Sand Casting of Metals
Topics covered:
Defects in cast components and their remedies
1
Casting Defects
A defect may be the result of single clearly defined cause or of a combination of factors,
in which case necessary preventive measures are more obscure.
Defects or flows affect appearance or the satisfactory functioning of the casting and
the casting do not come up to the quality standard applied.
Any irregularity in molding process causes defects in cast components which may be
sometimes tolerated, eliminated with proper molding practice or repaired using
methods such as welding and metallization.
2
Classification of Defects
Defects found in casting are divided into three classes:
1.Defects which
can be noticed on visual examination or measurement of the casting.
2.Defects which exist
under the surface and are revealed by machining, sectioning and
radiography.
Material defects discovered by mechanical testing of the casting.
3.
3
Classification of Defects
1.
Defects caused by patterns and molding box equipment
2.
Defects due to improper molding and core-making materials.
3.
Defects due to improper molding sand mixing anddistribution.
4.
Defects caused by molding, core making , gating, etc.
5.
Defects due to improper mold drying and core baking.
6.
Defects occurring while closing and pouring the molds.
7.
Defects caused by molten metal.
8.
Defects occurring during fettling
9.
Defects due to faulty heat treatment.
10.
Defects due to cast metal
11.
Warpage
4
Defects caused by Patterns and Molding Box Equipment
1.
Mismatch or Mold Shift
2.
Variation in Wall Thickness of the Casting
3.
Fins, Flash and Stain
4.
Crush
5
Mismatch or Mold Shift
It produces castings which does not match at the parting line.
There is mismatching of the top and bottom parts of the casting at partying line.
Causes:
Worn or loose dowel pins in the patterns
Faulty placement of the two halves of the pattern.
Worn out, loose molding box clamping pins.
Remedies:
Remedies involve removing the causes listed above.
6
Variation in Wall Thickness of the Casting
Causes:
Worn out core boxes making oversized core.
Worn out core prints allowing core to move or float.
Inadequate core print sizes
Remedies:
Remedies involve removing the causes listed above.
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Fins, Flash and Strain
They usually occur at the parting line and result in excess metal which needs to be ground off.
It is the displacement of the sand while closing a mold, thereby deforming the mold surfaces.
Causes:
Bottom board of the casting too flexible.
Pattern plates are not sufficiently rigid to keep straight during ramming
Mold halves not properly fitting together due to improper fastening of the cope to the drag.
Movement of the mold makes a casting appreciably thicker than the pattern.
Patterns having insufficient taper and thus requiring excessive rapping for their withdrawal results in fins at the joint.
High metal pressures due to too large down sprue.
Remedies:
Remedies involve removing the causes listed above.
8
Crush
It is displacement of the sand while closing a mold, thereby deforming mold surfaces.
A crush shows itself as an irregular sandy depression in the casting.
Causes:
Excessive weight of the green sand mold.
Core prints too small for the cores.
Careless assembly of the molding boxes.
Remedies:
Remedies involve removing the causes listed above.
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Defects due to Improper Molding and Core making Materials
1.
Blowholes
2.
Drop
3.
Scab
4.
Pin-holes
5.
Metal Penetration and Rough Surface
6.
Hot Tears
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Blowholes
Blowholes are smooth, round holes.
Blowholes visible on the surface of a casting are called as open holes whereas blowholes occurring below the surface of the casting and not visible from outside
are termed as blowholes.
Blowholes may occur in clusters or they may be one large smooth depression.
Blowholes are entrapped bubbles of the gas with smooth walls.
Causes:
Not adequately vented mold.
Excess moisture in the molding sand.
Low permeability and excessive fine grain sand.
Rusted damp chills, chaplets and inserts
Cores neither properly baked nor adequately vented.
Presence of gas producing ingredients in the mold or core sands
Extra hard rammed sands
Remedies:
Remedies involve removing the causes listedabove.
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Drop
A drop occurs when cope surface cracks and breaks, thus the pieces of the sand
fall into the molten metal.
Causes:
Low green strength
Low mold hardness
Remedies:
Remedies involve removing the causes listed above.
12
Scab
The metal penetrates behind the surface layer of sand in a thin layer.
Causes:
Too fine sand.
Sand having low permeability.
High moisture content of the sand.
Uneven mold ramming
Intermittent or slow running of molten metal over the sand surface thereby producing intense local heating.
Remedies:
Remedies involve removing the causes listed above.
13
Pinholes
Pinholes are numerous very small holes revealed on the surface of the casting.
Causes:
Sand with high moisture content.
Sand containing gas generating ingredients.
Remedies:
Remedies involve removing the causes listed above.
14
Metal Penetration and Rough Surface
Molten metal enters into the space between the sand grains which causing metal penetration.
This defect appears as an uneven and rough external surface of the casting.
Causes:
High permeability
High grain sized sand
Low dry strength of the sand
Soft ramming
Remedies:
Remedies involve removing the causes listed above.
15
Hot Tears
Hot tears are internal or external ragged discontinuities or crack on
the
casting surface.
Immediately after solidification the metal have low strength; at this stage solid
shrinkage of the casting develops sufficiently high stresses and causes hot tears.
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Hot Tears
Causes:
Too much shrinkage of the metal while solidifying.
Very hard ramming and therefore excessive mold hardness.
High dry and hot strength of the mold.
Insufficient collapsibility of the core and sand mold.
Poorly designed casting and abrupt sectional changes.
Proper fillets or corner radii are not provided and chills are inappropriately placed.
Too low pouring temperature
Improper placement of gates and risers can also create hot tears.
High Sulphur content promotes Hot Tears.
Remedies:
Method to prevent hot tears may involve improving the casting design, achieving directional solidification and
even rate of cooling all over, selecting proper mold and controlling the mold hardness in relation to other ingredients of sand.
17
Defects due to Improper Sand Mixing and Distribution
The defects due to improper sand mixing and distribution give rise to faulty sand
conditions and various defects as follows:
1.Blowholes
2.Drop
3.Scab
4.Pin-holes
5.Metal Penetration and
6.Hot Tears
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Rough Surface
Defects due to Molding, Core making, gating, etc.
1.
Hot Tears
2.
Shifts
3.
Fins and Flash
4.
Crush
5.
Cold Laps or Misrun
6.
Slag holes
7.
Shrinkage Defects
19
Cold Laps or Misrun
If the molten metal has low temperature or casting section is too thin, the entire mold cavity may not be filled during
pouring before molten metal starts solidifying thus result in Misrun.
If molten metal enters the mold cavity through two or more ingates and two streams do not fuse together in the mold
cavity, they develop cold shut.
Causes:
Low temperature of the molten metal.
Too this casting sections.
Too small ingates.
Too many restrictions in the gating system.
Metal lacking fluidity.
Intermittent floe of the metal from ladle or intermittent metal pouring.
Remedies:
Remedies involve removing the causes listed above.
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Slag Holes
They are smooth depressions on the surface of the casting.
They usually occur near the ingates.
Causes:
Slag holes occur when the slag enters the mold cavity.
Remedy:
They can be avoided by entering the slag traps in the gatingsystem
The runner extension helps to accumulate the slag and avoids the slag holes.
Use of skimmer core.
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Shrinkage
Shrinkage can reduce the 5-10% volume of the cast.
Gray cast iron expands upon solidification due to phase
changes. Need to design part and mold to take this amount
into consideration.
If the shrinkage is not compensated by providing
risers, the
voids will occur on the surface or inside the casting.
Remedy:
Shrinkage defect can be reduced by decreasing the
number of walls and increasing the draft angle.
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Defects caused by molding, core making , gating, etc.
1.
A layer of moisture collected under the impervious layer of mold paint causes sand
and paint to scab, peel, creates blowholes, etc.
2.
Oil sand cores, if over-backed, are not strong enough to resist the flow of molten
metal
which cause sand washes, rough surface, metal penetration and
undersized holes.
3.
Oil sand cores, if under-backed, absorb moisture from atmosphere or green sand
mold and cause defects like core sand wash, blowholes, etc.
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Defects occurring while Closing and Pouring the Molds
1.
Shifts or Mismatch
2.
Misrun
3.
Cold Laps or Cold Shuts
4.
Crush
5.
Run Out
6.
Inclusions
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Run Out
Run out occurs when molten metal leaks out of the mold during pouring and
result in incomplete casting.
Causes:
Faulty molding boo equipment.
Faulty molding procedure.
Remedies:
Remedies involve removing the causes listed above.
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Inclusions
Any undesirable foreign material present in the casting is known as inclusion.
An inclusion may be oxide, slag, refractory materials, sand, dirt, etc. which
enters the mold cavity along with the molten metal during pouring.
Remedies:
Skimming off or screening of the molten metal before pouring into the mold
cavity .
The provision of choke in the gating system and the pouring basin at the
top of the mold can prevent this defect.
Molding sand should possess adequate hot strength
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Defects caused by molten metal
1.
Misruns
2.
Cold Shuts
3.
Excessive Penetration
4.
Tin and Lead Sweat
5.
Hot Tears
6.
Sand cuts and washes
7.
Fusion
8.
Gas porosity, Gas holes, Sponginess
9.
Shot Metal
10.
Rattails and buckles
11.
Swell
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Porosity
Gas porosity is caused by gases absorbed by the molten metal.
The main gases dissolved by practically all metals are – oxygen, nitrogen and hydrogen.
Molten metal may absorb hydrogen from, unburnt fuel gas, moisture in the air, dampness in the furnace and green sand mold.
Porosity occurs in material as they change state, from liquid to solid during the casting process.
Causes:
Hydrogen and Sulphur Dioxide dissolved in molten metal
Excessive high pouring temperature.
Damp ladles
Low permeability of sand
High moisture content of the mold
Slow solidification rate
Remedies:
Removal of dissolved gases from mold.
The poring temperature should be maintained properly to reduce porosity.
Adequate fluxing of metal and controlling the amount of gas-producing materials in the molding and core making sand mixes can help in minimizing this defect.
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Swell
It is an enlargement of the mold cavity due to molten metal pressure on the mold
wall and the sides.
Causes:
Soft ramming
Low strength of mold
Mold being not adequately supported.
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Warpage
Castings warp, misalign or deform because of the stresses set up in them internally.
Stresses are generated due to differential solidification rates experienced by
different sections of large, long castings.
Causes:
Faulty casting design.
Absence of directional solidification.
Low green strength of the sand mold
Insufficient gating system that may not allow rapid pouring of metal or due to low green
Inappropriate draft allowance on the pattern
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