Expendable mould processes
- 1. Other Expendable Mold Processes • Shell Molding • Vacuum Molding • Investment Casting • Plaster Mold and Ceramic Mold Casting 1
- 2. Shell Molding Casting process in which the cavity (& gating system) is a thin shell of sand held together by thermosetting resin binder Steps in shell-molding: (1) a match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin. part 2
- 3. • Preparation of the metal match plate cope and drag type patterns : Cu, CI, Al or steel • Mix the investment material : dry, fine and thorughly washed silica & 5 to 8% thermosetting resin Phenolic resin ( phenol formaldehyde) acting as binder • Heat the pattern : 200 to 3000 C • Invest the pattern : dump box 30 to 45 s get 5 to 8 mm thick shell • Curing the shell 1 to 3 min at 250 to 4500 C • Remove the shell • Assemble the shells • Pour the mould • Remove the casting
- 4. Steps in shell-molding: (2) box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; (3) box is repositioned so that loose uncured particles drop away; 4
- 5. Steps in shell-molding: (4) sand shell is heated in oven for several minutes to complete curing; (5) shell mold is stripped from the pattern; 5
- 6. Steps in shell-molding: (6) two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished; (7) the finished casting with sprue removed. 6
- 7. Advantages and Disadvantages • Advantages of shell molding – High precision and accuracy of casting with tolerance of ± 0.002 to ± 0.005 mm/mm is possible – Smoother cavity surface permits easier flow of molten metal, better surface finish and reduced machining allowance possible. The roughness is in the range of 3.2 μ m Ra -Good dimensional accuracy - machining often not required – Complex parts can be made even of difficult to machine metals and alloys – Mould can be stored until required. – Permeability of thin shell moulds is higher compared with other types of moulds and thin shell does not have great chilling effect as a sand mould resulting better quality of casting 7
- 8. – Less sand is used compared to sand casting – Cleaning is considerable reduced and in some cases eliminated – Mold collapsibility minimizes cracks in casting – Can be mechanized for mass production – Saving of metal through use of smaller gates, sprues and risers resulting in increased yield. • Disadvantages: – More expensive metal pattern and binder, it is economical for mass/batch production , Difficult to justify for small quantities – Size of casting is limited - 10 to 13.5 kg – Serious dust and fume problems during snad and resin mixing – Carbon pickup in case of steels
- 9. Investment Casting (Lost Wax Process) A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal Pattern made of wax is melted out and gets destroyed – “lost wax method” • "Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers to coating of refractory material around wax pattern • It is a precision casting process (called Precision Investment Casting) - capable of producing castings of high accuracy and intricate detail Suresh Mayilswamy 9
- 10. Step-by-step Procedure 1. Making a master pattern of the part to be cast :made of metal such as brass, Al alloy or steel or a fusible alloy (Alloy of Sn, Pb and Bi) considering shrinkage of wax 2. Making a composite die to the master pattern for casting the wax/plastic pattern ; die matrial : low melting point material Bi or Al and Cast Iron. Cavities Can be machined directly from die-blocks of steel 3. Making of wax/plastic patterns: Molten wax/plastic injected into mater die cavity under pressure 4. Assembling the wax patterns to a wax gating system: several small vax patterns are assembled together to a wax gating system connected to a central sprue by wax welding using heated tools to form a cluster or Tree of wax pattern
- 11. • 5. Investing the wax patterns: Pre-coating of wax tree by dipping it in an extremely fine refractory slurry or spraying : slurry coating determine the surface quality of castings • 6. Melting of wax patterns and baking the mould: After the investment has set around the wax tree, the formed investment mould is placed in inverted form in an oven kept between 90 to 1750 C to dry the investment and to melt out wax pattern • 7. Melting the metal and pouring the mould: Mould is further dried by heating to 800 to 1050 0C to remove wax completely - preheated mould is filled with molten metal and allowed to solidify • 8. Shaking out the casting, removing gates and feeder: fragile mould material is broken to get casting and gates and runners are removed • 9. Cleaning and inspecting the casting
- 12. Steps in investment casting: (1) wax patterns are produced, (2) several patterns are attached to a sprue to form a pattern tree 12
- 13. Steps in investment casting: (3) the pattern tree is coated with a thin layer of refractory material, (4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid 13
- 14. Steps in investment casting: (5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity, (6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies 14
- 15. Steps in investment casting: (7) the mold is broken away from the finished casting and the parts are separated from the sprue 15
- 16. Advantages and Disadvantages • Advantages of investment casting: – Parts of great complexity and intricacy can be cast – Close dimensional control (± 0.003 mm/mm) and good surface finish (1.5 to 2.25 μm Ra (no parting line) – Undercuts and other shapes difficult to withdraw normal pattern easily obtained – Wax can usually be recovered for reuse – Additional machining is not normally required - this is a net shape process (unmachinable alloys –heat resisting steels or nimonic alloys- can be cast • Disadvantages – Many processing steps are required - Relatively expensive process – Limitation in use of and location of holes – Parts are limited in size to a few kg 16
- 17. Product Applications • Parts for Aerospace industry, in aircraft engines, frames, fuel systems and instrument • Computers and data processing equipment • Food and beverage machinery • Machine tools and accessories, scientific instruments, sewing machine, • Nozzles, buckets, vanes and blades for gas turbines • Costume jewellery • Rock drill thread chaser holder blocks • Dentures and special metal implants for orthopedic surgery • Parts for movie cameras and projectors • Radar wave guide • Parts such as reciprocates slides for cloth cutting machines
- 18. Vacuum Casting • In Conventional melting, reducing and pouring metals the molten metal comes in contact with O2, H2 and N2 of the air – metals have gases and non-metallic inclusions in their masses – free from contaminants metals held in closed chamber – Ingot mould is poured within the vacuum furnace and maintained until ingot is completely solidified – • produces high quality metals and alloys. – especially for Ti having high affinity for N2 and other elements – vacuum poured steel ingots forged to produce turbine spindles – more uniform and free from contaminants
- 19. Used for thin walled (0.75 mm) complex shapes with uniform properties ex gas-turbine components from superalloys. Process uses slight vacuum (2/3 atm) also reduces porosity. Automated, and production costs similar to green-sand casting. Mold cavity is prepared in metallic dies – mould material : mixure of fine grain sand and urethane as binder – gate provided at the bottom of the mold – metal melted in induction furnace – mould is held with a robot arm and is partially immersed in molten metal. When vacuum is drawn in the mould the molten metal is drawn into mould cavity through the bottom gate. The temp. of molten metal only about 550C above the solidus temperature. So, it solidifies and cools very quickly in the mould cavity – when the cavity is filled mould is withdrawn from the furnace
- 22. As the metal enters the mould cavity through the bottom gate under vacuum, the method is called Counter gravity low pressure process (CL) CLA – metal melted in air, parts are made at a high volume and low cost CLV - Metal melted in vacuum, involve reactive metals such as aluminum, titanium, zirconium, and hafnium – super alloys for gas turbines (as thin as 0.50 mm)
- 23. Plaster-Molding process – Slurry moulding 1) Made of Plaster of Paris (gypsum or calcium), talc, & silica flour 2) Mix Finer grained refractory with water- slurry Mixture poured over pattern (made of brass and phenolic resin plastics) and allowed to harden 3) Plaster sets and is removed, the mold dried at high temps to remove moisture and avoid steam formation 4) Mold halves assembled, preheated and molten metal is poured. • *Gases evolved during solidification cannot escape, so molten metal is poured under a vacuum or under pressure.
- 24. • Yellow brass, Mn and Al bronzes, Al and Mg alloys • Al pistons, locks, propellers, aircraft parts, plumbing fixture fittings, ornaments and tire and plastic moulds • Low Thermal Conductivity allows casting to cool slowly, allowing a more uniform grain structure is obtained with less warpage • More accurate and smooth surfaces and more faithful reproduction of details than sand casting • In some cases, machining and finishing operations are eliminated • Very close tolerances obtained ± 0.005 mm/ mm and surface roughness 0.75 to 3 μ m.
- 25. Ceramic Mold casting Similar to plaster mold process EXCEPT it uses refractory materials suitable for HIGH TEMPS. – fine grain refractory powders of Zircon ( Zr Si O ) or fused silica (Si O2) with bonding agents – the slurry is applied as thin coating to the pattern and is backed by a less expensive fire clay – mould baked at 10000C and melt is poured while the mould is hot Suitable for all materials due to the use of better ingredients for the slurry. High-temperature resistance of refractory molding materials allows for casting of high temp alloys, stainless steel, and tool steels. *Expensive Process but castings have good dimensional accuracy and surface finish
- 26. Permanent Mold Casting Processes • Economic disadvantage of expendable mold casting: a new mold is required for every casting • In permanent mold casting, the mold is reused many times • The processes include: – Basic permanent mold casting – Die casting – Centrifugal casting Suresh Mayilswamy 27
- 27. The Basic Permanent Mold Process Uses a metal mold constructed of two sections designed for easy, precise opening and closing • Molds used for casting lower melting-point alloys (Al, Cu, Brass) are commonly made of steel or cast iron • Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures Suresh Mayilswamy 28
- 28. Permanent Mold Casting Steps in permanent mold casting: (1) mold is preheated and coated Suresh Mayilswamy 29
- 29. Permanent Mold Casting Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies. 30
- 30. Advantages and Limitations • Advantages of permanent mold casting: – Good dimensional control (0.015 mm/mm to .25 mm / mm), better appearance and surface finish – surface roughness 2.54 to 6.35 μm – Very economical for mass production – Repeated use of mould (upto 25000) – More rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are stronger - – further by heat treatment – Low scrap loss – Low porosity – Lesser floor space required Suresh Mayilswamy 31
- 31. • Limitations: – Generally limited to lower melting point non-ferrous alloys. – Complex part geometries can not be made because of need to open the mold (0.1 kg to 225 kg; 2.38 mm to 50 mm thickness) – High cost of mold and its maintenance (3000 to 10000 castings for C.I. 10000 to 25000 castings for Al) – Not suitable for low-volume production
- 32. Applications • Automotive Al alloy pistons, cylinder heads, aircraft and missile castings, pump bodies, cooking utensils, refrigerator compressor cylinder blocks, head and connecting rod, flat iron sole plates, kitchenware, washing machine gear blanks of CI
- 33. Die Casting A permanent mold casting process in which molten metal is injected into mold cavity under high pressure • Pressure is maintained during solidification, then mold is opened and part is removed • Molds in this casting operation are called dies; hence the name die casting • Use of high pressure (7-35MPa) to force metal into die cavity is what distinguishes this from other permanent mold processes 34
- 34. Die Casting Machines • Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity • Two main types: 1. Hot-chamber machine 2. Cold-chamber machine Suresh Mayilswamy 35
- 35. Hot-Chamber Die Casting Metal is melted in a container, and a piston injects liquid metal under high pressure into the die • High production rates - 500 parts per hour not uncommon • Injection pressure: 7-35MPa • Applications limited to low melting-point metals that do not chemically attack plunger and other mechanical components • Casting metals: zinc, tin, lead, and magnesium Suresh Mayilswamy 36
- 37. Hot-Chamber Die Casting Cycle in hot-chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification. Permanent Mold Processes Because the die material does not have natural permeability (like sand has), vent holes at die cavity needs to be made Suresh Mayilswamy 38
- 38. Cold-Chamber Die Casting Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure (14-140MPa) into die cavity • High production but not usually as fast as hot-chamber machines because of pouring step • Casting metals: aluminum, brass, and magnesium alloys Permanent Mold Processes Suresh Mayilswamy 39
- 39. Cold-Chamber Die Casting Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber Permanent Mold Processes Suresh Mayilswamy 40
- 40. Cold-Chamber Die Casting Cycle in cold-chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification. Permanent Mold Processes Suresh Mayilswamy 41
- 41. Molds for Die Casting • Usually made of tool steel or mold steel • Tungsten and molybdenum (good refractory qualities) are used to make die for casting steel and cast iron • Ejector pins are required to remove part from die when it opens • Lubricants must be sprayed into cavities to prevent sticking Permanent Mold Processes Suresh Mayilswamy 42
- 42. Advantages and Limitations • Advantages of die casting: – Economical for large production quantities – Good accuracy (±0.076mm)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, so very complex parts can not be casted – Flash and metal in vent holes need to be cleaned after ejection of part Permanent Mold Processes Suresh Mayilswamy 43
- 43. Centrifugal Casting A family of casting processes in which the mold is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavity • The group includes: – True centrifugal casting – Semicentrifugal casting – Centrifuge casting Suresh Mayilswamy 44
- 44. Centrifugal Casting Molten metal is poured into a rotating mold to produce a tubular part • In some operations, mold rotation commences after pouring rather than before • Rotational axes can be either horizontal or vertical • Parts: pipes, tubes, bushings, and rings • Outside shape of casting can be round, octagonal, hexagonal, etc , but inside shape is (theoretically) perfectly round, due to radially symmetric forces Suresh Mayilswamy 45
- 45. Shrinkage allowance is not considerable factor Suresh Mayilswamy 46
- 46. Assignment No. 1 Propose the best suitable casting process to make an aluminum cup. During selecting a process, keep the following points in view: 1. No of cups= 4 2. Product cost= as low as possible 3. Surface quality= good. Quality is not as important as cost 4. Defects= some defects are acceptable 5. Processing time= not important Draw an analysis for each major type of casting process with reference to above conditions. Then choose one casting process and write a report in its support . Suresh Mayilswamy 47
Editor's Notes
- #2: Shell molding, also known as shell-mold casting,[1] is an expendable mold casting process that uses a resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements. It is used for small to medium parts that require high precision Vacuum molding, commonly known as vacuforming, is a simplified version of thermoforming, whereby a sheet of plastic is heated to a forming temperature, stretched onto or into a single-surface mold, and held against the mold by applying vacuum between the mold surface and the sheet. Expanded Polystyrene is a packing or cushioning material Investment casting is an industrial process based on and also called lost-wax casting. Plaster mold casting is a metalworking casting process similar to sand casting except the molding material is plaster of paris (Gypsum plaster – Calcium Sulphate) instead of sand
- #29: 1. Riser and gating system can also be machined in the mold.
- #30: Preheating is done to improve fluidity
- #31: 1. If difficult to remove core from casting, the sand-made core is used. Such a process is called Semi-permanent mold casting.
- #32: Mass production: To produce parts in large quantity Low volume production: To produce parts in low quantity
- #37: The pressure is maintained during liquid cooling and solidification Because the die material does not have natural permeability (like sand has), vent holes at die cavity needs to be made
- #39: Injection pressure: 7-35MPa
- #40: Injection pressure: 14-140MPa
- #43: Maraging steel -> are iron alloys which are known for possessing superior strength and toughness without losing malleability. 'Aging' refers to the extended heat-treatment process. The common, non-stainless grades contain 17–19% nickel, 8–12% cobalt, 3–5% molybdenum, and 0.2–1.6% titanium
- #44: Formation of flash: During injection, the molten metal (called flash) sticks to the surface between two halves of die, also around core. On solidification, this flash needs to be removed.
- #46: 1. Shrinkage allowance is not considerable factor in centrifugal casting because centrifugal force causes the metal to flow to compensate shrinkage 2. Horizontal axis centrifugal casting is more common. Because, in vertical axis, gravity causes more metal to flow towards bottom of mold. As a result, bottom becomes thicker than the top of casting.