I cast these replica stove parts in aluminium today for a friend, they are being used in a decorative stove so they won't be getting hot, in fact they'll be painted black like the rest of the stove. I've had a few failures to date, mainly from cutting the runner wrong and pinching off the metal flow due to not realizing the runner needed to cut into the mould slightly just as the runner touched the green sand mould impression. The aluminium is a mix of all kinds of scrap motor housings, car wheels, ingots and reused runners and some bits (the vented blobs) ring like a bell when hit. I used the dreaded soup can runner to give some hydraulic head as the castings measure 3.125 mm or a bit under 1/8". I poked vents with 1/8" wire at the corners of both castings and in the case of the part with the half circle cutouts the vents almost reached the surface 75mm or 3" high before freezing and you can see the top of the runner contracted and fed the casting a bit. What was more interesting to me was the smaller square door had one corner vent freeze near the surface (not pictured) and three vents made it all the way to the surface with the two vents farthest away from the pouring hole flowing until I had the sense to stop pouring or they would have flowed over the side of the flask: I'm impressed at how a soup can extension could keep metal flowing 75mm along a 3.2mm vent hole for so long and judging by the contraction kept feeding until the pressures equalized a bit. Both replica parts are a success and needed a bit of grinding here and there where sand broke off during the ramming process but are perfectly fine, I was even able to glue a sliver of wood to the original door to replace a broken off section. This casting had all four vents fill and freeze before the surface and the soup can pouring spout fed the casting a bit. You can see here most of the soup can height fed into the casting The amount of contraction of the metal as it fed the casting:
If I may, what was your pouring temp? So the soup can additional height increases pressure to overcome the overall thin section of the part without increasing the height of your flask correct? Pouring height (head) is something I'm researching now.
Hi Matt, I was pouring in bright sunlight so it was a bit hard to judge pouring temperature: when I do it indoors there's a very dull red glow almost nonexistent in the silver molten metal but that can't be seen in daylight. I'd say it wasn't that hot at all as the metal will flow into the sand and give a rough texture if too hot and can also have excess shrink if really hot. The soup can height extension of 12cm or about 5" increases the pressure of the molten metal once it's full to the top and can compensate for thin sections to some extent despite it being considered bad practice by some. The can is just resting on the top of the green sand with nothing to seal it except the weight of the green sand in the can. I poured as fast as I dared and poured this casting first out of several to be done so as to have the hottest metal with a larger A25 crucible as the volume of metal will have a slower cooling rate than smaller crucibles.
The can sits on top of the cope so it does make it taller. I used this method for years but there are disadvantages; The increased pressure tends to make the sand detail stand out more on the casting. The velocity of the incoming metal increases the risk of sand-wash into the casting. Since I converted to using a pouring-basin and narrower sprue, both of the above have disappeared from my castings, Cheers Charlie
I should point out the original parts were fairly old and had rust sand blasted off, so had a rough lumpy texture despite a couple of coats of spray filler laid on wet to try and level the surfaces to some extent. Charlie is right about his points above, you can see sand in a few parts of the flower on the door casting if you look closely. The larger diameter 20mm diameter sprue in my case is fairly hot from the flow of metal and (in the first photo) has a depression on top from feeding the casting to some extent.
Thank you sirs. I'm preparing to attempt an 8" (200mm) diameter ring thin wall pour. I saw your vents. You sure didn't pour hot at all. It sounds like 1300F max.
I have the luxury of a larger crucible so the thermal mass slows down the cooling, even so I poured as fast as I dared. The smaller your crucible, the hotter it'll have to be and the faster you'll need to pour. I even did a preliminary skim in the furnace and ran for it ten minutes or so to get it hot again to minimize heat loss. There is amazing amounts of crud stuck to the crucible sides below the surface so my skimmer is half round to match the crucible curve to allow the crucible wall to be lightly scraped to get all the crud to the top. Not too long ago I melted some painted alloy wheels and skimmed off the dross getting it nice and clean, then remembered to get below the surface and easily doubled the amount of dross removed. That's what happens when melting scrap instead of clean ingots.
Adding five inches can make a pretty big difference. Using depressurized gating systems and extra venting will help with the metal penetration and give you a better surface finish. Moisture and mold density also can lead to disasters in surface finish and are often the culprits to rough casting finish. Aluminum usually does not penetrate the mold as bad as iron or brass with its lower temps, density and the extra height probably doesnt hurt as much as with heavier metals. We like to drop our iron about 27.5 inches here so we can make our gating people lose their minds....which is me.LOL
27.5 inches is a long way for iron to fall before hitting the mould . I was discussing casting round bronze bar and hollow and asked a guy why he never went longer than about 16 inches and he said it was cast vertical and the bronze would have extruded into the sand grains if much longer from the pressure. Must be the reason for the mould wash on bigger castings to better seal the sand as well as finish.
Just dropping it 6.5 to the top of the mold... it is 27.5 from the nozzle to the bottom of the sprue...the sprues are generally between 17 and 21 inches long. The height of metal, inside the mold, above the casting creats extra weight and velocity increasing the penetration. We will see much more roughness and penetration on the bottom cavities vs the top cavities. I have been utilizing a new gating technique that feeds from the side to help eliminate the unequal pressure. While it has added extra grinding due to the longer thinner ingates we have seen a dramatic reduction in roughness and have almost eliminated gate wash completely. There are situations that it can not be applied so we still struggle with some jobs but on the castings that have been poured with this method we do not have to ( but still do for some reason..lol) introduce the castings into our media bath to reduce roughness and have in fact had issues with seasoning oil not sticking as well due to the smoothness of our surface finish. Our castings are very lightweight compared to most and require fast filling due to the thin walls. With the side filling method we can fill at a slower rate and still make a complete castings without all the turbulence of a fast filling bottom gate.
That's what amazed me at the factory. The metal is "shot" in so fast, just seconds to fill. I could never pour that fast. The DISA system is fun to watch.
Right...my grandpa was completely blown away by them. He had never seen an automatic molding machine and hand molded his whole life. They just shove 1000 pound molds around like they are nothing and every new machine is faster than the last. One of the smaller machines can make over 600 molds an hour with certain patterns and conditions. As for pouring fast...most of our pouring starts around 14 lbs a second but as soon as the sprue is full it levels out to around 4 to 6 depending on the job. We do have some that average closer to 2 lbs per second but 4 is about where we stay.
My friend with the stove sent me a photo today of the restored stove using the iron and aluminium parts I cast for it. It has four castings I made for it: the two in aluminium mentioned here and another two iron feet that bolt on to the stove:
Apparently it's got gargoyles cast into it, I'll have to get some detailed photos. One guy said that the patternmaker for this stove was really good.
Good job Mark. The need to cast stove parts is what got me started on my casting journey. I collected salesman sample cookstoves and always ended up buying them with missing parts. I started with aluminum, but if a stover picked one up to complete their stove they dropped it like a hot rock. So, I switched to cast iron and even though I corrected for shrink I wasn't able be compensated for foundry time. Thus my avatar in cast iron.
