On person inquired about whether there is any danger of the investment exploding when using the fast burnout process we have been discussing.
I have had only one occasion when the investment “exploded,” so to speak, and spread investment all over the inside of the burnout kiln. On that occasion a student had mixed his investment with improper proportions and the flask was filled with blobs of investment instead of a smooth pour with cream-like consistency. I speculate that, but cannot prove, that the lumpy investment left large cavities in the flask that trapped steam during the burnout. The steam probably blew out the investment when sufficient pressure built up. With a normal pour there are no cavities to trap steam so fast burnout usually works fine.
Are there any other experiences out there to add to this scenerio?
A few years ago I was talking on the telephone with the retired Director of Research at Ransom and Randolph. I cannot recall the exact topic of discussion but it had something to do with burnout time. He suggested that I could burn out a flask much faster if I put it into a hot oven rather than ramping the temperature up slowly over a period of hours. I remember remarking that I thought that the sudden thermal shock might cause the investment to crack but I tried his suggestion.
Ever since then I seldom put flasks in a cold oven for burnout and I generally don’t use a computer controller to ramp up the kiln temperature. Instead I pre-heat the oven to 1350 F (about 732 C). I have marked my manual oven control to hold that temperature and while teaching the oven remains at that temperature throughout the week.
“This is crazy!” you say, but let me explain what is going on an why I use this unusual burnout procedure.
First, I was cautioned to make sure that the flask was not partially dried. That might cause cracking. If the flask had set for several hours or over the weekend it should be immersed in water until the investment is thoroughly moistened. Then if the flask is placed in a hot oven, the moisture in the pores of the investment will turn to steam and escape. As long as water or steam is present, the flask internal temperature cannot rise above 212 F (100 C) the boiling point of water. In the case of small 2 x 2.5 inch flasks the core temperature remains at 212 F (100 C) for about 20 minutes until all of the water is vaporized. At the same time the wax melts and runs out of the sprue and pouring cup. The investment does not crack and I have done this fast burnout many times.
There is another side benefit of having a very hot oven. We don’t get any significant amount of vaporized wax to smell up the shop. Instead the wax turns to carbon and then combines with oxygen in the air to form carbon dioxide (CO2) a colorless, odorless gas. Be sure to leave the oven door cracked to permit air to enter. I still use a vented exhaust hood over the burnout oven, but there is still little odor to the exhausted gasses. I would suggest this completely eliminates the need for a steam dewaxer.
Once all the water leaves the pores in the investment, the flask rapidly rises to the final burnout temperature where I allow it to remain until all the wax is completely eliminated. This is determined by looking at the sprue opening and the pouring cup in the flask. As long as the surface around the opening is gray we know the burnout is incomplete. The gray coating is incompletely vaporized carbon residue. When all wax or carbon residue is gone the burnout is complete. The opening of the flask is chauky white. Total burnout time is as long as it takes to obtain a white flask and is not dependent on any particular time-temperature program. Small flasks will burnout rapidly in an hour or two while large flasks (or an oven full of small flasks) will take longer. The test of how long is the white color of the sprue opening and pouring cup.
Try it. You’ll like it.
Do you believe me? Any discussion out there?