I’m very new in casting and recently put Formlabs Castable Resin into test.
But the burnout schedule seems too long for large production.
Or maybe I misunderstand the time schedule. It took me 12 hr to go through burnout schedule. After, maintain oven at 732°C during casting process.
Here some picture follow official burnout schedule.
The broken one it’s because metal didn’t go down the pipe due to the operator error.
Does every temperature point have it’s meaning? Can someone explain to me?
Also I read about some people use air pump to increase chance of successful casting. But how? I don’t understand theory behind it. Is that air flow some how increase the temperature of oven, help burnout process smoothly?
First, a minor disclaimer that my experience is primarily in sand casting and machining. I have yet to do any investment casting, but looking forward to it.
For being new at casting, I would be very proud of those results if I were you. Casting isn’t as easy as pouring metal into a mold. Well, I mean it is…but it isn’t. One comment is that your mold may have been too hot depending on what metal those are made of. Generally speaking you want as cold of a mold as practical, and is usually a few hundred degrees below the melting point of the metal.
As for the purpose of each burn out stage, the first slow ramp and low temperature hold, this is to remove all of the moisture in the investment and the pattern. If done too quickly or at too high of a temperature, you can get a steam bubble causing the investment to explode. In a wax pattern, the wax is usually melted out at this point too.
The ramp up to the high temp, and usually has a hold at about 700°F, is to make sure that all of the wax, in in this case plastic is burned out. I’m not sure if the castable resin will actually melt and “flow” out of the mold, but it is these temperatures that the plastic is burned out. This temp also helps cure the investment and make the mold stronger.
The high temperature hold is your carbon conversion temp. Wax or plastic investment will leave a carbon buildup in your mold. The high heat will cause the carbon to react with oxygen to form carbon dioxide, essentially “burning” out the carbon. With a gypsum based mold (read plaster) it’s generally not advisable to exceed 1400°F because you can start to degrade the investment creating sulfur compounds that will have a negatively affect your part. This is also why people add air pumps to their kilns. Increased air flow means more oxygen to help with this conversion. A small amount of air flow also creates a more even heat in your kiln, think convection oven.
The final step is a cool down to “casting temp” This is what I mentioned earlier where the mold should be cooler than the melting point of metal you are pouring. If memory serves me correctly, this has something to do with more or less flash freezing the metal that comes in contact with the mold almost instantly creating an insulating metal “tube” for the rest of your pour to flow through. This is suppose to help minimize surface defects.
Yes, the 12 hour burnout schedule is long, and for casting a single piece at a time is not good for large production…this is why you cast many parts on a single tree. There are methods of putting a wet flask directly into a 1350°F kiln and burn it out for a fast burn out. But there are precautions to follow, and though the method is used even in industry, the 12 hour slower burn out is accepted as the “right” way.
I also recommend the 12 hour burnout. That is what we use and so far every casting we have done with the formlabs castable resin has been perfect. We have followed the formlabs recommended burnout procedure and our own that we mostly use for wax, and the result have been identical.
I also want to add that in the case of resins, the slow initial ramp and low temp hold, also aids in curing the resin through and through incase we didn’t post cure it enough.
But we also try to post cure the print really well for at least 3 hours under strong UV lights or 5-6 hours next to a sunny window.
Thank you for your advice.
Do you mean if I post cure the print then I can shorten burnout schedule?
My casting machine is built for small wax tree. Not much space I can put parts into. Machine belong to company. Change a bigger casting machine is not a option.
Next week I’ll try again, will post result and burnout schedule here.
No, post cure doesn’t shorten the burnout schedule. A good post cure ensures the resin deep inside the print is also cured really well. If your print is not cured through and through, you are likely to get bad quality castings.