Manufacturing Processes Casting (주조) Associate Professor Su-Jin Kim School of Mechanical Engineering Gyeongsang National University 1

Outline Casting Physics Sand Casting Investment Casting Die Casting Cost/Quality

Casting Casting(주조) is a manufacturing process by which a liquid material(용융금속) is poured into a mold(주형), which contains a hollow cavity of the desired shape, and then allowed to solidify. First casting: 4000 B.C. Bronze, iron age, light metal age? http://www.youtube.com/watch?v=yXVLbzI3xTE Engine block 5min

Casting Casting is Versatile Many types of metals Wide range of shapes and sizes Complex parts as an integral unit Disadvantage Poor dimensional accuracy and surface quality http://www.youtube.com/watch?v=mx1qteRUYwI Sand, Investment, Lost form SME 6:16

Cast structure (주조조직) of alloying Solidification begins when temperature drops below the liquidus(액상점), TL, and complete when solidus(고상점), TS, called freezing range(응고범위) For normal cooling cored dendrites(유심구조 수지성장) are formed. Surface has a higher concentration of alloying elements. Solidification begins when temperature drops below the liquidus(액상점), TL, and complete when solidus(고상점), TS. Freezing range(응고범위) Columnar dentrite(주상 수지상정) For high cooling rates, structure becomes finer with smaller dendrite arm spacing. For higher cooling rates structures developed are amorphous. The developed structures and grain size influence the properties of the casting. When alloy cooled slowly dendrite develops a uniform composition. For normal cooling cored dendrites(유심구조 수지성장) are formed, which have a surface composition that is different from that a their centers. Surface has a higher concentration of alloying elements. Segregation(편석)

Fluid flow (유동) of mold filling Gravity casting: pouring cup→sprue →well →runner →riser → cavity →open riser Bernoulli’s equation Velocity of the molten metal leaving the gate Core (sand) Vent Pouring basin (cup) Open riser Cope Drag Mold cavity Runner Well Sand Gate Parting line Sprue Flask Blind riser h = elevation p = pressure at elevation v = velocity of the liquid ρ = density of the fluid ©정하림GNU

Fluid flow of mold filling Reynolds number, Re Ordinary gating system: Re = 2,000 ~ 20,000 mixture of laminar and turbulent flow v = velocity of the liquid D = diameter of the channel ρ = density n = viscosity of the liquid.

Heat transfer (열전달) Heat flow (열유동) depends on casting material and the mold and process parameters. Temperature distribution (온도분포) in the mold-liquid metal interface is shown below. Distance Temperature Room temperature Melting point Air Mold Solid Liquid

Solidification time Solidification time (응고시간) is a function of the volume of a casting and surface area (Chvorinov’s rule). Ex) Sand casting C = constant n = 2 if Sand casting n = 1 if Die casting 4 2 .5 2 2 4

Shrinkage Shrinkage (수축) in casting causes dimensional changes and sometimes cracking. Thermal expansion Metals 주물재료 Contraction 수축률(%) Aluminum 7.1 Zinc 6.5 White iron 4.0-5.5 Copper 4.9 Brass(70-30) 4.5 Magnesium 4.2

Expendable Mold Casting Process (소모성 주형 주조 공정) Sand casting Shell mold casting Plaster mold casting Investment casting (lost wax casting) Lost foam casting

Sand casting (사형주조) The sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern. The sand mold separates along a parting line and the solidified casting can be removed. The steps in this process are described in greater detail in the next section.

Sand casting Use expendable sand molds Low production rate(1–20 pieces/hour-mold) Low cost, Recycle sand Small quantity production Complex entire parts at one time Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases. http://www.youtube.com/watch?v=rgL2Jn5mk1A Manual http://ma.gnu.ac.kr/vod/casting/casting_tap.wmv Tap

Shell mold casting (셸주조) Fine sand mixed with a resin is heated by the pattern and hardened into a shell around the pattern Can produce castings with close dimensional tolerances, good surface finish and low cost. Shell molding is similar to sand casting, but the molding cavity is formed by a hardened "shell" of sand instead of a flask filled with sand. The sand used is finer than sand casting sand and is mixed with a resin so that it can be heated by the pattern and hardened into a shell around the pattern. Because of the resin and finer sand, it gives a much finer surface finish. The process is easily automated and more precise than sand casting. Common metals that are cast include cast iron, aluminium, magnesium, and copper alloys. This process is ideal for complex items that are small to medium sized.

Plaster mold casting (석고주형 주조) Plaster is mixed with water and poured over the pattern. Accurate, good surface and low melting point alloy Plaster-mold, ceramic-mold and investment casting is also known as precision casting. http://www.youtube.com/watch?v=xPSI1wUOBnk&p=9B6D9EAE75875D9D

Investment casting (인베스트먼트 주조, lost wax casting) A wax pattern is coated by ceramic slurry that hardens into the mold. One pattern creates one part, which increases production time and labor costs. Complex geometries, good surface and high melting point alloy. Investment casting is one of the oldest manufacturing processes, dating back thousands of years, in which molten metal is poured into an expendable ceramic mold. The mold is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mold. Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mold after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be created. Investment casting can make use of most metals, most commonly using aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries. SME: http://www.youtube.com/watch?v=PIneaDTZ16g

Investment casting (lost wax casting) Wax pattern  Mold  Pouring Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding. The process steps include the following: Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity. Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting. Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used. Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. The ceramic mold is typically broken using water jets, but several other methods exist. Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen). Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part. Coin: http://www.youtube.com/watch?v=4Yvm6fUXf1U Aerospace: http://www.youtube.com/watch?v=f7FXtnXVqzY

Lost foam casting (소실모형주조) Polystyrene patterns evaporate with molten metal to form a cavity for the casting. Molding (금형) Patterns Coating ©samhwacast.com First, a pattern is made from polystyrene foam, which can be done many different ways. For small volume runs the pattern can be hand cut or machined from a solid block of foam; if the geometry is simple enough it can even be cut using a hot-wire foam cutter. If the volume is large, then the pattern can be mass-produced by a process similar to injection molding. Pre-expanded beads of polystyrene are injected into a preheated aluminum mold at low pressure. Steam is then applied to the polystyrene which causes it to expand more to fill the die. The final pattern is approximately 97.5% air and 2.5% polystyrene. Pre-made pouring basins, runners, and risers can be hot glued to the pattern to finish it.[1] Next, the foam cluster is coated with ceramic investment, also known as the refractory coating, via dipping, brushing, spraying or flow coating. This coating creates a barrier between the smooth foam surface and the coarse sand surface. Secondly it controls permeability, which allows the gas created by the vaporized foam pattern to escape through the coating and into the sand. Controlling permeability is a crucial step to avoid sand erosion. Finally, it forms a barrier so that molten metal does not penetrate or cause sand erosion during pouring. After the coating dries, the cluster is placed into a flask and backed up with un-bonded sand. The sand is then compacted using a vibration table. Once compacted, the mold is ready to be poured.[1][2] Automatic pouring is commonly used in LFC, as the pouring process is significantly more critical than in conventional foundry practice.[citation needed] Compact in sand Casting Shakeout Manhole cover

Non-expendable Mold Casting Process (영구주형 주조) Die casting Centrifugal casting Continuous casting

Die casting (다이캐스팅) The molten metal is forced into the die cavity by high pressure. Ejector platen (moves) Ejector die half box Ladle Stationary Shot sleeve Plunger rod Metal sleeve Cover disk Closing cylinder Pouring hole Plunger rod Shot sleeve Die casting is a manufacturing process that can produce geometrically complex metal parts through the use of reusable molds, called dies. The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal, typically a non-ferrous alloy such as aluminum or zinc, is melted in the furnace and then injected into the dies in the die casting machine. There are two main types of die casting machines - hot chamber machines (used for alloys with low melting temperatures, such as zinc) and cold chamber machines (used for alloys with high melting temperatures, such as aluminum). The differences between these machines will be detailed in the sections on equipment and tooling. However, in both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting. The steps in this process are described in greater detail in the next section. ©갈창우 http://www.youtube.com/watch?v=1AgDGLNE6Es SME 90s

Die casting Can be automated for large production runs. Strength-to-weight ratio of die-cast parts increases with decreasing wall thickness. Good surface finish and dimensional accuracy. It is used for aluminium, magnesium and copper alloys. The castings that are created in this process can vary greatly in size and weight, ranging from a couple ounces to 100 pounds. One common application of die cast parts are housings - thin-walled enclosures, often requiring many ribs and bosses on the interior. Metal housings for a variety of appliances and equipment are often die cast. Several automobile components are also manufactured using die casting, including pistons, cylinder heads, and engine blocks. Other common die cast parts include propellers, gears, bushings, pumps, and valves.

Centrifugal casting (원심주조) Molten metal is solidify in rotating mold. It uses centrifugal force to form cylindrical parts. Centrifugal force = mv2/r Gravitational force = mg Centrifugal casting, sometimes called rotocasting, is a metal casting process that uses centrifugal force to form cylindrical parts. This differs from most metal casting processes, which use gravity or pressure to fill the mold. In centrifugal casting, a permanent mold made from steel, cast iron, or graphite is typically used. However, the use of expendable sand molds is also possible. The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available) and includes the following steps: Mold preparation - The walls of a cylindrical mold are first coated with a refractory ceramic coating, which involves a few steps (application, rotation, drying, and baking). Once prepared and secured, the mold is rotated about its axis at high speeds (300-3000 RPM), typically around 1000 RPM. Pouring - Molten metal is poured directly into the rotating mold, without the use of runners or a gating system. The centrifugal force drives the material towards the mold walls as the mold fills. Cooling - With all of the molten metal in the mold, the mold remains spinning as the metal cools. Cooling begins quickly at the mold walls and proceeds inwards. Casting removal - After the casting has cooled and solidified, the rotation is stopped and the casting can be removed. Finishing - While the centrifugal force drives the dense metal to the mold walls, any less dense impurities or bubbles flow to the inner surface of the casting. As a result, secondary processes such as machining, grinding, or sand-blasting, are required to clean and smooth the inner diameter of the part. Centrifugal casting is used to produce axi-symmetric parts, such as cylinders or disks, which are typically hollow. Due to the high centrifugal forces, these parts have a very fine grain on the outer surface and possess mechanical properties approximately 30% greater than parts formed with static casting methods. These parts may be cast from ferrous metals such as low alloy steel, stainless steel, and iron, or from non-ferrous alloys such as aluminum, bronze, copper, magnesium, and nickel. Centrifugal casting is performed in wide variety of industries, including aerospace, industrial, marine, and power transmission. Typical parts include bearings, bushings, coils, cylinder liners, nozzles, pipes/tubes, pressure vessels, pulleys, rings, and wheels.

Continuous casting (연속주조) Developed for casting metal slab or strip continuously at low cost. Continuous casting Strip casting hot rolling strip Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the finishing mills. It allows lower-cost production of metal sections with better quality, due to the inherently lower costs of continuous, standardised production of a product, as well as providing increased control over the process through automation. This process is used most frequently to cast steel. Aluminium and copper are also continuously cast. Continuous casting. 1: Ladle. 2: Stopper. 3: Mold. 4: Roll support, Turning zone. 5: Slab. slab http://www.posco.com/homepage/docs/eng/html/company/product/s91c5010010c.html

Casting process comparison (비교) Advantages Disadvantages  Sand Casting Almost any metal is cast; Small to large parts; Low tooling cost Low dimensional accuracy & surface quality Plaster Cast Good dimensional accuracy and surface finish ; Fine details including thinner walls; Large parts cost less to cast than by Investment process More costly than Sand or Permanent Mold-Casting Limited to nonferrous metals Investment Cast Good dimensional accuracy and surface finish; Complex shape, fine detail; Ferrous and non-ferrous metals Costs are higher than Sand, Permanent Mold or Plaster process Castings Die Casting Excellent dimensional tolerances and surface finish; Parts require a minimal post machining; High production rate High die cost; Part size limited; Limited to nonferrous metals

Cost of casting (비용) Process Tooling & Equipment Direct labor Material utilization Sand Casting Low High Investment Cast Moderate & Low Die Casting Sand casting Tooling and equipment costs are low Direct labor costs are high Material utilization is low Finishing costs can be high Investment casting Tooling costs are moderate depending on the complexity Equipment costs are low Material costs are low Die casting Tooling and equipment costs are high Direct labor costs are low to moderate Material utilization is high Sand casting: Finishing costs can be high

Quality of Casting (품질) Process Tolerance (mm) Mechanical Property Surface finish Sand Casting 0.7~2.0 Material property is poor Generally have a rough grainy surface Investment Cast 0.08~0.2 Mechanical property and microstructure depends on the method Good to excellent surface detail possible due to fine slurry Die Casting 0.02~0.06 Good mechanical property and microstructure due to high pressure Excellent surface detail

Rate of Casting (생산성) Process Development time (weeks) Production rate Sand Casting 2~10 OO min, t~(V/A)2 Investment Cast 5~16 Die Casting 12~20 OO s, t~(V/A)1 Development time is also depending on the size and complexity. Production rate is depending on the cooling time.

Economics of Casting (경재성) Cost of equipment per casting decreases as number of parts increases. 100 101 102 103 104 105 106 Number of pieces Cost per pieces 1 2 3 4 5 6 7 8 9 Sand casting Investment Die casting

Design for Casting (설계) Poor Good Use radii or fillets to avoid corners and provide uniform cross-section Wall sections should be uniform Sloping bosses can be designed for straight parting to simplify die design Side cores can be eliminated with this hole design Ribs and/or fillets improve bosses Core in Cover half Ejector half Deep cavities should be on one side of the casting where possible Corner (X) Uniform section Draft direction ©이성규

Defects (주조결함) Slag inclusion Sand inclusion Surface blow holes Surface pinholes Crack

References POSCO: http://www.posco.com http://www.dhcasting.co.kr http://www.edaehan.co.kr http://www.taechangpi.co.kr http://www.donglimms.co.kr http://www.yeonghwa.co.kr http://www.himet.co.kr Casting: http://www.custompartnet.com