Author Topic: Short sprues and air vents on sand molds  (Read 1218 times)

Offline airmodel

  • Full Member
  • ****
  • Posts: 79
Short sprues and air vents on sand molds
« on: September 26, 2016, 12:48:26 AM »
Hi everyone.

When I pour cast iron I use very short sprues and risers also no vents on my sand molds. So I melted some aluminium which is so much lighter than cast iron and see if the mold will still fill up using the same method as cast iron. Have a look at the video to see what happens.

Online b.lindsey

  • Global Moderator
  • Full Member
  • *****
  • Posts: 9794
  • Dallas, NC, USA
    • Workbench-Miniatures
Re: Short sprues and air vents on sand molds
« Reply #1 on: September 26, 2016, 01:03:22 AM »
Even having never done any casting, I find these videos fascinating. Thanks so much for taking the time to post them.

Bill

Offline airmodel

  • Full Member
  • ****
  • Posts: 79
Re: Short sprues and air vents on sand molds
« Reply #2 on: September 26, 2016, 01:45:48 AM »
Your welcome.

Offline Tjark

  • Full Member
  • ****
  • Posts: 317
  • Appingedam, Netherlands
Re: Short sprues and air vents on sand molds
« Reply #3 on: September 26, 2016, 08:30:23 AM »
Thanks for the video’s
I use most off the time oil bounded sand and have to vent always.
For piston castings I use green sand and was venting always but you have shown that this is not required,
As you mentioned this saves a lot of time, thanks for the tip.

   Tjark.

Offline airmodel

  • Full Member
  • ****
  • Posts: 79
Re: Short sprues and air vents on sand molds
« Reply #4 on: September 27, 2016, 06:33:50 AM »
Oil bonded sand was developed because oil expands so much less into a gas than water to steam therefore a much finer sand can be used to give a better surface finish. Also oil burns taking the oxygen from the metal surface and gives a better surface finish.

Offline fidlstyks

  • Full Member
  • ****
  • Posts: 29
Re: Short sprues and air vents on sand molds
« Reply #5 on: December 22, 2016, 12:50:08 PM »
It is an interesting subject to not have vents. I know a foundry that  use's no vents. He has hot coupola iron, it pours much better, the steam goes out the sprue hole.Then the trapped steam boils up out the sprue. However I also know people who machine his castings , and the model cylinders have holes in them, and the flywheels have holes in them.
   I have made castings since 1980, I always use vents. I some times on larger castings allow the steam to vent through a riser, or the sprue if a large part where I pour in the top, like full scale mufflers.
   I have forgotten to vent, and small castings did not fill to the end. You can look at the failed casting and see the air pushed bubbles into the casting, when it chills and stops flowing, it makes a smooth rounded end. I also have seen castings with odd shaped holes all the way across, steam. So I make sure I vent.
   I also sand vent the top and bottom, (cope and drag), with a 3\32 hardened steel vent rod. I jab evenly all around, and circle the sprue . It does help dispell moisture , hole causing steam.
    I use powdered coal ( sea coal) in the sand. Not sure on the science ,but I thought it made a film of gas between the molding sand and the casting.I never thought of it as burning the oxygen. I know the coarseness is burn in, where the sand melts into the casting, so making a gas film separating the sand from the casting makes more sense to me.
  I use the petro bond on aluminum, and some brass. I also like the sea coal on the brass. The petro bond sand is finer .
  I also use wood flour in the iron sand.

Offline fidlstyks

  • Full Member
  • ****
  • Posts: 29
Re: Short sprues and air vents on sand molds
« Reply #6 on: December 22, 2016, 03:58:57 PM »
Another steam factor is the hydrostatic type preasure created, from steam, it raises the mold, just enough to allow metal to flow out. Many mistakenly believe that the metal raises the molds, and allows seepage. Stream does raise the mold first.     I use mold glue, helps  prevent this. On larger molds , where the casting is far away from the edge, I use the trowel to just make a knife like cut. Thus helps seal the mold. Of course I also use weights and clamps.
    Pictured is a mold I made this morning. I show the mold glue, and as applied, and the trowl, how it cuts. I also show vents thar are on top, and bottom, which do not go in the cavity, and the vent rod from the edge of cavity, out.
    Steam from green sand  is like steam in a cylinder of a steam engine .It pushes, only in this case it pushes back on the molten iron and raises the mold to allow seapage.
   I pour a lot of iron with short sprues, but the amount of iron standing a couple of inches taller is not much. Velocity with a taller mold, gives the iron the nessessary flow to fill some molds.
   The plaque looks very nice. I suspect you had very hot iron.
     Steam damage resembles ice that is partially melted. Sometimes you see ice like it along a creek, that is hallowed out, and looks like some one melted part of it with a torch. Some times fingers with very thin, hard sharp points appear.
  When I lived in Indianapolis, we would make large molds like flywheels,10 hp IHC, etc , and long boiler grates. We would leave the molds open for a day or two, and let them dry out some.
   I do not like  putting vents on top of small protrusions. Some times the steam  collects there . It also does something that makes a hard spot. Not sure why.Especially on a flywheel ring. I like to make the vents on the inside edge, or make slots on the center out, and riser on each spoke, for venting, and feeding.