Remedy for casting flaws????

[Chet]

I'm experimenting with a change in the way I do foam castings using regular dry sand.
The attached photos show the problem... seams or creases in the TOP half of the casting at the end FURTHEST from the pouring sprue.
I melt to 1400 degrees, remove slag, and then add salt [remove gas] and sodium bicarbonate [increase fluidity] and remove new slag.
The chemicals are added per recommendations of myfordboy's videos on youtube.
I'm suspecting the creases are the result of 'fold back' when the molten aluminum reaches the far end of the pattern and starts to fill up.
IF that is the case, then I'll add a 'pouring well' or two to the far sides of the casting to act as a 'bumper'
If you know it's either of the chemical additives, I'll eliminate them.
But I'd sure like to know if any of you guys have run into this problem and how you solved it.
Thank you for your insights and suggestions!
Chet

[jpfalt]

It's trapped gas and cold metal. It's very similar to the condition you get if the mold is too wet and cools the metal surface as well as blowing the metal way from the mold surface with steam.

As you burn out the foam with the poured metal it gives off gas. If the sand doesn't vent off the gas, this is the surface you get.

The best approach to solve this is to put a pop-off in the mold for the gas to escape through and to provide a reservoir at the end of the casting for the initial metal in the mold to flow out of the desired casting into a slug that will get sawed off and remelted for the next casting..

If you are using dry, unbound sand for the mold, the pop-off won't work. If this is the case, go to a pertobond or green sand mix that will hold shape and add lots of vents and pop-offs to get the gas out of the mold.

[Chet]

Some good thoughts, JP.
Since the sand is dry, I'm suspecting it's the addition of the sodium carbonate. It creates a bright orange flame when mixed in, and quite possibly the combustion is continuing during the pour.
I'll skip it on the next pour and see how things work out.
I'd love to use green sand, but I'm still working on perfecting the art of re-conditioning it...
I really found the following video interesting:

http://www.youtube.com/watch?v=U7ut_2nqiGk

It sort of sets the standard I'm trying to perfect.
Chet

[steamin10]

Conditioning sand was traditionally done on the foundry floor. being covered overnight with a plastic or tarp, to prevent moisture loss( green sand), the mass was rolled over by flat shovel, chopped several times, like slicing bread, and then stomped on with the foundry shuffle. This process was repeated several times mimicing a Mull, adding a touch of water, if needed, until the sand was broken down and smooth, and held a nice mass if squeezed by hand. Simple, fast, and with a little skill, very dependable.

It is my opinion that carved foam for Aluminum is not well worth the effort. That detail of effort is taken from Iron casting, where the heat available is greater, and the wieght is greater, to accomplish the mold fill. Aluminum that is heated or liquidous will readily pick up porosity and oxides, during a pour, and you are feeding it through plastic. I think its shooting yourself in the foot.

I would suggest reading some authoritive books on the process, and trying that route, rather than going and trying to reinvent the wheel. I have been down that road of disappointments, and soon learned to mimic a commercial process. Much better success, and quality, than a blob of stuff that may be in a shape, but flawed, and gassy.

No offense meant. Carry on.

[Chet]

Hi Steamin10, thanks for the feedback.
I respect your experience and value your suggestions.
But I've generally had good experience with foam and dry sand casting.
It's just that I experimented with a new method to make the production a little more reliable, and ran into some unexpected results.
Today's pour turned out much better -- excellent detail with no crevases.
The big difference is I stopped using the sodium bicarbonate.
There was a slight shift in the pattern near the pouring sprue, but I suspect that was simply me attempting to remove the sprue before everything had solidified.
Yes, it's really annoying to do something that stupid!
So now it's back to re-cutting the foam pattern on my homebuilt CNC router.
It's not difficult, just time-consuming...
Chet

[steamin10]

I am happy to hear of your successes, and am not trying to poo-poo your efforts. Most often guys with little foundry knowledge, try all sorts of wild things, reinventing the wheel, and get discouraged when all the details and calamities take hold. It really isnt that hard, although aluminum is more touchy than say bronze, to cast. Casting in sand of course goes back thousands of years to China, where horse bits, and battle armor pieces were cast en masse, as well as other household pieces.

There is a U-tube of a guy tha casts a bronze ingot to be worked ---right through a container of water. Very clean ingot. Very fine grain, because of the rapid cooling. It is mind bending, but illustrates process, to an end.

In short, in todays world, a zerox copy of the basic needs is relatively easy to make in a garage, with gas from the house and vacuum cleaners and leaf blowers, just a little creative spirit, can make it all happen. Please continue on, and post your results. It is still of interest to me. There is Good, Best, and just getting it done. Getting it done opens many doors.