Mould and core washes

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I cracked a 3D printed pattern, and so decided to cast permanent pattern halves in bronze.

I could not get the bronze hot enough (did not understand how to correctly adjust an oil burner yet), and so I did not get a complete fill.

I have several boxes full of failed castings.

I think these were sodium silicate molds.

I didn't understand how to do sprues, basins, runners, and gates at this time.


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Poured this base too cold, and got cold joints where the metal fronts joined but were too cool.
This mold did not fully fill.
Pour temperature for aluminum 356 is about 1,350 F.

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I poured this casting much hotter, in order to get a complete fill, and got the melt way too hot, which caused a lot of incursion into the sand (Petrobond).

I think this was actually when I decided to purchase a pyrometer.

The surface finish it terrible when you overheat aluminum, and overheating aluminum generally causes a lot of gassing issues/porosity.

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This was an early attempt at casting a flywheel in aluminum.
This was a successful casting, and I think my first successful aluminum flywheel casting.
I went on to cast two of these in gray iron successfully later.

Something was going on with the lighting, and I think the fluorescent bulbs gave the aluminum a golden appearance.

I had a lot of excessive flash on this flywheel, and some inclusions, but it did clean up.
This was my best aluminum casting up to this point, and so at least I felt like I was making some progress.

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As my techniques and materials improved, I started getting good castings every time.
This was before I discovered ceramic mold coat, and so I had some burn-on/cleanup to do.

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Here are some cylinder molds.

The coreboxes for the cores are pieces of PVC pipe that are slit on one side with a thin saw.

The cores were rammed with a dowl rod in the center for venting.
The cores were released from the coreboxes by prying the PVC open slightly at the cut line.

There is no need to cut the PVC completely in half, as many/most folks do.

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My main mistake (fault) is lack of patience. When I made a core, either by using sodium silicate and sand, or epoxy resin and sand, I was in too much of a hurry to use them. I should have done what you did, leave them alone for a day or so, or put the ss cores in an oven. Another branch of my lack of patience was not waiting long enough after my electric foundry reached temp, I should have given it a lot more time. The "dipping" temp gauge for testing the temp of the aluminium in the crucible was so slow I could lose over 100deg C before the meter showed the metal temp.
That's why I like threads like this, my mistakes are writ large.
 
I haven't done any green sand moulding for many years , once I had discovered lost wax investment casting there was no going back.
However I do remember that I used a bottle of CO2 to gas cure my sodium silicate cores , this was very rapid and no catalyst was required.
Baking the cores in a domestic oven also made them tougher for handling.
My local public bar sold me a cylinder of gas at the cost price which was very cheap. I also note that many of the metal founders who post here use Petrobond or it.s equivilent to make their moulds.
I have always understood that petrobond , once used , is not re-useable.
It should be used as a thin covering of the pattern , usually applied through a sieve , and then backed with green sand.
The petrobond not only imparts a better surface finish to the castings but insulates from the greensand if it contains too much moisture.
After knocking out the used petrobond mixes with the green sand to become backing up sand.
Dan.
 
I haven't done any green sand moulding for many years , once I had discovered lost wax investment casting there was no going back.
However I do remember that I used a bottle of CO2 to gas cure my sodium silicate cores , this was very rapid and no catalyst was required.
Baking the cores in a domestic oven also made them tougher for handling.
My local public bar sold me a cylinder of gas at the cost price which was very cheap. I also note that many of the metal founders who post here use Petrobond or it.s equivilent to make their moulds.
I have always understood that petrobond , once used , is not re-useable.
It should be used as a thin covering of the pattern , usually applied through a sieve , and then backed with green sand.
The petrobond not only imparts a better surface finish to the castings but insulates from the greensand if it contains too much moisture.
After knocking out the used petrobond mixes with the green sand to become backing up sand.
Dan.

I have not hear of anyone mixing oil-based greensand with water-based greensand.
Seems like that would contaminate the water-based sand with petrobond oil, but perhaps the facing petrobond oil gets burned out by the heat of the casting.

Of course you can always just use 100% petrobond for the entire mold, but for large molds, that could require a lot of petrobond.

When I first started making sodium silicate cores, I used too much sodium silicate, since I was overgassing them and making them weak, and was trying to compensate.
Excessive sodium silicate makes the core very difficult to remove after the casting work is complete.
I finally figured out that a 5 second gassing of sodium silicate molds with the manufacturer's recommended percentage (I think 5%) made cores that would last a very long time even if left out on the shelf.

Putting sodium silicate bound cores in a bag of CO2 to cure them destroys the core.
I made that mistake early on.

Less is more when it comes to hardening sodium silicate with CO2.

I have studied investment casting, but am not ready to get set up for that process, and am basically to impatient to go through the multiple steps that investment casting required.

For small intricate parts, I think the investment casting process can pay off, since you can tree the parts and cast many of them at the same time.

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In the early days of my casting attempts, I used to harden my sodium silicate cores with bicarb and vinegar in a container that kept the cores away from the liquid. Perhaps the co2 produced was weaker, giving me very good cores. When I switched to small bottles of co2 I must have used too much as my cores were never as good.
 
At first I thought that more MUST be better, and that was a blunder as far as overgassing sodium silicate cores.

Gassing SS cores more than 5 seconds with CO2 causes the cores to crumble, sometimes within an hour or less.

I saw a video online from someone who said "Do NOT overgass SS cores".
I did not believe it, but I tried a 5 second gas, with 5% SS, and some of those cores have lasted over a year on the shelf, with no signs of degradation.

And at 5% SS, you can add water and break the core down relatively easily.

I am not sure what the chemical process is, but too much CO2 ruins the bonding power of sodium silicate.

The 5 second gassing time is one of many things about foundry work that I found counterintuitive.

More often than not, LESS is more in foundry work, such as less mass in the furnace greatly improves warmup times, and very slight amounts of ferrosilicon in iron are all that are needed.
Excessive amounts of ferrosilicon causes excessive shrinkage and hot tears.

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"Of course you can always just use 100% petrobond for the entire mold, but for large molds, that could require a lot of petrobond."

I guess that the cost of petrobond sand (about £4 per kilo) is nothing to our wealthy US cousins .
Here in the UK we are much more frugal.
The small amount of burnt petrobond mixes in very well with the backing green sand.
When all's said you do whats suits you.
 
I guess that the cost of petrobond sand (about £4 per kilo) is nothing to our wealthy US cousins .
They must have skipped over me when they were passing out all that money over here.

I hear what you are saying though.

I have one buddy/advocate who uses greensand only (running a commercial foundry), and he does not have the money for bound sand, but always has plenty of time to redo castings over and over again, with a success rate of about 25%, and lots of quality control issues.
Is he saving money by using the less expensive sand ? No, he is losing his shirt recasting things over and over again.

One thing I have learned over the years in doing backyard casting is that if you are serious about consistently making quality castings, you really cannot do that on the cheap. You can get away with using substandard materials for a little while, but in the end, using long-lasting materials gives the highest yield, and is ultimately the cheapest way to go in the long term. If you want to trade off quality/quality control, you can go the cheap route.

We have a local art-iron group that does both iron, aluminum, and bronze work.
They use resin-bound sand with cupola iron, but with aluminum they have a large commercial muller, and they use Petrobond exclusively.
I asked them about the cost of that much Petrobond (tons of it), and they said they actually make their own, using I think OK85, and bulk-purchased clay and oil/alcohol. They purchase multiple 3,500 lb bags of OK85 at the same time. Not sure where they get their clay, but they purchase it in bulk too.

There are recipes out there for home-brewed Petrobond, with "Petrobond" being a trademark for a brand of oil-based foundry sand.
The home-made oil-based sand seems to be quite reasonable in price, and probably a fraction of the official brand.
I have posted one home-made recipe for oil-bound sand somewhere here.

One can use 100% oil-based sand, and still be on the frugal side, if you bulk-purchase the materials, and make your own oil-based sand; assuming you have a muller.
I have seen some hand-mull their oil-based sand, but that is pretty rough going in my opinion for an oldster like myself.
Young wippersnappers could easily hand-mull their sand.
I have seen one person put their oil-based sand in a tarp, put it on the floor, and roll and step on it.

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Edit:
I wish some folks would set up some model engine casting non-profit groups, like the art-iron folks do.
The art-iron folks can purchase bulk materials at wholesale prices, and the foundry supplier folks support these groups.
If I as an individual walk into the same supplier that the art-iron folks use, I am lucky if they don't slam the door in my face, and it is often a battle to get them to sell you anything.

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