CASTING PROCEDURES AND DEFECTS Presented by: Dr. Pallavi B. Gopeshetti, CODS, Dvg.

CONTENTS CONTENTS 

      

Introduction & History Definition Dimensional changes in lost wax technique Accuracy of lost wax technique Steps in metal casting procedure Direct & Indirect wax patterns Impression making & Dies Preparing the wax pattern for investing -variables & principles of sprue design -crucible former -casting ring -ringless casting system -casting ring liners.



  

  

Investing the pattern -investment materials -hand investing -vacuum investing -high & low heat technique -controlled water- added technique Burnout of the wax pattern & heating of the ring Accelerated casting method Casting procedure Gas –torch flame -melting of the alloy Electrical energy -casting crucibles -casting machines -a) air pressure casting machines b) vacuum pressure casting machines c) centrifugal casting machines -casting technique Recovery & cleaning the casting Finishing the casting Cementing the casting



 

CASTING DEFECTS -distortion -surface roughness & surface irregularities -porosities -discoloration - incomplete casting References Conclusion

INTRODUCTION

HISTORY

WILLIAM H.TAGGART(1855-1933), He demonstrated on fabrication of cast inlay restoration developed in 1905 before the New York Odontological Society in 1907.

CASTING is defined as the process in which molten material is poured into a mold and allowed to hardened into the shape of the mold.

LOST WAX TECHNIQUE is so named because wax pattern of a restoration is invested in a investment material, then the pattern is burned out (lost) to create a space into which molten metal is placed or cast.

DIMENSIONAL CHANGES IN LOST WAX TECHNIQUE Wax shrinkage + metal shrinkage(1.25-2.5%)= setting expansion+ hygroscopic expansion + thermal expansion

ACCURACY OF THE LOST WAX TECHNIQUE Tolerance limit ±0.05% (± 2µm)

STEPS IN METAL CASTING PROCEDURES STEP 1: cavity preparation in the tooth STEP 2: Direct wax pattern OR impression of the prepared tooth STEP 3: Preparation of the die from impression & indirect wax pattern on the die STEP 4: Preparing the wax pattern for investing, includes sprue design, crucible former, casting ring & liners

STEP 5: Investing the pattern STEP 6: Burnout of the wax pattern STEP 7: Casting procedures STEP 8: Cleaning the casting STEP 9: Finishing the casting STEP 10: Cementing the casting.

STEP 1 CAVITY PREPARATION

STEP 2 A) DIRECT WAX PATTERN – Type I medium wax Indication….

B) IMPRESSION OF THE PREPARED TOOTH

STEP 3

A) PREPARING A DIE FROM IMPRESSION Most commonly used…dental stone IV & V Disadvantage…. Modifications….DIVESTMENT  Other die materials….  Recent advances 

B) INDIRECT WAX PATTERN : type II soft wax Die spacer: <0.5mm/ 10-30µm Lubricants

Methods for removal of wax pattern 

MOD PATTERN

MO/ DO PATTERN  CROWN PREPARATIONS 

STEP 4: PREPARING WAX PATTERN FOR INVESTING VARIABLES & PRINCIPLES OF SPRUE DESIGN SPRUE is defined as… Purpose of the sprue… 



SPRUE FORMER

Type of sprue formers

waxes

metals

plastics

SPRUE DESIGNS

5 PRINCIPLES OF SPRUE DESIGN

SPRUE DIAMETER :   



  

Diameter range: 6- 12 gauge( 4.1- 2mm) Too large??? Too small??? Narrow sprues indicated in….

Large inlay- 14 gauge( 1.6mm) Small inlay- 16 gauge (1.2mm) Large crowns, molars or metal ceramic crowns- 10 gauge (2.5mm) Small crowns or premolars or partial coverage restorations- 12 gauge (2.0mm)

SPRUE LENGTH:   

 



Average length… Too long sprue??? Too short sprue??? Large inlays: 4-5mm Small inlays: 3-4mm Large & small crowns: 4-5mm

SPRUE POSITION: 

Bulkiest portion of the pattern because….



Largest non-centric cusps.

SPRUE ATTACHMENT: 

Flared attachment/ constricted attachment.



At the largest cross-section of the pattern.



Direct spruing



Indirect spruing

Reservoir:

SPRUE DIRECTION: 

Away from thin delicate parts of the pattern



45˚ angulations…



Not at 90˚ angulations…

CRUCIBLE FORMER:

VENTING:

CASTING RING: 

Average size: 29mm(1 1/8 “) diameter 38mm(1 ½”) in height



Split rings, flexible rubber rings.

“RINGLESS CASTING SYSTEM.” 

Phosphate-bonded investments



Paper & plastic casting rings



“Powercast ringless system”- whip-mix

CASTING RING LINERS: Asbestos liners Non-asbestos liners

Aluminosilicate ceramic liner Cellulose liner

Application – wet - dry

Advantage??

STEP 5: INVESTING THE PATTERN INVESTMENT MATERIALS GYPSUM-BONDED INVESTMENTS: -casting of gold metal alloys, melting point below 1200˚c for crowns & bridges & inlays

Properties: -expansion -porous -strength -heating never >700˚c

PHOSPHATE- BONDED INVESTMENTS: -alloys with higher melting range(>1200˚c) like  metal ceramic restorations,  higher melting gold alloys,  base metal( chrome- cobalt),  pure titanium & titanium alloys.

Properties: -

Expansion Porosity Strength Temperature

SILICA- BONDED INVESTMENTS: -

Alloys with high melting range like



Base metal alloys Chrome- cobalt alloys to make cast removable partial dentures



Disadvantage…..

3.25mm

6mm

Wax pattern

Casting ring Ring liner sprue

Crucible former

Investment material 3.25mm

INVESTING TECHNIQUE  



Preparing the pattern- wetting agents… Pattern should not stand for >20-30 mins Methods for investing-hand -vacuum

Preweighing of powder & liquid

HAND INVESTING: Mechanical mixer-100 turns

Paint investment on the pattern

For gypsumbonded....

Fill the casting ring

For phosphatebonded investment…

VACUUM INVESTING:

VACUUM MIXING:

VACUUM INVESTING: Technique…vac-u-spat investor Advantage ???

LOW HEAT TECHNIQUE: NORMAL SETTING EXPANSI ON (0.7%)

LOW-HEAT TECHNIQUE HYGROS COPIC EXPANSI ON (1.5%)

HIGH HEAT TECHNIQUE: NORMAL SETTING EXPANSIO N (0.7%)

HIGH-HEAT TECHNIQUE THERMAL SETTING EXPANSION (1.25%)

CONTROLLED WATER ADDED TECHNIQUE: 

Technique….



Why rubber rings ???



Purpose ???

STEP 6: BURNOUT OF THE WAX PATTERN

•

Burnout ovens: manual, semiautomatic, fully programmable

•

Position of the ring in furnace…

Gypsum-bonded investments: 

Temp.- 650˚c-700˚c in 60mins & held at higher temp. For 15-30mins



-2 stage: keep in 200˚c preheated furnace for 30 mins, later 480˚c for hygroscopic & 650˚c for thermal expansion & held for 45mins.

Precautions: Avoid rapid heating of furnace  Never heat above 700˚c CaSo4+ 4C CaS+4CO 3CaSo4+CaS 4CaO+4So4  Burnout wax when invst. is still wet.. 

Phosphate-bonded investment: Temp: ranges from 750˚c - 1030 ˚c  Method: -slow till 315 ˚c & rapid thereafter 

-held in upper temp. for 30mins.

After Burnout: 

Heating the ring ???



Immediately cast…



Cherry Red color

ACCELERATED CASTING METHOD Conventional: 1 hr bench setting for investment 1-2 hrs wax elimination

Accelerated casting method: 15 mins bench setting 15 mins wax burnout (815˚c)

STEP 7: CASTING PROCEDURE MELTING OF THE ALLOY USING TORCH FLAME: Types of gases: 1) natural gas( propane) & air 2) natural gas & oxygen 3) air & acetylene/oxygen & acetylene

Types of tips…. Temp…. 870˚-1000˚c Typical flame….

CONICAL FLAME

USING ELECTRICAL ENERGY Types: -electrical resistance -induction melting -direct-current arc melting

ELECTRICAL RESISTANCE Principle…. Advantage…

INDUCTION MELTING MACHINE Principle… Used for base metal, co- ch alloys, titanium alloys

DIRECT CURRENT ARC MELTING MACHINE Principle… Temp…exceeds 4000˚c Disadvantage…

CASTING CRUCIBLES

Types…. 1) Clay, graphite- crown & bridges, alloys of high noble metals & noble types 2)

Carbon- high noble crown & bridges, higher fusing gold based metal ceramic alloys.

3)

Quartz, alumina, silica, ceramic - high fusing alloys of any type and the sensitive to carbons.

CASTING MACHINES

AIR PRESSURE CASTING MACHINE: Principle: -pneumatic forces -CO2 or nitrogen.. - pressure of 10-15 psi - thin sprue

Used for metal ceramic casts

VACCUM CASTING MACHINE Principle: -pneumatic forces Used for base metal alloys

CENTRIFUGAL CASTING MACHINE Types: *motor driven or spring driven *vertical rotating casting machine or horizontal casting machine.

principle... ”broken arm” principle pressure- 30-40 psi (0.21- 0.28 Mpa) Crucible Balancing weight Metal Casting ring

Locking arm

Handle

Amount of driving energy depends on: Density ii. Porosity iii. No. of sprues iv. Length of the sprue v. Diameter of the sprue vi. Size of the pattern vii. Amount of metal viii. Angulations & funneling of the sprue ix. Configuration of the pattern & details. i.

CASTING TECHNIQUE

STEP 8: RECOVERY & CLEANING OF THE CASTING “Quenching” Advantage…

“pickling”- Gypsum bonded investments: 50% HCl- disadvantage… method of pickling… precautions… other pickling solutions

Galvanic cell

Phosphate bonded investments: Ultrasonics Cold hydrofluoric acid Sand blasting.. AlO(50µ), 100psi

STEP 9: FINISHING OF THE CASTING Initial stage:

rubber, rag or felt wheels impregnated with abrasives like diamond, silicone carbide, emery, AlO, granet, sand, tripoli, rouge etc…

Final stage: “beilby layer”… various oxides of tin & Al, small rag or chamois buffing wheel, iron oxide rouge

Finally… “polishing compound removal”

STEP 10: CEMENTING THE CASTING Shrinkage of 1.4- 1.5%

CASTING DEFECTS CLASSIFICATION:

ALLOY

Nodules Surface roughness

1. 2.

3. 4.

INVESTMENT

Distortion Surface roughness & irregularities Porosity Incomplete or missing details

Ridges or veins Fins

Internal porosity

Gas bubble Incomplete margin

DEFECT AFFECTING THE SHAPE DISTORTION Causes… Prevention…

DEFECT AFFECTING SURFACE SURFACE ROUGHNESS is defined as.. SURFACE IRREGULARITIES is defined as…

ALLOY

INVESTMENT Nodules

Surface roughness

Ridges or veins Fins

Causes & prevention AIR BUBBLES:

WATER FILMS: ALLOY

INVESTMENT

Ridges or veins

RAPID HEATING RATES: fins ALLOY

INVESTMENT

Fins

UNDERHEATING:

PROLONGED HEATING: LIQUID/ POWDER RATIO:

TEMRATURE OF THE ALLOY: CASTING PRESSURE: air pressure- 0.10-0.14MPa centrifugal- 3-4 turns

COMPOSITION OF THE INVESTMENT: ADA No. 2 FOREIGN BODIES:

IMPACT OF THE MOLTEN ALLOY: PATTERN POSITION: 3mm btn patterns CARBON INCLUSIONS:

POROSITIES CLASSIFICATION: I) Solidification defects -localized shrinkage porosity -microporosity

II) Trapped gases -pin hole porosity -gas inclusion porosity -subsurface porosity

III) Residual air -back pressure porosity

SOLIDIFICATION DEFECTS LOCALISED SHRINKAGE POROSITY: “ suck-back porosity”

MICROPOROSITY Fine grain alloys

TRAPPED GASES PIN- HOLE POROSITY

Internal porosity

Gas bubble

GAS INCLUSION POROSITY

SUB- SURFACE POROSITY

Incomplete margin

RESIDUAL AIR ENTRAPPED AIR/ BACK- PRESSURE POROSITY

DISCOLORATION OF THE ALLOY

DEFECT AFFECTING SIZE INCOMPLETE CASTING: causes..a) insufficient venting, b) incomplete wax elimination prevention..

LARGE CASTING SMALL CASTING

COMPLETE CASTING FAILURE

REFERENCES 1. 2.

3. 4. 5.

Phillip`s Science Of Dental Materials-11th Edition, Kenneth J Anusavice Restorative Dental Materials-11th Edition, Robert G Craig, John M Powers Dental Materials & Their Selection-3rd Edition, William J O`brien Materials In Dentistry. Principles & Applications2nd Edition, Jack L Ferracane Essentials Of Dental Materials- S H Soratur

6. Contemporary Fixed Prosthodontics- 3rd Edition, Rosenstiel, Land, Fujimoto 7. Fundamentals Of Fixed Prosthodontics-3rd Edition, Herbert T Shillingburg 8. Textbook Of Operative Dentistry- 2nd Edition, Vimal K Sikiri 9. www.google.com 10. Department of prosthodontics, casting laboratory. CODS, Davangere.

CONCLUSION “A DENT IN YOUR MERCEDES, A CRACK IN YOUR MOBILE, A TEAR IN YOUR BRAND NEW LOUIS PHILLIPPE, A SCAR ON A BEAUTIFUL FACE, A FRACTURE IN A TOOTH, NOT SO DESIRABLE… RIGHT??? NOR ARE CASTING DEFECTS…………..”