Friday, August 23, 2013

casting laws



         Laws of casting
·        Ingersoll & Walding (1986) formulated an expanded set of 17 separate recommendations for spruing, investing, burnout, melting & casting procedures. Collectively these guidelines are referred to as “the laws of casting”.
 
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1st Law of Casting

ü Attach the pattern sprue former to the thickest portion of the wax pattern:
         This provides the molten metal to flow from larger diameter to thinner sections
         Penalty: cold shuts, short margins and incomplete casting

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2 nd Law of Casting

Ø Orient wax patterns so all the restoration margins will face the trailing edge when the ring is positioned in the casting machine:
         Add a wax dot to the crucible so that, it will guide us in placing the ring in casting machine
         Penalty: cold shuts and short margins

3rd Law of Casting

ü Position the patterns in the “cold zone” of the investment and reservoir in the “heat center” of the casting ring:
         Adherence to this law causes porosity in the reservoir
         Penalty: shrinkage porosity

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4th Law of Casting

ü A reservoir must have sufficient molten alloy  to accommodate the shrinkage occurring within the restorations:
         Molten metal shrinks and creates a vacuum, for complete casting vacuum must be able to draw additional metal from adjacent source.
         Penalty: shrinkage porosity and/or suck-back porosity.


5th law of casting

ü Do not cast a button if a connector bar or another internal reservoir is used:
         With indirect spruing the largest mass of metal should be the reservoir
         A button can draw available molten alloy from the bar, shift the heat centre and reduce the feed of the metal to the restorations
         Wax patterns should not be larger than the connector bar
         Weigh the sprued patterns and use the wax pattern-alloy conversion chart
Penalty: shrinkage porosity and suck back porosity (potential distortion during porcelain firing).

6th law of casting

ü Turbulance must be minimized, if not totally eliminated
         Eliminate sharp turns, restrictions, points or impingements that might create turbulance and occlude air in the casting
         Restrictions or constrictions can accelerate the metals flow and abrade the mold surface (mold wash)
         Penalty- voids and /or surface pitting

7th law of casting

ü Select a casting ring of sufficient length and diameter to accommodate the patterns to be invested
         The casting ring should permit the patterns to be ¼ inch apart and ¼ inch from the top of investment and 3/8 inch of investment between pattern and ring liner
         Penalty- mold fracture, casting fins and shrinkage porosity

8th law of casting

ü Increase the wettability of wax pattern
         Wetting agent should be brushed or stained on the patterns and dried before investing
         Too much wetting can weaken the investment and produce bubbles or fins on the casting
         Penalty- bubbles (due to air entrapment)

9th law of casting

ü Weigh any bulk investment and measure the investment liquid for precise powder liquid ratio
         A thick mix of investment increases investment expansion and produces loose fitting castings
         Thinner mix yields less expansion with tighter fitting castings
         Penalty- ill fitting casting



10th law of casting

ü Eliminate the incorporation ofair in the casting investment and remove the ammonia gas by –product of phosphate bonded investments by mixing under vacuums
         Vacuum mixing removes air and gas providing an uniform mix without large voids
         Entrapped air can affect the expansion at various sites of the investment
         Penalty- small nodules, week mold and distortion of the casting

11th law of casting

ü Allow the casting investment to set completely before initiating the burn out procedure
         The mold may not withstand steam expansion during burnout if the mold is not set
         Advise to wait till the recommended setting time by the manufacturer
         Penalty- mold cracking/ blowout or fins on the casting

12th law of casting

ü Use a wax elimination technique that is specific for the type of patterns involved and recommended for the particular type of casting alloy selected
         Plastic sprues should be heated slowly, so they can soften gradually and not exert pressure, so use a two stage burnout
         Recommended atleast a 30 min heat soaking at 800F for the first burnout
         Penalty- cold shuts, short margins, cold welds, mold cracks and/or casting fins

13th law of casting

ü Adequate heat must be available to properly melt and cast the alloy
         Selected heat source should be capable of melting the alloy to the point of sufficient fluidity
         Too much heat or high temperature can burn off minor alloying elements and /or oxidation (burned metal)
         Penalty- cold shuts, short margins and cold welds (too little heat) or rough castings and investment breakdown (too much heat)

14th law of casting

ü When torch casting, use the ‘reducing zone’ of the to melt the alloy and not the oxidizing zone
         Melt achieved by the exclusive use of reducing zone minimizes the likelihood of metal oxidation and gas absorption and ensures the proper melt
         Penalty- gas porosity and/ or a change of alloys quotient of thermal expansion



15th law of casting

ü Provide enough force to cause the liquid alloy to flow into the heated mold
         Low density metal generally needs four winds of a centrifugal casting arm as compared to higher density, gold based alloys
         Do not over wind
         Penalty- cold shuts, short margins, cold welds (insufficient force), or mold fracture and fins (too much force)

16th law of casting

ü Cast towards the margins of wax patterns
         Place the heated ring in the casting cradle using the orientation dot, so the pattern margins face the trailing edge
         Penalty- cold shuts, short margins and otherwise incomplete castings

17th law of casting

ü Do not quench the ring  immediately after casting:
         Uneven cooling and shrinkage between alloy and investment can apply tensile forces to the casting dot
         It can reduce strength
         Penalty- hot tears