Soldering Process and Techniques

Soldering Process

• The components involved in the soldering process are:
  • Metals to be joined (parent metals)
  • Solder alloy
  • Flux and Antiflux
  • Heat source

Parent Metals

• The composition of the parent metals in soldering is determined by their melting range.

Ideal Solder Alloy

• Desirable qualities of an ideal solder alloy include:
  • Ease of flow at low temperature
  • Ability to wet the parent metal with sufficient flow
  • High strength properties compatible with that of the structure being soldered
  • Acceptable color
  • High resistance to tarnish and corrosion

Solder Alloys

• Types of solder alloys:
  • Soft Solders (lower melting point, 260°C)
  • Hard Solders (higher melting temperature, high strength, and hardness)
• Specific examples of solder alloys include:
  • Lead-tin eutectic alloy (commonly known as Plumber Solder)
  • Gold solder (fusion temperature range: 700°C to 900°C)
  • Silver solder (formation of silver-copper eutectic alloy responsible for low melting range)

Flux and Antiflux

• The role of flux in soldering is to:
  • Facilitate the flow of solder
  • Dissolve any oxides present on the surface and protect the surface from oxidation during heating, providing a clean and non-oxidized metal surface
• The role of antiflux in soldering is to prevent the flow of solder

Other

• Low fusing solder is used in dentistry to avoid carbide precipitation and structural changes.
• The fineness or carat of the gold solder should be slightly less than the carat or fineness of the parts being joined.

Crown and Bridge Alloys

• Crown and bridge alloys are made of silver-palladium alloys.
• Copper is avoided in Type IV systems when gold is not present.

Gypsum

• Manipulation steps for gypsum include:
  • Proportioning
  • Mixing
  • Pouring
• Vigorously mixing gypsum increases powder wetting and removes air bubbles.
• Mechanical mixers under vacuum pressure give gypsum superior properties.

Impression

• The impression should be held firmly against the vibrator to remove bubbles and encourage flow into impression details.
• Stone should be added from one side slowly to allow small amounts to flow from tooth to tooth around the arch until all teeth are filled.

Setting Time

• Setting time is the time needed for the reaction to be completed.
• Mixing time is the time taken from the addition of the powder to water until a homogeneous workable mix is obtained.
• Working time is the time available to use a workable mix with even consistency.
• Initial setting time is the time elapsed from the beginning of mix until partial setting occurs.
• Final setting time is the time elapsed from the beginning of mix until complete setting.

Sprue

• Sprue is used to create a channel and act as a reservoir.
• Rules for sprue attachment include:
  • Attaching to the thickest portion
  • Attaching at a 45° angle to the proximal surface
• Sprue materials include:
  • Wax
  • Plastic
  • Metal filled with sticky wax

Crucible Former

• Crucible former functions as a base of the ring and increases input concentration.
• Types of rings include:
  • Metallic
  • Rubber
  • Split metal

Liner

• Liner functions include:
  • Allowing expansion space
  • Providing water for gypsum to undergo hygroscopic expansion
  • Allowing easy removal

Distortion

• Distortion is due to the release of internal stresses.
• Distortion is affected by time and temperature.

Wetting Agent

• A wetting agent is used to increase wetting of the wax pattern.

Vacuum Investing

• Properties of vacuum investing include:
  • Low porosity
  • Smoother surface
  • Increased mechanical properties

Melting and Casting

• Melting and casting requires:
  • Heat source
  • Casting pressure
• Underheating or overheating of the investment can cause problems.

Casting Machines

• Types of casting machines include:
  • Centrifugal casting machine
  • Air pressure machine
  • Both centrifugal and air pressure vacuum machine

Porosity

• Causes of surface porosity include:
  • Under packaging
  • Sandy or sticky stage
  • Increased monomer
  • Decreased separating medium
• Causes of gaseous porosity include:
  • Rapid heating
  • Decreased thermal conductivity of the investment
• Shape and location of surface porosity:
  • Irregular shape
  • Surface location
• Shape and location of gaseous porosity:
  • Round, regular, uniform shape
  • Covered with a thin layer of skin investment
  • Bulkiest portion location

Non-Metalic Denture Base

• Presentation and proportioning of non-metalic denture base:
  • Powder/liquid or gel form
  • 3/1 ratio
• Effects of increasing or decreasing the powder/liquid ratio:
  • Insufficient wetting or granular cured
  • Flowy mix, polymerization shrinkage, porosity
• Stages of mix:
  • Sandy
  • Sticky
  • Dough
  • Rubbery
  • Stiff

Separating Medium

• Rules of separating medium:
  • Prevent passage of water from gypsum to resin
  • Decrease passage of monomer to plaster
  • Decrease porosity
  • Ease separation of flask
• Mould requirements:
  • Over-filled
  • Dough stage
  • Sufficient pressure
• Effects of early or delayed packaging:
  • Decreased viscosity or increased porosity
  • Increased viscosity or fracture and movement of the tooth

Curing Cycles

• Curing cycles:
  • 74°C for 8 hours
  • 74°C for 1 hour
• Effects of increasing or decreasing temperature or curing time:
  • Gaseous porosity
  • Inferior properties, porosity, denture stomatitis

Polishing

• Methods for stone removal and acrylic excess removal:
  • Shell blasting
  • Acrylic burs
• Polishing method:
  • Wet polishing wheel and slurry of pumice in water

Gold Alloys

• Functions of zinc in gold alloys:
  • Improves castability and fluidity of the alloy
• Functions of indium and tin in gold alloys:
  • Produces oxides in porcelain alloys, which help bonding with porcelain
• Classification of gold alloys based on gold content:
  • Carat and Fineness
• Difference between carat and fineness:
  • Carat refers to the parts of pure gold in 24 parts of alloy
  • Fineness refers to the parts of pure gold per thousand parts of alloy
• Use of carat or fineness classification in dentistry:
  • Not suitable, as it only refers to the amount of gold and does not determine the other metals which may have a pronounced effect on properties
• Description of yellow and white colored alloys:
  • Yellow colored alloys have a prominent yellow color
  • White colored alloys have a prominent yellow gold color
• ADA requirements for dental gold alloys:
  • Not less than 75% noble metal content
• Economy gold alloys:
  • Gold content: 42-58%
  • Comparable to Type III and Type IV gold alloys
• Replacement for reduction in gold in dental gold alloys:
  • Palladium, silver, and other metals
• Critical ratio between palladium and silver-copper:
  • 1% palladium for every 3% silver
• Effects of unbalanced silver-copper ratio:
  • Tarnish of the alloy