise, the cost of the casting increases. In commercial practices, tolerances are usually in the range of ± 0.8 mm for small castings. For large castings, tolerances may be as much as ± 6 mm.

12. Design Considerations in Casting - Design of cast parts Lettering and markings : it is common practice to include some form of part identification (such lettering or corporate logos) in castings. These features can be sunk into the casting or protrude from the surface. Machining and finishing operations : should be taken into account. For example, a hole to be drilled should be on a flat surface not a curved one. Better yet, should incorporate a small dimple as a starting point. Features to be used for clamping when machining.

13. Casting Defects Some defects are common to any and all process. These defects are illustrated in figure 13.22 and briefly described in the following: a) Misruns : A Misruns is a casting that has solidified before completely filling the mold cavity. Typical causes include 1) Fluidity of the molten metal is insufficient , 2 ) Pouring Temperature is too low, 3 ) Pouring is done too slowly and 4 ) Cross section of the mold cavity is too thin . b ) Cold Shut: A cold shut occurs when two portion of the metal flow together , but there is lack of fusion between them due to premature freezing, Its causes are similar to those of a Misruns .

14. c) Cold Shots: When splattering occurs during pouring, solid globules of the metal are formed that become entrapped in the casting. Pouring procedures and gating system designs that avoid splattering can prevent these defects. d) Shrinkage Cavity: This defects is a depression in the surface or an internal void in the casting caused by solidification shrinkage that restricts the amount of the molten metal available in the last region to freeze. It often occurs near the top of the casting in which case it is referred to as a pipe. The problem can often be solved by proper riser design.

15. e) Microporosity : This refers to a network of a small voids distributed throughout the casting caused by localized solidification shrinkage of the final molten metal in the dendritic structure. The defect is usually associated with alloys, because of the protracted manner in which freezing occurs in these metals. f) Hot Tearing: This defect, also called hot cracking, occurs when the casting is restrained or early stages of cooling after solidification. The defect is manifested as a separation of the metal (hence the terms tearing or cracking) at a point of high tensile stress caused by metal’s inability to shrink naturally. In sand casting and other expandable mold processes, compounding the mold to be collapsible prevents it. In permanent mold processes, removing the part from the mold immediately after freezing reduces hot tearing.

16. Economics of Casting There are also major costs involved in making patterns for casting . The cost of the cast part (unit cost) depends on several factors: including materials, tooling, equipment, and labor . Costs also are involved in melting and pouring the molten metal into molds and in heat treating, cleaning, and inspecting the casting. Heat treatment in an important part of the production of many alloys groups (especially ferrous castings) and may be necessary to produce improved mechanical properties. The equipment cost per casting will decrease as the number of parts cast increase. Sustained high-production rates, therefore, can justify the high cost of dies and machinery. However, if the demand is relatively small, the cost-per-casting increases rapidly . Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

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