Crown and Bridge

• Soldering


  The joining of metals by fusion of filler metal between them, at a temperature below 450°C
• Bond in soldering


  Created by wetting the parent metals with the solder, not by melting or diffusion the parent metals
• Solder


  A metal filler alloy that, when melted, flows into intimate contact with (wets) the metal surfaces to be joined
• Ideal properties of a dental solder

  • Lower fusing: 50°C-100°C lower than melting point of the components
  • Free-flowing: flow temperature well below melting point of alloys being joined
  • Strong: as strong as the components being melted
  • Corrosion resistant: to prevent tarnishing and discoloration
  • Same color: match the alloy
  • Compatible with the metal, but not necessarily similar composition
• Solders for joining noble metal components are formulated from mixtures of gold, silver and copper designed to have low fusion temperatures
• Base metal alloys can be difficult to solder because they oxidize; this must be controlled with special fluxes
• Dental solders are supplied in a variety of shapes, such as strips, rods, wires, or cubes, each of which is convenient for certain operations
• Soldering investments should not expand as much as casting investments. They are similar in composition to casting investments
• Correct gap between units


  Units must not touch, as this causes distortion and porous inadequate joints. Excessive gap spaces cause undersized restoration widths due to solder solidification shrinkage
• Solders are susceptible to oxidation during the melting/softening procedure and the resulting oxides can weaken the soldered joint
• The metal components being joined are often coated with a thin oxide film which can limit the ability to achieve proper joining
• Fluxes


  Employed to break down the surface oxide layers on metals and to prevent oxidation of the solder
• Fluxes commonly used


  • Fluoride salts
  • Borax
• Ideal properties of Flux

  • Melting point lower than solder
  • Lies quietly on the work while being fused and does not increase in volume
  • Spreads evenly and remains on the parent metal without volatization
  • Dissolves metallic oxides or other surface impurities
  • Easily removable after soldering
• The selection of flux and solder depends on the alloys being joined and their intended application
• Torch (flame) soldering


  • Access and visibility are maximal
  • The laboratory technician can heat differentially to the work
  • Oxidation and reductions can be controlled directly
  • Heat can be removed immediately after solder flow
  • Addition of solder to the partially completed joint can be made readily
• Disadvantages of torch soldering

  • Uneven distribution of heat can warp or damage portions of the prosthesis
  • Overall control of temperature is imprecise
  • Pressurized supply of necessary gases must be available
• Oven Soldering


  • Temperatures are more nearly uniform throughout the work
  • Temperatures are known at any point during the procedure
  • Application of vacuum can control oxidation
  • Close monitoring of the procedure is feasible if the work is visible in the furnace
• Oven soldering is particularly suitable for joining porcelain-to-metal prosthesis after applying porcelain (post soldering)
• Discolors porcelains rapidly, changing even temperatures causes cracking and pitting
• Soldering Technique


  • Ensure the surface of the alloy is free from dirt and oxides
  • Place the components as close together as possible without touching
  • If using soldering investment use the smallest amount possible
  • Place flux in the joint to be soldered
  • Select a suitable solder which melts 50°C-100°C below the melting point of the components
  • Heat the components evenly by either using the reducing zone of the flame or a furnace
  • When the flame is evenly cherry red, apply the solder
  • The parts being joined are not melted during soldering but must be thoroughly wettable by liquefied solder
  • The operation should be completed in the shortest time possible to avoid oxidation and prevent damage to the microstructures
• Soldering defects


  • Porosity or pitting: Incorrect fluxing, flaming, cleaning, or spacing of parts
  • Distortion of parts: Overheating, thermal expansion of metal parts
• Welding


  The process by which two pieces of similar metal are joined together without the addition of another metal
• The material used for welding is always the same composition and the same color as the alloy to be welded
• Welding is generally done at higher temperatures than soldering
• Methods of welding


  • Fusion welding: Parts are melted and joined, but pressure is not applied (e.g. Gas welding, Laser welding)
  • Pressure welding: Parts are heated and pressed, but not melted (e.g. Spot welding)
• Spot welding


  Two clean metal surfaces are placed together under pressure, current is passed through to fuse the metals at that point
• Applications of spot welding

  • Fusing stainless steel strip for making bands
  • Securing attachments to bands
  • Attaching springs to a rigid bow wire, or to bands
  • Construction of fixed restorations
• Pressure welding


  Also called cold welding, does not require heat application. Gold foil is welded by pressure at room temperature
• Laser welding


  A laser generates a high intensity pulse of light that can be focused to melt the opposing surfaces at the joint, which then form a weld on solidification
• Welding current and time

  Important for heat production - if current is allowed to pass for too long it will burn and produce a rough result, but insufficient time will result in non-union