Castings

65tons, 7m high, 43cm dia,

1600years old

At 500/550 pound in weight .

The biggest Gravity Die Casting in The World at the Timehttp://www.rainwater.demon.co.uk/bertha.htm

CASTING AND RELATED PROCESSES

Fundamentals: 6 steps are involved in the casting process:

1. A mold/ cavity

• A mold for each cast part (Single-Use Molds)

• Permanent mold (Multiple-Use Molds, Metal or graphite)

• Shrinkage allowance

2. Melting (proper temperature, desired quantity& quality, reasonable cost)

3. Pouring (gas escaping, free of defects)

4. Solidification (the nucleation, the growth, free of cracks)

5. Mold removal (draft angle, extractor)

6. Cleaning, finishing & inspection

Casting : Forming the metal in liquid state by solidification in molds of desired

shapes.

Inexpensive, economic (no chips formed)

SAND CASTING – for any metal

Pattern, Flasks, Molds, Core, Cope, Drag, Risers, Gating, Runners, Sprue

Sand : a refractory material with additives will solidify (for molds)

-The pattern simulates the part when making the mold and packing the sand

-The pattern has to be next removed form the mold to leave place for metal

-The mold has to be made such that

it could be divided

-An inlet is required to bring the metal

to the mold – sprue hole

- System of channelsrunners, are required to bring metal to every place of

the mold :Gating system (controls the metal flow)

- Sometimes, a Core is required (for hollow parts) – cavity inside the mold

- PINS are required to position the two holding boxes: Flasks: DRAG & COPE

Solidification: 1. Nucleation

Undercooling

Impurities, Grain refinement

2. Growth

Directional Solidification

Cooling rate:

Faster Finer grains

Slower Larger grains

Solid-liquid phase cooling curve: shows the type of alloy

(Eutectic alloy)-Alloy with freezing range

High super heat Larger time to fill delicate shape

Thermal arrest plateau of latent heat of fusion

• Undercooling recalescence

Prediction of total solidification time: Chvorinov’s rule

ts =B (V/ A)n

V- volume of casting

A- surface area

B- mold constant

n- [1.5, 2]

Ts of Riser >> Ts of casting (In general 1.25 times)

Different Cooling rates, Ts Different grain and material properties

Faster Cooling Finer grains

Slower Cooling Larger grains

Cast Part Zones: Rapid Zone, Columnar Zone, Equi-axed zone

Pattern is not identical with the part to be cast

size modification ,

shape modification and

shape modifications according to the requirements of casting process

- made of wood, plastic on certain easy-to-machine metals

Pattern Design

- Dimensions of the pattern have to incorporate shrinkage allowances differ,

depending on the kind of metal, typically 1-2%

- Special shrink rules are used by the pattern makers

- Large holes and Surface are tapered to allow its withdrawal Draft angles (1%-4%)

- Draft angle requirement affect the thickness of the wall weight of the casting

- If machining is followed some finishing allowance have to be provided,

depending on the casting experience Shrink Allowances must be considered.

- Shrinking can cause a distortion of the casting allowances may be required as per

experience of designer to avoid such distortions.

Length:196mm, dia: 96mm

Different ways of doing it:

Machining allowance: 2mm

Melting point: 1520C, Room temp:20C

LCT: 20ppm 20um/m/C

Shrinkage Allowance on length = (196+2*2)*20e-6*1500=6mm

3mm on both sides (3%)

Shrinkage allowance on dia = (96+2*2)*20e-6*1500=3mm on dia.

Different Ways of Designing

Gating Flow System:

Reduce Gas Absorption (Less Turbulence, No vortex)

Regulate Flow ( larger area, sprue well, runner well)

Gas Release, Reaction with mold gas release porosity, voids

TYPES OF PATTERNS: For removable patterns

A. Solid Pattern – simple shapes – made of one piece only

B. Split Pattern – More complicated , made of two parts.

- half of the pattern will rest in the lower part of the mold (drag)

- the second half – in the upper part (cope)

- the split is at the parting line

C. Loose piece pattern – pattern must be divided to allow its removal

D. Gated pattern for serial production – made of metal and there are multiple

pattern on the same main branch

E. Match plate pattern for production – they include runners and gate systems

F. Follow Board – for patterns difficult to split this avoids making a cavity for a

solid pattern – for symmetrical patterns

G. Sweep Patterns- for rotary parts & straight sweep pattern for grooves

•Simple Shapes

•No. of castings is small

•Needs follow board to position the pattern

Split – Pattern

•Moderate quantities

•Pattern is split along the parting line

•Pattern halves are aligned with tapered pins

•Cope and drag molds are made with split patterns

Match Plate Pattern

•Large quantities of duplicate castings

•Split patterns are fastened to wood/metal match-plate

•Pattern plate is aligned to flasks with alignment pins

•Cope and drag molds are made separately and assembled

•Independent molding of cope and drag

•Gating, runner, riser can be part of match plate.

Loose Piece Pattern

•Sand castings of complex shapes

•When pattern removal is difficult with protruding sections

•Loose patterns are held by beveled pins or grooves

•Once the mold is formed, main segment is removed followed by loose

pieces

•Expensive

•Substitute for full-mold or investment casting

SAND MOLDS:

Made of SiO2 Traditional molding material

Requirement for Sand: in molds

1. Refractoriness – withstanding high temperature - basic to sand

2. Bond- retaining of shape, strength

3. Permeability – allowing the gases to escape

4. Collapsibility – permitting the metal to shrink – mold would not collapse

For molds, Green Sand is used and reused, (recycled)

Additives: mixed with the sand to obtain the required properties (mainly 2,3,4)

(clay (9%), Water (3%))

For Uniform Mixture, sand put into a muller

Other additives: Silicate of Sodium

(Na2 SiO3) 4%

- hardened when exposed to CO2 gas

SAND TESTING

STANDARDIZED METHODS to control the sand quality (size of grains,

moisture content, clay content, mold, hardness, permeability and bond ).

Size of grains – controlled by 11 standard series in decreasing mesh size –

shaken for 15 min and weighed.

Moisture – by electrical conductivity of small samples or by weight loss of 50g

sample after heated @ 1100 C

Clay- by washing the clay out of a standard amount of foundry sand (50g)

Permeability- with special equipment

Strength test – Special equipment (moisture gives the strength)

Hardness – Penetration of a ball loaded by a spring the some Brinell hardness test

Sand mold preparation

• Machines are used + help of an operator

JOLT – SQUEEZE, Roll – over molding machine

Two halves of a flask they can be slightly opened

• Highest density of sand should be obtained close to the pattern

• Special flexible diaphragm squeezing process can be applied for very

accurate casting.

• Before pouring metal, heavy weights are placed on top of the mold to prevent

separation of mold sections

• Great importance is attached to the hardness of the skin of the molds. There are skin

drying process involving heating of the surfaces and adding of some binders.

• Large floor molds – made on the floor – sand slings have been developed for

tamping of the sand (up to 12 m long molds. Pneumatic sand rams in helping in

extra tamping Larger molds can be in Pits – on the floor Sections of dried sand

can be used to make modular mold

Sand conditioning before the use (properties of conditioned sand)

• Sand is uniformly mixed with additives and has uniform moisture content

• Sand is aerated at uniform room temperature such that sand can be

processed through machines to make a mold

CORES: Allow producing holes and cavities inside the casts.

• Different methods in using of cores

• Cores may be classified as

- green sand cores – made of the same sand as the molds

- dry sand cores – with binders of increasing strength and are

dried more expensive sand

Green sand – cheaper good enough Made in special core boxes, sometimes

using special machines

Cores strength increased by adding silicate of soda Na2SiO3 followed by CO2 process

Na2SiO3 + CO2 Na2CO3 + SiO2 (colloidal)

Core making process – the same as for molds

Cores – need to be installed in molds

molds require recesses to accommodate cores, called CORE PRINTS.

Core require sometimes support, which is done by chaplets –

which become a part of the mold – (not removed)

• Core manufacturing – mechanised machines for very complicated cores,

for engine blocks

• Cores can be avoided by making the patterns

and molds in a special way shell molds

SHELL MOLDING

For smaller, more precise or 2 sizes much large than the third one parts

• MOLD:mixture of sand and resin (thermosetting plastic) stronger but more expensive

• Shell Mold has a form of two half – mold shells, which are clamped together before

pouring of metal

• Metal Pattern is required

Procedure to fabricate the shell:

1. Sand mixed with resin 6. Pattern builds 3-5 mm thick sand

2. Sand + resin slurry dump box 7. Shell placed in oven for 1 min

to cure the shell

3. Metal pattern is heated 8. The second shell is obtained by 1-6

4. Placed on top of dump box 9. Molds are assembled

5. Box inverted & kept 30 sec (few times) 10. Molds (more than 1) put in to a

flask reinforcement

Advantages:

• Close tolerances

• Smooth surfaces

• Low sand cost, as compared to metal dies

Disadvantages

• High cost of metal pattern, resin and equipment

FULL – MOLD PROCESS – one piece mold.

(LOST-FOAM CASTING)

Disposable pattern are used – made of materials which vaporises upon pouring of

metal Styrofoam or polystyrene made of one piece with sprue, gates and runners –

fixed to pattern

• No taper walls are required

• Used in case of only few castings (prototypes made around 20numbers)

• Very large casting could be made

• Complicated molds shapes used.

INVESTMENT CASTING PROCESS

• Single use patterns

• High precision, very good surface quality (0.2%)

• Good permeability – ceramic molding

STEPS

• Produce a master pattern

• Produce a master die

• Produce wax patterns

• Assemble wax patterns on a single sprue

• Coat the cluster with investment material (dip or spray)

• Produce the final investment

• Leave dry the investment material

• Heat the cast to remove the wax

• Set the mold in flask

• Preheat the cast (11000 C)

• Pour the melted metal

• Remove the casting from the mold

• Permeability assured by small cracks in the surface of the mold.

PERMANENT MOLD CASTING

PROCESS :

• Introduction in early 1900’s. Known for

Higher quality and larger quantity

• Uses gravity only to introduce metal to

the mold (made of steel)

• Cores – made usually of dry sand – used

for quite complicated shapes

• Better accuracy and surface finish than

sand casting

• Special vent holes have to be made for

gases no permeability in metal mold

• Very effective in production

• Materials : Al, Mg, & Zn alloys

– more recent:cast iron & steel

• Smaller, complicated parts, engine

pistons (large quantities)

• Certain shapes can be done (difficult for

large parts such as engine blocks)

Low Pressure Permanent Molds casting Process

Uses besides the gravity, some pressure to pour metal into permanent mold

- Easy to automate

• Vacuum Permanent Mold

- variation of the permanent mold casting

- Thin wall casting can be produced

- Max size: 10 lb

- Good mechanical properties of the component

DIE CASTING PROCESS

• Metal injected in to steel mold at high pressure and held under pressure during

solidifying period. Heavy machines are used

• Usually – non ferrous metals are used Al, Cu, Zn

• Very high accuracy and good surface finish are obtained

• The internal part of the work is porous

• Dies are very complicated – mold of many parts experience

• Must be cooled & ejectors must be included

• Only for high quantity production pieces, to justify the investment

STEPS:

1. Closing & locking the die

2. Metal injection and the die pressurized for a while

3. Metal has to solidify in the die

4. Opening of the die

5. Ejection of the casting + cleaning of the die

The pressure of the melted metal is variable during the pouring process (injection)

can be controlled improves density & reduces porosity

Pressures 2000 bar

Parts up to 80 lbs (size up to 600 mm)

Engine blocks, transmission housings

Die Casting machines : two types

A. Hot chamber die casting machine

- Hot metal held in a goose neck chamber

- The plunger forces the metal out, into the die

- It cannot handle metals at higher melting point temperatures, used mainly for

Zn, Pb, Sn based alloys

- Fast operation – up to 6 cycle/ min

B. Cold chamber die casting machine

- Cycle of operation – longer than for hot chamber machine (2 cycle /min)

- Operator used to pour metal and handle the parts

- Hydraulically operating machines (automated cycles)

- Metal cores are used and they have to be backed off when the die is opened

- Small vent and overflow holes are also made in the die

- Hard skin (good) & porous interior (bad)

- Excellent accuracy and finish

- Very thin wall and small draft (small weight)

Comparison: permanent mold – die casting

CENTRIFUGAL CASTING – for symmetrical & rotary parts

- Mold – spun up to 2500 RPM – depending on the cast part (vertical or horizontal axis)

metal introduced into the cavity is spread along the wall where it solidifies

- Molds are made of dry sand or metals

- Round cylindrical or similar shapes can be cast

- Only outside surface must have a mold cheaper

- Inner surface : cylindrical or parabolic – centrifugal force

- Light impurities tend to appear on the inner surface due to the centrifugal effect

improved purity of the metal good for bearings

- Dense external surface of the casting which solidifies first

Equipment – not expensive for small parts bearings, bushing, pipes, cylindrical

shapes are cost

SEMICENTRIFUGAL CASTING

-When the metal cast is achieved by both – gravity and centrifugal forces (low speed)

- A core is required for hollow shapes

- Gives more dense structures of metal

- Several casting in one cast

Centrifugal Casting Process: several casting are made in multi cavity mold

- To obtain more dense grain structure low rotational speed

CONTINUOUS CASTING

- Long continuous shapes, with shaped cross section can be cast by continuous casting

- Pieces are then cut after solidifying (steel, Al, Cu)

- Eliminates energy losses from re-heating, cost of ingots, molds, handling,

transportation

- Surface defects occur (cracks)

MELTING & POURING

- Melting in a furnace

- Requirements for furnaces:

* Provide the required temperature, quantity and quality

* Do not contaminate the metal

* Economical

* To be capable to control the melting process when additives are introduced

TYPES OF FURNACES

CUPOLAS – refractory lined, vertical cylindrical with a sand bed at the bottom

- wood placed at the bottom – a bed of coke over it

- After ignition and burn, alternate charge of coke and iron are added from the top

- 10% by weight of coke is used

- Limestone is added to increase the fluidity

- When iron melts, it drops down through the coke

- Blasting of air accelerates melting

- When enough metal is collected, the tap hole is opened an the metal flows out into

a ladle

- Simple, economical, but metal is contaminated with the coke ingredients

ARC FURNACES

- Do not introduce contamination from the heat source

- Can produce a very pure metal

- Melting is rapid

- Good melting and anti-pollution

- Can hold the molten metal for long time

- Heating occurs by lowering the electrodes so that an arc is created between them &

metal

- Melting in vacuum removes gases and assures higher purity

- The induction furnace and the mold enclosing are done directly

INDUCTION FURNACES

- Similar advantages as arc furnace

- Can be high frequency or low frequency induction furnace

- It has only one alternating – current primary coil

- A channel of molten metal forms secondary coil

- Low voltage, high amperage current is include in the secondary coil

- Some molten metal has to be used as a starter

- Capacity – up to 65 tons

POURING PROCESS + Transportation

- To bring metal from the furnace to the molds, the LADLES are used

- They can be hand or lift operated ladles

- The slag is prevented to enter the molds

- Automatic pouring systems can be used in automatic line foundries