Spruce design for lost wax slip investment casting?

Discussion in 'Lost wax casting' started by John Gaertner, Oct 10, 2021.

  1. John Gaertner

    John Gaertner Copper

    I have a project I am returning to after working on it several years ago. It's an axle cover with a very detailed winged logo on it. I have made at least 6 by sand casting with PetroBond and 4 with lost wax slip investment. I have a good silicone mold of the part for making my wax patterns.

    I am seeking advice on the spruing of these wax patterns. When I cast them in sand I had a riser, a gate that fed the part and another riser on the opposite side. (See second try 2 ) These worked well but I did not get the very best detail for the logo. Sometimes it was good, sometimes it was not. Time involved to fail the ramming of the mold was not worth it.

    First ever investment casting I put the sprue right in the middle of the back of the part and gave it a vent (see picture Investment). 2 of the 4 cast parts had issues with the fine detail that I attribute to the direct pour of the aluminum charge. Maybe the aluminum was not fully clean of dross, I don't know.

    So when I go to remake these parts I am wondering whether I should make a wax sprue and gate to one side to fill the cavity and maybe a vent as well? (See Logo-hub-investment) I did not at the vent cause I will use just plain rod wax. Thanks! Jgaertner

    Attached Files:

  2. Jason

    Jason Gold

    I tend to over vent the hell out of my stuff. Is it necessary? Not really, but a failure with lost wax isn't the same as screwing up something that took just an hour to ram up.

    When I vent a part, I tend to run it back up to the top of the pouring cup. It adds a little strength to the piece and catches the metal when it comes to the top.
    Like so! btw, what is your dewax procedure? If you had detail problems with shell, I would say your aluminum might be to blame. Overheating the metal (which is easy to do) and also overheating your shell will also cause problems.

  3. John Gaertner

    John Gaertner Copper

    Hello Jason, Thanks for your help. I use an electric burn out oven with digital PID temperature controller. So I am hoping that the shells are pretty close to optimum temp. I have to check my notes but I think I had it set at 1200 F. I put them in a sand box for stability just before pouring. I use a digital pyrometer for checking my aluminum charge temperature prior to pouring. I use fresh foundry aluminum, no scrap or reclaimed.

    I like your idea of venting, venting, venting. Your picture shows your sprue pretty much like my part in my question's picture. I have to make the wax patterns using the silicone mould I made years ago. I preheat the silicone to 150 F and I put the filled mould in my pressure pot. I use another silicone mould for the sprue. I think I will try both a side fill sprue and a straight in sprue.
  4. Jason

    Jason Gold

    SO let me get this straight. You take your shelled waxes and just put them in the burnout oven? And this has worked for you in the past with no crazy shell cracking?
    Ya see John, the Achilles heel with ceramic shell is getting the wax out of the shell. I use low expanding wax, I run a dewax procedure which rids 99% of the wax and then after inspection and repair of any cracks, I go back in the kiln for final burnout and heat up to casting temp.

    If I put a shelled wax into my kiln, I'd have a big fire and black smoke you'd be able to see from VIRGINIA! Even after my procedures, I still get cracks which is why I'm curious what your method is.
  5. John Gaertner

    John Gaertner Copper

    Hello Jason, Yes that is how I perform my shell burn out. I have a steel shallow cookie sheet at the bottom of my burn out oven and and it catches the wax. There is some smoke, but its not crazy or anything. I have had very few shells crack. I was told to burn your wax out as fast as possible and I drill a large hole down the middle of my wax pattern, prior to burn out to give some room for expansion and a cavity for the wax to burn out from. I then usually heat the shell to it is finished. Works for me. I have a video of this process. Jgaertner
  6. Jason

    Jason Gold

    Got a link?
  7. John Gaertner

    John Gaertner Copper

    I will have to upload it to my Youtube channel and I will try and post a link.
    Jason likes this.

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