Alloys
Locate the problem and then select the possible cause to find a probably solution.
Heat Tears (Intergranular cracks)
Caused by overheating alloys during casting. Adjust fuel mixture so that the dark blue inner cone is approximately ½ inch long. Lower noble, high palladium or platinum containing alloys are particularlysusceptible to oxidation and extra care should be exercised in their melting and casting. Overheated alloys can result in large grain size castings and may contain excessive quantities of impurity elements absorbed from the torch flame.
Do not quench ceramic alloys; instead wait 25 minutes or more before removing the alloy from the investment.
Overheated investments can lead to the growth of metal grains and the subsequent formation of hot tears. Use the recommended temperatures and times specified in the technical sheets for each alloy.
The melting, casting or recasting of buttons can cause the vaporization of certain components of the alloy and thus slightly change its composition. Each time the alloy is melted, a quantity of the interstitial elements (carbon, hydrogen, and oxygen) are absorbed from the torch flame. These elements tend to migrate and concentrate in the grain boundaries. Excessive amounts of interstitial elements result in brittle grain boundaries. Compensation by adding new alloy will assure that the metal retains its composition and reduce the probability of heat tears.
Using a separate crucible for each type alloy metal and insuring that the crucibles are not discolored or glassy will assure proper casting.
Incomplete Castings
Be sure to follow the instructions for the investment allowing sufficient time at temperature to assure all the investment is at equilibrium with the furnace temperature.
- Oxidation of alloy especially the higher platinum and palladium containing alloys. Please see section on brittle alloys above.
- Incomplete burnout of the investment resulting in gas formation when in contact with the molten metal.
- Sprue gauge sizes too small resulting in the entrapment of gases during casting. Patterns should be placed above the heat zone center of the ring. Spues should be a minimum of 8 gauge and use reservoirs when casting larger restorations and especially with larger pontics.
- Dirty or contaminated crucibles that allow the entrapment of impurities that cause out gassing in the metal or when in contact with the investment.
- Too high a burnout temperature or excessive overheating of metal can cause a reaction at the metal/investment interface with the possible evolution of gaseous products. Use the recommended burnout temperatures specified in the alloy technique sheets.
Suck Back
Overheated alloy remains fluid for too long a period of time, causing excess gases to form between the investment and the alloy, resulting in a suck back.
There is a minimum and maximum alloy ratio. The minimum amount should be equal amount of alloy in the button as in the casting. The maximum is three times the alloy in the button as in the casting. If the button is too large, the alloy will stay molten too long causing suck back.
Sprues and runner bars must be of sufficient gauge to assure that the molten alloy will flow freely during casting. Too narrow a sprue gauge, too long a sprue, or pattern walls too thin may cause insufficient flow to all parts of the casting resulting in voids and potential porosity. Sprues should be placed at the thickest portion of the wax-up and the reservoirs 1.5-2mm (1/4 inch) from the wax-up.
Porosity or Pitting
Overheating can cause the volatile elements in an alloy (e.g., silver, zinc, indium, gallium, etc.) to vaporize resulting in the areas of pores or voids in the casting. If extensive colored fumes are seen on melting, it is likely that the alloy is being over heated.
Overheated molds can interfere with proper cooling of the alloy affecting casting density. Overheating furthermore may increase hydrogen or oxygen absorption by platinum group metals and in palladium-silver alloys. Calibrate burn-out ovens periodically to confirm temperatures. Use the suggested burnout temperature(s) listed in the alloy technique sheet. When using multiple rings, add an additional 10 minutes or more for each ring in the oven.
Underheating the mold might result residual wax or plastic pattern not being completely burned out. This residual material can react with the molten alloy causing porosity.
Periodically calibrate burn-out ovens to confirm temperatures. Use the suggested burnout temperature(s) listed in the alloy technique sheet. When using multiple rings, add an additional 10 minutes or more for each ring in the oven.
