Permanent mold casting• The permanent mold process is a castingprocess where the mold is manufacturedfrom metal instead of sand. This allows themold to be reused to produce a large volumeof castings. The molds are machined fromgrey iron or tool steel.• The molds are heated to approximately 500degrees Fahrenheit and are filled usinggravity pressure. The molds are coated witha ceramic coating to prevent wetting of themold and ease of part removal.
Investment casting• Investment casting is a process that hasbeen practiced for thousands of years, withlost wax process being one of the oldestknown metal forming techniques.• Investment casting derives its name from thefact that the pattern is invested, orsurrounded, with a refractory material. Thewax patterns require extreme care for theyare not strong enough to withstand forcesencountered during the mold making. Oneadvantage of investment casting it that thewax can be reused.
Centrifugal casting• Centrifugal casting is both gravity- andpressure-independent since it creates its ownforce feed using a temporary sand mold heldin a spinning chamber at up to 900 N (90 g).Lead time varies with the application. Semi-and true-centrifugal processing permit 30-50pieces/hr-mold to be produced, with apractical limit for batch processing ofapproximately 9000 kg total mass with atypical per-item limit of 2.3-4.5 kg.
Continuous castingContinuous casting is a refinement of the casting processfor the continuous, high-volume production of metal sectionswith a constant cross-section. Molten metal is poured intoan open-ended, water-cooled copper mold, which allows askin of solid metal to form over the still-liquid centre. Thestrand, as it is now called, is withdrawn from the mold andpassed into a chamber of rollers and water sprays; therollers support the thin skin of the strand while the spraysremove heat from the strand, gradually solidifying the strandfrom the outside in. After solidification, predeterminedlengths of the strand are cut off by either mechanical shearsor travelling oxyacetylene torches.
Sand casting• Sand casting is one of the most popular and simplest types of castingthat has been used for centuries. Sand casting allows for smallerbatches to be made compared to permanent mold casting and a veryreasonable cost. Not only does this method allow for manufacturers tocreate products for a good cost there are other benefits to sandcasting such as there are very little size operations. From castings thatfit in the palm of your hand to train beds (one casting can create theentire bed for one rail car) it can be done with sand casting. Sandcasting also allows for most metals to be cast depending in the thetype of sand used for the molds.• Sand casting requires a lead time of days for production at high outputrates (1-20 pieces/hr-mold), and is unsurpassed for large-partproduction. Green (moist) sand has almost no part weight limit,whereas dry sand has a practical part mass limit of 2300-2700 kg.Minimum part weight ranges from 0.075-0.1 kg. The sand is bondedtogether using clays (as in green sand) or chemical binders, orpolymerized oils (such as motor oil.) Sand in most operations can berecycled many times and requires little additional input.
MoldA mold is a hollowed-out block that is filled with a liquidlike plastic, glass, metal, or ceramic raw materials. Theliquid hardens or sets inside the mold, adopting itsshape.• Mold materials:– Wood - common material because it is easy to work,but it warps– Metal - more expensive to make, but lasts muchlonger– Plastic - compromise between wood and metal
Desirable Mold Properties• Strength to maintain shape and resist‑erosion• Permeability to allow hot air and gases to‑pass through voids in sand• Thermal stability to resist cracking on‑contact with molten metal• Collapsibility ability to give way and allow‑casting to shrink without cracking thecasting
Making the Sand Mold• The cavity in the sandmold is formed bypacking sand arounda pattern, thenseparating the moldinto two halves andremoving the pattern• The mold must alsocontain gating andriser system
• Molding sand, also known as foundry sand, is sand that when moistenedor oiled tends to pack well and hold its shape. It is used in the process of sandcasting.
TYPES OF SAND1. Green sand2. Dry sand:3. Loam sand:4. Parting sand:5. Facing sand:6. Backing sand:7. System sand:8. Core sand:
Green sand: It is sand used in the wet condition formaking the mold . It is mixture of silica sandwith 15-25per cent clay and 6-8 per cent water Green sand molds are not dried and metal ispoured in them in the wet condition Being damp the sand can be easily workedwith hand to give it any desired shape LIMITATIONS:o This sand is used for producing small tomedium sized molds which are not verycomplex.
2. Dry sand:• Dry sand is the green sandthat has been dried or bakedafter preparing the mould.• Drying sand gives strengthto the mould so that it canbe used for larger castings
3. Loam sand:• Loam sand is sandcontaining up to 50 % claywhich has been worked tothe consistency of buildermortar.• This sand is used for loamsand moulds for makingvery heavy castings usuallywith the help of sweeps andskeleton patterns.
4. Parting sand:• Parting sand consists of finegrained clay free dried silica sand,sea sand or burnt sand with someparting compounds.• This sand is used during making ofthe mould to ensure that greensand does not stick to the patternand the cope and drag parts canbe easily separated for removingthe pattern without causing anydamage to the mould.• The parting compounds usedinclude charcoal, ground bone andlimestone, groundnut shells, talcand calcium phosphate.
5. Facing sand: Facing sand is the sand which covers the pattern all around it.The remaining box is filled with ordinary floor sand. Facing sand forms the face of the mold and comes in directcontact with the molten metal when it is poured. High strength and refractoriness are required for this sand. Graphite, mollases, plumbago etc. may be added to the facingsand. Thickness of the sand layer varies from 20 to 30 mm.
6. Backing sand: Backing sand is the bulk ofthe sand used to back upthe facing sand and to fillup the volume of the flask. It consists mainly of old,repeatedly used moldingsand which is generallyblack in color due toaddition of coal dust andburning on contact withhot metal. Because of the colorbacking sand is alsosometimes called blacksand. The main purpose for theuse of backing sand is toreduce the cost ofmolding.
7. System sand: This is the sand used inmechanized foundries forfilling the entire flask. No separate facing sand inused in a mechanizedfoundry. Sand, cleaned andreactivated by the additionof water and binders is usedto fill the flask. Because ofthe absence of any freshsand, system sand musthave more strength,permeability andrefractoriness compared tobacking sand.
8. Core sand: Core sand is the sand used for making cores. This is silica sandmixed with core oil. That is why it is also called oil sand. The core oil consists of linseed oil, resin, light mineral oil with somebinders. For larger cores, sometimes pitch or flour and water may also beused to save on cost.
ADVANTAGES The sand made can be used to pour liquid metal without drying, which hasthe advantages of low production cost and short production cycle. Wet clay sand used in sand casting production accounts for about 60%. Asthe wet sand has a high moisture content, low strength and air permeability,the castings can easily have the porosity, coarse, sticky sand and sandexpansion defects. In mechanical modeling, the castings have high dimensional accuracy, so itis widely used for high-volume production of castings. Facing sand is made of silica sand and clay without the addition of any usedsand. The main purpose for the use of backing sand is to reduce the cost ofmolding.
DISADVANTAGES• In hand molding, the dimensional accuracy is low, so it is generally usedonly for the production of small and medium sized iron castings and non-ferrous alloy castings• The clay used to make dry sand clay is ordinary clay, and its moisturecontent is high. The sand made should be dried in the temperature of about250 ~ 400 and then be combined cast, and it is generally used for steel℃castings. Because of the high energy consumption, long production cycle,low dimensional accuracy, dry sand is gradually phased out.
Properties of molding sands:o 1. Strength:o 2.Grain size and shapeo 3. Thermal stabilityo 4. Refractorinesso 5. Flow abilityo 6. Sand textureo 7. Adhesivenesso 8. Reusabilityo 9. Easy of preparation and controlo 10. Conductivity
Furnaces for Casting Processes• Furnaces most commonly used infoundries:– Cupolas– Direct fuel fired furnaces‑– Crucible furnaces– Electric arc furnaces‑– Induction furnaces
CupolasVertical cylindrical furnace equipped withtapping spout near base• Used only for cast irons– Although other furnaces are also used, the largesttonnage of cast iron is melted in cupolas• The "charge," consisting of iron, coke, flux,and possible alloying elements, is loadedthrough a charging door located less thanhalfway up height of cupola
Cupola zones• Combustion or Oxidizing zone• It is the zone where combustion takes place. It extends from the top of the tuyeres to a surface boundarybelow which all the Oxygen of air is consumed by combustion, chemical reaction that takes place in thezone is• C(coke) + 02 (from air) -> C02 + Heat• The temperature in this zone is about 1800°C.• Reducing zone• It extends from the top of the combustion zone to the top of the initial coke bed. The CO2 produced in thecombustion zone moves up and is reduced to CO. The temperature also drops to 1650°C.• C02 + C2 -» CO – Heat• Melting zone• It includes the first layer of pig iron above the initial coke bed. In this zone, the pig iron is melted. Thefollowing reaction takes place.• 3 Fe + 2 CO -» Fe3C + C02• Preheating zone• It includes all the layers of cupola charges placed above the melting zone to the top of the last charge.The layers of charges are heated by the out-going gases. The temperature in the zone may be up to1050°C.• Stack:• It is the zone beyond the pre-heating zone, through which the hot gases go to the atmosphere.
Advantages• The cupolas is one of the only methods ofmelting which is continuous in its operation• High melt rates• Relatively low operating costs• Ease of operation
Disadvantages• In more recent times, the use of thecupola has declined in favour of electricinduction melting, which offers moreprecise control of melt chemistry andtemperature, and much lower levels ofemissions.
Limitations• Not possible to produce iron below 2.8%carbon in this furnace
Charge is melted by heat generated from anelectric arc• High power consumption, but electric arc‑furnaces can be designed for high meltingcapacity• Used primarily for melting steel
Working• The roof which can normally swing away to facilitatecharging, generally contains three carbon electrodes (orwater cooled graphite electrodes) operating on a hightension three-phase power supply. These electrodesprotrude vertically through the roof and an electriccurrent passes directly through them and into the metalbath. The distance between the electrodes and the metalbath is automatically controlled and determines thepower input into the bath. So that it produces a stablearc. Temperature of the melt can be easily controlledand regulated.
• Expensive alloying elements such as Chromium, Nickel, andTungsten etc. can be easily added without any loss byoxidation.• There is complete absence of fumes and gases which arepresent in fuel fired gases. This ensures excellent control onthe quality of the melt and leads to production of very highquality castings.• These furnaces generally have a door at the back for alloying,oxygen lancing and slag removal purposes, and a pouringspout at the front. The entire unit is capable of being tilted fordischarge of the melt through the pouring spout. Some of theadvantages of direct arc furnaces include high melt rates, highpouring temperatures and excellent control of melt chemistry
Induction FurnaceUses alternating current passingthrough a coil to develop magneticfield in metal• Induced current causes rapidheating and melting• Electromagnetic force field alsocauses mixing action in liquidmetal• Since metal does not contactheating elements, environmentcan be closely controlled toproduce molten metals of highquality and purity• Melting steel, cast iron, andaluminum alloys are commonapplications in foundry work