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”.

 

            

1^st 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

                                

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

3^rd 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

4^th 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.

5^th 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).

6^th 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

7^th 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

8^th 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)

9^th 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

10^th 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

11^th 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

12^th 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

13^th 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