Manufacturing Technology Casting PDF

Manufacturing Technology Casting Classification of casting process Shell, investment, plaster, ceramic casting Reusable mold -...

Manufacturing Technology Casting Classification of casting process Shell, investment, plaster, ceramic casting Reusable mold - Gears, Pistons, Wheels  Manufacturing Technology Casting Pouring Metal casting Quickest Least expensive Near net shape product  Manufacturing Technology Metal Casting Process Casting is one of the oldest manufacturing process. It is the first step in making most of the products. It 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 to build part. Steps: - Making mould cavity - Material is first liquefied by properly heating it in a suitable furnace. - Liquid is poured into a prepared mould cavity - allowed to solidify - product is taken out of the mould cavity, trimmed and made to shape We should concentrate on the following for successful casting operation: (i) Preparation of moulds of patterns (ii) Melting and pouring of the liquefied metal (iii) Solidification and further cooling to room temperature (iv) Defects and inspection  Manufacturing Technology Casting Gating system: Assembly of sprues, runners, gates and individual casting cavities in the mold. Gate: An opening between the runner and part cavity that starts the molten metal into the cavity and feeds the casting as it solidifies. Riser: A reservoir built into a metal casting mold to prevent cavities due to shrinkage. Runner: directs the molten metal towards the individual part of mold cavity. Blind (closed/blink) Riser: A blind riser is a venting system used in casting to feed different parts of casting and improve casting yield. They are usually located in the lower sections of the casting or hidden beneath the top risers Core: A core is a sand or metal insert used to shape any part of a casting that cannot be shaped by the primary removable pattern.  Manufacturing Technology Important casting terms Flask: A metal or wood frame, without fixed top or bottom, in which the mould is formed. Depending upon the position of the flask in the moulding structure, it is referred to by various names such as drag – lower moulding flask, cope – upper moulding flask, cheek – intermediate moulding flask used in three piece moulding. Pattern: It is the replica of the final object to be made. The mould cavity is made with the help of pattern. Parting line: This is the dividing line between the two moulding flasks that makes up the mould. Moulding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions. Facing sand: The small amount of carbonaceous material (kerogen, and crystalline graphite) sprinkled on the inner surface of the mould cavity to give a better surface finish to the castings.  Manufacturing Technology Important casting terms Core: A separate part of the mould, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings. Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten metal is poured. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mould cavity. In many cases it controls the flow of metal into the mould. Runner: The channel through which the molten metal is carried from the sprue to the gate. Gate: A channel through which the molten metal enters the mould cavity. Chaplets: Chaplets are used to support the cores inside the mould cavity to take care of its own weight and overcome the metallostatic force (lift force to avoid swell). Riser: A column of molten metal placed in the mould to feed the castings as it shrinks and solidifies. Also known as “feed head”. Vent: Small opening in the mould to facilitate escape of air and gases. Dross: non-metal inlusion; accompanied by a gas bubble. (Turbulence can cause mold erosion)  Gating System In-gate - Ingate is the end of the path and where the mold cavity begins. It leads the liquid metal that flows from the runner into the mold cavity. Depending on the characteristics of the casting, there are different number of ingates. There are two types of gates: big gate and small gate. The small gate is used for slowing solid casting, while the big gate is for fasting solid casting. The gate should not have sharp edges as they can crack during pouring so that the sand can be caught in the molten metal into the mold cavity. Types of Gate in Casting Top gate: the gate is in the cope mold part. The disadvantages of top gate are high metal flow turbulence, poor casting surface. Bottom gate: the gate is in the drag mold part. In the bottom gate, liquid metal fills the lower part of the mold cavity and gradually increases into the mold wall. The bottom gate has the advantage of less chaos and sand erosion than the top gate. The downside of the bottom gate is that the metal flow can be clogged due to solidification before the mold is full. The temperature range generated by the bottom gate is difficult to reach the standard causing uneven solidification. Parting line side gate: is the gate located along the parting line. The compartment below the parting line is filled with liquid metal through top gating, while the compartment above the parting line is filled with the bottom gating. This gate type solves the disadvantages of the two types above. Splash Core: It prevents damage to the mold by the inflowing metal. Skim Bob: It helps in trapping impurities of eroded sand and thus prevents them from going into the mould cavity.  Gating System Spure-base well - The sprue well or base changes the direction of molten metal by right-angle and sends it to the runner. It is also known as a choke. It is a reservoir at the bottom of a sprue in a casting gating system that holds molten metal and controls the rate at which it flows. The runner takes the metal from the sprue to close to the casting. It is the part of a sprue that's rotated at a right angle to the runner to prevent the liquid metal from falling freely. A sprue is a vertical channel that's part of the gating system in gravity diecasting, and it's used to pour molten metal into a mold. The sprue is wide at the top, where it receives liquid metal from the pouring basin, and narrows as it goes down to the runners. The sprue's cross section can be circular, square, or rectangular, but circular is preferred.  Runner extension - The runner extension (blind end) has multiple functions and they are as follows: Runner extension should trap the slag, dross and dirt in the mould cavity. The other function of the runner extension is to absorb all kinetic energy built giving a smoother flow to the molten metal entering the mould cavity. The extension is usually 1–12 in (2.5–30.4 cm) long. When molten metal flows through the sprue to the runner, it hits the extension first, trapping slag that's lighter than the metal. The slag then can't flow into the mold cavity. With the Unpressurized Gating System, this system is called a “Choke control system” because the choke controls the flow of the metal.  (Outside air will rush inside to balance pressure) This system is called a “Gate control system” because ingates control the flow of the metal. Gating ratio for various materials Materials Gating ratio 1:2:1 1:1.2:2 1:2:4 Aluminum 1:3:3 1:4:4 1:6:6 Aluminum bronze 1:2.88:4.8 1:1:1 Brass 1:2:3 1.6:1.3:1 2:8:1 Copper 3:9:1 1.15:1.1:1 Ductile iron 1.25:1.13:1 1.33:2.67:1 Casting  Casting – Components Typical metal match plate pattern used in sand casting Taper used in patterns for ease of removal from the sand mold Casting Cores – made of sand aggregates  Manufacturing Technology Casting Cores – made of sand aggregates › Possess: Strength Permeability Ability to withstand heat Collapsibility › Anchored by core points › Chaplets are used to keep the core from moving *Permeability: ability for fluids (gas or liquid) or specific chemicals to flow through the material C > 2% C -.75% Cracks Casting Type of base sand Binders or Binding Agents Silica sand The binding agents are the glue that Olivine sand holds the sand particles together. The following are the most frequent types of Chromite sand binders: Zircon sand Clay and water Chamotte sand Oil Resin Sodium silicate Casting Sand making Plastic mixture of sand grains, clay (bentonite - soft plastic clay consisting predominantly of montmorillonite, a fine particle-sized hydrous aluminum silicate), water and other materials such as graphite powder: molding sand, Silica sand. Green sand: 70% sand + 30% clay + moisture Bentonite powder Sand muller few times depending on particular standard, like three times, etc. The quantity of heat necessary to change 1g of a solid to a liquid with no temperature change (latent heat/enthalpy of fusion of substance) Specific heat: The quantity of heat required to raise the temperature of 1gm of a substance by 1 degree celsius. Casting Sand casting steps  Manufacturing Technology Casting Steps for sand casting (Fundamentals) 1. Pattern making 2. Sand making 3. Mold making 4. Core making 5. Metal making Sand casting 6. Pouring 7. Solidification 8. Machining 9. Quality investigation Interestingly, every steps boils down to solidification…..and everything is intimately related…  Manufacturing Technology Casting Complex, 3-D shapes Near net shape Low scrap Relatively quick process Near Net Shape Intricate shapes Large hollow shapes No limit to size Reasonable to good surface finish Near Net Shape  Manufacturing Technology Casting Engine manifold Turbine blade Dhokra art form Casting - Cooling curve for pure metal Isotherm: same temperature at all point Recalescence: increase in temperature occurs while cooling metal when change in structure with increase in thermal energy/entropy occurs.  Casting Microstructure Columnar grains are long, thin, Equiaxed crystals are coarse grains created when a crystals that have axes of metal solidifies approximately the same length in all direction. Casting Dendrite microstructure It is a tree-like structure of crystals growing as molten metal solidifies. The process is too quick to form dendrites. Rapid dendrite growth is realized by the rapid movement of the liquid/solid interface toward the undercooled melt.  Casting Gibbs free energy: is a quantity used to measure the maximum amount of work done in a thermodynamic system when the Undercooling/Sup temperature and ercooling: When a pressure are constant. substance is G = H – TS (kilojoules per cooled below its mole) freezing point Enthalpy, H (sum of without it internal energy + Pressure becoming a solid. x Volume) of the system – Temperature x times the entropy (randomness) H = E + PV If G ~ Negative = Spontaneous Reaction If G ~ Positive = Not Spontaneous Reaction  Casting - Solidification The probability of nucleation occurring at point in the parent phase is same throughout the parent phase In heterogeneous nucleation there are some preferred sites in the parent phase where nucleation can occur *Nucleation: phase change in metal Casting Direction solidification In order to avoid shrinkage problem Furthest section solidify first and riser solidifies last Directional solidification aspect of freezing and methods by which it is controlled……..(chills are used to incorporate directional solidification) Chills – internal or external heat sinks that cause rapid freezing in certain regions of the casting (thin section where V/A ratio is highest)  The solidus is the highest temperature at which an alloy is solid. Freezing range The liquidus is the (Slurry State) temperature at which an alloy is completely melted.  Phase change and shrinkage Shrinkage cavity: This defect 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 “pipe”. The problem can often be solved by proper riser design.  Manufacturing Technology Casting Shrinkage Graphitization is a process that transforms carbonaceous materials into graphite through heat treatment. The process can involve heating materials to temperatures of over 2,500°C in a controlled environment. It converts the graphene layers into three-dimensionally ordered crystalline graphite. Vacuum molding, is a manufacturing process that uses vacuum pressure to create plastic parts by stretching a heated sheet of plastic over a mold. The process is often used to make lightweight, three-dimensional objects like trays, packaging, and automotive parts. Clamp: A plastic sheet is placed in an open frame and clamped in place. Heat: The plastic is heated until it becomes soft and pliable. Vacuum: The frame with the plastic is lowered over a mold and a vacuum on the other side pulls the plastic into the mold. The vacuum holds the plastic against the mold until it takes the desired shape. Eject: The plastic is then ejected from the mold. Vacuum molding  Other Casting Process Investment casting Investment casting is a metalworking process that uses a ceramic shell built over a wax pattern to produce parts with extraordinarily uniform and smooth surfaces. The wax pattern is produced from an aluminum die. The final products of investment casting have very small parting lines or mold half-marks that aggregates, a binder, and water are removed as part of the after- casting processes. The final parts require a certain amount of secondary finishing to ensure their smoothness and uniformity. Autoclave machine  Other Casting Process Permanent mold casting 1. Mold is preheated and coated 2. Cores (if used) are inserted and mold is closed 3. Molten metal is poured into the mold, where it solidifies 4. Mold is opened 5. Finished part received  Other Casting Process Die casting Die casting is a widely used permanent mould metal casting process, in which molten metal is forced into the mould called Dies. Pressures ranging from 0.7 to 700 MPa, where it solidifies into a metal cast. Die castings, sometimes known as pressure die casting, are used in automotive housings, appliance components, and toys. Impact (injection) molding  Casting Defects in casting (large cavity or hole) (Erosion of mold parts by liquid metal) (Shallow blow due to vent issue) (irregular line or crack) (contaminants) (Surface blowholes) (Low strength mold, push away the walls) (cracking on sand (unable to flow due to low temperature) upper surface) (molten material begun to solidify (Slag inclusion) before fully filling the mold) Due to residual stresses, material fail as it cools (long, broad shallow depression) Mismatching of the top and bottom parts of a casting *Scab: Rough, thin layer of a metal, protruding above the casting surface, on top of a thin layer of sand. Defects in casting  Manufacturing Technology Casting Metals processed by casting Sand casting – 60% Permanent mold casting – 11% Die casting – 9% Investment casting – 7% Centrifugal casting – 7% Shell mold casting – 6%  Manufacturing Processes References… 1. A. Ghosh, and A. K. Mallik, Manufacturing Science, Affiliated East-West Press. 2. P.N. Rao, Manufacturing Technology-Metal Cutting and Machining, Tata McGraw Hill, Publishing Company. 3. S. Kalpakijain, Manufacturing Engineering and Technology, Pearson Education India Edition, 2013.