TOPIC: INSERTION, RELINING AND REBASING
🎯 KEY POINTS:
🔹Lost wax technique was first used in casting of alloys by W.H. Taggart (1906).
🔹Gypsum-bonded investment material is used for casting gold alloys.
🔹Pickling is a method of cleaning gold casting by hot acid solution for several minutes.
🔹Phosphate-bonded investment is used for casting metal–ceramic alloys having high-melting temperature.
🔹The minimum fineness required for dental solder to be corrosion resistant is 580 fine.
🔹Beryllium added to base metal alloys to control oxide formation is a carcinogen.
🔹Rounded margins on the casting may be caused by wax which is not completely eliminated during burn out procedure.
🔹Expansion of the investment by heat during elimination of the wax is called thermal expansion.
🔹There are two techniques in which the impression can be poured with dowel pins, namely, prepour and postpour techniques.
🔹Tamping effect, i.e. the heavier particles settle at the bottom and the liquid part comes to the top.
🔹Metamerism is a phenomenon of an object which appears different under different sources of light’.
📌 CLASSIFICATION OF DIE SYSTEMS
✳️ On the Basis of Their Design:
(i) Working cast with a separate die
(ii) Working cast with a removable die
- Dowel pin systems – straight and curved
- Pindex system
- Di-Lok system
- Accu-trac system
📌 CLASSIFICATION OF DENTAL CASTING ALLOYS
✳️On the Basis of Total Noble Metal Content given by American Dental Association (1984):
📍High Noble: Must contain ≥40% wt Au and ≥60% wt of noble metal elements (Au, Pt, Pd, Rh, Ru, Ir, Os); also called precious alloys.
📍Noble: Must contain ≥25% wt of noble metal elements (Au, Pt, Pd, Rh, Ru, Ir, Os); also called semiprecious alloys.
📍Predominantly Base Metal: Must contain <25% wt of noble metal alloys; also called nonprecious alloys.
✳️On the Basis of Mechanical Property Requirements given by ISO Draft International Standard 1562 for casting gold alloys (2002):
📍Type 1: Low strength – Casting which can tolerate very less stress (e.g. inlays; minimum yield strength is 80 MPa, and minimum percentage elongation is 18%).
📍Type 2: Medium strength – Casting which can tolerate moderate stress (e.g. inlays, onlays, complete crowns; minimum yield strength is 180 MPa and minimum percentage elongation is 10%).
📍Type 3: High strength – Castings which can tolerate high stresses (e.g. onlays, thin coping, pontics, crowns and saddles; minimum yield strength is 270 MPa and minimum percentage elongation is 5%).
📍Type 4: Extra-high strength – Castings which can tolerate very high stresses (e.g. saddles, bar, clasps, certain single units and partial denture frameworks; minimum yield strength is 360 MPa and minimum percentage elongation is 3%).
📌 CLASSIFICATION OF CASTING DEFECTS
(i) Distortion
(ii) Surface roughness
(iii) Porosity
(iv) Incomplete casting
📌 CLASSIFICATION OF POROSITY
(i) Solidification defects
- Localized shrinkage porosity
- Suck-back porosity
- Microporosity
(ii) Entrapped gases
- Pinhole porosity
- Gas inclusions
- Subsurface porosity
(iii) Residual air
🔶 TYPES OF INVESTMENT MATERIAL
💡 Three types of investment material commonly used in fixed prosthodontics:
(i) Gypsum-bonded investment
(ii) Phosphate-bonded investment
(iii) Silica-bonded investment