Why did this hole in the top appear ?

Discussion in 'Sand Casting' started by metallab, Apr 18, 2023.

  1. metallab

    metallab Silver

    I made this bronze / brass (brass diluted with at least equal amount of copper to prevent zinc fumes) little egg cup (here upside down as it was in the mold).
    The pouring sprue was left and right is a riser and in the center a small riser where a just poked a nail foir air venting, but it did fill. But next to the latter small riser, there is still a hole in the metal. Despite I heated the metal at least 200 C over the melting point.

    What went wrong here ?

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  2. mytwhyt

    mytwhyt Silver

    My guess would be, the riser cased the hole.. The bottom of the cup was completely filled when the metal reached that point. The small riser was forced to fill with the decreasing head pressure.. What became the hole, was probably the last source of liquid metal to draw on, and it did.
     
    Mark's castings likes this.
  3. Petee716

    Petee716 Gold Banner Member

    It sure does look torn. I try to avoid contacting the casting directly with vents. It often creates shrink defects right at the point of contact as you can see on this casting where the vent necks down at the contact point. This one would have probably cleaned up fine, but sometimes it directly affects the casting. I’d be more inclined to think that the thicker cross section of the outer circle cooling pulled on the flat bottom to the point of failure like over-tightening a drum head.
    Or could be aggravated by crud in the melt or interrupted pour?
    All guesses.

    Pete
     
    HT1 likes this.
  4. metallab

    metallab Silver

    I can understand, but is was not torn and no cracks appeared in the casting. I casted this earlier with also a vent hole in the thinner part of the bottom (top in the mold), but the hole was thinner. Next time I just pierce narrow holes as venting holes.
     
  5. Billy Elmore

    Billy Elmore Silver

    Looks like metal filled the vent before before all the air/gas escaped. Happens in thin wall castings. Metal takes the path of least resistance. Id take off the vent or whatever is on the side of the casting that is pulling hot metal from the casting and either make the vent big enough to flow all cold metal out or just make lots of pinhole vents around the top that metal will not actually fill due to its small size but will let gas out.
     
    HT1 likes this.
  6. HT1

    HT1 Gold Banner Member

    where to start?
    Billy touched on it, vents are NOT part of the casting if they fill with metal they are not working as vents (Unless your fill sequence is WELL planned) someone help me outI know I did a post on how to vent properly

    why do you have no pouring cup, did you dribble metal down the tube on the right??? if so, you did not develop proper head pressure, in short you poured too slowly

    risers will never work properly unless they contact the casting with the same or a larger cross sectional area than the riser ( excluding exothermic riser sleeves and hot topping ) but that is not hobbiest SH1t

    your riser is WAY too Small as a side riser, a top riser that size might have worked just barely


    V/r HT1
     
  7. Tops

    Tops Silver Banner Member

  8. HT1

    HT1 Gold Banner Member

    Sawyer massey likes this.

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