Metal Casting Processes

TopicsIntroduction

Sand casting

Shell-Mold Casting

Expendable Pattern Casting

Plaster-Mold Casting

Metal-Casting ProcessesFirst casting were made during 4000 3000 BCIn time many casting processes have been developedMany parts can be made from castingsEngine blocksCamerasGun barrelsCook wareEtc

Introduction

The Polaroid PDC-2000 digital camera with AZ91D die-cast, high purity magnesium case. (b) Two-piece Polaroid camera case made by the hot-chamber die casting process

Die Casting Examples

Ex: Typical gray-iron Castings used in automobilesTrends impacting industry

Continuing mechanization & automation of the casting process

Increasing demand for high-quality castings

Types of MoldsExpandable molds mixed with various types of binders or bonding agentsSandPlasterCeramicsNote: these molds are able to withstand high temperatures and mold is broken up to remove the casting

Permanent molds - Made of metalSubjected to a higher cooling rateAffects grain sizeUsed repeatedlyCasting can be removed easily

Composite molds - Made of two or more materialsSandGraphiteMetalCombines advantages of each materialUsed to:Control cooling ratesImprove mold strengthOptimize economics of the process

Sand CastingTraditional way to cast metalsHow to make a sand castingPlacing a pattern having the shape of the desired casting in to the sand to make an imprintIncorporating a gating systemFilling the resulting cavity with molten metalAllow the metal to coolBreak away the sand mold

Type of sand to useMost common silica sand (SiO2)There are two types of sand:Naturally bonded (bank sand)Synthetic (lake sand)Preferred by foundries b/c its composition can be controlled

Steps in Sand Casting-Pattern making-Core making-Gating systems

Furnaces Solidification Shakeout Additional heat Defects Removal of risers treatment Pressure tightness and gatesMoldingPouringinto moldInspectionCleaning & FinishingHeat TreatmentCastingSandMelting Of MetalMold

Schematic illustration of sand mold features

SAND MOLD FEATURES

Sand Casting ContSelection of sandFine round grainsSmooth surface finishesCan be closely packedHave lower mold permeabilityEnhances mold strengthCourse grainsAllow gases and steam escape

Sand moldsShould have good collapsibilitySand should be mixed thoroughly withClay (bentonite) cohesive agentZircon, olivine, and iron silicate sands are used in steel foundries for low thermal expansionChromites is used for its high heat transfer characteristicsTypes of sand moldsGreen mold sandingMixture of sand, clay, and watergreen sand in the mold is moistLeast expensive method of making molds

Sand Casting ContSkin-dried moldsMold surfaces are driedUsed for large castingsHave higher strength than green-sand moldsBetter dimensional accuracy and surface finishDrawbacks:Distortion to the mold is greaterCastings susceptible to hot tearingSlower production rate

Cold-box moldsVarious organic and inorganic binders are blended into the sand for greater strengthDimensionally more accurate

No-bake mold Synthetic liquid resin is mixed into the sandHardens at room temperature

Sand Casting ContMajor components of sand molds:Flask supports the moldPouring basin in which molten metal is poured in toSprue through which molten metal flows downwardRunner system channels that carry molten metal from the sprueRisers - supply additional metal to the casting during shrinkage. Cores Inserts made of sandUsed to make hollow regions Used to form letterings on the casting and other featuresVents used to carry off gases that are produced and exhaust air from the mold cavity as metal flows on to the mold

PatternsUsed to mold the sand mixtureMade fromWoodPlasticMetalThey must be tough enough to be used repeatedlyMade of different materials to reduce wearCoated with a parting agent to facilitate there removalPatternsOne-piece patterns for simple low cost productionSplit patterns each part forms a portion of the cavityMatch-plate patterns popular type of patternTwo-piece patterns are constructed by securing each half of one or more split patterns to the opposite sides of a single plateUsed in large production runs and in molding machines

Rapid prototypingUsed to make a mold quicklyReduces cost

Patterns Taper in patterns for ease of removal from the sand moldTypical metal match-plate pattern used in sand casting

Cores made of sand aggregatesPossess: Strength Permeability Ability to withstand heatCollapsibilityAnchored by core pointsChaplets are used to keep the core from moving

Sand molding machines:Vertical flaskless moldingSandslingerImpact moldingVacuum moldingThe sand-casting operation

Shell-Mold CastingDeveloped in the 1940sProduces close dimensional tolerancesGood surface finishLow cost process

Common methods of making shell molds.(a) Pattern rotated and clamped(b) Pattern and dump box rotated

(c) Pattern dump box in position for the investment

(d) Pattern and shell removed from dump box

(a) Schematic illustration of a semi permanent composite mold. (b) A composite mold used un casting an aluminum-alloyComposite MoldsMade up of two or more different materialsUsed in shell molding Used in casting complex shapes

Sodium silicate processMixture of sand and 1.5% - 2% sodium silicateThe mixture is packed around the patternHarden by blowing CO2 gas though itFirst used in the 1950sReduces tendency for parts to tear

Rammed Graphite MoldingRammed graphite is used to make molds for:TitaniumZirconiumSand cannot be used because sand will react vigorously with silicaMolds are pack ed like sand moldsAir driedBaked at 175 CStored under controlled humidity and temperatureCasting process similar to those for sand molds

Polystyrene beads are placed in a preheated dieThe polystyrene bead expand to fix the dieThe die is cooled and the polystyrene pattern is removedThe polystyrene pattern is placed in a mold with support sandThe polystyrene pattern evaporates with contact of molten metal to form a cavityExpendable-Pattern CastingSchematic illustration of the expendable pattern casting process also known as lost foam or evaporative casting.

Plaster-Mold CastingMold is made of plasterMixed with water and additives and poured over a patternAfter plaster sets, pattern is removed and the mold is dried at 120 CHave low permeability gases can not escapePatterns are made of:Al alloys, Thermosetting plasticsBrass or Zinc alloysHave fine details and good surface finishForm of precision casting

Ceramic-Mold CastingSimilar to plaster-mold process

Uses refractory mold materials

Suitable for high temperature applications

Mixture made of:Fine grained zirconAluminum oxideSilica

Mixture is mixed with bonding agents and poured over pattern

Mixture is mixed with bonding agents and poured over pattern

Molds are baked in an ovenMolds can be used to cast high-temperature alloysCastings have good surface finishesGood dimensional accuracySome what expensive to make