Casting Defects

CASTING CASTING DEFECTSDEFECTS

Dr.Kamran AsadullahDr.Kamran AsadullahDem/LecturerDem/Lecturer

Science of Dental MaterialsScience of Dental Materials

DEFECTS IN CASTINGDEFECTS IN CASTING• Defective casting may be due to various

reasons and can be avoided or minimized by strictly following the fundamental rules and procedures.

CLASSIFICATION OF CASTING DEFECTS

1. Distortion2. Surface roughness3. Porosity4. Incomplete casting

• In addition, a casting can be contaminated due to:

— Over heating the alloy.

— Use of oxidizing zone of the flame.

— Failure to mix flux.

5. Sulfur compounds, formed by the breakdown of the investment when the ring is overheated.

DISTORTIONDISTORTION• Distortion of casting is usually due to

distortion of wax pattern.• Some distortion of wax occurs when the

investment hardness or due to hygroscopic & setting expansion.

• Some distortion of wax occurs during manipulation, b/c of the release of stresses.

Minimize distortion by: — Manipulation of wax at high temperature.— Investing pattern within 1hr after finishing.— if storage is necessary, store in refrigerator.

SURFACE ROUGHNESSSURFACE ROUGHNESS• Surface roughness can be usually be

traced to:

1. Air bubbles on wax pattern cause nodules on the casting.

Avoid by:— Proper mixing of investment.

— Vibration of mix or by vacuum investing.

— Application of wetting agent.

2. Too rapid heating cracks the investment resulting in fins.

Avoid by:— Heat the casting ring gradually 700

OC (in at

least 1hr).

3. W/P ratio: Higher W/P ratio gives rougher casting. Large particle size of investment has the same effect.

Avoid by:— Using correct W/P ratio & select investment

of correct particle size.

4. Prolonged heating causes disintegration of mould cavity liberating sulfur or its compounds.

Avoid by:— Complete the casting as soon as the ring is heated

& ready.5. Over heating gold alloy has the same effect. They combine with the metals in gold alloy forming

a sulfide film producing a dark casting, which cannot be cleaned by pickling.

6. Too high or too low casting pressure.

Avoid by:— Using 15lbs/sq inch of air pressure or 3 to 4 turns

of centrifugal casting machine.

7. Composition of the investment, proportion of the quartz & binder influences the surface texture of casting. Coarse silica will give coarse casting.

8. Foreign body inclusion shows sharp, well- defined deficiencies. Inclusion of flux show bright concavities.

INCOMPLETE CASTINGINCOMPLETE CASTING• Many factors are involved in incomplete

casting.

A. Insufficient amount of alloy:— If the quantity of the molten alloy is small

the molten metal or alloy will not reach to every part of the mould & causes in complete casting.

Overcome:— Correct quantity of molten alloy is used.

B. Number & Size of sprue:— If the no. & size of sprue are small, molten

metal will not reach in every part, causing incomplete casting.

Overcome:— Thick & more than one sprue are used.

C. Attachment of sprue:— If the sprue attached to the thin part of the

pattern may cause incomplete casting.

Overcome:— Sprue should be attached to the most bulky

part of the pattern.

D. Back pressure of Gases:— Gases present in the mould may cause in

complete casting.

Overcome:— By using the investment material which is porous

& allow the gases to escape.

E. Centrifugal force:— Molten metal enters into the mould slowly & may

become hard before reaching every part of mould f the arm of casting machine is small.

Arm small = Centrifugal force small

Arm Large = Centrifugal force large

F. Presence of dust particle:― Dust particle & investment particle present

in the mould cavity may block the way of entrance of molten metal causes incomplete casting or may be embedded on the surface of the casting.

G. Temperature:— If the mould is cool the metal become hard

causes incomplete casting.

Overcome:— Mould should be warm at the time of

insertion of metal or alloy.

POROSITYPOROSITY• Porosity may be internal or external.

• External porosity can cause Discoloration of casting.

• Severe porosity at the tooth restoration junction can even causes secondary caries.

• Internal porosity weakens the restoration.

• Porosities are classified as:1. Those caused by solidification shrinkage:— Localized shrinkage porosity irregular— Suck back porosity in— Micro porosity shape2. Those caused by gases:— Pin hole porosity usually— Gas inclusions spherical — Subsurface porosity in shape3. Those caused by air trapped in the mould (Back pressure porosity)

A. Irregular voids: Cause:1. Due to shrinkage on cooling of alloy.2. Inclusion of particles of investment

material.

Overcome:— By using the sprue of correct

thickness.— By Placing sprue at the bulkiest

portion of the pattern.

B. Spherical voids:

Cause:1. Occluded gases in the molten alloy &

released when set.

Overcome:― Heat mould upside down so that the

gases come out from the mould.

― Ovid over heating & prolonged heating of alloy.

C. Rounded margins: Regular large voids.

Causes:1. Back pressure effect: air unable to escape

from the mould.

Overcome:― Using adequate casting force.― Use investment of adequate porosity.― Place pattern not more than 6 to 8 mm

away from the end of the ring.― Providing vents in large casting.

D. Contamination: Causes:1. Due to oxidation when molten alloy is over

heated.2. Use of oxidizing zone of the flame.3. Failure to use flux.4. Due to formation of sulfur compounds.

Overcome:— Not overheating the alloy.— Use reducing zone of the flame.— Use flux.

DIMENSIONAL ERRORS IN CASTINGDIMENSIONAL ERRORS IN CASTING

PROBLEM CAUSE PRECAUTIONS

Casting too large

Excessive expansion

Use correct temperature

Use correct investment

Casting too small

Too little mould

expansion

Heat the mould sufficiently

Distorted casting

Distorted wax

pattern

Correct handling of wax

ROUGH SURFACE & FINS ON CASTINGROUGH SURFACE & FINS ON CASTING

PROBLEM CAUSE PRECAUTIONRough surface

Investment breakdown

Air bubbles on wax

(nodules on casting)

Weak surface of investment

•Avoid overheating of mould & alloy.•Correct use of wetting agent.•Vacuum investing•Avoid too high W/P ratio.

•Avoid dilution of investment from application of too much wetting agent

Fins on casting

Cracking of investment

•Avoid rapid heating of investment

POROSITYPOROSITYPROBLEM CAUSE PRECAUTION

Irregular voids Shrinkage on cooling of alloy

Inclusion of foreign particles

•Use correct sprue thickness.

•Attach sprue at thickest part of wax pattern

•Use reservoir.

•Heat mould upside down so that particles fall out.

Spherical voids Occluded gases in molten alloy

•Avoid overheating & prolonged heating of alloy.

Rounded margins: regular large voids

Back pressure effect: air unable to

escape from mould.

•Use adequate casting force.

•Use porous investment.

•Avoid wax residue in mould.

•Place pattern 6 to 8 mm away from the end of the ring.

•Use vents.

Porosity Turbulent flow of molten alloy into

the mould.

•Correct placement of sprue.