Casting DefectsNumerous opportunities to go wrong in a casting operation

Defects may occur due to one or more of the following reasons: Fault in design of casting pattern Fault in design on mold and core Fault in design of gating system and riser Improper choice of mould and core material Moulding and core making equipments Inadequate melting temperature and rate of pouring Metal composition

Casting Defects1. Blow holes2. Porosity3. Shrinkage4. Hot tears5. Drop6. Metal Penetration7. Fusion8. Shot metal9. Shift10. Swells11. Hard Spots12. Warpage

Classification of casting defectsCasting defects

SurfaceDefect

Internal Defect Visible defects

BlowScar

BlisterDropScab

PenetrationBuckle

Blow holesPorosityPin holesInclusions

Dross

WashRat tailSwell

MisrunCold shutHot tear

Shrinkage/Shift

SurfaceDefects

Poor design and quality of sand moldsCasting Defects

SurfaceDefects

Poor design and quality of sand molds

Blow is relatively large cavity produced by gases which displace molten metal from convex surface

Scar is shallow blow generally occurring on a flat surface A scar covered with a thin layer of metal is called blister

These are due to improper permeability or venting or excessive gas forming constituents in moulding sand

Casting Defects SurfaceDefects

Casting Defects

Drop is an irregularly-shaped projection on the cope surface caused by dropping of sand

Penetration occurs when the molten metal flows between the sand particles in the mould (fluidity of liquid metal is high). These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it. Lower pour temperature or decrease sand permittivity

SurfaceDefects

Casting Defects

A scab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold

Buckle is a vee-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face. A proper amount of volatile additives in moulding material could eliminate this defect by providing room for expansion

SurfaceDefects

Casting Defects

Internal Defects

The internal defects found in the castings are mainly due to trapped gases and dirty metal. Gases get trapped due to hard ramming or improper venting. These defects also occur when excessive moisture or excessive gas forming materials are used for mould making

Blow holes are large spherical shaped gas bubbles Pin holes are tiny blow holes appearing just below the casting

surface Inclusions are the non-metallic particles in the metal matrix Lighter impurities appearing on the casting surface are dross

Decrease pour temperature or increase sand permittivity

Casting Defects

Internal Defects

Porosity is pockets of gas inside the metal caused by micro-shrinkage

Microporosity -Network of small voids caused by localized solidification shrinkage. Caused by the freezing manner of the alloy

Casting Defects

Visible Defects

Insufficient mould strength, insufficient metal, low pouring temperature, and bad design of casting are some of the common causes

Wash is a low projection near the gate caused by erosion of sand by the flowing metal

Rat tail is a long, shallow, angular depression caused by expansion of the sand (compression failure of the skin of the Mold Cavity due to high expansion of sand mould wall)

Increase hot strength of sand

Casting Defects

Swell is an enlargement of casting surface because the mould wall moves backward due to liquid metal pressure and thus rise of cavity

Casting DefectsVisible Defects

Causes :Soft ramming of mouldLow strength of mould coreMould not properly supportedRemedies :Provide harder rammingIncrease strength of mould and coreProvide adequate support to mould

Misrun metal solidified before completely filling cavity

Casting DefectsVisible Defects

Increase pour temperature or increase height of downsprue

Cold shut metal has flowed together but failed to fuse or blend into a solid mass.

Cause - insufficient superheat provided to the liquid metal

Hot tear is the crack in the casting caused by high residual stresses due to inability to shrink naturally

Resolve by mold collapsing or removing from the mold immediately after freezing.

Casting DefectsVisible Defects

Causes:Lack of collapsibility of core and mouldFaulty designHard rammingRemedies:Improve collapsibilityModify designProvide soft ramming

A hot crack is a crack formed during cooling after solidification because of internal stresses developed in the casting

Casting DefectsVisible Defects

Shrinkage is essentially solidification contraction and occurs due to improper use of Riser

Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze

Increase riser volume to surface area ratio

Casting DefectsVisible Defects

Shrinkage porosity

Shift is due to misalignment of two parts of the mould or incorrect core location

Mold shift - shift of the cope relative to the drag Core shift - shift of the core, usually verticalCauseImproper support and location of core MisalignmentWorn out clamping pinsFaulty core boxesRemediesRepair or replace the pinsProvide adequate support to coresLocate the core propertyRepair or replace the core boxes

Casting DefectsVisible Defects

Mold crack - mold strength insufficient, liquid metal form a fin of the final casting

Casting DefectsVisible Defects

Casting Defects

Hard spots on surfaces are generally developed on iron castings rich in silicon content due to local chilling of those spots by moulding sandCauses :Faulty metal compositionFaulty casting designToo much moisture in molding sand

Improper design of die i.e. metallic mould

Remedies :Suitable change in the metal compositionModify the casting design

Hard spots make themachining difficult andincrease cracking tendency

General Defects

Casting Defects

Warpage

Causes : Improper directional solidification. Continuous large flat surfaces.Remedies : Provide harder ramming. Increase strength of mould and core. Provide adequate support to mould.

General Defects

Casting Defects

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

Remedies :

Redesign of pouring procedure or gating system is needed

Increase diameter of pouring cup or slow down pour

General Defects