47. Investment process of the wax prototype of the restoration. Casting. Soldering and welding

whas. init

• sprueing

• investing

• burnout

• casting

• soldering

• soldering requirements + steps

• welding

this all happens after wax pattern is fabricated

spreuing, Investing + burnout and castign

ipruing (Before Investing):

• Attach a wax sprue (sprue former) to the wax pattern to fix it in space.

• One end of the sprue is attached to the wax pattern at its greatest bulk (thickest part).

• The other end is connected to the crucible former—this will be the channel through which molten metal is poured later.

• Investing: (Material can be gypsum bonded, phosphate, or ethyl silicate

  • Surround the wax pattern and attached sprues with investment material to produce a negative mold capturing all details.

  • This also creates the “path of elimination” where wax will be removed, and pathways for molten metal to flow.

• Burnout:

  • Heat the invested mold to remove (eliminate) the wax, leaving behind a hollow mold with sprue channels intact.

• Casting:

  • Introduce molten alloy through the crucible former and sprue channel into the mold to fill the cavity left by the wax.nvesting;

  • Molten alloy is introduced into the mold via the sprue channel.

  • This can be done by centrifugal force, vacuum pressure, or air pressure.

  • The metal fills the space left by the wax pattern.

functions of the sprues

To keep the wax prototype firmly into place
To provide channel for molten material to flow into mold

To compensate for alloy shrinkage during solidification (as more molten alloy will flow through sprue when alloy shrinks)

rules for sprueing

• Sprue must be large enough so it remains open until the casting solidifies and short enough for rapid filling of the mold cavity. Large and small inlays requires sprues 4-5mm long and 3-4mm long respectively.

• wax sprue is preferred to a plastic sprue - melts away more easily and leaves a good channel - and has better thermal expansion (5x more than plstic) again to compensate for shrinkage

• sprue must be 6mm away from investment ring → further than that and there will be air bubbles trapped by the time casting material sets. closer than that and the molten alloy doesnt have enough space and mould may break

• 
  a liner is placed between the investment mould and its ring to allow 3mm of lat. expansion

soldering

Joining of two or more metal components by filler metal, or solder, which is fused to each of the parts being joined. This is done at temperature below the solidification temperature of the metals being joined.

(its called solidification temp bc even though it means below the melting point of these two metals, the base metals stay solid after their melting point is introduced)

requirements for soldering

Requirements for solders are:

• -  Solders fusing temperature must be lower than that of metal parts.

• -  Solders melting temperature must be lower than the solidus temperature of metal

  parts.

• -  Solder must have lower gold content than the metal parts.

• -  Copper is added to lower the fusing temperature, improve the strength of solder

• -  Nickel is added to also lower the fusing temperature, improve the strength of solder,

  as well as providing white colour to the solder.

• -  Silver is added to improve wetting and colour of solder

steps of soldiering

1. Metal parts must be free from contamination by cleaning and applying flux to

   prevent the formation of oxide layer

2. Both solder and metal parts must be bought to the solders melting point

3. Solder wets the surface of the metal parts; spreads and provides intimate contact

   between the parts.

WELDING

WELDING
Welding – Joining of two or more metal parts by applying heat, pressure or both, with or without a filler metal to produce localized union across the interface.