investment procedure

Question 1

what is back pressure porosity

Answer 1

occurs on the inner surface of the casting as large concave depression
appears usually on the surface of crown
this porosity exerts pressure on sprue and button
cause ; when molten metal is pushed into the mould the porous investment pushes the air out but if the bulk of investment is too heavy the air remains trapped inside and the air pressure is increased in the moulds
this defect can be avoided by
placing sprue not more than 6-8 mm away from the casting ring
using vets for large investments
using an imvestment material of adequate porosity
this back pressure preents the liquid alloy to oocupy the entire casting surface

Question 2

list down the names, cause , and remedies of all type of porosities present in the dental castings

Answer 2

localized shrinkage porosity appear large irregular voids presnt at the bulkiest portion close to the sprue attachment at the bulkiest portion
cause : due to incomplete feeding of casting
corrected by using spure of appropriate thickness
remedy attach spure to the thickest portion of wax pattern
suck back porosity : at occuoaxial or insisoaxial line angle
present due to the impinging effect of molten metal on the mould wal due to whch a hot spot is created and freezes the last as the rst of the molten metal has already solidified this spot appears as a defect
remedy include reducing the temp diff between the mould and the metal alloy
microporosity appear as small irregular voids throughout the casting
occurs due to rapid solidfication when casting temp is too low
remedy heat the molten metal properly
pinhole porosity arishing due to entrapent of gas by liquid molten metal eg cu and ag dissove oxygen in .liquid stage in solidification absorbed gas is expelled and porosity develpos
remedy the liquid molten metal should not be kept for a longer period of time
aslo temp should not be low
gas inclusion porosity : occurs due to trapping of gas mechanically due to reasons such as usibng oxidising flame instead of reducing flame
remedies investment porous proper positioning of flame
subsurface porosity these are regular air bubble voids throughout the surface of the casting arising due to simultaneous nucleation of solid grains and gas bubbles below casting , remedy temp of molten meatl shoul be high

Question 3

short note on pickling

Answer 3

after removing the investment the cast appears dark due to oxides such impurities are removed by method of pickling
for gypsum bonded investment pickling solution consists of
hydrochloric acid but its fumes are a health hazard may corrode laboratory furnishings in place uv light could be used for cleaning or an acid salt could be used

Question 4

note on sprue or sprue former

Answer 4

1 sprue is the channel created in the investment through which molten metal enters the mold space created due to burnout of wax pattern
2 sprue former- material through which sprue is formed
1.1 functions of sprue former
1.1a forms a mount for the wax pattern
1.1b channel for elimination of wax pattern
1.1c channel for entry of molten metal
1.1d reservoir of molten metal which compensates for alloy shrinkage

Question 5

casting ring liner

Answer 5

placed inside the casting ring should be 3mm short
made up of fibrous- ceramic aluminous silicate
cellulose
ceramic cellulose
1 function of casting ring liner
1.1 acts as a cushion allows mold expansion
1.2 acts a thermal insulator preventing heat loss when

Question 6

casting crucible

Answer 6

crucible former is the heat resistant container in which the molten metal is melted before casting 
material: clay noble alloy
carbon noble crown and bridge 
zirconia alumina high fusing alloy 
quarts or silica higfusing alloy