CHAPTER THREE Metal Casting Casting, shown in Fig. 3.1, is the process of pouring molten metal into a mould containing a cavity, which represents the required product shape. It is one of the most commonly used processes for making parts is casting. Fig. 3.1: Casting process Steps in casting are three: 1. Melt the metal 2. Pour it into a mold 3. Let it freeze MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 27 -
Advantages of casting Casting process most often is selected over other manufacturing methods because the following reasons: Can create complex part geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can produce very large parts Some casting methods are suited to mass production. It is economical to use. Disadvantages of casting Different disadvantages for different casting processes: Limitations on mechanical properties Poor dimensional accuracy and surface finish for some processes; e.g., sand casting Safety hazards to workers due to hot molten metals Environmental problems Parts Made by Casting Big parts: engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, and pump housings Small parts: dental crowns, jewelry, small statues, and frying pans MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 28 -
All varieties of metals can be cast, ferrous and nonferrous 1. Types of Casting Casting classification is related to: Mold material; Molding processes Methods of feeding the mold with molten metal The mold in casting Contains cavity whose geometry determines part shape Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling Made of a variety of materials, including sand, plaster, ceramic, and metal The mold can be in two forms as shown in Fig. 3.2 : open mold, simply a container in the shape of the desired part; and closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 29 -
Fig. 3.2: Forms of molds The two major categories are ; expandable and permanent mold casting. 3.1.1 Expandable molds Expandable molds are made of sand, plaster, ceramics and similar material. After the casting has solidifies, the molds in these molds are broken up to remove the casting. Advantage: more complex shapes possible Disadvantage: production rates often limited by time to make mold rather than casting itself 3.1.2 Permanent molds Permanent molds are made of metals that maintain their strength at high temperature. The casting can be easily removed and the mold used for the next casting. Advantage: higher production rates Disadvantage: geometries limited by need to open mold MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 30 -
The choice of casting types depends on several factors such as complexity, surface finish and cost. 2. Solidification of Metals Solidification is the transformation of molten metal back into solid state. Solidification differs depending on whether the metal is a pure element or an alloy. Solidification process is shown in Fig. 3.3. Fig. 3.3: Solidification process Solidification shrinkage A casting with a higher volume-to-surface area ratio cools and solidifies more slowly than one with a lower ratio. Since riser and casting mold constants will be equal, design the riser to have a larger volume-to-area ratio so that the main casting solidifies first. This minimizes the effects of shrinkage. Shrinkage effect is shown in Fig. 3.4. MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 31 -
Fig. 3.4: Shrinkage of casting product during solidification The shrinkage shape due to further reduction in height and diameter due to thermal contraction is shown in Fig. 3.5. Fig. 3.5: Cavity caused due to shrinkage effect 3. Sand Casting This process relies on the mold maket creating a cavity in a block of sand. This cavity must have the desired configuration so that when molten metal is poured into it and MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 32 -
allowed to solidify, the solid metal will take on the desired shape and have dimensions that match, or can be machined to match, a finished parts. Steps in Sand Casting 1. Pour molten metal into sand mold 2. Allow metal to solidify 3. Break up the mold to remove casting 4. Clean and inspect casting 5. Heat treatment of casting is sometimes required to improve metallurgical properties 3.3.1 Properties required in sand Permeability :النفاذیة posrosity of the sand enables the escape of gas and steam formed in the mold. Strength: sand must be cohesive to the extent that it has sufficient bond; both water and clay content affect the cohesive properties. Refractoriness: Sand must resist high temperatures without fusing. Grain size and shape: Sand must have a grain size commensurate with the surface to be produced, and grains must be irregular to the extent that they will have sufficient bonding strength. 3.3.2 Elements of Sand mould The elements of sand casting, shown in Fig. 3.6 can be summarized as follows:- MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 33 -
Fig. 3.6: Elements of sand mould Molten metal is poured into the mould cavity. Sand is packed into the casting boxes around the pattern. Pattern an exact copy of the required casting made from wood and sometimes from metal. It is removed to leave the mould cavity. Casting removed from sand after cooling. Cores الدلیك are placed in the cavity to form holes. Riser is an extra void created to contain excessive molten metal. The purpose of this is to feed the molten metal to the mold cavity as the molten metal solidifies and shrinks. The cavity in the sand is formed using a pattern. In the two-part mould, which is typically of sand casting, the upper half, flask is called cope and the lower half is called drag. Parting line is the surface that separates the mould parts. MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 34 -
3.3.2.1 Pattern The pattern is a full-sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting A. Types of patterns According to the type of pattern used, there are two different methods by which sand casting can be produced Removable : The mold is made in a flask that has two parts, or in a cheek that has three or more parts. The top part of the flask is called a cope while the lower part is called a drag. Fig. 3.7: Types of patterns used in sand casting: (a) solid pattern (b) split pattern (c) match-plate pattern (d) cope and drag pattern Disposable : In this case, the pattern is usually one piece including the gate. The pattern is placed on a follow board and the drag is molded in the conventional way. MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 35 -
B. Pattern allowance Three pattern allowances, shown in Fig. 3.8, should be considered during pattern making; shrinkage, draft and machining. Shrinkage : When any pure metal and most alloys cool, they shrink. To compensate for shrinkage a shrink rule must be used in laying out the measurements for the pattern. Draft : When a removable pattern is drawn from a mold, the tendency to tear away the edges of the mold in contact with the pattern is greatly decreased if the surfaces of the pattern, parallel to the direction it is being withdrawn, are slightly tapered. This tapering of the sides of the pattern, known as draft Machining (Finish): The finish of the cast surface is fairly bad. In most cases these surface have to be machined. The Machining allowance is added to compensate the loss of material as chip during machining process. Fig. 3.8: Pattern allowances MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 36 -
2.4.2.2 Core Core as a full-scale model of interior surfaces of part It is inserted into the mold cavity prior to pouring The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces May require supports to hold it in position in the mold cavity during pouring, called chaplets Fig. 3.9: Core Core can be place in different orientation as shown in Fig. 3.10. Horizontal Vertical Complex Fig. 3.10: Different orientation of core MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 37 -
4. Preparation of mould Manufacturing: Chapter 3 Casting The steps as shown in Fig. 3.11 include not only the casting operation but also pattern-making and mold-making Fig. 3.11: Mould preparation steps Steps required to cast the following sample product Pattern making Core Making MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 38 -
Pattern made with its three allowances; shrinkage, machining and draft. A core prints are added to hold the core in place. Core boxes produce core. The core will be used to produce the hollow area of the part shown. Molding Preparation 1 Drag preparation Description Half of the pattern have been mounted on plate inside the drag. The drag is filled with casting sand. 2- Drag inverted Description The drag is inverted. The half pattern is removed 3- cope preparation Description Drag is filled with sand. The full pattern is inserted The cope is ready for sand MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 39 -
. 4- hole; riser, sprue and vent holes; making Description Cope and drag are filled with sand. The pattern is ithdrawn. The riser, sprue and vent holes are made inside the sand in the cope 5. Cope and drag assembly Description Core is set in place. Cope and drag assembled ready for pouring. السباكة باسطمبات. Die Casting 5 Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity Die-casting, shown in Fig. 3.12, is employed where many items of the same form are to be manufactured. In this process, molten metal if forced into the cavity between dies under high pressure. After the metal has been injected, the pressures is held for a short time whilst the metal solidifies. The die blocks are then opened and the casting is injected. MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 40 -
Fig. 3.12: Die casting 3.4.1 Molds for Die Casting Usually made of tool steel, mold steel Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron Ejector pins required to remove part from die when it opens Lubricants must be sprayed into cavities to prevent sticking 3.4.2 Die Casting Steps Fig 3.13 shows the steps of die casting process. Fig. 3.13: Die casting steps MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 41 -
Advantages: Economical for large production quantities Good dimensional accuracy and surface finish Thin sections are possible Rapid cooling provides small grain size and good strength to casting Disadvantages: Generally limited to metals with low metal points Part geometry must allow removal from die cavity MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 42 -
Q1: Define the casting process? Review Questions Q2: Describe three properties required in casting sand? - - - Q3: State three differences between final product and the pattern - - - Q4: State three properties required in casting sand - - - Q5: State two disadvantages of casting process - - - Q7: Discuss the correctness of the following statements. ( State true of false, then describe the reasons in detail) 1 The riser has to be solidified before the product solidification. 2 Sand casting is required when good surface quality is required 3 Draft allowance is added to facilitate product extraction 4 In die casting, the metal fills the cavity under gravity force. 5 Core is used in die casting to form the internal cavities MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 43 -
Q8: Draw the sand mould? 9. Identify each part of the following sand mould 10. Identify the function of each of the following sand mould elements: Core Flask Drag Cope Riser Pouring basin vent cavity MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 44 -
11. Compare between sand and die mould Comparison Item Sand Die Mould material No. of product per mould Mould cost Product cost Product accuracy Product complexity Product volume Force required Q12: State the three pattern allowances 1-2- 3- Q13: Write the allowance name which match each of the following purposes No. Pattern Purpose allowance type 1 To facilitate pattern withdrawal 2 To compensate volume reduction due to shrinkage during solidification 3 To compensate volume loss due to chip formation MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 45 -
Q14: Discuss the correctness of the following statements. ( State true of false, then describe the reasons in detail) 1 The riser has to be solidified before the product solidification. 2 Sand casting is required when good surface quality is required 3 Draft allowance is added to facilitate product extraction 4 In die casting, the metal fills the cavity under gravity force. 5 Core is used in die casting to form the internal cavities 6 Core is used in die casting to form the internal cavities 7 Complex shapes could be produced by casting technique. 8 Core is positioned in the mould cavity during pouring of the melt 9 Core must be made from the same material of the cast. 10 Pattern may be made from one, or two, or three pieces 11 Pattern is positioned in the mould cavity during pouring of the melt 12 Pattern must be made from the same material of the cast MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 46 -
Q15: Explain the reasons of each of the following: Using pattern instead of final product in sand molding Adding draft in sand molding Using die mould to produce high no of products Q16: Identify the steps required to cast the following sample product (fill the missing fields) Molding Preparation MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 47 -
Description Description Description Description Description MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 48 -
Q17: Write the Suitable Word(s) in the shown spaces: 1. Sand casting mold is made from a mixture of,, and. 2. The process of cleaning the cast, and removing the unwanted projection is called 3. The process of extracting the cast from the mold is called 4. To produce a hollow cast, a must be positioned in the mold during pouring of the melt. 5. The rise of molten metal in the indicates that the mold is completely filled. 6. Riser is made to compensate for during solidification of the cast. Q18: Choose the best answer and make a circle around the number: 1) Comparing the size of the pattern and the cast, the pattern should be made---- a. Smaller than the cast b. Exactly equal to the cast. c. Bigger than the cast. 2) The last part of the mould filled with molten metal is called----------- b. Mold cavity. c. Riser. d. Pouring cup. 3) -------------------------is better for products made from cast iron. a. Machining technique b. Forming technique c. Casting technique d. Welding technique MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 49 -
Q19: Put a check mark ( ) against the correct answer, and cross mark ( ) against the wrong answer: 1 ( ) Complex shapes could be produced by casting technique. 2 ( ) Core is positioned in the mould cavity during pouring of the melt. 3 ( ) Pattern is positioned in the mould cavity during pouring of the melt. 4 ( ) Pattern is used to generate the mould cavity. 5 ( ) Core is used to generate the mould cavity. 6 ( ) Pattern is used to generate the cast cavity. 7 ( ) Core is used to generate the cast cavity. 8 ( ) Pattern must be made from the same material of the cast. 9 ( ) Core must be made from the same material of the cast. 10 ( ) Pattern may be made from one, or two, or three pieces. MDP024: Prepared by: Dr. Mohamed Ahmed Awad Page - 50 -