Aluminum casting problem

I'm new to this "art" and have searched the Internet for info on my problem but haven't yet found much help so I turn here.
My problem is what appears to be sand inclusions due to breakdown of the sand when hot. I'm trying to cast some relatively thin parts and found the only way to get the mold full is to pour a bit hotter than I normally do. Don't have a pyrometer yet so I'm kind of guessing.
The inclusions are on the top side of the casting (floating) and appear to just be in the flow stream above the entrance to the mold. The last parts of the mold filled are defect free, which makes me think that it is sand cooking and flaking of the runners after upstream sand has had a chance to heat.
I'm using western bentonite, around 10%, and relatively coarse sand (70). My muller isn't able to properly mix Petrobond or K-bond sand so until I can upgrade I'm stuck with water bonded sand.
What I would like to know is if there is some kind of modification I can make to my Gates, runners, chokes, etc that might eliminate the problem, or trap the floating sand before it gets to the mold? Is there some readily available additive I can use to increase the hot strength of the sand? More watter? I have no steam problems - yet.
========================Leon McAtee
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On 28 Feb 2004 09:36:34 -0800, snipped-for-privacy@caspercityauto.com (Leon McAtee) wrote:

If you mixed your greensand correctly I would not fool with it. When you cut your sprue and gate and runners are you slicking them down and making it so there are no sharp projections into the mold cavity that can get washed off. At the bottom of the sprue you need to make a deeper impression so the turbulance of the molten aluminum does not cause the sand in this area to get erroded away. Simply make a deeper cavity below the bottom of sprue and also below the bottom of any gate or runner. I think the 10% of bentonite to sand is in the ball park. 70 mesh is pretty coarse so you may want to add just a touch but not all that much more bentonite.
Another last ditch effort is to make a vent riser equal in size to the fill sprue, and tilt your mold so this vent riser is at the higest portion so any loose floating sand is floated up and out this vent riser. Its not a cure but a possible work around. Make sure your ramming the mold sufficiently and slick down the edges of the pattern / mold at the parting line so it does not contribute to loose sand in your mold.
Pouring thinner sections hotter is the normal proceedure. Visit my website: http://www.frugalmachinist.com Opinions expressed are those of my wifes, I had no input whatsoever. Remove "nospam" from email addy.
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check your gating system. Are you using a pressurized or non-pressurized gating system? What's your ratio for downsprue: runner : ingate?
Tricks to help can include: * reduce pouring height * put bigger well at bottom of downsprue * go to non-pressurized system * put traps at end of runners * dog legs in runners/gates * minimize turbulance and metal veloceties in gating system
You can get some good books/training courses at the following: http://www.afsinc.org

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Thanks for the link. I suspect the last parameter is the one giving me the problems. My ingate is quite small due to the thiness of the casting. Metal velocity at that point is probably high. I also was trying to keep the hydrostatic pressure high to help fill. My first attempt only filled partialy - but had no sand includions.
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check your venting.
What is the smallest cross section of your casting? Can you move the ingate? Faster fill doesn't always make a better fill.
What's the height of your cope?

non-pressurized
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The venting on this part has consisted of several small(3/32") vents at the top most portions. They have never fully filled. One can see that the ones closest to the ingates fill to a greater height (around 2 inch) than the colder ones.

Size and general shape of a bread pan - about 4mm thick the piremeter has a section only about 1.5mm thick which has been limiting my gate size. There is a way I can gate to a more substantial portion of the patern but it will take a bit more creativity - and my first attempt at making a core.

Yes. Tried one at one edge and then one on each end. The attempts with 2 had sand inclusions (if that is what they really are) on only one side, and seemed to be of less total volume.

That has been my limited experience. The slower and the colder I can pour and still have a fully filled mold seems to give the best results.

About 6 inches total - 3 inches above the top of the casting. I also tried swapping the cope and drag relative to the pattern with little differance.
Will probably try again this weekend with larger runners, bigger sump under sprue, a sand trap before the gate, and larger gates. I've had good luck with other patterns using a slanted sprue. Seems to allow the metal to flow in with less turbulance. But the flask I have been using is a bit small to try that with this pattern. I have made a slightly larger flask and will use it next time.
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On 2 Mar 2004 18:14:07 -0800, snipped-for-privacy@caspercityauto.com (Leon McAtee) wrote:
<snip>

<snip>
What about using a runner and multiple ingates around the edge?
This advice is worth what you paid for it... my foundry work, so far, comprises reading about it
Mark Rand RTFM
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