Failed Casting - Any thoughts

Discussion in 'Investment casting Block method' started by 0maha, Nov 18, 2021.

  1. 0maha

    0maha Silver

    Despite my success in reducing the melt time, last night's pour was a failure.

    I'm using Everdur for all this.

    I had a fair amount of this on the resulting parts:


    (For scale, that circle is about 1/2" in diameter)

    Is there a word for those voids? It is probably worth noting that the trouble spots were all on the upper side of the parts (closest to where the metal goes into the flask).

    When I cut off the sprues, I saw more voids. This is the cross section of the main sprue (right after where the metal enters the flask):


    I've had a good number of successful pours before this. This failure is new to me.

    The only things that I know were different with this one are:

    1. The flask didn't have much time at all to cool down to casting temperature (the speed of the melt kind of caught me off guard).
    2. This is the first time I re-melted left-overs/sprues/scrap from earlier pours. About half of the total weight I melted was reclaimed.
    3. I might have poured a little hot. My last reading with the pyrometer was 2180 F or so. I pulled the crucible at that point, then took however long getting the flask ready to go, then poured. I didn't bother taking a final reading with the pyrometer.
    End to end, my process is:

    - Make wax molds (I'm using R&R Limerick wax)
    - Invest using R&R Rancast
    - Dewax (upside down, letting the wax run into a pan) in a household oven: 250 F for about five hours
    - Move the flask to the PID controlled kiln: Ramp up to 1350 F over five hours, hold there for five hours, then drop to 1000 F and hold until I'm ready to pour
    - Pour using a vacuum table to draw air out of the bottom of the flask

    Pending a better idea, my current thinking is that my flask was way too hot. Normally, it is several hours (five, six or more) after the PID goes into "hold at 1000 F" mode before I am ready to pour. Last night, the kiln had only cooled down to an indicated 1100 F or so before the metal was ready, so I went ahead and poured.

    My guess is that even after the thermometer on the kiln is reading 1000 F during the hold cycle, it will be hours before the internal temperature of the flask cools to that level. The internal temperature of the flask was probably still close to the 1350 F peak temperature.

    Does that seem like a reasonable guess? Any other ideas?
  2. 0maha

    0maha Silver

    PS: In case this helps, I cut the part in half along the point of failure. This is what it looks like inside:


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