![EME3016
WORKSHOP TECHNOLOGY
TRIMESTER2 (2013/2014)
CHAPTER 2: Casting
[TEXTBOOK CHAPTER 10 & 11]
1](https://image.slidesharecdn.com/c2casting-150507160759-lva1-app6892/85/C2-casting-1-320.jpg)
C2 casting
- 1. EME3016 WORKSHOP TECHNOLOGY TRIMESTER2 (2013/2014) CHAPTER 2: Casting [TEXTBOOK CHAPTER 10 & 11] 1
- 2. Learning Outcome of Subject • LO3 - Explain the principle of hot & cold working processes and apply to the forming processes and casting. 2
- 3. OFFICIAL SYLLABUS: Casting • Introduction to casting processes. Sand casting: castability, quality of sand for casting. Casting process. Other processes: permanent mould die casting, precision investment casting, shell and plaster moulds. 3
- 4. OVERVIEW A. FUNDAMENTALS OF METAL CASTING B. METAL-CASTING PROCESSES AND EQUIPMENT 4
- 5. INTRODUCTION • Casting was first used about 6000 years ago, to make ornaments and copper arrowheads. • Casting basically involves pouring molten metal into a mold cavity where (upon solidification) it takes the shape of the cavity. The part is then removed from the mold. • A wide variety of products can be cast, including one piece complex shapes with internal cavities such as engine blocks. • Casting processes are most often selected over other manufacturing methods, for the following reasons: – Casting can produce complex shapes and with internal cavities or hollow sections. – Very large parts can be produced in one piece. – Casting can utilize materials that are difficult or uneconomical to process by other means. – The casting process is competitive with other manufacturing processes. 5
- 6. Examples of cast parts. (a) A die-cast aluminum transmission housing. (b) A tree of rings produced through investment casting. Source: (a) Courtesy of North American Die Casting Association, (b) Courtesy of Romanoff, Inc. 6
- 7. Cast parts in a typical automobile. 7
- 8. Outline of metal-casting processes 8
- 9. A. FUNDAMENTALS OF METAL CASTING. 9
- 10. • Important considerations in casting operations are as follows: – Flow of the molten metal into the mold cavity. – Solidification and cooling of the metal in the mold. – Influence of the type of mold material. FLUID FLOW • Fluid flow is important to achieve good casting. • The solidification process is affected by the fluid flow. • The design of a proper gating system will affect: – Contaminants to be trapped (oxides) and removed from the molten metal when they adhere to the walls of the gating system before reaching the mold cavities. – Avoidance or mimimize premature cooling, turbulence or gas entrapments in the system. 10
- 11. Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification. 11
- 12. FLUIDITY OF MOLTEN METAL • The capability of the molten metal to fill mold cavities is called fluidity. The following characteristics of molten metal influence fluidity: – Viscosity – fluidity decreases when viscosity increases. – Surface tension – fluidity will be reduced when the liquid metal has a high surface tension. – Inclusions – insoluble particles will lower the fluidity of the liquid since the viscosity would have increased. – Solidification pattern of the alloy – fluidity is inversely proportional to the freezing range. The shorter the range (pure metal), the higher the fluidity. 12
- 13. HEAT TRANSFER • The heat transfer during the complete cycle (from pouring, to solidification, and to cooling to room temperature) is an important consideration in metal casting. • Heat flow at different locations depends on many factors such as cast material, mold material and process parameters. • For instance, in casting thin sections, the metal flows rates must be high enough to avoid premature chilling and solidification. • On the other hand, the flow rate must not be so high as to cause excessive turbulence 13
- 14. Aluminum piston for an internal combustion engine: (a) as cast and (b) after machining. The part on the left is as cast, including risers, sprue, and well, as well as a machining allowance; the part on the right is the piston after machining. Source: After S. Paolucci. 14
- 15. B. METAL-CASTING PROCESSES AND EQUIPMENT 15
- 16. • Various casting processes have been developed over time, each with its own characteristics and applications, to meet specific engineering and service requirements. • Two trends have had a major impact on the casting industry. – The first is the mechanization and automation of the casting process, which has led to significant changes in the use of equipment and labor. Advanced machinery and automated process-control systems have replaced traditional methods of casting. – The second major trend has been the increasing demand for high-quality castings with close dimensional tolerances. 16
- 17. • The 3 major classifications are: – Expendable molds • Typically are made of sand, plaster or ceramics and generally mixed with various binders (bonding agents) for improved properties. • Typical sand mold contains sand, clay and water. These materials are refractories (capable of withstanding the high temperatures of molten metal). After the casting is solidified, the mold is broken up to remove the casting. – Permanent molds • Made up of metals that maintain their high strength at high temperatures. • Used repeatedly. • Designed that castings can be removed easily and the mold used again. • Metal molds are better heat conductors than expendable nonmetallic molds. The solidifying process is at a higher rate of cooling which then affects the microstructure and grain structure of the casting. 17
- 18. – Composite molds • Made of two or more different materials (such as sand, graphite and metal) which combines the advantage of each material. Have permanent and expendable portion. • Used in processes to improve mold strength, control cooling rates and optimize overall economics of the casting process. 18
- 19. (i) EXPENDABLE-MOLD CASTING PROCESSES (a) Permanent -pattern SAND CASTING • Traditional method of casting. • Still the most prevalent form of casting. • Typical applications include machine bases, propellers, plumbing fixtures and industry equipment. • The basic processes of sand casting are: – Placing a pattern (having the shaped of the desired casting) in sand to make an imprint. – Incorporating a gating system. – Removing the pattern and filling the mold cavity with molten metal. – Allowing the metal to cool until it solidifies – Breaking away the sand mold. – Removing the casting. 19
- 20. • SANDS – Most sand casting operations use silica sand (SiO2) as mold material. – Sand in inexpensive and suitable for mold materials because it has high-temperature characteristics and high melting point. – For proper functioning, sand must be clean and preferably new. – There are 2 general types of sand: • naturally bonded (bank sand) • synthetic (lake sand) – preferred for most foundries since its composition can be controlled more accurately. – Fine and round sand grains can be packed closely and form smooth mold surface. Although this enhances mold strength, the fine-grained sand lower the mold permeability (penetration through pores). Good permeability of molds allows gases and steam to escape easily. 20
- 21. • TYPES OF SAND MOLDS – Sand molds are categorized by the types of sand and by the methods used to produce them: • Green-sand mold process – Most common mold material which is a mixture of sand, clay and water. – The term ‘green’ refers to the fact that the sand in the mold is moist or damp while the material is being poured into it. – Least expensive method of making mold. – Sand is recycled easily for subsequent use. • Cold-box mold process – Various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. – More accurate dimensionally but more expensive than green-sand molds. • No-bake mold process – Synthetic liquid resin is mixed with the sand, and the mixture hardens at room temperature. 21
- 22. – MAJOR FEATURES OF THE SAND MOLD are: • Flasks – supports the mold itself. Two-piece molds consist of a cope on the top and a drag on the bottom. The parting line is the seam between these 2 pieces. When more than two pieces are used in a sand mold, the additional parts are called cheeks. • Pouring basin (cup) – into which molten metal is poured. • Sprue – through which molten metal flows downwards. • Runner system – which has channels that carry the molten metal from the sprue to the mold cavity. • Gates – inlets into the mold cavity. • Risers – which supply additional molten metal to the casting as it shrinks during solidification. There are two types of risers: blind riser and open riser. 22
- 23. • Cores – which are inserts made from sand. There are placed in the mold to form hollow regions or otherwise define the interior surface of the casting. Cores are also used on the outside of the casting to form features such as lettering on the surface of a casting or deep external pockets. • Vents – which are placed in molds to carry off gases produced when then molten metal comes into contact with the sand in the mold and the core. Vents also exhaust air from the mold cavity as the molten metal flows into the sand. 23
- 24. Schematic illustration of a sand mold, showing various features. 24
- 25. • PATTERNS – Patterns are used to mold the sand mixtures into the shape of the casting and may be wood, plastic or metal. – Coated with parting agents to facilitate the removal of the casting from the molds. – Patterns can be designed for specific applications: • One-piece patterns – generally used for simpler shapes and low-quantity production. • Split patterns – 2-piece patterns that individually form each portion of the cavity for the complex-shaped casting. • Match-plate patterns – 2-piece patterns that are mounted on either side of a single plate. Gating system is mounted on the drag side. It is used with molding machines and large production runs to produce smaller castings. – The development of rapid prototyping applications and equipment have enabled faster (thus cheaper) patterns can be fabricated for castings. 25
- 26. A typical metal match-plate pattern used in sand casting. 26
- 27. • CORES – Cores are utilized for castings with internal cavities or passages. – Placed in the mold cavity to form the interior surfaces of the casting and removed from the finished part during shakeout. – Made out of sand aggregates that have strength, permeability, ability to withstand heat and collapsibility. – Core prints and Chaplets can be used to support and anchor the core in place in the mold cavity. 27 Examples of sand cores, showing core prints and chaplets to support the cores.
- 28. Schematic illustration of the sequence of operations for sand casting. (a) A mechanical drawing of the part is used to generate a design for the pattern. Considerations such as part shrinkage and draft must be built into the drawing. (b–c) Patterns have been mounted on plates equipped with pins for alignment. Note the presence of core prints designed to hold the core in place. (d–e) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins and attaching inserts to form the sprue and risers. (g) The flask is rammed with sand, and the plate and inserts are removed. (h) The drag half is produced in a similar manner with the pattern inserted. A bottom board is placed below the drag and aligned with pins. 28
- 29. (i) The pattern, flask, and bottom board are inverted, and the pattern is withdrawn, leaving the appropriate imprint. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and securing the assembly with pins. The flasks are then subjected to pressure to counteract buoyant forces in the liquid, which might lift the cope. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled, and the casting is cleaned, inspected, and heat treated (when necessary). Source: Courtesy of Steel Founders’ Society of America 29
- 30. 30
- 31. Outline of production steps in a typical sand-casting operation. 31
- 32. SHELL MOLD • Produce castings with close dimensional tolerances and good surface finish at low costs; such as high precision molding cores, gear housings, cylinder heads and connecting rods. • A mounted pattern (ferrous metal or aluminum) is heated (range 175 C to 370 C) and coated with a parting agent (silicone). • It is then clamped to a box and chamber which contains fine sand, mixed with 2.5 to 4% of a thermosetting resin binder that coats the sand particles. • The box is then rotated upside down allowing the sand mixture to coat the pattern. The assembly is then placed in an oven to complete the curing of the resin. Two half-shells are then bonded or clamped to make a mold. 32
- 33. The shell-molding process, also called the dump-box technique. 33
- 34. PLASTER MOLD • This is a precision casting process due to its high dimensional accuracy and good surface finish. Typical parts made are valves, fittings and tooling. Castings usually weigh less than 10kg. • The mold is made of plaster of paris with the addition of talc and silica flour to improve strength and to control the plaster setting duration. Water is mixed with the mold materials mentioned and the slurry produced is poured over the pattern. • After the plaster sets, the mold is dried at higher required temperatures to remove moisture. The mold halves are then assembled to form the mold cavity. 34
- 35. (b) Expendable-pattern EVAPORATIVE-PATTERN CASTING (LOST-FOAM PROCESS) • It is unique in that a mold and pattern must be produced for every casting. Earlier casting methods utilized reusable patterns. • Typical applications are cylinder heads, engine blocks and brake components. • This lost-foam casting process uses a polystyrene pattern which evaporates upon contact with molten metal to form a cavity for the casting. This process has become important for casting ferrous and nonferrous metals in the automotive industry. • The polystyrene pattern is made in die cavities and after post- preparation, is coated with slurry and placed in the flask. Sand is then filled into the flask and compacted. • When molten metal is poured into the mold, it would evaporate the pattern and fill the mold cavity. 35
- 36. • Advantages over other casting methods: – Relatively simple process – no parting lines, risers or cores are required. – Inexpensive flasks – Polystyrene is inexpensive and easily processed to produce complex pattern shapes. – Casting requires minimum finishing and cleaning operation. 36
- 37. Schematic illustration of the expendable-pattern casting process, also known as lost- foam or evaporative-pattern casting. 37
- 38. INVESTMENT CASTING • The pattern is made of wax, plastic (polystyrene) or rapid prototyping techniques. It is then coated with refractory material such as fine silica and binders. • The term ‘investment’ derives from the fact that the pattern is invested (surrounded) with the refractory material. • Unlike plastic patterns, wax can be recovered and reused. 38
- 39. Schematic illustration of the investment-casting (lost-wax) process. Castings produced by this method can be made with very fine detail and from a variety of metals. Source: Courtesy of Steel Founders’Society of America. 39
- 40. (ii) PERMANENT -MOLD CASTING PROCESSES PERMANENT-MOLD CASTING • In hard-mold casting, two halves of a mold are made from materials with high resistance to erosion and thermal fatigue, such as cast iron, steel, bronze, graphite or refractory metal alloys. • The mold cavity and gating system is machined into the mold and is an integral part of it. • To produce castings with internal cavities, cores made of metal are placed in the mold prior to casting. • The molds are clamped together and preheated before molten metal is poured through the gating system. After solidification, molds are opened and casting is removed. • To increase the life of permanent molds, the mold cavity surfaces are usually coated with a refractory slurry or sprayed with graphite every few castings. These also serve as parting agents and thermal barriers. • Mechanical ejectors (pins) can be used for removal of complex castings. vii. This process is used mostly for aluminum, magnesium and gray iron which have generally lower melting points. 40
- 41. VACUUM CASTING • This is an alternative to investment, shell-mold and green-sand mold casting and is suitable for thin-walled (0.75mm) complex shapes with uniform properties. • The vacuum system will draw the molten metal into the mold cavities through a gate in the bottom of the mold. 41 Schematic illustration of the vacuum-casting process. Note that the mold has a bottom gate. (a) Before and (b) after immersion of the mold into the molten metal. Source: After R. Blackburn.
- 42. DIE CASTING • In this process, molten metal is forced into the die cavity . • Although equipment costs are high but automation has made this process economical. 42 Various types of cavities in a die-casting die. Source: Courtesy of American Die Casting Institute.
- 43. SQUEEZE CASTING • This is a combination of casting and forging processes. • The process involves the solidification of molten metal under high pressure. 43 Sequence of operations in the squeeze-casting process. This process combines the advantages of casting and forging.
- 44. Summary of Casting Processes 44
- 45. General Characteristics of Casting Processes 45