Lecture 2 casting full

CASTING

Definition

The casting process basically involves (a) pouring molten metal into a mold patterned after the part to be manufactured, (b) allowing it to solidify, and (c) removing the part from the mold.

• Important considerations in casting operations are as follows:1. Flow of the molten metal into the mold cavity2. Solidification and cooling of the metal in the mold3. Influence of the type of mold material.

Components involved in making a casting:

Steps involved in making a casting:

• Pattern Making

• Moulding

• Melting

• Pouring and solidification

• Cleaning and Inspection

Pattern

A Pattern is a model or the replica of the object to be cast with some modifications.

Modifications are:

Pattern AllowancesProvision for core printsElimination of fine details

Master pattern

Master pattern is the name given to a pattern having a double contraction or shrinkage allowance.

For the general purposes of lower cost and of speed in the manufacture of a master pattern, poplar is the most logical material to use.

Poplar (Liriodendron tulipifera)

Types of Patterns:Single piece pattern.

Split pattern

Loose piece pattern

Match plate pattern

Sweep pattern

Gated pattern

Skeleton pattern

Follow board pattern

Cope and Drag pattern

(a)Split pattern

(b) Follow-board

(c) Match Plate

(d) Loose-piece

(e) Sweep

(f) Skeleton pattern

Figure 11.3 ‑ Types of patterns used in sand casting: (a) solid pattern(b) split pattern(c) match‑plate pattern(d) cope and drag pattern

Single piece pattern•generally are used for simpler shapes and low quantity production; •they generally are made of wood and are inexpensive.

Split Pattern•Castings with complicated shapes can be produced.

•When a one piece solid pattern has projections or back drafts which lie above or below the parting plane, it is impossible to with draw it from the mould. •With such patterns, the projections are made with the help of loose pieces. One drawback of loose faces is that their shifting is possible during ramming.

Match plate pattern•In such constructions, the gating system can be mounted on the drag side of the pattern. •This type of pattern is used most often in conjunction with molding machines and large production runs to produce smaller castings.•Piston rings of I.C. engines are produced by this process

Sweep pattern:

•A sweep is a section or board (wooden) of proper contour that is rotated about one edge to shape mould cavities having shapes of rotational symmetry. •This type of pattern is used when a casting of large size is to be produced in a short time. Large kettles of C.I. are made by sweep patterns.

castings

Gating system

GATED PATTRN•A gated pattern is simply one or more loose patterns having attached gates and runners.•Because of their higher cost, these patterns are used for producing small castings in mass production systems and on molding machines.

Cope and drag pattern

Follow board pattern

Materials for making patterns:WOOD

METAL

PLASTIC

PLASTER

WAX

The pattern material should be:1. Easily worked, shaped and joined.2. Light in weight.3. Strong, hard and durable.4. Resistant to wear and abrasion .5. Resistant to corrosion, and to chemical

reactions.6. Dimensionally stable and unaffected by

variations in temperature and humidity.7. Available at low cost.

Types of Pattern Allowances:The various pattern allowances are:

1. Shrinkage or contraction allowance.2. Machining or finish allowance.3. Draft of tapper allowances.4. Distortion or chamber allowance.5. Shake or rapping allowance.

1.Shrinkage Allowance: All most all cast metals shrink or contract

volumetrically on cooling.

The metal shrinkage is of two types:

1. Liquid Shrinkage: 2. Solid Shrinkage:

2. Machining Allowance: A Casting is given an allowance for

machining, because:i. Castings get oxidized in the mold and during

heat treatment; scales etc., thus formed need to be removed.

ii. It is the intended to remove surface roughness and other imperfections from the castings.

iii. It is required to achieve exact casting dimensions.

iv. Surface finish is required on the casting.

3. Draft or Taper Allowance:

It is given to all surfaces perpendicular to parting line.

Draft allowance is given so that the pattern can be easily removed from the molding material tightly packed around it with out damaging the mould cavity.

Taper in Design

4. Distortion or cambered allowance:

A casting will distort or wrap if :i. It is of irregular shape,ii. All it parts do not shrink uniformly i.e., some

parts shrinks while others are restricted from during so,

iii. It is u or v-shape

5. Shake allowance:A pattern is shaken or rapped by striking the same with

a wooden piece from side to side. This is done so that the pattern a little is loosened in the mold cavity and can be easily removed.

In turn, therefore, rapping enlarges the mould cavity which results in a bigger sized casting.

Hence, a –ve allowance is provided on the pattern i.e., the pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping.

Example• A job shown in the Figure is to be made of steel by

casting process. The mould for this job is made from a wooden pattern. Determine the dimensions of the wooden pattern. Assume machining allowance of 2 mm on each side, shrinkage allowance of 2% and a taper allowance of 1 degree.

Solution

After machining allowance

Solution

Example A job shown in Figure is to be made from steel by

casting process. The mold for this job is made from wooden pattern. Determine the dimensions of the wooden pattern assuming machining allowance of 3 mm on each side, shaking allowance of 1 mm on length and width, shrinkage allowance of 3%

After machining allowance

After 3% shrinkage allowance

After rapping allowance

Classification Of

Moulding Sands

A)Natural sand is the one which is available from natural deposits. Only additives and water need be added to it to make it satisfactory for molding.

B)Synthetic sand is prepared by mixing a relatively clay free sand having specified type of sand grain, with specified type of clay binder as well as water and other additives.

1. Green sand: It is sand used in the wet condition for making the mould. It is mixture of silica sand with 15-25 % clay and 6-8 % water.The sand can be easily worked with hand to give it any desired shape.This sand is used for producing small to medium sized moulds which are not very complex.

Color is blackWe can maintain its porosity

2. Dry sand:Dry sand is the green sand that has been dried or baked after preparing the mould.Drying sand gives strength to the mould so that it can be used for larger castings

3. Loam sand:Loam sand is sand containing up to 50 % clay which has been worked to the consistency of builder mortar.This sand is used for loam sand moulds for making very heavy castings usually with the help of sweeps and skeleton patterns.

4. Parting sand:-This sand is used during making of the mould to ensure that green sand does not stick to the pattern and the cope and drug parts can be easily separated for removing the pattern without causing any damage to the mould.-Parting sand consists of fine grained clay free dried silica sand, sea sand or burnt sand with some parting compounds.-The parting compounds used include charcoal, ground bone and limestone, groundnut shells, talcum and 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 mould and comes in direct contact with the molten metal when it is poured.-High strength and refractoriness are required for this sand.-It is made of silica sand and clay without the addition of any used sand.

6. Backing sand:-Backing sand is the bulk of the sand used to back up the facing sand and to fill up the volume of the flask.-It consists mainly of old, repeatedly used moulding sand which is generally black in colour due to addition of coal dust and burning on contact with hot metal.Because of the colour backing sand is also sometimes called black sand.

7. System sand:-This is the sand used in mechanized foundries for filling the entire flask.-No separate facing sand in used in a mechanized foundry.-Sand, cleaned and reactivated by the addition of water and binders is used to fill the flask. Because of the absence of any fresh sand, system sand must have more strength, permeability and refractoriness compared to backing sand.

8. Core sand:-Core sand is the sand used for making cores. --This is silica sand mixed with core oil. That is why it is also called oil sand.-The core oil consists of linseed oil, resin, light mineral oil with some binders.-For larger cores, sometimes flour and water may also be used to save on cost.

GENERAL PROPERTIESOF

MOLDING SANDS

1. Green strength: The green sand, after water has been mixed into it, must have adequate strength and plasticity for making and handling of the mold.

2. Dry strength: As a casting is poured, sand adjacent to the hot metal quickly loses its water as steam. The dry sand must have strength to resist erosion, and also the pressure of the molten metal, or else the mold may enlarge.

3. Hot strength. After the moisture has evaporated, the sand may be required to possess strength at some elevated temperature.

4. Permeability/Porosity. Heat from the casting causes a green‑sand mold to evolve a great deal of steam and other gases. The mold must be permeable, i.e. porous, to permit the gases to pass off, or the casting will contain gas holes.

5. Thermal stability. Heat from the casting causes rapid expansion of the sand surface at the mold‑ metal interface. The mold surface may then crack, buckle, or flake off (scab) unless the molding sand is relatively stable dimensionally under rapid heating.

6. Refractoriness. Higher pouring temperatures, such as those for ferrous alloys at 2400 to 3200 F, require greater refractoriness of the sand. Low‑ pouring‑temperature metals, for example, aluminum, poured at 1300 F, do not require a high degree of refractoriness from the sand. 7. Plasticity or flow-ability : It is the measure of the molding sand to flow around and over a pattern during ramming and to uniformly fill the flask. 8. Cohesiveness: It is the property of sand which hold grains together. 9. Collapsibility: Heated sand which becomes hard and rock like is difficult to remove from the casting and may cause the contracting metal to tear or crack.

10. Adhesiveness: This is the property of sand mixture to adhere to another body (here, the molding flasks). The molding sand should stick to the sides of the molding boxes so that it does not fall out when the flasks are lifted and turned over.

11. Offers ease of sand preparation and control.

12. Removes heat from the cooling casting.

13.Produces good casting finish

14.It is reusable.

FLOW DIAGRAM FOR CASTING

Core-Core prints : When a casting is required to have a hole, through or blind, a core is used in the mould to produce the same.It is made up of sand ,wood, or metal body, which is left in the mould during casting and it remove after casting. This core has to be properly seated in the mould extra projections are added on the pattern surface at proper places. These projections are known as core prints.

Use of chaplets to avoid shifting of cores

Possible chaplet design and casting with core

•It must be strong to retain the shape while handling, •It must resist erosion by molten metal, • It must be permeable to gases, •It must have high refractoriness, •It must have good surface finish to replicate it on to the casting.

Core properties

Element Of Gating System

Casting Terms:

2. Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.

3. Parting line: This is the dividing line between the two molding flasks that makes up the mold.

Pattern

4. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.

5. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold.

6. Runner: The channel through which the molten metal is carried from the sprue to the gate.

7. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as feed head.

8. Gate: A channel through which the molten metal enters the mold cavity.

9. Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.

10.Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.

11. Vent: Small opening in the mold to facilitate escape of air and gases.

12. Chill: Chills are metallic objects, which are placed in the mould to increase the cooling rate of castings.

Types of Gate or In-gate

Top gate: Causes turbulence in the mould cavity, it is prone to

form dross, favourable temperature gradient towards the gate, only

for ferrous alloys.

Bottom gate: No mould erosion, used for very deep moulds, higher

pouring time, Causes unfavorable temperature gradients.Parting Gate: most widely used gate, easiest and most economical in preparation.Step Gate: Used for heavy and large castings, size of ingates are normally increased from top to bottom.

The goals for the gating system

• To minimize turbulence to avoid trapping gasses into the mold

• To get enough metal into the mold cavity before the metal starts to solidify

• To avoid shrinkage• Establish the best possible temperature gradient in the

solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part.

• Incorporates a system for trapping the non-metallic inclusions.

Types of Gating Systems

The gating systems are of two types:

– Pressurized gating system

– Un-pressurized gating system

Pressurized Gating System

• The total cross sectional area decreases towards the mold cavity

• Back pressure is maintained by the restrictions in the metal flow

• Flow of liquid (volume) is almost equal from all gates• Back pressure helps in reducing the aspiration as the sprue

always runs full• Because of the restrictions the metal flows at high velocity

leading to more turbulence and chances of mold erosion.

Un-Pressurized Gating System

• The total cross sectional area increases towards the mold

cavity

• Restriction only at the bottom of sprue

• Flow of liquid (volume) is different from all gates

• Aspiration in the gating system as the system never runs

full

• Less turbulence.

Risers and Riser Design• Risers are added reservoirs designed to feed liquid metal

to the solidifying casting as a means of compensating for solidification shrinkage.

• To perform this function, the risers must solidify after the casting.

• According to Chvorinov's rule, a good shape for a riser would be one that has a long freezing time (i.e., a small surface area per unit volume).

• Live risers (also known as hot risers) receive the last hot metal that enters the mold and generally do so at a time when the metal in the mold cavity has already begun to cool and solidify.

Types of Risers

Gating ratio

• Gating ratio is defined as: Sprue area: Runner area:

Ingate area.

• For high quality steel castings, a gating ratio of 1: 2: 2 or

1: 2: 1.5 will produce castings nearly free from erosion,

will minimize oxidation, and will produce uniform flow.

• A gating ratio of 1: 4: 4 might favour the formation of

oxidation defects.

Chvorinov’s rule• Total solidification time (ts) = B (V/A) n where n = 1.5 to 2.0[Where, B = mould constant and is a function of (mould

material, casting material, and condition of casting] n = 2 and triser = 1.25 tcasting

2 2

riser casting

V V1.25A A

2

2

V D H /4DA DH 2 4

For cylinder of diameter D and height H

or

Padding• Tapering of thinner section towards thicker section is

known as 'padding'. • This will require extra material. • If padding is not provided, centre line shrinkage or

porosity will result in the thinner section.

Cooling Curve

Self study

• Moulding practices: • Green, dry and loam sand moulding, pit

and floor moulding; shell moulding; permanent moulding; carbon dioxide moulding.

Furnaces

• Melting is an equally important parameter for obtaining a quality castings. A number of furnaces can be used for melting the metal, to be used, to make a metal casting. The choice of furnace depends on the type of metal to be melted. Some of the furnaces used in metal casting are as following:

1. Crucible furnaces2. Cupola3. Induction furnace4. Electric arc furnace5. Rotary furnace6. Pit electric

Crucible furnace•The crucible is made of either a clay-silicon-carbide or a clay- graphite mixture. •The furnace can either tilt for pouring or the crucible can be lifted out

ROTARY FURNACE

OPEN HEARTH FURNACE DIAGRAM

Cupola• Cupola has been the most widely used furnace for melting cast

iron.• In hot blast cupola, the flue gases are used to preheat the air

blast to the cupola so that the temperature in the furnace is considerably higher than that in a conventional cupola. Coke is fuel and Lime stone (CaCO3) is mostly used flux.

• Cost of melting low.• Main disadvantages of cupola is that it is not possible to produce

iron below 2.8% carbon.• Steel can be also prepared in cupola by employing duplexing and

triplexing operations.

Description of CupolaThe cupola consists of a vertical cylindrical steel sheet and lined inside with acid refractory bricks. The lining is generally thicker in the lower portion of the cupola as the temperature are higher than in upper portion.There is a charging door through which coke, pig iron, steel scrap and flux is charged The blast is blown through the tuyeresThese tuyeres are arranged in one or more row around the periphery of cupolaHot gases which move up from the bottom (combustion zone) preheats the iron in the preheating zoneCupolas are provided with a drop bottom door through which debris, consisting of coke, slag etc. can be discharged at the end of the meltA slag hole is provided to remove the slag from the meltThrough the tap hole molten metal is poured into the ladleAt the top conical cap called the spark arrest is provided to prevent the spark emerging to outside

Operation of CupolaThe cupola is charged with wood at the bottom. On the top of the wood a bed of coke is built. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. The purpose of adding flux is to eliminate the impurities and to protect the metal from oxidation. Air blast is opened for the complete combustion of coke. When sufficient metal has been melted that slag hole is first opened to remove the slag. Tap hole is then opened to collect the metal in the ladle.

The Electric Arc

Furnace (EAF)

•The Electric Arc Furnace (EAF) uses only scrap metal.

•The process was originally used solely for making high quality steel. Modern electric arc furnaces can make up to 150 tones of steel in a single melt.•The electric arc furnace consists of a circular bath with a movable roof, through which three graphite electrodes can be raised or lowered.

• At the start of the process, the electrodes are withdrawn and the roof swung. The steel scrap is then charged into the furnace from a large steel basket lowered from an overhead travelling crane. When charging is complete, the roof is swung back into position and the electrodes lowered into the furnace. •A powerful electric current is passed through the charge, an arc is created, and the heat generated melts the scrap.

Direct Arc Furnace

• In the direct-arc method, there are two arcs, one from an electrode to the metal and another from the metal to the second electrode.

•In the indirect-arc method, the arc extends from one electrode to another and the heat is transferred to the metal by radiation.

Casting Processes

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

• "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 - capable of producing castings of high accuracy and intricate detail

Investment Casting

Figure. Steps in investment casting: (1) Wax patterns are produced, (2) Several patterns are attached to a sprue to form a pattern tree

Investment Casting

Figure 11.8 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.

Investment Casting

Figure 11.8 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

Investment Casting

Figure 11.8 Steps in investment casting: (7) The mold is broken away from the finished casting and the

parts are separated from the sprue.

Investment Casting

Figure 11 9 A one piece compressor stator with 108 separate ‑airfoils made by investment casting

(photo courtesy of Howmet Corp.).

Advantages and Disadvantages

• Advantages of investment casting:– Parts of great complexity and intricacy can be cast– Close dimensional control and good surface finish – Wax can usually be recovered for reuse – Additional machining is not normally required this is a ‑

net shape process• Disadvantages

– Many processing steps are required– Relatively expensive process

CONTINNOUS CASTING

• In this process the molten metal is continuously poured in to a mold cavity around which a facility for quick cooling the molten metal to the point of solidification.

• The solidified metal is then continuously extracted from the mold at predetermined rate.

In reciprocating process, molten metal is poured into a holding furnace. At the bottom of this furnace, there is a valve by which the quantity of flow can be changed.

The molten metal is poured into the mold at a uniform speed. The water cooled mold is reciprocated up and down. The solidified portion of the casting is withdrawn by the rolls at a constant speed.

The movement of the rolls and the reciprocating motion of the rolls are fully mechanized and properly controlled by means of cams and follower arrangements.

CONTINNOUS CASTING

CONTINNOUS CASTING

Continuous casting 1: Ladle. 2: Tundish. 3: Mold. 4: Plasma torch. 5: Stopper. 6: Straight zone.

Advantages & Application Advantages

• The process is cheaper than rolling• 100% casting yield.• The process can be easily mechanized and thus unit labor cost is

less.• Casting surfaces are better.• Grain size and structure of the casting can be easily controlled

Application• It is used for casting materials such as brass, bronzes, zinc,

copper, aluminium and its alloys, magnesium, carbon and alloys etc.

• Production of blooms, billets, slabs, sheets, copper bar etc.• It can produce any shape of uniform cross-section such as round,

rectangular, square, hexagonal, fluted or gear toothed etc.

Centrifugal Casting• In centrifugal casting process, molten metal is

poured into a revolving mold and allowed to solidify molten metal by pressure of centrifugal force.

• It is employed for mass production of circular casting as the castings produced by this process are free from impurities.

• Due to centrifugal force, the castings produced will be of high density type and of good strength.

Centrifugal Casting• The castings produced promote directional solidification as

the colder metal (less temperature molten metal) is thrown to outside of casting and molten metal near the axis or rotation.

• The cylindrical parts and pipes for handling gases are most adoptable to this process.

• Centrifugal casting processes are mainly of three types which are discussed as under.

(1) True centrifugal casting(2) Semi-centrifugal casting and(3) Centrifuged casting

True Centrifugal Casting• In true centrifugal casting process, the axis of rotation of

mold can be horizontal, vertical or inclined. Usually it is horizontal.

• Molten metal is poured into rotating mold to produce a tubular part

• In some operations, mold rotation commences after pouring rather than before

• 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

True Centrifugal CastingFigure Setup for true centrifugal casting.

Semicentrifugal Casting

• It is similar to true centrifugal casting but only with a difference that a central core is used to form the inner surface. Semi- centrifugal casting setup is shown in Fig. Below.

Semicentrifugal Casting

• This casting process is generally used for articles which are more complicated than those possible in true centrifugal casting, but are axi-symmetric in nature.

• A particular shape of the casting is produced by mold and core and not by centrifugal force. The centrifugal force aids proper feeding and helps in producing the castings free from porosity.

• Symmetrical objects namely wheel having arms like flywheel,gears and back wheels are produced by this process.

Centrifuge Casting

Mold is designed with part cavities located away from axis of rotation, so that molten metal poured into mold is distributed to these cavities by centrifugal force

• Used for smaller parts • Radial symmetry of part is not required as in

other centrifugal casting methods

Centrifuging casting setup

Shell Molding Casting process in which the mold is a thin shell of

sand held together by thermosetting resin binder

Figure. 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.

Shell MoldingFigure. 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;

Shell MoldingFigure. 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;

Shell Molding

Figure. 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.

Advantages and Disadvantages• Advantages of shell molding:

– Smoother cavity surface permits easier flow of molten metal and better surface finish

– Good dimensional accuracy - machining often not required

– Mold collapsibility minimizes cracks in casting – Can be mechanized for mass production

• Disadvantages:– More expensive metal pattern – Difficult to justify for small quantities

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

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 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.

Permanent Mold Casting

Figure Steps in permanent mold casting: (1) Mold is preheated and coated.

Permanent Mold Casting

Figure 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.

Advantages Advantages of permanent mold casting:• Good dimensional control and surface finish• More rapid solidification caused by the cold metal mold results

in a finer grain structure, so castings are stronger • No blow holes exist in castings produced by this method.• The process is economical for mass production.• Close dimensional tolerance or job accuracy is possible to

achieve on the cast product.• Casting defects observed in sand castings are eliminated.• Fast rate of production can be attained.

• The process requires less labor.

Limitations– Generally limited to metals of lower melting point – Simpler part geometries compared to sand casting

because of need to open the mold – High cost of mold

Applications of Permanent Mold Casting

• Due to high mold cost, process is best suited to high volume production and can be automated accordingly

• Typical parts: automotive pistons, pump bodies, and certain castings for aircraft and missiles

• Metals commonly cast: aluminum, magnesium, copper base alloys, and cast ‑iron

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 to force metal into die cavity is what distinguishes this from other permanent mold processes

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 ‑

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

• Applications limited to low melting point metals that do not chemically attack plunger and other mechanical components

• Casting metals: zinc, tin, lead, and magnesium

Hot-Chamber Die Casting

Figure Cycle in Hot chamber casting: ‑

(1) With die closed and plunger withdrawn, molten metal flows into the chamber

(2) Plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.

Cold Chamber Die Casting ‑Machine

Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure 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

• Advantages of hot chamber process favor its use ‑on low melting point alloys (zinc, tin, lead) ‑

Cold Chamber Die Casting‑

Figure Cycle in cold chamber casting: ‑(1) With die closed and ram withdrawn, molten metal

is poured into the chamber

Cold Chamber Die Casting‑

(2) ram forces metal to flow into die, maintaining pressure during cooling and

solidification.

Molds for Die Casting

• Usually made of tool steel, mold steel, or maraging steel (carbon free iron-nickel alloy with additional of cobalt, molybdenum titanium and aluminum)

• Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron

• Ejector pins required to remove part from die when it opens

• Lubricants must be sprayed into cavities to prevent sticking

Advantages and Limitations• Advantages of die casting:

– Economical for large production quantities– Good accuracy 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

Applications

1. Carburetor bodies2. Hydraulic brake cylinders3. Refrigeration castings4. Washing machine5. Connecting rods and automotive pistons6. Oil pump bodies7. Gears and gear covers8. Aircraft and missile castings, and9. Typewriter segments

ADVANTAGES OF DIE CASTING OVER SAND CASTING

1. Die casting requires less floor space in comparison to sand casting.

2. It helps in providing precision dimensional control with a subsequent reduction in machining cost.

3. It provides greater improved surface finish.4. More true shape can be produced with close tolerance in die

casting.5. Castings produced by die casting are usually less defective.6. It produces more sound casting than sand casting.7. It is very quick process.8. Its rate of production is high as much as 800 casting / hour.

Casting Quality

• There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product

• The defects can be classified as follows:– General defects common to all casting processes– Defects related to sand casting process

Probable Causes and Suggested Remedies of Various Casting Defects

Metal splatters during pouring and solid globules form and become entrapped in casting

Figure 11.22 Some common defects in castings: (c) cold shot

General Defects: Cold Shot

Balloon shaped gas cavity caused by ‑release of mold gases during pouring

Figure 11.23 Common defects in sand castings: (a) sand blow

Sand Casting Defects: Sand Blow

Formation of many small gas cavities at or slightly below surface of casting

Figure 11.23 Common defects in sand castings: (b) pin holes

Sand Casting Defects: Pin Holes


Open blows are smooth cavities or voids on the surface of the casting.

Blow holes are bubbles of gas entrapped inside the casting. Both are caused by gases carried by hot metal.


Shrinkage faults are faults caused by improper directional solidifications.


Porosity : It is caused by gases absorbed by the molten metal.Practically all metals absorb oxygen, hydrogen and nitrogen.Oxygen and nitrogen form oxides and nitrides respectively. Hydrogen is responsible for causing pin hole porosity.Molten metal picks up hydrogen from fuel, moisture in air and moulds.As the metal solidifies solubility of hydrogen decreases considerably. The hydrogen thus comes out forming a number of small holes distributed throughout the metal.


A Misrun is caused when the section thickness of a casting is so small or the pouring temperature so low that the entire section is not filled before the metal solidifies.


Hot tears are ragged irregular internal or external cracks occurring immediately after the metal have solidified. Hot tears occur on poorly designed castings having abrupt section changes or having no proper fillets or corner radii. Wrongly placed chills.


Penetration : If the sand grains used are very coarse or the metal poured has very high temperature the metal is able to enter the spaces between sand grains to some distance. Such sand becomes tightly wedged in the metal and is difficult to remove.


Cuts and washes are caused by erosion of mould and core surfaces by the metal flowing in the mould cavity. These defects are avoided by proper ramming, having sand of required strength and controlling the turbulence during pouring.


Cold shuts: It is an interface within a casting that is formed when two metal streams meets without complete fusion.


Inclusions : they are due to any foreign materials present in the cast metal.• These may be in the form of oxides, slag, dirt, sand or nails.


Fusion: When the mould sand does not have enough refractoriness or the metal is poured at very high temperature or the facing sand is of poor quality, the sand may melt and fuse with casting surface.• This makes it difficult to clean the castings and gives them a rough glossy appearance.


Shift: A mismatch is caused by the cope and drag parts of the mould not remaining in their proper position.


Drops : Drop in a Mould is an irregularly shaped projections on the cope surface of a casting.

Rat tails and buckles are caused by the expansion of a thin outer layer of moulding sand on the surface of the mould cavity due to metal heat.• A rat tail is caused by depression of a part of the mould under compression which appears as an irregular line on the surface of the casting.• A buckle is a more severe failure of the sand surface under compression.• The mould must provide for proper expansion instead of forming compressed layers to avoid this defect.



Buckles is a long, fairly, shallow, broad, vee depression that occurs in the surface of flat casting.

Rat- tail is a long, shallow, angular depression in the surface of flat casting.


Swells: • A swell is an enlargement or bulging of the casting surface resulting from liquid metal pressure.

Additional Steps After Solidification

• Trimming• Removing the core• Surface cleaning• Inspection• Repair, if required• Heat treatment

Trimming

Removal of sprues, runners, risers, parting line ‑flash, fins, chaplets, and any other excess metal from the cast part

• For brittle casting alloys and when cross sections are relatively small, appendages can be broken off

• Otherwise, hammering, shearing, hack sawing, ‑band sawing, abrasive wheel cutting, or various ‑torch cutting methods are used

Removing the Core

If cores have been used, they must be removed • Most cores are bonded, and they often fall out

of casting as the binder deteriorates.• In some cases, they are removed by shaking

casting, either manually or mechanically • In rare cases, cores are removed by chemically

dissolving bonding agent • Solid cores must be hammered or pressed out

Surface Cleaning

Removal of sand from casting surface and otherwise enhancing appearance of surface

• Cleaning methods: tumbling, air blasting with ‑coarse sand grit or metal shot, wire brushing, buffing, and chemical pickling

• Surface cleaning is most important for sand casting– In many permanent mold processes, this step can be avoided

• Defects are possible in casting, and inspection is needed to detect their presence

Heat Treatment

• Castings are often heat treated to enhance properties

• Reasons for heat treating a casting:– For subsequent processing operations such as machining– To bring out the desired properties for the application of

the part in service

Foundry Inspection Methods

• Visual inspection to detect obvious defects such as misruns, cold shuts, and severe surface flaws

• Dimensional measurements to insure that tolerances have been met

• Metallurgical, chemical, physical, and other tests concerned with quality of cast metal.

Lecture 2 casting full

Engineering

Transcript of Lecture 2 casting full