Thermal Methods of Change of Form

1. Thermal Methods of Change of Form Chapter 9 IT 208

2. Competencies • Identify the unique characteristics for each casting process • Describe the Investment Casting process • List the advantages of Investment Casting over Sand Casting • Describe how to select a casting process • Identify a given casting process as either expendable mold or permanent mold IT 208

3. Thermal Methods of Change of Form Metal Casting: Solidification of molten metal poured into a mold or injected into a die. 4 Most Popular Casting Processes: • Sand casting • Investment casting • Permanent mold casting • Die casting IT 208

4. Thermal Methods of Change of Form SAND CASTING (Expendable Mold classification) – mold is produced around a pattern that is withdrawn to leave a cavity. Used only once and must be broken up to free the solidified casting There are two significant restrictions on the shape of an object that can be sand cast. • There must be a plan that will become the plane of separation for the two halves of the mold • The pattern should slip cleanly out of the sand without disturbing the sand. IT 208

5. Thermal Methods of Change of Form Important factors in casting operations: • The flow of the molten metal into the mold cavity • Heat transfer during solidification and cooling of the metal in the mold (The metal grain sizes are smaller near the mold than in the center) • Influence of the type of mold material • Solidification of the metal from its molten state IT 208

6. Thermal Methods of Change of Form Parts of a Sand Casting: • Pattern is the shape that is to be cast, about which the Mold is made. • Mold is the container into which liquid metal, plastic, or other material is poured in making a casting. • TheCope is the top half of the casting flask. The Drag is the bottom part of the flask. • Pouring Basin - Molten metal is poured through a pouring basin (cup) and flows through the gating system IT 208

7. Thermal Methods of Change of Form • Sprue - vertical channel through which the molten metal flows downward into the mold • Runners - channel to carry molten metal from sprue to gate • Gates - portion of the runner through which the molten metal enters the mold cavity, traps contaminate, prevents gas buildup and provides for ease of separation • Riser (Feeder head) – let out trapped air, reservoirs to supply the molten metal necessary to prevent shrinkage during solidification. IT 208

8. Thermal Methods of Change of Form Procedure • Sand is Mulled • The pattern is place flat side down on a smooth surface • The empty drag is placed around it • Pattern is dusted with dry parting sand • Mulled sand is riddled into the drag and tamped about the pattern • After complete packing around pattern, the bottom of drag is screeded • Packed drag is carefully lifted from the patter and inverted • The cope is similarly made with cores forming the sprue and riser for filling • Cope is placed atop drag and secured if necessary • Mold is ready to pour. IT 208

9. Thermal Methods of Change of Form Melting and pouring • Charge - the mixture of alloying elements that will be at the specified composition upon melting • Fluxes - various compounds that can be mixed with the metal to react with the melt • Melt - the molten metal that will be used to produce the casting • Tapping - when melt reaches proper temperature, method used to extract the metal from furnace • Pouring - transferring the tapped material to the casting IT 208

10. Thermal Methods of Change of Form Casting techniques: • Mulling is the mixing of the sand with the bonding agent. • Riddling is the sifting of the sand into the flask. • Green-sand casting uses water as the bonding agent for the sand. When the water dries out, the mold falls apart. • Shell molds use a resin or other organic bonding agent and are baked to harden the mold. Since they do not dry out, they last a long time. IT 208

11. Thermal Methods of Change of Form Casting Alloys • Cast steels - readily welded to build up components of unusually large size and complexity • Railroad equipment (wheels, truck, frames, couplers) • Construction and mining equipment • Metal working machinery IT 208

12. Thermal Methods of Change of Form White cast irons - hard and brittle, wear resistant parts (virtually unmachineable except by grinding) • Grinding balls • Liners for ore-crushing mills • Some agricultural machinery parts Gray Iron - (low cost) is preferred choice in all fields where ductility and high strength are needed • Weights • Motor gears • Pump and pipe fitting • Machine tool bases (damping capacity) IT 208

13. Thermal Methods of Change of Form INVESTMENT CASTING (Expendable Mold) • Lost wax– Pattern made of wax or polystyrene. Dipped into refractory material, then wax is melted out and metal is poured in. Advantages of investment casting versus sand are greater detailed parts, better surface finish, less machining, less tool wear. IT 208

14. Thermal Methods of Change of Form Full-mold – lost-form/expanded polystyrene/evaporative patter – pattern is made of expanded polystyrene rather than wax. Stryrofoam is vaporized and molten metal takes its place. • Minimum wall thicknesses - are 2.5 mm • Tolerances - can be held to .3 % on dimensions • Surface finish - can be held from 2.5µm to 25µm (0.1µin to 1.0 µin) rms. • Size limits- are from 400 g (1 lb) to several tons. No draft allowance is required. • Typical materials - aluminum, iron, steels, nickel alloys, copper alloys. • Types of parts - pump housings, manifolds, and auto brake components. IT 208

15. Thermal Methods of Change of Form Plaster Mold – investment material is plaster, works only with plastics or low-melting-point nonferrous metals or alloys. In plaster mold casting, a plaster, usually gypsum or calcium sulfate, is mixed with talc, sand, asbestos, and sodium silicate and water to form a slurry. • This slurry is sprayed on the polished surfaces of the pattern halves (usually brass). The slurry sets in less than 15 minutes to form the mold. The mold halves are extracted carefully from the pattern, and then dried in an oven. IT 208

16. Thermal Methods of Change of Form The mold halves are carefully assembled, along with the cores. The molten metal is poured in the molds. After the metals cools down, the plaster is broken and the cores washed out. • Minimum wall thicknesses- section thickness can be as small as 0.6 mm (0.025 in) • Tolerances - 0.2 % linear • Surface finish- is 1.25 µm to 3 µm (50 µin to 125 µin) rms. • Size limits - usually small to medium size, ranging in weight from 30 g (1 oz) to 7 kg (15 lb) the draft allowance is 0.5-1.0 degree. • Typical materials - Low temperature melting materials such as aluminum, copper, magnesium and zinc. • Types of parts -used to make quick prototype parts as well as limited production parts. IT 208

17. Thermal Methods of Change of Form Ceramic Mold – uses a refractory slurry instead of plaster as the investing material, can withstand higher temperatures than plaster. • Minimum wall thicknesses- as small as 1.25 mm (.050 in) • Tolerances - can be held to 0.4 %, • Surface finish- can be better than 2 - 4 µm (.075 - .15 µin). Add 0.3 mm (.012 in) for parting line tolerances. • Size limits - from 60 g (2oz) to a ton. Draft allowance of 1° is recommended. • Types of parts - Typical parts made from this process include impellers made from stainless steel, bronze, complex cutting tools, plastic mold tooling This process is expensive, but can eliminate secondary machining operations. IT 208

18. Thermal Methods of Change of Form Shell Mold - higher throughput due to reduced cycle times. A heated (200 ºC / 392 ºF) metal pattern is covered with a mixture of sand and thermoset plastic. • Minimum wall thicknesses- Minimum thicknesses can be as low as 1.5 mm (0.062 in) to 6.25 mm (0.25 in), depending on the material. • Tolerances - and dimensional tolerance of 0.5 %. • Surface finish- complex parts with good surface finish 1.25 µm to 3.75 µm (50 µin to 150 µin) rms • Size limits – Size limits of 30 g to 12 kg (1 oz to 25 lb). • Typical materials – cast irons, and aluminum and copper alloys • Types of parts -connecting rods, gear housings, lever arms etc. IT 208

19. Thermal Methods of Change of Form Shell Mold con’t • A good surface finish and good size tolerance reduce the need for machining • A fairly high capital investment is required, but high production rates can be achieved. The process overall is quite cost effective due to reduced machining and cleanup costs. IT 208

20. Thermal Methods of Change of Form Centrifuged- flask is rotated containing the liquid metal where the centrifugal force drives the material into the inner recesses of the mold cavity. Vacuum Casting Molding – vacuum draws the material into the mold. Can be used as the final steps in the lost-wax process or with shell molds • Production Rate: 15-25 parts each week. • Vacuum costs money and takes tooling time. • Parts are of higher quality than with Gravity Casting, with thinner walls and fewer voids. • Materials can simulate ABS, Nylon, Polycarbonate, etc. IT 208

21. Thermal Methods of Change of Form PERMANENT MOLD • Instead of using sand as the mold material, a metal is used as a mold. Typically cast iron or Meehanite (a dense cast iron) is used as the mold material and the cores are made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate. IT 208

22. Thermal Methods of Change of Form Permanent mold castings permanent mold castings produce a lower price per piece • Minimum wall thicknesses-(such as 3mm for lengths under 75 mm), radius (inside radius = nominal wall thickness, outside radius = 3 x nominal wall thickness), draft angles (1 to 3º on outside surfaces, 2 to 5º on inside surfaces) etc all apply. • Tolerances - Typical tolerances are 2 % of linear dimensions. • Surface finish- ranges from 2.5 µm to 7.5 µm (100 µin to 250 µin). • Size limits – Typical part sizes range from 50 g to 70 kg (1.5 ounces to 150 lb). • Typical materials – small and medium sized parts made from aluminum, magnesium and brass and their alloys. • Types of parts - gears, splines, wheels, gear housings, pipefittings, fuel injection housings, and automotive engine pistons. IT 208

23. Thermal Methods of Change of Form Low-pressure feed - Here, instead of using gravity to assist in the metal pour and flow in the mold, a low pressure of up to 1 atmosphere gas is applied to the molten metal. This maintenance of pressure on the melt causes complete fill of the mold and compensates for any shrinkage on cooling. IT 208

24. Thermal Methods of Change of Form Vacuum feed - This is similar to the low-pressure permanent mold casting, where a vacuum is used instead of a pressure. Thin wall castings can be made as in the low-pressure permanent mold casting. Castings range in size from 200 g to 4.5 kg (6 oz to 10 lb). IT 208

25. Thermal Methods of Change of Form Die casting - Made of iron or steel. Molten aluminum or magnesium is forced into die cavity by hydraulic ram (1000-30,000 psi). Die is cooled by circulating oil or water. • High cost of manufacturing and maintaining a permanent mold. • Used for high-volume production runs. • Hot-chamber die casting is used for low-melting temperature casting materials. • Cold-chamber die cast is used for higher melting temperature alloys. IT 208

26. Other Permanent Mold Castings Slush Casting: Slush Casting is a special type of permanent mold casting, where the molten metal is not allowed to completely solidify. After the desired wall thickness is obtained, the not yet solidified molten metal is poured out. This is useful for making hollow ornamental objects such as candlesticks, lamps, statues etc. IT 208

27. Other Permanent Mold Castings Corthias Casting: Corthias Casting is another variation of the permanent mold casting, where a plunger is used to pack down the molten metal form the sprue hole. This allows for thinner walls and greater details to be produced. IT 208

28. Other Permanent Mold Castings Centrifugal Casting and Semi-Centrifugal Casting. • Centrifugal Casting: In centrifugal casting, a permanent mold is rotated about its axis at high speeds (300 to 3000 rpm) as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. IT 208

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30. Other Permanent Mold Castings • Minimum wall thicknesses-. 2.5 mm to 125 mm (0.1 - 5.0 in). • Tolerances - OD can be as good as 2.5 mm (0.1 in) and on the ID can be 3.8 mm (0.15 in). • Surface finish- from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms. • Size limits –up to 3 m (10 feet) diameter and 15 m (50 feet) length • Typical materials –iron, steel, stainless steels, and alloys of aluminum, copper and nickel. Two materials can be cast by introducing a second material during the process. • Types of parts - pipes, boilers, pressure vessels, flywheels, cylinder liners and other parts that are axi-symmetric Only cylindrical shapes can be produced with this process IT 208

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