Investing and Burnout
Somewhere in any conversation about casting the subject of mold making will come up. Whether its types of investments, the mixing and pouring process, or the burnout schedule; there’s bound to be question about making molds. One of the most important commitments we make as casters is to faithfully reproduce the surface and detail of the wax pattern. You can only do this with a controlled investing process. In this discussion I will share my experience on what is important to getting consistent, successful results from jewelry investments. Here is a separate article when casting for karat golds.
Investment
Make sure it has been stored properly and is in good condition. Investment is hydroscopic and absorbs moisture from humidity, a damp concrete floor, or rapid changes in temperature. The smaller particles can also settle out to the bottom of the drum affecting the chemistry of binder and the silica. So it’s a good idea to roll your investment container as you go to open it, to make sure the powders are evenly distributed. If stored improperly, or over a year old, I wouldn’t use it.
Successful casting operations can be found using a number of quality investment manufacturers. Besides cost, investment will be chosen for the characteristics most needed to produce that maker’s particular castings. He could need a strong investment because he uses rapid prototype pattern materials. He might need an investment that breaks out easily to produce delicate parts, or alloys that are sensitive to cracking during quench operations. Or, he might need a good all-purpose investment that holds up to a wide variety of detailed parts and pattern types. Your wax
Mixing
I can only insist that everyone that still mixes by eye, to splurge on a small scale and a graduated cylinder. You will make that money back quickly in your finishing labor and reducing your metal loss. Most jewelry investments require a ratio of 38 or 40 grams of powder to 100 ml. of water. Always add the powder to the water. Most operations will mix investment about three minutes, vacuum their mixing bowl, pour their flask, vacuum the air from the flask, then remove the flask to a counter where it can set up. This counter-top or table should be free from vibration or movement.
We religiously time our gloss-off on every new lot of investment to make sure we have the correct mixing steps in place. Gloss-off should happen about ten minutes after you add the powder to the water. When you first view the top of an invested flask it will be glossy wet in appearance. Within one and a half to two minutes, this should go from glossy wet to a frosty, matt look with no reflectivity. If this gloss off is faster than a minute, reduce your mixing time. If this gloss off takes longer than two minutes, increase your mixing time. A flask that takes too long to set could allow the water and solids to start separating, resulting in watermarks on your castings. Changes in the number of flask you are investing will obviously extend or shorten your total work time. Manipulate your mix time to account for this.
Burn-out Cycles
I intentionally headed this section as cycles, meaning plural. There are as many burnout schedules as there are casters. You can get good castings using many of them, but you will see customizations depending on time constraints, size of flask, non-traditional pattern materials, types of oven, investments, etc. I feel there are five critical steps to correctly process investment molds for consistent results. I will briefly discuss each step and follow that with some possible defects that could result from cutting corners.
Curing- I talk to folks that have done their investing and load the flask too quickly after the investment has set. Investment needs to bench cure after hardening to achieve full green strength. This allows the investment to be hard enough to take the expansion of the pattern material when the heating starts. Small flasks need at least an hour and larger flask should be held for a minimum of two hours to get good green strength. Bad surfaces and investment breakdown could result from investment molds that are not adequately cured.
Low temp cycle- The first step in the actual burnout is the low temp removal of the bulk of the pattern material. This is most commonly done at 250-300F for one and a half to three hours depending on how large the flask and how much pattern material you have to move out. Do this too aggressively, by being too hot in temp or not giving it enough time, and the pattern boils and erodes investment from the surfaces of the pattern cavity. This will give you rough, raised surfaces on your castings called spalling.
Ramp up- The next step is to get your molds up to the high temperature dwell needed to remove the carbon. There are a few things going on during this cycle that require attention. In this ramp up you finish removing the water from the mold, and you pass through some thermal expansion zones where your investment expands. I was taught that 4F/minute is a good average speed to avoid steam and thermal shock. Many burnouts have a suggested dwell or hold at 700F where the steepest expansion curve takes place. Moving too quickly to your high temp will usually cause fractures in your molds and fins, or flashing, on your castings.
High temp cycle- So now we get to the business of removing the carbon from the investment. There is some debate about maximum temperature because much work has been done studying the decomposition of jewelry investments and their components in the presence of carbon. The jewelry world commonly uses 1350F as a maximum temperature. The duration at this temp ranges between three and five hours, dependent on size of flask and airflow. You should see clean, white investment, free of any grey discoloration inside the investment. Any grey carbon residue left in the mold could potentially lead to a buildup of gas porosity in the recycled portion of your casting alloys, as you repeatedly cast into badly prepared molds.
Ramp down- Realize that investment molds retain their heat very well. Even though the oven may indicate you are at casting temp it will take a mold one to two hours, dependent on flask size, to stabilize at a lower temperature. We wait two hours on tall, four inch diameter flask before casting. Embedded thermocouples, inside an invested flask, have shown it can take two hours to stabilize going down to casting temp. Not waiting long enough, slow cooling might cause hot spots where the sprue and shank intersects. Hot spots could manifest as porosity, enlarged grain structure, or roughness near the sprue-gate that will increase labor in finishing.
In sharing this I hope I can pay it forward to others that might be struggling to work out defects in their daily production. I had a lot of folks that helped me along the way. I still rely on them for input as we meet the next set of challenges. Be it new pattern materials, investments, alloys, or cast technologies.
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