If you are experiencing rough surfaces, discolored metal or no-fills, the problem may be in the burnout. When casting, the burnout cycle is an important step that is often treated with indifference. The burnout is not just a means to remove wax from the investment; it also cures the investment in preparation for the thermal shock of the metal entering the pattern cavity.
The burnout cycle is determined by the size and number of flasks in the oven. For example, four 2 1/2 inch flasks need less time than two 6 x 4 flasks.
The flasks should be moist, but not wet, when placed in the oven. A wet flask will explode. The moisture in the flask turns to steam during the burnout and helps to push the wax away from the walls of the pattern cavity. If the flask is dry when it is burned out, the investment will act like a sponge and absorb the wax leaving a rough surface texture.
The first step of the burnout should not exceed 300 degrees Fahrenheit and should be held at that temperature for at least 30 minutes.
The next step is determined by the type of wax in the flask. If there are patterns carved from hard carving wax, there should be a stop at400 degrees F. to allow the hard wax to flow out. If the temperature rise is too rapid it will cause the wax to boil inside of the pattern cavity and destroy the surface.
The next step of the burnout is 1350 degrees F. This is where the investment is cured. Most investments do not need to exceed 1350 degrees and should never reach 1500 degrees F. At 1500 degrees, the gypsum binder begins to break down and discolor the metal.
The 1350 F temperature must be reached gradually. A controller can be programmed to raise the temperature 300 degrees at a time, but if the oven is operated manually then there will have to be stops made at 700, 900 and 1200 degrees.
The 1350-degree temperature should be held until the investment is chalky white. Dark gray coming out of the sprue opening indicates an improper burnout. If your flask is gray, place it back in the oven and take it back to 1350 degrees. Please note: At this point the flask can go straight to 1350 F because it has already been through a burnout and is partially cured. Hold it at 1350 F until the gray is gone. It will not be chalky white because it cooled down before curing; instead the surface will be a light brown.
The last step is to gradually take the flask down to the desired casting temperature and allow it to stay at that temperature for at least one hour. This amount of time allows the center of the flask to cool down to the same temperature as the oven. If there is more than one casting temperature, cast the high temp flasks first, then lower the oven temp and allow the flasks to sit for at least 30 minutes before casting.