CASTING DEFECTS
K.PRIYADHARSHINI
Senior Lecturer
Department Of Prosthodontics
INTRODUCTION
DISTORTION
SURFACE ROUGHNESS
IRREGULARITY
DISCOLOURATION
AIR BUBBLES
WATER FILMS
RAPID HEATING RATES
UNDER HEATNG
PATTERN POSITION
POROSITY
LOCALIZED SHRINKAGE POROSITY
MICROPOROSITY
PINHOLE POROSITY
GAS INCLUSION POROSITY
SUBSURFACE POROSITY
CONTENTS
DISTORTION
PROPER MANIPULATION OF WAX AND HANDLING THE PATTERN
CASTING FITS BEST WHEN THE PATTERN IS INVESTED IMMEDIATELY AFTER ITS REMOVAL FROM THE PREPARATION
NONUNIFORM OUTWARD MOVEMENT OF INVESTMENT [SETTING AND HYGROSCOPIC EXPANSION ]
DISTORTION THICKNESS OF THE PATTERN DECREASES
WHEN HOT WAX COOLS IT SHRINKS TOWARDS THE CENTER,PULLED AWAY FROM LUBRICATED SURFACE
SURFACE ROUGHNESS, IRREGULARITIES AND DISCOLORATION
PARTICLE SIZE OF INVESTMENT AND ITS ABILITY TO REPRODUCE THE WAX PATTERN IN MICROSCOPIC DETAILS
AIR BUBBLES AND WATER FILMS
TO USE VACUUM INVESTING TECHNIQUE
USE OF A MECHANICAL MIXER WITH VIBRATION BOTH BEFORE AND AFTER MIXING
APPLY THIN LAYER OF WETTING AGENT –AIR DRIED-EXCESS LIQUID DILUTES THE INVESTMENT [ SURFACE ROUGHNESS]
WATER FILM FORM IRREGULARLY OVER THE SURFACE
IT OCCURS IF THE PATTERN IS SLIGHTLY MOVED OR VIBRATED AFTER INVESTING
TOO HIGH LIQUID POWDER RATIO
RAPID HEATING
FINS OR SPINES ON CASTING
FLAKING OF THE INVESTMENT WHEN THE WATER OR STEAM POURS INTO THE MOULD [ SURFACE ROUGHNESS]
MOULD SHOULD BE GRADUALLY HEATED – 60 MINS
Greater the bulk of the investment more slowly it should be heated
UNDER HEATING
INCOMPLETE ELIMINATION OF WAX RESIDUES- GAS FORMED WHEN HOT ALLOY COMES IN CONTACT WITH CARBON RESIDUES
INCOMPLETE CASTING
POROSITY
SOLIDIFICATION DEFECTS
LOCALIZED SHRINKAGE POROSITY
MICROPOROSITY
TRAPPED GASES
PINHOLE POROSITY
GAS INCLUSIONS
SUBSURFACE POROSITY
RESIDUAL AIR
LOCALIZED SHRINKAGE POROSITY
PREMATURE TERMINATION OF FLOW OF MOLTEN METAL DURING SOLIDIFICATION
LINEAR CONTRACTION OF NOBLE METAL ALLOY-1.25%
Continuous feeding of molten metal through sprue should must occur to compensate of metal volume during solidification
Sprue freeze before last portion of casting that solidifies
Small chill-set sprue ensure that solidification begins within the sprues-cooling pins to carry heat away from the pontic
Generally occur near sprue cast junction
SUCK-BACK POROSITY
MICROPOROSITY
SOLIDIFICATION IS TOO RAPID FOR THE MICROVOIDS TO SEGREGATE TO THE LIQUID POOL
MOLD OR CASTING TEMPERATURE IS TOO LOW
PINHOLE AND GAS INCLUSION POROSITY
METALS DISSOLVE OR OCCLUDE GASES WHILE THEY ARE MOLTEN
[EG copper and silver dissolve oxygen , platinum and palladium dissolve hydrogen and oxygen.
On solidification absorbed gases are expelled.
Large voids occurs as the gases are mechanically trapped by the molten metal.
BACK PRESSURE POROSITY
SUBSURFACE POROSITY
INCOMPLETE CASTING
MOLTEN ALLOY HAS BEEN PREVENTED FROM COMPLETELY FILLING THE MOLD
-INSUFFICIENT VENTING –BACK PRESSURE POROSITY-CANNOT FILL BEFORE IT SOLIDIFY-INSUFFICIENT CASTING PRESSURE
-HIGH VISCOSITY OF THE METAL
EG – ROUNDED INCOMPLETE MARGINS
INCREASE IN TEMPERATURE-DECREASES SURFACE TENSION AND VISCOSITY
SPRUE POSITION
REFERENCES
Philips’ science of dental materials,anusavice, 11th edition
Craig’s restorative dental materials,Power and sakaguchi, 12 th edition
THANK YOU