By J. Tyler Teague • Sponsor: Ransom and Randolph
In my last MJSA Tech Sheet (#39, in the August 2014 MJSA Journal), I wrote about how the addition of boric acid can strengthen casting investments, improving the casting results of resin-based models. I also promoted the use of a low cristobalite, high calcium-sulfate investment, primarily because of the strength that calcium sulfate (CaSO4) provides at the beginning of the burnout process.
Since then I’ve done further research, and I’ve made a few discoveries that I’d like to share. First, while a high CaSO4 binder does indeed make investment strong, I failed to recognize the advantages of a high-cristobalite investment. Cristobalite expands naturally during the burnout process between 200oC and 300oC (392oF and 572oF), and the pressure of the expansion causes the crystals to lock up. Further, I failed to fully consider the limitations that an investment heavy in calcium sulfate would have under certain high-temp conditions: In the presence of carbon (wax and resin residue), at around 750oC/1,382oF, the binder decomposes quickly and forms sulfur trioxide, which is highly corrosive to all copper-containing alloys of gold and silver and is a main contributor to gas porosity. Although this temperature is not normally reached during a burnout cycle (which typically peaks at 730oC), it’s always reached when the molten metal makes contact with the investment, especially in white-gold casting.
To avoid this, I now recommend a high cristobalite investment (I use Ultra-Vest Maxx by Ransom & Randolph, but others might work as well) in a 38 percent water-to-powder ratio. In addition, while I still advise adding boric acid to your investing water, I have increased my boric acid addition, from 15 grams of boric acid per liter of water to 20 grams. Try heating only 25 percent of your investing water to 85oC/180oF, dissolving the full amount of boric acid into that, then adding the remaining water back to cool down the mix.
Besides mold strength, another challenge is getting enough oxygen into the flask during burnout to fully combust the resin parts. The solution here is to generate oxygen inside the flask. To do that, I also now add 20 grams of calcium nitrate—2Ca(NO3)2—per liter to my boric-acid-and-water solution. Quality calcium nitrate can be purchased from chemical supply companies in a crystalline form. (It’s sold as calcium nitrate tetrahydrate and it absorbs water quickly, so keep your crystal container tightly closed.) Calcium nitrate starts to decompose during burnout at around 500oC/932oF) by the following equation: 2Ca(NO3)2 → 2CaO 4NO2 O2. A great deal of oxygen is set free during that reaction. Let your resin parts cook at 500–550oC (932–1,022oF) for at least 90 minutes during your burnout cycle and you should have cleaner results. Because the cristobalite expansion phase-change took place between 200oC and 300oC, and the investment’s quartz phase-change expansion happened around 500oC/932oF, the investment (especially with the addition of boric acid) is about as strong as it can possibly get
Here is the other big benefit you get with these changes: Your top burnout temperature can now climb to 850oC /1,562oF without fear of damaging your investment mold—and that additional heat really helps to remove any remaining residue of resin in your mold. Make your temperature changes up and down at about 2.2oC (4oF) per minute, but do not cast your parts at this temperature: Always cast at the lowest metal and flask temperatures possible to properly fill the mold. Also, try blowing a stream of air across (not down into) the button hole of your flask just before casting—this will help to draw out minor ash residue.
If you follow these updated suggestions—and, as recommeneded before, cure and clean your parts well properly—your casting of resin materials should improve.
J. Tyler Teague is the founder of JETT Research (jettresearch.com) and Proto Products (protoproducts.com) in Nashville, Tennessee. The author would like to thank Mike Stover of Ransom & Randolph for his assistance with this article.