CAN BE CLASSIFIED AS:
 1) Distortion
2) Surface roughness and irregularities
3) Porosity
4) Incomplete or missing detail
1. Distortion: Caused mainly by distortion of wax pattern due to – Improper manipulation of wax and handling of pattern. Setting and Hygroscopic expansion of investment. Distortion increases as thickness of pattern decreases.
Surface roughness/ irregularities / Discoloration:
2. Surface roughness : relatively finely placed surface imperfections with their height/ width / direction producing a distinct surface pattern.
CAUSES:
a) Air bubbles:
May become attached t o pattern during or subsequent to investment.
Produces small nodules on casting
Nodules on margins/internal surfaces alters fit of casting
Avoided or overcome by:
Proper mixing of investment using vaccum investing tech/mechanical mixer with vibration
Application of a thin layer of wetting agent
Water Films:
– Formed due to loss of intimate contact of investment with wax pattern of slight jarring of pattern / vibration after investing / improper painting procedure / increases L:P ratio
– Appears as minute ridges or veins
Avoided or overcome by:
Proper application of wetting agent
Optimal L:P ratio
C) Rapid heating:
– Causes due to flaking of investment or modifier salts carried into mold and left as deposit on walls.
Avoided or overcome by:
Gradual heating of mold (60 mins elapse from room temp to 7000C
D) Under heating:
Results in incomplete elimination of wax (carbonaceous) residues.
Voids or porosity occurs in casting / tenacious carbon coating on casing surface
Avoided or overcome by:
Employing optimal burnt out temperature
E) Liquid Powder ratio:
Higher liquid: Powder ratio / Larger particle size – rougher casting
Low L:P ratio – investment unmanageably thick and improperly applied to pattern.
Avoided or overcome by:
– Using correct L:P ratio and selecting investment of correct appropriate particle size
F) Prolonged heating
Causes disintegration of investment and decomposed sulphur compounds contaminates alloy.
Produces rough and irregular castings
Avoided or overcome by:
– Heating mold to casting temperature and casting immediately
G) Casting pressure
– Too high / too low casting pressure avoided, produces rough surface.
In air pressure casting machine – 0.10 – 0.14 MPa gauge pressure maintained.
Centrifugal machine – 3-4 turns of spring sufficient.
 H) Foreign bodies
Bits of investment carried into mold with molten alloy/ bits of carbon from flux
Appears as sharp, well-defined deficiencies or bright appearing concavities.
Avoided or overcome by:
– Use of clean crucible former / ring
Elevated temperature avoided (prevents investment disintegration)
I) Impact of molten alloy:
Improper direction of sprue former /undesirable turbulence of alloy
Molten alloy strikes a weak portion of mold surface
Causes fracture or abrasion of mold surface
Avoided or overcome by:
Proper sprucing (450 angle)
J) PATTERN POSITION:
Many patterns if placed close together in same plane
Expansion of wax – than investment
Cracking / breakdown of investment
Avoided or overcome by:
Spacing between patterns – not less than 3mm
K) Investment composition:
Proportion of quartz and binder influences casting surface texture
Coarse silica – produces surface roughness.
Avoided or overcome by:
Using material that meets requirements of A.D.A No. 2
III Porosity
Internal – weakens casting
External – May cause roughness / discoloration.
I Solidification Defects
Localized shrinkage / Shrink spot porosity
Occurs near sprue-casting junt
Due to incomplete feeding of molten metal during solidification continual feeding of metal imp (to compensate for shrinkage of metal volume due to solidification)
Over come by –
– Making sprue thicker than pattern to be cast
b) Microporosity
Generally not considered as a serious – defect
Seen in fine grain alloy castings
Occurs from rapid solidification if mold / casting temp is too low
Defect usually not detectable unless casting sectioned
Hot spot / suck back porosity
Often occurs at occlusoaxial incisoaxial line angles that is not well rounded – entering metal impinges at this point – creates increased localized mold temp – ‘Hot spot’ – retains molten metal after solidification of other areas – creates shrinkage void – ‘Suck back porosity’
Overcome by :
Flaring point of sprue attachment / lowering casting temp by about 300C
II : Trapped Gases : 3 types:
Pinhole porosity
Subsurface and
Gas inclusion porosity
– Formed due to gas entrapment during solidification
– Spherical in contour
– Gas inclusion porosity larger than pinhole porosity
PIN HOLE POROSITY
Occurs when metals dissolve or occlude gases when molten, and during solidification – gas expelled to form blebs / pores in metal
Porosity extends to surface in form of small pinpoint holes
GAS INCLUSION POROSITY
Caused by gas mechanically trapped by molten metal or gas occluded from a poorly adjusted torch flame / use of mixing or oxidizing zones of flame
Large voids produced
Over come by:
Correct positioning and adjusting of torch flame
Premelting alloy on graphite block / crucible (If alloy had been used before)
Subsurface porosity
Caused by simultaneous nucleation of gas bubbles and solid grains when metal freezes at mold walls
Overcome by:
– Controlling rate of entry of molten metal into mold
III: RESIDUAL AIR:
Back pressure porosity / entrapped air porosity
Caused due to incomplete venting of gases from mold
Produces large concave depressions
Incidence of entrapped air increased – in dense investments
– Low heat technique
Overcome by:
Proper burnout
Adequate mold and casting temp
Higher casting pressure
Proper L:P ratio
IV) INCOMPLETE CASTING:
Causes:
Insufficient venting of mold – backpressure exerted by air
High viscosity of fused metal (due to insufficient heating)
Premature solidification of alloy
Sprue’s blocked with foreign bodies
Insufficient casting pressure
Insufficient alloy used
Surface irregularities: Isolated imperfections do not characterize the total surface area.
Eg: Nodules