Modern Technology for Casting, Lecture notes of Mechanical Engineering

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GATE

2016 anulaClring Science

F UN DAM rALS OF CASTING

Contents

Previous Ye ar Prob l€ ms

Gate Forum

Date:^ Kan^ or

Chapter- Fu ndamental of Casting

GATE Syll abus

• Design of patterns
• Moulds and cores
• Solidi fica tion and cooling
• Riser and ga ting design
• Design considerations

Year wise Marks

Year 2005 2006 200 7 2008 2009 2010 2011 2012 2013

Marks lx2 lxl, l x2 2xl,lx 2 lx2 Ixl,lx2 lxl lxl lxl 1

(1) Introduction of Casting .:. In casting process molten me tal flows into a mold where it solidifies in the shape of the mold cavity. The part produced is also called casting.

• :. Basically it is combination of following four processes; (i) Preparation of patterns and moulds,

(ii) MeIting and pouring of liquid metals,

(iii) Solidification and cooling u pt o roo m temperature, (iv) Removal of solidified component from the mould, cleaning and inspection for defects.

• :. Generally casting process is foll owed by secondary processes such as machining, surface finishing etc. ** Difference between mould ing and c asting

Advantage of Casting (a) Casting can produce very compl ex g eometry parts with internal cavities and hollow sections. (b) It can be used to make sma ll (few h undred grams) to very large size parts (thousands of kilograms) (c) It is economical, with very little wastage: the extra metal in each casting is re-melted and re-used (d) Cast metal is isotropic - it has the sam e physical/mechanical properties along any direction. (e) Can create both external and internal shapes (f)Near net shaping process fo r verity of material as ferrous and non-ferrous materials (g)Mass production capability Disadvantage of Casting (a)Poor dimensional accuracy a nd su rfa ce finish: Conventional casting process like sand casting delivers a poor dimensional accuracy and surface finish. Some advanced casting processes like die casting, investment casting, vacuum-sealed molding process etc. gives high level of dimensional accuracy and surface finish.

(x) Ris er : A reservoir of mol ten metal provided to compensate the material volumetric shrinkage during solidification. It is used as a buffet st ck of liquid metal. (xi) Co pe: U pper m ulding pact con taining pouri ng basin, sprue, vent etc. (xii) Drag: Lower mo ulding part c on ta ining sprue well, runner, ga te s atc. (xiii) Fal sk: A combina ti on o f c ope, drag, cheek (if used), 'clamps etc. (xiv) P arting line: E dg of surface w hich separates cope and drag is termed as parting line. (xv) Cheek: Inte rm ediate pact o f mo ul ding flask used for three piece casting. Generally used for making grooved shape casting like gro 0\1ed wh eel, p ully etc. (xvi) Dead Weig ht: H eavy weights on co pe to avoid the upward motion of cope due to high gaseous pressure. (xvii) Chills and E x othermic Padding: Small metallic objects placed in the mould to increase the rate of cooling. Heat capacity of these materials IS very high. Padding (thick layers of chill material) is the other technique used for solidifica tion of critic al sections.

(xviii) Chaplets: Chaplets are used 0 support the cores inside the mold cavity. They prevent the motion of core due to its own weight and ma tallos tatic force s. (xix) Ladle: Equipment used for fee di ng high temperature molten metal into pouring basin is termed as ladle. (xx) Gaggers: Extra support provided within the cope flask to prevent the sand from falling off when lifted for pattern removal. (Drop)

(3) Patterns .:. Patterns are the replica of the object to be made by casting process, with some modifications. Main modifications are the addition of p at tern allowances and provision of core prints.

• :. Generally patterns are made woo d, metals and alloys, plastics, plaster of paris etc. however in some cases patterns are made of was, resins or so me special kind if organic materials. Types of Pattern I~ Rise r patt m I Pattern

()~ B^ ~~^ --=~^ ~^ ~g:~Km

Loose Sp lit Match Plate

Pattern Pattern Pattern

Match Plate Pattern L.^ paDrag ttern

with Gating System

(i) Loose Pattern: It is generally one piece pat tern and used for very small scale production or prototype development.

(ii) C op e and Drag Pattern: T heses patterns are also kno w n as split pattern. One half of the pattern is molded in dr ag flask ho wever o th er pa rt is molded in cope part of flak. Two halves of the pattern mu st be aligned properly by using dowel pins (fitted in cope flask ). (iii) Match Pl a te Pattern: This type of pattern is similar to split pattern but the b as ic difference is the use of match plate and locating pin s for p r p er alignment. In this, cope and drag portions of the pattern are mounted on oppo site sides of wood or m et al plate confirming the parting line. GeneraUy match plate patterns are used for large scale production of unsymmetrical casting. (iv) Gated Pattern: It is also known as follow board pattern. In this, large number of loose patterns is connected with a comm on runner t hro ugh small pass way s. known as gates. Theses gates are internally connected with runner and wh ole system is known as follow board. Patterns rRunner

Gated Pattern Sweep Pattern (v) Sweep Pattern: It is also kn own as revolutionary patterns. They are made of wood and used to generate surfaces of revolution in large casting. Use of paste like material for moulding is the most import a nt feature of sweep pattern casting. (vi) Skeleton Pattern: The se pa tterns are made of wooden frame which gives outline of the casting. They are used for large casting having simpl ge om etrical shape ( transformer casing, hollow box like structure etc .). Pattern Allowances It is a dimensional characteristic of ca sting that may be po sitive (for external dimension s) or negative (for internal dimensions). G enerally allowances are present in the form excess in dimensions which reduces machining cost and dimensional inaccuracie s. Pattern allowances are pre s ent in three form s as: (i) Shrinkage allowance: Also kno wn as contraction allowance. In ca sting process, there are two type of shrinkage takes place as liquid shrinkage and solid shrinkage.

Liquid Shrinkag~ Liquid Shrinkage (^) '0^ i!' c: LiquidState Solid'"'"' "C'"U State (^) Solid !E Shnnkage

\ ,

\ (^) <Il~

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TO{ltal = Liquid + Solid'--- solid Shrink~ Shrinkage Shrinkage Time

Solidificationshrinkage

Shrinkage of liquid Time^ nme (a) Liquid shrinkage: It occurs due to reduction in temperature from pouring temper ature (liquid state) to freezing temperature (liquid state) a nd phase change from liquid state to solid state.

(c) As piration e ffe ct .:. G as absorbi ng na ture of mo uld is kn own as aspiration effect. It causes po rou casting.

• :. Aspiration effect gener te due to pressure difference between outside atmos ph ere and inside mould. It can be avoided by using tapered spm e. .:. To avoid aspiration effec t, ratio o f top and bottom^ Ideal sprue area can be gi ven as;

Where R is the aspiration ratio, A2 is the t op sprue area, A3 is the bottom sprue area, he is the cup height and h, is the total head. (d) Design of Ga ting Syste m (i) Top (vertical) ga ting s yste m

Gate Velocity (Vg) = ~

Filling time (t f) = A 9 Vv 9 Ideal Actual where Ag= Gate area Vg= Gate velocity ht= Total head V = Mould cavity volume

(ii) Bottom gating system Pouring Basin

L----'--"------'-.::.----:-------' 1

Where t f = Filling time Mould Am^ =Mould^ cross section^ area r h Ag^ =^ Gate^ area m (^) ht = Total height hm ~ Mould height Am (5) Moulding Sand Moulding sand is mixture of si lica (sand), clay, water (moisture) and additives. Relative properties of integrands depend upon type of casting (green, dry or any other type), casting metal (ferrous or non-ferrous) and quality of final product.

Types of M oul di ng Sand: Based on relative fra ction o f di fferent integrands, important types of moulding sand can be given as; (i) Green sand: Sand 70-85%, Clay 10-20%, Water 3-6%, Additives 1-6%. This type of moulding sand is used for medium si ze casting. (ii) Dry sand: Sand 70-85%, Clay 10-20%, W ater 0%, Additives 1-6%. This type of moulding sand is used for large size castin g due t i ts co mp act and strong nature. ** Skin dried san d moulding: All inter/tlediate option between green sand and dry sand. (iii) Loam sand : Sand 40-50%, Clay 40-50%, Water 0%, Additives 1-6% (fire clay). This type of moulding sa nd is u se d for irregular sh ape casting as drum, large bell etc. (iv) Facing sand : Fine particles o f silica and cl ay are mixed in equal ratio s wjth some amount of additives. It is used directly next to surface of the pattern and comes into contact wjth molten metal. It provides smooth surface to final product. (v) Baking sand: Black sa nd that is repeatedly used as facing layer. It reduces the cos't of new moulding sand. (vi) Shell sand: Synthetic sand wjth phenol or urea formaldehyde s resins. This type of material has properties like thermosetting plastic and us ed in special casting methods like investment casting, sheil mould casting etc.

Properties of Moulding Sa nd (i) Strength: Ability of sand to mainta in its shape under high compressive load condition. Higher strength provides large loading capacity wjthout fracture. Large size grains reduce strength due to poor compaction. Irregular size grains provide mo re strength however angular grains causes lower strength.

(ii) Deformation: Change in dimensions of standard specimen at the point of failure. It should be high to

avoid quick fracture. (iii) Permeability: Ability to allow escapmg of trapped gases through the sand voids. Permeability is determined by the size and shape of sand grains. Bigger size of moulding sand gives higher permeability however irregularity in size of moulding sand reduces permeability. Angular grains also reduces permeability of moulding sand. Higher permeability gives low porosity however lower perme ability causes poor surface quality. (iv) Refractoriness: Ability of moulding sand to retain its properties at high temperature. The refractoriness of silica sand is very high. (v) Flowability: Ability of sand to flow around and over the pattern so that complex shape can be formed easily. It depends upon shape and size of moulding sand particles.

~Perm~abillty

o % of water Clllllenl 12

(ii) Columnar zo ne

• Crystals tend to grow in a ll directions with preferred growth. The columnar shape of metal is known as **columnar de ndrites.
• Highly directional property** due to unidirectional cooling. (iii) Equiaxed zone
• Cooling rate decreases si gni fi can tl y towards the center, leading to randomly oriented spherical crystals with no preferred direction of grow th. Size of grains is relatively higher as compared to chilled zone.
• Inner zone of equiaxed grains can be extended throughout the casting by adding nucleating agents such as Na, Mg, Bi.

(7) Solidification T ime According to Chvorinov's rule, solidi fi ca tion time is a function of ratio of volume and total surface area of casting object. Mathematically it can be given as;

2 So li dification Time = C ( i)

Where, C is the solidification c on stant of casting material. It depends on the characteristics of the metal being cast (its density, heat capacity, and heat of fusion), the mould material (its density, thermal conductivity, and heat capacity), the mould thickness, and t he amount of superheat. Generally metal contracts during solidification but gray cast iron expands during solidifi ca ti on.

(8) Riser

• Riser is a source of extra metal which flows from riser to mould cavity during solidification to compensate the liquid shrinkage. Without riser, heavier parts of the casting will have shrinkage defects due to lack of material. A riser must stay in liquid state until complete conversion of molten metal into solid state. (Law of directional solidification)

Riser Design

• Riser should be designed with a minimum possible volume while maintaining a cooling rate slower than casting. One way to calculate the minimum size of riser is to use Chvorinov's rule by setting the longer solidification time for riser than casting.
• According to Chvorinov's rule, solidification time of riser must be 25% higher than solidification time for casting. Hence it can be given as; flV fl (i) = 1.25 (A). riser casting Value of n (Solidification exponent) dep ends upon casting material and casting operation. Generally n is taken as 2 for normal operation.

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(9) C asting d efec ts Casting defects ca n be b ro a dl y classified into following six categories: (i) Gas de fe cts: Th ese de fects are generated due to lower gas pa ss ing tendency of mould which may be due to lower permeabili ty, p oo r ven ting and impr oper c as ting design. Important types of gas defects can be given as; (a) Blow hole s: Irregular cavity inside the surface caused by gases and water vapor inside the mould cavity. When these large dep th irregular cavity c omes on surface, termed as blow. (b) Open holes (Scar): Low de pt h irregular cavity on casting surface (When open hole is covered with a thin layer of metal, t rmed as blister). (c) Air inclusion: Small bo bbles o f gases caused due to high pouring temperature that absorbs atmospheric air. Aspiration effect due to po or gating sys t em design is the main reason for air inclusions. (d) Pin hole porosity : Caused by hydrogen due to its high penetration capacity.

Blow holes Blow (^) Scar ~~ Pin holes Blister Inclusions (ii) Shrinkage cavities: Caused due to poor feeding of liquid metal. Lack of material during solidification is the main reason for this kind of defect. It can be avoided by use of proper riser design and shrinkage allowance.

(iii) Moulding material de fects: These defects are related with the characteristics of moulding material as expansion properties, strength and th ermal properties. Important defects can be given as; (a) Cuts and washes: Rough spots and excess material areas caused due to erosion of moulding sand at higher liquid metal velocity. (b) Metal penetration: Rough casting surface caused due to coarse sand particles which allows the penetration of molten metal. It can be avoided by using fine moulding sand or facing sand (c) Rat Tails and buckles: Zigzag crossed lines and v shaped cuts on the casting surface caused by the compression failure on the skin of the mould cavity.

~ ~ ~~~

lo-.a....;a...::Ilo...Jlo...JoJ

Rat tail^ Buckle^ Swell

:^ ·:· .. :e^ ... :^ ··^ · -.^ ·^ : ·^ ·^ ~~-^ ~'..^ ... m^ •^ R^ ..^6 "..^ •^ •^ •^ .. P rosily Scab Sand Blow (d) Fusion: Brittle and glassy surface due to fusion of moulding sand with molten metal. (e) Swell: Swell in the dimensions of casting caused by backward movement of mould cavity walls due to high metallostatic force.

(c) Mo ld crack: O ccurs whe n mold strength is insufficient and a crack develops, into which liquid metal can seep to form a fin on th fi na l cas ti ng.

Moald Shin (^) Core Shift Mould^ Crack

(10) Casting Metho ds Important types of casting pro cess can be given as; (i) Shell Mould C asting Shell-mold casting yields bett er surface quality and tolerances. The process is described as follows: Hat pattern

Shell

Ha ndles

resin binderSand with Ejector

Box andinvened pattern

)

pin t

~"m'oo'~rema"ed / ~ .. , '/" j )

\ CI~mp^ , Flask

• Two piece pattern (made of metal as aluminum or steel) is heated to between 175°C-370°C and coated with a lubricant (silicone spray).
• Each heated half-pattern is covered With a mixture of sand (thermoset resin / epoxy binder). The binder glues a layer of s and to the pattern and fo rms a shell. The process may be repeated to get a thicker shell. Thickness of shell depends upon dwell period.
• The patterns are removed and the two half-shells joined together to form the mold. Metal is poured into the mold and when the metal solidifies shell is broken to get the part. Advantages Disadvantages Area of application
• Good surface finish
• Good dimen sional accuracy
• Suitable for mass production
• Ex pensive metal pattern (^) • Mass production of steel casting but less than 10 kg.

(ii) Investment casting (Lost wax pr ocess) Sand p"1ICIcad ~ aroord .-.om Mold bw<

(1) (Zl (Z)

• T his is an old pr oc ess, and has been used since ancient times to make jewellery. It is also used to make other small comp o ne nt s (few gram , tb oug h it can be used for parts up to a few kilograms).
• An advantage of th is p rocess is that the wax can carry very fine details so the process gives good dimensional tolerances and exce ll e nt surface finish. Almost o any s urface texture a nd logos can be reproduced with very high level of detai l.
• Expandable pa tterns (made of wax r special type of organic polymers as plas tics) are used.

(iii) Die casting

• Die casting is a very c omm onl y used type of pennanent mold casting proce·ss.
  • It is used for producing many co m ponent s of home appliances (rice cookers, stoves, fans, washing and drying machines, fridges), mo tors, toys and hand-tools.
  • Surface finish and tolerance o f die cast parts is so good that there is almost no po st-processing reguired.
• Thinner sections can be easily formed due to pressurized pouring of molten metal.
  • Injection pressure under wh ic h so li difica tion takes place varies from 2 to 300 MFa.
  • In gravity die casting, liguid metal is poured into basin and it comes into mould cavity under the action of gravity.
  • There are two common types of die casting: hot- and cold-chamber die casting. (a) Hot Chamber Die Cas ti ng
  • In hot chamber die-ca sting, the metal is melted in a container attached to the machine, and a piston is used to inject the liguid metal under high pressure into the die.
  • The hot chamber process is used fo r metals that (a) have low melting points and (b) do not alloy with the die material, steel; common examples are tin, zinc, and lead. Advantages Disadvantages Area of ~plication
• High^ productivity^ (u^ p^ to^500 parts per hour)
• Close tolerances
• Good^ surface finish
• Th^ e^ injection^ system^ IS submerged in the molten metal
• O nly^ simple^ shapes^ can^ be fonned
• Mass^ production^ of^ non ferrous alloys with very low melting point (zinc, tin, lead).

(b) Cold Chamber Die Casting

• In cold-chamber die-casting, molten metal is poured into the chamber from an external melting container, and a piston is used to inject the metal under high pre ss ure into the die cavity.