Dental Casting Procedures

• Seminar 7 is about Casting Procedures
• Casting is a widely used method for making metallic restorations
• The lost wax casting technique was first described at the end of the 19th century for making dental castings
• The process involves encasing a wax pattern in a heat-resistant investment material, melting out the wax, and injecting molten metal into the resulting cavity via a channel called "Sprue"
• Resulting dental casting must be an accurate replica of the wax pattern, including surface details and dimensions
• Small variations in investing or casting can affect the final restoration quality

Steps in Casting Procedures

• Wax pattern removal
• Spruing
• Investing
• Burnout
• Casting

Wax Patterns

• Initially, a dental wax pattern is prepared for an inlay or crown using the lost-wax process
• Direct Wax Technique: Pattern is created directly within the tooth
• Indirect Wax Technique: Pattern is made indirectly, prepared within a die
• Specialized dental wax for direct and indirect pattern preparation on a die surface
• Specified by American National Standards Institute/American Dental Association Sp No 4, with:
  • Type I (Medium Wax) for direct techniques
  • Type II (Soft Wax) for indirect techniques

Sprue

• Once the wax pattern is complete and margin re-flowed, it's evaluated for smoothness, finish, and contour, then inspected under magnification for flash removal
• A sprue is attached to the pattern, removed from the die, and fit to a crucible former
• Sprue Definition: channel or hole used to pour or cast plastic or metal into a gate/reservoir into the mold

Basic Sprue Requirements

• Must allow molten wax escape from the mold
• Must enable molten metal to flow into the mold with as little turbulence as possible
• Molten metal must be retained long enough for the alloy to fill the mold, providing a reservoir to compensate shrinkage during solidification

Types of Sprue

• Can be made of wax, plastic, or metal
• Sprue wax is often preferred
• As they melt at the same rate as the pattern and thus allow easy escape of the molten wax
• Solid plastic sprues soften at a higher temperature, potentially blocking wax escape, which increases casting roughness
• Hollow plastic sprues can permit the escape of max
• Useful for casting fixed partial dentures (FPDs) in one piece because their added rigidity minimizes distortion
• Metal sprues should be non-rusting to avoid wax contamination
• Metal sprues are often hollow, increasing surface area and strengthening attachment to the pattern
• Usually removed during crucible former placement
• Care must be taken to examine the orifice for investment particles that may cause an incomplete casting

Sprue Former Diameter

• Depends on type/size of pattern, casting machine used, and the ring size
• Prefabricated sprue formers are available in a variety of gauges (6 to 18)
• Sprue diameter should match the thickest wax pattern portion
• Use 10-gauge (2.5mm) for molar and metal-ceramic restorations
• Use 12-gauge (2.0mm) for premolars and partial coverage restorations
• Large Diameter Sprue: improves molten metal flow into the mold
• Small Diameter Sprue: can solidify prematurely causing shrinkage porosity

Length of the Sprue Former

• Should position the pattern within 6mm of the ring's end, and be short enough to avoid premature alloy solidification
• Use 6mm for gypsum-bonded investments and 3-4mm for phosphate-bonded
• Average sprue length: 4-5mm for large inlay; 3-4mm for small inlay
• Short Sprue: moves the pattern away, preventing gas venting
• Long Sprue: Solidifies early causing shrinkage porosity

Sprue Former Location

• Should be attached to the bulkiest part of the pattern which is away from margins and occlusal contacts
• Normally the largest noncentric cusp is used
• The point of attachment should allow metal to flow in the direction of the casting force to all parts of the mold
• For full veneer crown, the sprue is attached to the maxillary buccal and mandibular lingual cusp
• For partial veneer crown, the sprue is attached to the cusp the encompasses

Sprue Former Direction

• It should be directed away from thin or delicate parts of the pattern, as molten metal can damage the investment
• It should be attached 45 degrees to the walls of the mold, which decreases the turbulence of molten alloy

Sprue Former Attachment

• Attachment should be smooth without pits/irregularities to prevent investment tags, leading to potential casting failure
• The sprue former connection to the wax pattern is generally flared for high-density gold alloys but often restricted for lower-density alloys.
• Design minimizes turbulence risk and the orientation should minimize metal flow onto flat areas and line angles

Reservoir

• A reservoir is placed approximately 1.5mm from the pattern
• Function Prevents localized shrinkage porosity, due to the large mass of alloy and heat centered on the ring

Venting

• Small auxiliary sprues/vents improve thin pattern casting
  • By improving: gas escape, decrease shrinkage during solidification, and begin solidification in critical areas

Spruing Technique

• Direct and indirect spruing techniques
  • Direct: The sprue former is a direct connection between the pattern area and crucible former
  • Indirect: A connector is positioned between the pattern and the crucible former

Spruing Principle for Removable Partial Dentures

• Requires larger quantity of metal
• Pay special attention to point of attachment, direction of the sprue, and direction of the flow of molten metal
• Heavy sprue is given, acting as a reservoir

Both Maxilla and Mandible

• A hole is placed in the center of the refractory cast
• To provide easier attachment for the sprue leads
• The hole should be 7 mm in diameter and should be as near to the center of the casting
• A base plate wax is rolled to diameter of 7 mm and it should extend 10 mm onto the pattern side
• 8-10 gauge, flat sprue wax should join one end of sprue to each junction of either major or minor connectors
• They are joined together away from the cast side upwards to the center
• The sprue is attached to the rest of the major and minor connectors

Crucible Former

• Sprue is attached to the crucible former to constitute base of casting during investing which
• Assists in holding sprue in desired ring
• Types:
  • Steep sided cone: for use with centrifugal force
  • Shallow cone: used to cast metal via stream/air pressure
• Available in rubber, metal, and plastic forms
• Exact shape depends on the casting machine used
• Modern machines are tall and allow for a short sprue and for the pattern to be positioned near the end of the ring
• They form a conical depression in investment, which guides flow of molten metal.
• It should be clean and petroleum is applied to prevent formation of rough investment tag.

Casting Ring

• Are used to confine the fluid investment material around the wax pattern
• Allows hardened investment to be safely handled
• Small rings (32mm diameter) for single crowns/inlays
• 63mm round/oval rings for large fixed partial dentures

Types of Casting Ring

• Classified by shape (round or oval) and structure (complete or split)
  • Complete = Rigid Metal or Plastic and Flexible Rubber rings
  • Split = Metal or Plastic rings

Ringless Casting System

• Designed to improve productivity through accurate, consistent results without time-consuming metal ring steps
• Employs reusable, tapered plastic rings for unimpeded investment expansion and easy mold removal prior to burnout
• Allows for quick clean-up and divesting
• Expansion control is facile, not limited by ring liner thickness

Casting Ring Liners

• Provide consistent room for expansion of investment
• Allow for freedom to expand, which would otherwise be restricted
• Offset contraction of cooling rings
• In a wet liner, there is some hygroscopic expansion, and greater expansion with a thicker liner

Liners of Asbestos

• Asbestos liner is temperature refractory and water absorbent
• Asbestos liner has several types:
  • White asbestos is least toxic and used in dentistry
  • Blue asbestos has the most toxicity
  • Asbestos is no longer used because of potential toxicity can cause cancer

Cellulose Liner

• Cellulose liner provides adequate water absorption
• It is burned during burnout
• The liner is kept 3mm short of ring ends, to keep investment in contact after burnout
• This restricts longitudinal and hygroscopic expansion

Ceramic Ring Liner

• Ceramic ring liner is fibrous alumino-silicate material
• Does not absorb water but retain water on its surface
• Resistant to high temperature
• Binders may be toxic
• Fibers 5.3-17.8mm long and 0.2-0.97 mm in diameter

Investing

• Process of covering or enveloping an object such as a denture, tooth, wax form, crown with suitable investment material before processing and casting
• Types:
  • Gypsum bonded
  • Phosphate bonded
  • Ethyl silicate bonded

Investing Procedure

• The wax pattern should be cleaned of any debris, grease, or oils.
• A commercial wax pattern cleaner or a diluted synthetic detergent may be used.
• Any excess liquid is shaken off, and the pattern is left to air-dry while the investment is being prepared.
• The thin film of cleanser left on the pattern reduces the surface tension of the wax and permits better “wetting” of the investment to ensure complete coverage of the intricate portions of the pattern
• Vacuum mixing is recommended for casting with minimal surface defects

Important Considerations

• Air bubbles may remain on flat/concave surfaces even with vacuum mixing
• Tilting the ring helps release air bubbles to the surface
• Avoid excessive vibration, which can cause settling and lead to surface roughness
• Excessive vibration may dislodge the pattern
• Immediately place the ring in a 37C water bath for hygroscopic technique with the crucible former side down
• Bench set undisturbed for the time recommended by the manufacturer for thermal expansion or high-heat technique

Wax Elimination

• Wax elimination or burnout is heating investment in controlled furnace until all traces of wax vaporize
• Requires setting investment for one hour to prepare for burnout
• The crucible former is removed, invested rigs are placed in a temperature furnace, and heated to prescribed max temperature

Burnout Ovens

• Manual is one type
• Semiautomatic is one type
• Fully programmable controls is one type

Considerations of Burnout Ovens

• Gypsum-bonded investments use 500 degrees C (hygroscopic) or 700 degrees C (thermal expansion)
• Phosphate-bonded investments use between 700 and 1030 degrees C, depending on alloy type
• Begin burnout while mold is wet
• Water in investment reduces wax absorption and flushes mold on vaporization
• In high-heat technique, carbon converts to carbon monoxide and carbon dioxide to escape pores

Casting

• Something that has been cast in a mold, this object formed by the solidification of a fluid that has been poured or injected into a refractory mold
• Metallic duplicate from pouring molten metal into a mold of a required form and that allows it to solidify

Ways Alloys are Melted

• Torch
  • Gas/Air
  • Gas/Oxygen
  • Air
  • Oxygen
• Electrical.
  • Resistance
  • Induction
  • Direct Current Arc

Types of Torch Flame

• Multi-orifice
• Single-orifice
• Zones of the blow torch flame
  • Colorless zone or Non combustion zone
  • Combustion zone
  • Reducing Zone
  • Oxidizing Zone

At Proper Casting Temperature

• Alloy has a spongy appearance
• Small globules of fused alloy appear
• Molten alloy assume a spheroidal shape.
• Molten alloy has a light orange color
• Molten alloy will spin or follow the movement of the flame slightly

Flux

• Enhances gold crown and bridge alloys by minimizing porosity and fluidity
• The film of flux formed prevents oxidation of alloy film
• Better flux is from equal parts of fused borax powder ground with boric acid
• Reduced fluxes are for cleaning old alloys

B. Electrical Resistance- Theated Casting Machine

• Heat energy is produced during current flow, depending on applied voltage
• The alloy melts electrically by resistance heating
• Current is passed through a resistance heating conductor, and automatic melting of the alloy occurs in a graphite or ceramic crucible
• Avoid using a carbon crucible for palladium, nickel-chromium, or cobalt-chromium alloys
• The crucible is located flush against the casting ring and therefore the button remains molten longer

C.Induction Melting Machine

• Alloy is melted by induction field, it develops with water-cooled metal tubing surrounding crucible
• The electric induction furnace is a transformer in which alternating current flows
• Transformer generates a variable magnetic field
• Forced with air pressure, vacuum, or centrifugal force
• Commonly used for melting base metal alloys

D. Direct Current Arc Melting Machine

• Direct current arc is produced between two electrodes: alloy and water-cooled tungsten electrode
• Alloys temperature with 4,000C exceed
• This quickly overheats the alloy

E. Vacuum- or Pressure-Assisted Casting Machine:

• Evacuates the metal chamber to reduce oxidation of the item being cast
• Used for titanium and titanium alloys
• Applies air pressure or vacuum to force molten alloy into mold
• Molten alloy is “PUSHED & SUCKED" into the mold by gravity or vacuum

Casting Crucibles:

• Melting the alloy requires a platform crucible where the heat is applied
• Available in:
  • Clay
  • Carbon
  • Quartz
  • Zirconia-alumina

Casting Crucible Characteristics

• High noble and noble alloys are best with clay crucibles.
• High-fusing alloys are best with quartz crucibles

Casting Force

• Type of forces: Vacuum pressure
• Air or Gas
• Centrifugal force

Cleaning the Casting

• Involves quenching, divesting, pickling, sand blasting, and finishing or polishing to clean

Quenching

• After gold alloy is hardened, the ring is quenched in water, after the button casts a dull-red glow
• Leaves alloy is an annealed condition
• Causes water contact to remove granular investment reaction to occur

Divesting

• Divesting is part of the process in removing the investment to gain the casting
• First the ring must be held at ¼ inch
• Bulk pressure to remove
• Sprue is removed from the restoration
• Use carborundum disc abrasive

Pickling

• Used when casting is dark because of oxides or tarnishing, removed by pickling processing
• Used for solutions such as Hydrofluoric acid, Phosphoric acid, or 50% of HCI
• Some precious solutions should be renewed with hydroflouric acid

Method of Cleaning

• Use Test tube with casting and pour acid over it
• Heating casting by dropping into pickling solution

Solution

• Gold and palladium based ceramic alloys and base metals are not pickled if cooled
• Should be held with steel tongs

Ultrasonic devices and abrasive blasting are used to clean surface casting Nickel-Chromium alloy does not use acid

Sand Blasting:

• Clean the casting, which may use a sand blasting machine
• Base metal alloys require light sandblasting
• Partial dentures should be sandblasting by removing investment instead of acid

Finishing and Polishing:

• Al203 stones that are White/Brown
• Use abrasive wheels to impregnate Final process with Chamois and aluminium oxide wheels Use hot and soapy water to remove residue wax