Wax Burnout Problems: Effects of Carbon
The effects of carbon in relation to inadequate burnout is a considerable problem with silicon alloys. Carbon residue may remain in the flask due to a lack of oxygen (air) at 1350 degrees F. The carbon residue may also result from too brief a time period at the desired burnout temperature.
Silicon alloys in yellow gold will show a tearing effect or crack where the carbon explodes on contact with the molten metal. White gold alloys tend to absorb the gases that are released and may result in hard spots (silicides). This conversion to silicides will be dependent on the amount of carbon residue and if sulfur dioxide gas forms when the combination of carbon, calcium sulfate (investment) and molten metal converge.
Conditions that may cause incomplete burnout of carbon:
- Too short a burnout cycle at 1350 degrees F.
- Lack of oxygen at 1350 degrees F.
- Type of wax
Elimination of carbon residue problems:
- Normally increasing the length of hold time at 1350 degrees F will be enough to reduce carbon residue. Recommended time may vary from 2-1 0 hours.
- Proper oxygen levels in the oven are critical to remove carbon residue. The recommendations are the same as for elimination of sulfur gas conversion.
The type of wax or plastic can have a major effect on the amount of carbon residue needed to be burned. The following are in ascending order by increased amounts of carbon.
- Standard injection wax, light in color, i.e. aqua color
- Darker colored injection wax, i.e. burgundy red color.
- Carving wax, dark green & purple.
- Plastics, typically polyethylene.
Steam de-waxing can be helpful in reducing the amount of carbon residue left behind in the flask. After investing, you should wait at least I hour (2 hours is safer) before putting flasks into steam de-waxer. You should allow a minimum of at least 45 minutes for the flasks to be in the steam, and then allow the flasks to air dry for 2 hours (outside of the dewaxer) before putting them into the oven. Steam de-waxing cannot be used for plastics or plastic hybrid waxes. The type of wax or plastic should be taken into consideration when burning out. It will take a much longer time at 1350 degrees F to remove carbon. When plastics are burned, it may also be necessary to burn-out with a direct temperature rise to 1350' F. Direct burn-out usually leaves slightly less carbon residue in the flask, however at the expense of a rougher surface texture.
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