Hot Tear Remedy

Discussion in 'Sand Casting' started by oldironfarmer, Jul 8, 2018.

  1. oldironfarmer

    oldironfarmer Silver Banner Member

    This is a small trivet I cast several of, 36 so far. I have noticed a small crack in this area a few times but nothing that prevented the trivet from being used. While cleaning up this one it came apart.

    IMG_3853.JPG

    The members are small (less than 1/4") and I have to pour it hot (1550F and above) to get a complete fill. After this failure I had to think back. I had poured two that morning, about 1570F and two the next morning, a little hotter. I'm using scrap, specifically an old TV antenna for a lot of this one, and maybe some window frame material. I wasn't sure whether it was the metallurgy, it was pouring practice, or other.

    I repoured this morning at over 1,600F with no cracks.

    I always assumed hot tears occurred from pouring too hot. on this pattern I'm assuming the small wheel spokes cooled first and the large feeder tried to pull material from the joint. I don't know why hotter worked and am looking for comments. I can't pour much cooler and still fill.

    IMG_3687.JPG

    In this picture MHR has been desprued and AFL has not. Gates are 3/16" thick, runners are about 1/2" wide by 3/4" deep, break is between the spoke between the A and F, or M and H.
     
  2. HT1

    HT1 Silver Banner Member

    Please tell me your are mistaken runners 1/2 wide and 3/4 deep in aluminum is plain ass backwards and a waste of metal in the extreme

    V/r HT1

    P.S. your Sprue is WAY too Big
     
    Last edited: Jul 8, 2018
  3. oldironfarmer

    oldironfarmer Silver Banner Member

    I'm guessing. But they're about that big.:rolleyes: When making the pattern I was concerned the metal would cool before it got all around the pattern. And I made them bigger after initial failure. I have three time the metal in runners and sprue as I do in the trivet. It is a waste of fuel but the runners make nice ingots.
     
  4. HT1

    HT1 Silver Banner Member

    OK... Long deep breath... the rule of thumb for Aluminum is wide and shallow ... concerning the gating system. in a horseshoe gate you REALLY need to get rid of the direction changes, those corners are causing the Metal to do all kinds of turbulent things. the runner should be an arch parallel to the casting... and all the gates should be exactly the same length. the runner should decrease in size after each ingate the exact amount of the Volume of metal the ingate is drawing that will theoretically put the same amount of head pressure on all the gates...
    Ingates should be as short as possible... this is doubly important here as that will add lots of heat to the casting... thus letting the metal flow better

    V/r HT1
     
  5. oldironfarmer

    oldironfarmer Silver Banner Member

    OK, thanks. So the gates are not too out of line? Or are you saying the runners should also be wide and shallow as part of the gating system?

    Those straight runners were really easy to make:rolleyes:

    Might be worth redoing the gating system on the match plate. When I set up the gating system the pattern was lots smaller than any guidelines I could find. The runners are intentionally large to try to get a lot of heat into the system.

    Logic tells me the gates should not be larger than the cross section of the sections being fed. That would make them really small.
     
  6. HT1

    HT1 Silver Banner Member

    Yes the runner should be wide and narrow... I like 1 inch wide 3/8 deep for most of the stuff I do... Give me A sec I'll do a doodle

    V/r Ht1
     
  7. HT1

    HT1 Silver Banner Member

    7BDCC31D-B39B-43C5-9B5F-CBB6BD33A20B.jpeg

    I don't recommend using this method, or the runner will get way to thin at the end take half from the width and half from the depth and keep everything smooth... Oh you may have to start with a larger runner then you think you really need so it does not get too small at the end... But right now you have near to twice the runner you need... Move it as close as possible to the casting... Use that heat from the runner to your advantage
     

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