Yesterday I needed to make a mold for the 18" straight edge I have been casting in iron but this time I am casting it in aluminum with the aluminum casting to be used as a pattern to be used instead of the wooden one I have been using to date. Background: Ordinarily the flasks for this mold require 2.5 five-gallon buckets of sand. My supply of petrobond is somewhat limited and I did not want to use that much sand on that mold if I could avoid it as I had six other parts to cast with the same melt. The pattern mold consists of 3 parts---a cope, cheek, and drag with the aluminum casting only in contact with the cheek portion of the mold. The cope makes up only the pouring basin and sprue and the drag is just a minimal splash basin and runner. The "top" surface of the mold is composed of a large silicate core. The solution seemed to be to fill the cope and drag with green sand and use oil-bound sand for the cheek. (My oil bound sand is neither pure Petrobond nor pure K-bond, but a mix of the two.) My plan was to segregate the oil sand from the green sand post-pour. That plan worked pretty well. I was able to retrieve 99% of my oil sand in clean condition. Here are some pics: The assembled mold post pour. The cope lifted off with good separation from oil-sand except for the area around the pour basin which caused mechanical interference with separation. But the green sand was easliy pushed off. You can see the oil sand in the cheek. Here is a blind riser made jsut the way I make it when pouring this pattern in iron. Interestingly it fed by collapse of the vertical sides of the riser rather than by the more typical collapse of the top surface when pouring iron. Today the top was intact on the aluminum pour. Popping the sodium silicate core loose. Above you can see that the molten aluminum made a nearly flat advancing surface as it filled the mold from bottom to top. I was a little surprised that the core was harder post-pour than it is when used on an iron pour. I'll have to revisit Tom Cobett's monograph on sodium silicate cores. I weakened the silicate by mixing in sugar in the percent recommended for iron. Perhaps a different weakening agent or percentage sugar is desirable. I think it will soften OK with soaking. But, when I use it on iron the core is pretty soft and brushes and scrapes out easily. Denis
I’ve used what might be called hybrid molds in the sense you’ve described a couple of times recently. The first time was when I cast a motor cover for a Delta Unisaw. I have an original cast iron one on my saw but cast one in aluminum for a friend with the same model. I didn’t have enough supply of either petrobond or green sand so I used every ounce of petrobond I had to fill the cope and most of the internal cavity. I finished filling the last inch or so of the cavity and the rest of the drag with greensand. The mold sat for several days and the greensand became very hard so after the pour it was very easy to separate the layers with very little commingling. I just threw the small amount that got mixed up away. Sorry, no picture of the casting. (It was successful). The other example was with a bronze cylinder I cast to use as turning stock for a new lead screw worm for my lathe. I used petrobond only as facing sand in both the cope and drag but then fully rammed both with greensand. That mold also sat for several days and again the greensand got quite hard. The petrobond stayed attached to the greensand just fine. No delamination. After separating the mold halves after the pour, the completely burned up petrobond just ran out onto the bench from the drag half, and was easily brushed from behind the casting which was still held captive in the cope. The petrobond was waste, but wasn’t anymore than a handful of sand-the cost of a cup of coffee. All that was left was the uncontaminated greensand which I shook out and mulled as normal. My motive for trying this was to prevent my petrobond heap from being unnecessarily weakened by the burnt up sand which would normally be almost unavoidable. I’d call it a success. I don’t know if I would do this with aluminum because the petrobond doesn’t get burnt as badly, but it was certainly a winner with the bronze. Pete
That looks like a sizable mold. Maybe a couple hundred pounds? Nice job. Getting that thing mold and pour properly is not an easy task. Did you gate it just on one side or with a runner around the outside and several gates? You had no room to spare(and I understand why.) Also good to hear that someone else has used the method successfully and my trial was not just a fluke. Denis
I've had success using petrobond as facing sand for small detailed areas of a pattern, and filling the rest with green sand. It's not like the flask falls apart after the pour, it just takes some finesse and time to segregate the two after pouring. Kind of annoying but if you only have so much petrobond it's the only way.
It was well over 200 lbs. It was too heavy to move outside so I winched it to the floor and cleared and liberally sanded an adequate area around it. From my pictures it probably doesn’t look like there’s sufficient room in my shop, but I was comfortable with it. I had another guy there with me on fire watch armed with extinguisher and dry sand. This was actually my second attempt with this mold. The first time I had tried to widen the flange of the part to make up for contraction by cutting sand away from the periphery but I got careless and muffed the mating surface of cope and drag, resulting in a runout. Disappointing. This time was successful though. It made for some extra cleanup, but I went extra big at my gating contact area and poured on the hot side. It used every drop of a #18 crucible. Pete
I had not finished cleaning the sand out of the mold yesterday. That is because the SS core was surprisingly hard (compared to when I use the same core with iron) and needed an overnight soak in water to soften it so that it was easier to remove. But, soaking it did soften it as hoped and expected. Here is how the casting looks after brushing out the sand and rinsing it in water: Lessons learned: SS cured with esther may not be an ideal core for aluminum castings. I wonder if I had used more or less sugar as a weakening agent or other additives if I could have made it more easily removed. While mildly lacking in convenience, the SS provided good rendition of the pattern and a very usable result. Lessons to be learned: Using epoxy as a binder for the sand might be an option worth exploring. I wonder how well it weakens with pouring and how easy it might be to remove. I think I will try using an epoxy spray paint to coat the aluminum pattern as recommended by Tom Cobett in another thread. I had not finished cleaning the sand out of the mold yesterday. That is because the SS core was surprisingly hard (compared to when I use the same core with iron) and needed an overnight soak in water to soften it so that it was easier to remove. But, soaking it did soften it as hoped and expected. Here is how the casting looks after brushing out the sand and rinsing it in water: Lessons learned: SS cured with esther may not be an ideal core for aluminum castings. I wonder if I had used more or less sugar as a weakening agent or other additives if I could have made it more easily removed. While mildly lacking in convenience, the SS provided good rendition of the pattern and a very usable result. Lessons to be learned: Using epoxy as a binder for the sand might be an option worth exploring. I wonder how well it weakens with pouring and how easy it might be to remove. I think I will try using an epoxy spray paint to coat the aluminum pattern as recommended by Tom Cobett in another thread. What is the Slickest Most Adhesion Resistant Prep for Sodium Silicate Core Boxes? Denis
