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• Dental Investment Materials


  Ceramic materials used for making a mold into which the metal or alloy is casted
• Tooth reduction and impression recording

  Casting procedures
• Pouring of a master cast

  1. Wax pattern fabrication and spruing

  2. Sprued wax pattern attached to crucible former
• Investment material poured around the sprued wax pattern


  Casted restoration after investment removal
• Investments


  • Ceramic materials that are used for making a mold into which the metal or alloy is casted
  • Surrounding the wax pattern with a material that accurately duplicates its shape & anatomical features
• Requirements of investment materials

  • Sufficient strength to withstand heat of burnout and actual casting of molten alloy
  • Provide sufficient expansion to compensate for thermal shrinkage and casting shrinkage
  • Sufficient porosity to allow gas to escape
  • Chemical stability at high casting temperature
  • Produce casting with smooth, accurate surface with fine details without nodules
  • Easily broken after casting
  • Easy manipulation & cheap
• Components of investment materials

  • Refractory Material (65%)
  • Binder (30%)
  • Modifiers (5%)
• Refractory Material


  • A material capable of withstanding the high temperature without significant degradation
  • Silica (silicone dioxide) used to regulate the thermal expansion
  • Cristobalite gives more expansion than quartz, used in crowns & bridges
  • Quartz used in soldering
  • Resist high temperature
  • Produce thermal expansion by displacive transformation of silica from alpha to beta form during casting
• Binder


  • Binds investment components together
  • Provides strength, rigidity to investment
  • Gives hygroscopic and setting expansion (in some types of investment)
• Types of binders


  • Gypsum-bonded investment
  • Phosphate-bonded investment
  • Silicate-bonded investment
• Modifiers


  • Chemicals added in small amounts to modify various physical properties
  • Provide non oxidizing atmosphere (prevent metal oxidation) in mold by graphite & copper powder as reducing agents in casting gold alloys
  • Prevent binder contraction caused by gypsum when it is heated above 300C
  • Increases expansion without presence of excessive amount of silica in gypsum bonded investment by boric acid
  • Regulate setting time and setting expansion by boric acid
  • Dissolve black color from surface
  • Provide smooth surface of investment
• Types of investment materials

  • Gypsum bonded investment
  • Phosphate bonded investment
  • Silica bonded investment
• Expansions of investment materials

  • Setting expansion
  • Hygroscopic expansion
  • Thermal expansion
• Setting expansion


  • As the gypsum investment sets after mixing, it expands and slightly enlarges the mold
  • Calcium hemi hydrate + water → Calcium di hydrate
• Factors that increase setting expansion

  • Decreased W/P ratio to increase molecules entry
  • Additional ring liner
  • Adding Silica
  • Increased mixing time (spatulation)
• Hygroscopic expansion


  • Submerge ring under water at 37°C (room temp) for up to 1 hr. immediately after investing
  • Add wet ring liner: amount of hygroscopic expansion is 6 times more than setting expansion
• Thermal expansion


  • The silica refractory material (Quartz, Cristobalite) is principally responsible for this type because of solid state phase transformations
  • Transitions make a change in crystal form → Increase volume of refractory components
  • Happens at heating during wax elimination
• Investing techniques


  • Brushing technique
  • Vacuum investing technique
• Brushing technique


  1. Paint wax pattern with surface tension reducer [debubblizer]

  2. Add powder to liquid in bowel & vacuum mixed

  3. Coat wax pattern with investment mix by brush to prevent incorporation of air adjacent to the pattern

  4. Under vibration, the investment is poured into ring

  5. Tilt the ring so investment flows slowly down to fill without air trapping
• Vacuum investing technique


  1. Mix powder and water are mixed by hand 15 sec

  2. Then by vacuum for 30 sec slowly

  3. Mixed investment is permitted to flow into ring and around wax pattern with vacuum present
• Two-step investing technique


  1. First step: Paint wax pattern with thick mix, left to complete setting. Immersed in water for about 10 mins. (1st coat)

  2. Second Step: Ring is applied over crucible former. Filled with mixed investment till ring is completely filled. Mix is left to set. (2nd coat)
• Advantages of two-step investing technique

  • Minimizes wax pattern distortion
  • Provides smooth surface of casting
• Wax burn out (wax elimination)

  1. The procedure of heating the investment in a temperature-controlled furnace until all traces of waxes are vaporized

  2. The thermal expansion of investment is determined by temperature

  3. The ring is re-examined for any residual particles & placed with sprue hole facing down in furnace which will allow molten wax to flow out freely by gravity

  4. The furnace is brought to 200°C for 30 minutes; most of the wax will be eliminated by this time

  5. The final maintained burnout temp. 480°C or 650°C → Heat soaked for 30 mins to prevent sudden drop in temp. between burnout & casting
• Alloy


  Two or more elements, at least one of which is metal, and all of which are mutually soluble in the molten state
• Casting alloy


  The choice of casting alloy largely determines the selection of investment and casting techniques
• Laboratory steps


  1. Cast and die wax pattern spruing

  2. Investing wax

  3. Elimination of wax

  4. Casting
• Ideal properties of casting alloys

  • Biological requirement: Biocompatible, nonallergic, nontoxic and noncarcinogenic
  • Chemical requirement: Chemically inert, good tarnish and corrosion resistance
  • Physical/mechanical requirements: High strength, wear resistance, ductile, resilient, sufficient hardness, minimum solidification shrinkage, high fatigue strength
  • Cost requirement: Inexpensive, good fluidity when molten, ease of melting and casting
  • Other requirements: Ease of soldering, accurate fit, amenable to heat treatment
• Requirements of casting alloys

  • High strength and hardness
  • Excellent tarnish and corrosion resistance
  • Possess acceptable fluidity and castability
  • Ease in soldering or welding
  • Compatible with porcelain
  • Biocompatible
  • Allow for easy casting, finishing and polishing
  • Can be burnished to fine margin
• Classification of dental casting alloys based on ADA specification or nobility

  • High noble: Noble metal content >=60%, gold content >=40%
  • Noble: Noble metal content >=25%
  • Predominantly base metals: Noble metal content <25%
• Classification of dental casting alloys based on description

  • Crown and bridge alloys: Noble metal and base metal alloys
  • Metal-ceramic alloys: Noble metal and base metal alloys for porcelain bonding
  • Removable partial denture (RPD) alloys: Gold alloy and base metal alloys
• Pure gold


  Soft and ductile, not used for casting dental restorations and appliances
• Dental casting gold noble metal alloys


  Alloyed with copper, silver, platinum, palladium, nickel, and zinc to improve physical and mechanical properties and reduce cost
• Applications of dental casting gold noble metal alloys

  • Inlays and onlays
  • Crowns and bridges
  • Metal-ceramic bridges
  • Resin bonded bridges
• Advantages of dental casting gold noble metal alloys

  • Excellent corrosion resistance
  • Disadvantages: Low melting range, low modulus of elasticity
• Classification of dental casting gold noble metal alloys

  • Type I Soft: Small inlays, class III and class V cavities
  • Type II Medium: Inlays subject to moderate stress
  • Type III Hard: Onlays, crowns and bridges
  • Type IV Extra Hard: Onlays subjected to very high stresses, partial denture frameworks and long span bridges
• Properties of gold alloys

  • Color: Golden to white
  • Chemical: Excellent tarnish and corrosion resistance
  • Melting range: 920-960°C
  • Density: Higher than base metal alloys, better castability but requires more material
  • Biocompatibility: Relatively biocompatible
  • Casting shrinkage: Less than base metal alloys
• Gypsum-bonded investments


  Used for gold alloys because of their lower fusion temperature
• Gold-palladium-silver alloy


  White colored alloy used as economical alternative to Au-Pt-Pd or Au-Pd alloys, with 50% gold content and increased palladium and silver
• Advantages of gold-palladium-silver alloy

  • Excellent tarnish and corrosion resistance
  • Excellent mechanical properties and porcelain adherence
• Disadvantages of gold-palladium-silver alloy

  • Green discoloration due to silver diffusion into porcelain
  • Additional processing step to grind away discolored region
  • Silver vapor contamination of porcelain furnace