Mold Preparation, Melting and Casting

Mold Preparation, Melting and Casting

Aim: To prepare a green mould for casting using only two boxes.

Tools And Pattern: Wooden pattern is made in two halves, dowelled together, the division passing through the center of the grooves; cope and drag moulding tools parting sand, brick dust etc.

Stage Sketches:
The mould can be prepared by using three boxes without any difficulty. However the same can be prepared using two boxes using an ingenious method known as false-core method.


Procedure:
1. One half of the pattern is molded in the bottom box, the parting being cut an incline as shown. The other half pattern is then placed in position and sand poured and rammed to form the second parting with a slope down wards from the upper flange of the pulley.

2. The top box is next placed on the bottom box and properly located. Sand is poured and rammed without damaging the false core.

3. The top box is gently removed; the upper half pattern is gently taken out from the top box.

4. The top box is replaced on the drag and the entire mould is turned upside down. The bottom box, which now is at the top, is gently lifted and the remaining half of the pattern is withdrawn.

5. The bottom box is replaced and the mould id inverted. The spruces are removed, pouring basin is cut and the mould is finished after piercing holes (vents).

Observations:
1. After ramming using moulds hardness tester check the mould hardness on all the four sides of the pattern.
2. Locate the rumen and riser 900 exactly.

Precautions:
1. Ramming should be uniform to impart uniform strength to the mould.
2. Apply parting sand at the partitions for esy separation of boxes.
3. Locate the two halves of pattern properly to avoid mismatch.

Result: Sand mould is prepared for the given pattern.

Melting Procedure For Aluminum Alloys:-

The charge materials, chemicals should be free from moisture, oil, and corrosion powder and should be preheated before charging. The calculation of charge should be done considering the melting loss of each element in the melting furnace for final desired analysis.
1. The furnace crucible should be clean and red hot for charging.
2. Aluminium alloys get readily oxidized and form dross, using 
proper covering top with flux and chemicals help to reduce this. Different proprietary chemicals are available for different alloys.
3. Melting should be done under steady conditions without agitation. Stirring is done to reduce gas pickup.
4. Once melting is complete, degassing using solid chemicals like hexachloro-ethane which evolves chlorine by purging with nitrogen or argon gas is done to remove the dissolved hydrogen. Hydrogen is evolved from moisture.
3H2O+2Al→Al2O3+6H
Hydrogen absorbed by liquid metal causes serious porosity in casting during solidification.
Degassing should be done in the temperature range 7300  to 7500 Degree centigrade.
5. Liquid metal after degassing is treated with sodium containing chemicals to improve mechanical properties.
6. Liquid metal once ready should not be super heated. Agitated or kept long in the furnace which will cause dressing and gas pickup. Dross should be skimmed properly before pouring.
7. Alloys containing magnesium should be melting carefully as it is highly reacting. Special fluxes and chemicals like sulphur are used to inhibit the reactivity and prevent spontaneous ignition, melting loss and dross.

Casting Defects Due To Improper Melting:

1. Improper chemical analysis: Incorrect charge, calculations, including wrong estimates of melting losses, metal recovery, excessive losses due to improper fluxing and slogging operations, improper covering of non-

Ferrous melt causes this defect.

2. Gassy metal/hydrogen pickup/pinhole porosity: unclean melting causes formation and absorption of hydrogen into liquid metal. As casting solidifies, the absorbed hydrogen losses solubility and forms cavities inside casting.
3. Oxygen absorption
Excessive oxygen furnace operations in atmosphere following oxidation during melting cause this defect. It also causes loss of costly metal added in the charge.
4. Slag inclusions
Improper fluxing and slag removal slag particles to be mixed in the metal being poured. Careless pouring, lip pouring for alloys with fluid slag causes slag particles to enter casting.
5. Cold shut, misrun, unfilled castings
Low pouring temp, delay in pouring, due to many folds being poured, loss of heat from lable, due to improper covering failure of ladle opening in the bottom pouring cause premature solidification of metal causing defects.
6. Sand fusion, metal penetration, rough surface
Excessive pouring temp of liquid causes damage to the casting surface by attacking mould surface.
7. Sand erosion sand inclusions
Uncontrolled high pouring rate from ladle into mould leads to erosion of mould/core
PRECAUTIONS:
1. The furnace crucible should be clean and red hot for charging
2. Charge material should be free from oil, moisture etc.,
3. Melting must be done under steady condition to reduce gas pickup.
RESULT:
Melting practice is observed.

Applications:

• Transport : Automobile, aerospace, railways and shipping
• Heavy Equipment : Construction, farming and mining
Machine Tools : Machining, casting, plastics molding, forging, extrusion and forming
• Plant Machinery : Chemical, petroleum, paper, sugar, textile, steel and thermal plants
• Defence : Vehicles, artillery, munitions, storage and supporting equipment
• Electrical Equipment Machines : Motors, generators, pumps and compressors
• Hardware : Plumbing industry pipes, joints, valves and fittings
• Household : Appliances, kitchen and gardening equipment, furniture and fittings
• Art Objects : Sculptures, idols, furniture, lamp stands and decorative items

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