Causes of Defective Casting

A.A. DistortionDistortion

B.B. Surface roughness and Surface roughness and irregularitiesirregularities

C.C. PorosityPorosity

D.D. Incomplete/missing detailIncomplete/missing detail

A.A. DISTORTIONDISTORTION1. Due to distortion of the wax pattern- Proper

handling of wax pattern

2. Setting and Hygroscopic expansionexpansion of investment can produce a nonuniform expansion of walls of the wax pattern

3. Distortion increases as the thickness of the pattern decreases .

4. Less the setting expansion of investment- less distortion.

B.B. Surface roughness, Surface roughness, irregularities & Distortionirregularities & Distortion

1. Surface Roughness - relatively finely spaced surface imperfections

2. Surface Irregularities - isolated imperfections

• Nodules that are not characteristic of entire surface area

3. Surface RoughnessRoughness of the casting is always greater than the wax pattern

- Due to particle size of the investment

- Its ability to reproduce the wax pattern in microscopic detail.

• Excessive roughness, irregularities needs additionaladditional finishing and polishing

• Irregularities on the cavity surface prevent prevent Proper Seating Proper Seating of castingof casting

Air bubblesAir bubbles

• Small Nodules on the casting

- by air bubbles that becomes attached to the pattern during investing.

• Removal of nodules on the margins and the internal surface alter the fit of the casting.

• Vacuum mixing should be done

• Surfactant should be used.

Water filmsWater films• Water film separates investment from wax

pattern

• Such irregularity appears as minute ridge/vein on the surface.

• Surfactant should be used

• Too high liquid/powder ratio can also produce these irregularities.

Rapid heating rates• Causes fins/spinesfins/spines on the casting

• Due to flaking of the investment when water/steam pours into mold

• Steam can also carry salts used as modifiers into the mold - these salts are left as deposits on walls after water evaporates

• Mold should be heated gradually (60min to heat the investment from room temperature to 700°c)

• Greater the bulk of investment - the more slowly it should be heated.

• Incomplete elimination of the wax residue

• Voids or porosity occurs in the casting

- due to gases formed when alloy comes in contact with carbon residues

• Casting can also be covered by carbon coating which cannot be removed by pickling.

UnderheatingUnderheating

Liquid/powder RatioLiquid/powder Ratio

• Higher the Liquid/powder ratio - RougherRougher

the casting

• Too thickToo thick investment - Cannot be applied

properly to the pattern

Prolonged heatingProlonged heating

• When high heat casting technique used

- Disintegration of gypsum bonded investment

- Walls of the mold will be roughened.

- Sulfur compounds released which contaminates

the alloy and does not respond to pickling.

Temperature of the alloyTemperature of the alloy

• Temperature of alloy is too high

- Investment breaks down

- surface roughness results.

• If Gas Air TorchGas Air Torch is used the alloy cannot be overheated

• With other methods alloy can be over heated

Casting pressureCasting pressure

• Too high a pressure produces rough

surface on the casting

• 3-4 turns of spring in centrifugal casting

machine and 0.10-0.14 MPA in an air

pressure casting machine is enough.

Composition of investmentComposition of investment

• Ratio of binder to quartz influences the

surface texture of the casting.

• Coarse silica causes surface roughness.

Foreign bodiesForeign bodies

• When foreign substances enter the mold

- cause surface roughness (investment from rough crucible former or sprue can be carried into the mold)

- it can also cause incomplete casting and surface voids.

• Bright appearing concavities are the result of flux being carried into the mold with the metal.

Impact of molten alloyImpact of molten alloy

• Direction of sprue former - should not

strike a weak portion of the mold surface

which can cause cracks in the investment

Pattern positionPattern position

• Positioning too many patterns in the same plane must be avoided.

• Expansion of wax is much greater than that of the investment - causing breakdown/ cracking of investment if spacing between patterns is less than 3 mm.

Carbon inclusionsCarbon inclusions

• Carbon from the crucible

• Improperly adjusted torch

• Carbon containing investment

can be absorbed by the alloy and lead to formation of

Carbides.

Other causes

• Certain surface discoloration and roughness appear later during service.

• Mixtures of casting alloys cannot be used which possess lesser physical properties with low corrosion resistance

• Gold alloy can be contaminated by mercury which causes loss of ductility and corrosion.

C.C. PorosityPorosity

• Can occur both on the interior and external

surface of the casting (surface roughness)

• Internal porosity weakens the casting and if

it extends to the surface can cause

discoloration.

I.I. Solidification defectsSolidification defects

a. Localized shrinkage porosity

b. Microporosity

II.II. Trapped gasesTrapped gases

a. Pinhole porosity

b. Gas inclusions

c. Subsurface porosity

III.III. Residual airResidual air

Localized shrinkage porosityLocalized shrinkage porosity

• Caused by premature termination of the flow of molten metal during solidification.

• Linear contraction of the noble metal alloys from liquid to solid is 1.25% so a continual feeding of the molten alloy through the sprue must make up for the shrinkage.

• If sprue freezes before the casting a localized shrinkage void will occur in the last portion of the casting that solidifies.

• localized shrinkage occurs near the sprue casting junction.

• If a hot spot is created by the hot metal impinging on the sprue channel on a point of the mold wall.This hot spot causes the local region to freeze last and results in suck-back porosity.

• It usually occurs at occlusoaxial line angle which is not rounded.the metal impinges onto the mold surface at this point and creates a higher localized mold temperature known as hot spot.

• It retains a localized pool of molten metal after other areas of casting have solidified.

• This in turn creates suck back porosity

• It can be eliminated by flaring the point of sprue attachment and lowering the casting temperature.

Microporosity

• Occurs from solidification shrinkage but is present in fine grain alloy casting when the solidification is too rapid for the micro voids to segregate into the liquid pool.

• Due to premature solidification if the mold or casting temperature is too low.

• They are irregular in shape.

• It is not a serious defect and is not detected until the casting is sectioned.

Pinhole and gas inclusion porosities

• Due to entrapment of gas during solidification

• Both are spherical in shape but different in size

• gas inclusion porosities are larger than Pinhole porosity

• Many metals dissolve gases (copper and silver dissolve oxygen)

• On solidification ,the absorbed gases are expelled leading to pinhole porosity

• Larger voids are caused due to mechanically entrapment of air in the molten metal from a poorly adjusted torch flame or by the use of oxidizing zone rather than reducing zone.

• Castings which are severely contaminated by gases are black and do not clean on pickling.

Subsurface porosity

• Caused by simultaneous nucleation of solid grains and gas bubbles at the first movement that the alloy freezes at the mold walls.

• Can be controlled by the rate at which the molten metal enters the mold

Back pressure porosity

• entrapped air porosity seen on the inner surface of the casting produce large concave depressions

• Inability of the air in the mold to escape through the pores in the investment

• The entrapment is found on the cavity surface of the crown

• It can also be seen on the outer surface of the casting when the casting temperature or mold temperature is so low that solidification occurs before the entrapped air can escape.

• Dense investments

• Vacuum investing

• Mold clogged with residual carbon slows the venting of gases.

Incomplete casting

• Molten metal is prevented in completely filling the mold

• Insufficient venting

• High viscosity of fused metal

• Back pressure develops in Insufficient venting and if insufficient casting pressure is used it cannot be overcome.the pressure should be applied for at least 4 sec.

• Mold is filled and alloy is solidified in 1 sec yet it is quite soft during early stages.

• Incomplete elimination of the wax residue from the mold

• Pores in the investment gets filled and air cannot be vented completely.

• Margins are rounded and quite shiny rather than dull ,this is due to the strong reducing atmosphere created by carbon monoxide left by the residual wax.

• Lower liquid/powder ratio - less porosity of the investment

• Increase casting pressure should be used

• Incomplete casting also results from too great a viscosity due to insufficient heating.

• Temperature should be raised above the liquidus temperature to reduce the viscosity and surface tension of the alloy.