Introduction: Greensand Aluminum Casting.
This instuctable is to cover some of what I know about greensand casting aluminum. In this example I will be casting a blank for a vacuum table for use in lost wax casting, your template, for the purpose of this process, could be anything that can be seperated in halves.
Please take note that I do not consider myself good at this. There is a chap named "myfordboy" on youtube who is awesome at it, but doesn't say much an I think some of the subtleties that are performed can be easily lost.
Youtube is filled with lots fo great casting videos, there is a chap in Alaska worthy of mentioning but I don't recall his name.
I will miss things here!
- water spraybottle
- sprew/riser pegs
- bottomless large soup cans
Step 1: Prep the Greensand
"Greensand" is a mix primarily of sand clay and water. Not enough sand and it will shrink wrong as it dries, and not vent properly, Not enough clay and it will not hold together properly, not enough water and it wont hold together, too much water and the steam will mess up the molten metal (and other chaos)
So by prep, I'm referring to making sure the lumps are out from the previous casting, and that the moisture level has been restored (the greensand loses lots of moisture after a casting, to be reused it needs to be de-lumped and have its moisture reset) Did I mention gets the lumps out?
I have taken to dumping my supply in a wheelbarrow to work on it.
Add water really slowly, and mix well, a bit too much and it all just turns to soup.
Step 2: Set Up Drag.
So, the 'flask' is (in this case) made of two halves, the bottom is called a drag, and the top is called a cope. So we are told, I been reading an OLD book ( https://archive.org/details/americanfoundryp00wes... ) and, maybe its the author, but he always calls the drag the 'bottom', makes me suspicious that 'drag' was really just a term referring to the factory floor and we got mixed up over the years. If there are intermediate sections, they are called "cheeks"...
Note that the drag is upside down, the aluminum brackets you see go in the middle between the two peices. In this flask set, my alignment pins are permanently mounted in the cope.
- Spray the inside of the drag with water. This will make the sand REALLY stick to it, which is a good thing.
- Powder down your work table. I'm using talc (baby powder). Make sure the take you have it really solid, you don’t want it changing shape while you work.
- Powder down the template. This template happens to have a flat bottom. Often, if the template does not, it will split into two pieces that do.
- Position template in drag. You need room for the 'sprew' (metal going in) and 'riser' (metal reservoir)
Step 3: Pack Drag.
There are a few goals packing the greensand around the mold. You want the greensand to be well packed against the walls of the flask so that it stays in when being handled later. You want intricate packing around your template details. You do not want the sand packed really hard against your template, its counter-intuitive. When the metal is poured in, the casting material needs to give off vapors and gasses, if you pack it really hard, it wont be porous enough to be able to do that, bad things will happen. How hard do you pack it? By your experience, only as hard as you need to. Expect that you may need to take atleast 2 stabs at setting up a casting while you learn.
- Using a siv or mesh, sprinkle fine sand over your template. This will help get your details into the final casting.
- Cover the template up with bulk greensand, not too deep. Realize that the hardest you could try to pack it, will only cause about the top 5cm or so to pack.
- From whatever directions are required, use your fingers to squeeze in the sand up against your template in any small corners. Tight corners will NOT want to pack well, and with no packing, they will just fall apart.
- Add sand in layers of about 5cm at a time, use the wedge end of a ram to pack the body of the sand, its important not to use the flat end, as it creates a separation layer that will bite you later when handling the flask. use the flat end of the ram around the edges to pack sand in at and angle (see pictures) towards the flask edges. Make sure not to hit the flask with the ram, it'll disconnect the sand from the flask. Pack the flask till its over-full.
- using a flat scraper, gradually shave the top down till its level with the flask. Don't take this all in one step or it will sheer the sand in evil ways that will leave you doing more packing and scraping.
- pat down any loose grains from the scraping.
Step 4: Prep for Work on Cope Half.
This is the first big test or your work so far. If the sand was right, if you packed it right, if your flask isn't flimsy, if your table was solid, if you don't bump it in this step.... Then it will stay in one piece. A well done mold will take a surprising amount of "abuse".
- Carefully, flip the drag over. I do this in two parts, I get it up on its edge, clean the table, and set it back down the other way. It will most likley need to be handled again before the pour.
- Powder it up. we are going to pack another layer of sand on it and do NOT want them to stick togethor. If your sand is too wet, you may run into that problem.
- spray down the inside edges of the cope.
- fit cope on drag. take note of the orange paint, when these come apart and go back togethor a few times before the pour, you do not want the orientation to get mixed up!
Step 5: Pack Cope
This is similar to the drag packing, with a few differences. First, we have to add channels for the aluminum going in and out. Second, the top does not need to be flat, its actually a bit of a good idea to have the top slightly concave, so that any aluminum spills will stay away from the wood.
- Place tapered pins for the sprew and riser
- sift fine greensand over template
- Pour in bulk greensand as before, packing as you go. (flat into the edges of the flask, wedge in the flask body)
- When done, pat down any loose sand.
Step 6: Inner Mold Prep
This is where any interconnecting channels get set up for metal to move around. In this project I moved the riser to the dead middle of the project, I would not normally do this, I would put it off to the side and channel over to it. This part has a big hub in the middle, and the shrinkage of the aluminum kept being a problem (these images are my 5th try ) to curb the shrinkage I put the riser right over the bad spot. As the metal cools, it pulls the surplus from the riser. the downside being a huge lump I have to cut off later.I did not need to on this shape, but be sure any pockets above the part being poured have vents to let the air out. This can be done with a 1/4inch bar, just gently poke holes where required to help air get out as the aluminum goes in.
One of the things I'v learned is to have two channels ("gates") for the metal to flow, if, for some reason one gets blocked off, you don't lose your part.
- Pull off the cope. set it carefully on its side. This template is pretty forgiving, I dont have to worry about pulling perfectly straight off, you might!
- Wiggle out the sprew and riser pins.
- By hand, gently round the edges of the sand around the hole. These edges present a danger of dropping off and ending up in your casting during the pour.
- Carve or impress your gates. These go from the sprew/riser to your part. I'v been pressing my channels into the sand, you have to do it carefully as you will push down a large area if you try this in one go. tapping it in is a good method. Be aware that pressing causes the sand to push sideways, and will break into any nearby edges. Carving the sand works, I find it a bit messy.
- Impress/carve a cavity below the channel, in line with the sprew and riser. Contaminates are usually either heavy or lighter than the aluminum your pouring in. This pocket provides a place for heavy stuff to settle that is not in your casting. (The sand is lighter than the aluminum (floats))
- Using fingers again, round off edges.
- Get a grip on your template. For me, this is a screw that was pre-threaded into a hole.
- Tap template. You want to knock the template around in all directions enough to see a minute separation around it. Here I'm using a metal spade and striking the screw. Can you see the separation in the image?
- SLOWLY wiggle and pull your template straight up. Your have a number of things working against you here. If you tip the template, you tear edges of the sand. There is a vacuum created under the tempate as you pull up, it will try to tear your sand apart too. Apologies as the photo of this step is from attempt 4 (see I tore 2 bits in the middle?)
- Get rid of any loose sand. The best way to do this, is to tip the flask on its side and blow the loose sand out, you have to trust the good stuff will stay put.
Step 7: Close the Flask.
Pay careful attention here to get the two halves oriented right, and don't bump them in the process of putting it all back together.
- REALLY carefully, set the cope back on the drag.
- lock/weigh down flask as required. This project threw me another problem I'd never had before with a casting. The cope here weighs about 26lbs, the diameter of that project is about 9 inches, an area of roughly 64 square inches. From the top of the aluminum as I'm pouring it, its about 6 inches down to the part. 64 square inches will lift 26 lbs at about 0.4psi a column of aluminum 6 inches high develops a pressure of 0.56psi, meaning that almost every time I tried to pour this, the flask would separate and leak aluminum all over my lawn. (do not pour aluminum over a concrete pad, a spill will cause the concrete to explode in chips, throwing molten aluminum everywhere with it.) This one I put screws into the flask and bailed it together. Its the first project where I'v had this be an issue.
Step 8: Let There Be Fire.
There is more to do on the flask, but its a good time to get the aluminum going. Remember to make more than needed for the part, you want to have enough to fill the sprew and risers up so they generate some pressure in the cavity.
- inspect crucible! (I'm using a crucible thats a mix and 6011 and stainless metal, the stainless gets attacked by something in aluminum and is constantly springing leaks) That aside, make sure there are no cracks, holes or almost-holes!!!
- start smelter
- add aluminum to pot.
Step 9: Final Flask Prep.
You want a sufficient amount of pressure on your mold to get the aluminum into all the details of your part. This usually requires a bit of "head pressure" and we can help this out a bit yet. I have two methods going here. One of these is a "swing away" device I use for the sprew, the other is the sand-and-can trick that myfordboy uses. In the can method, the sand lining is important, without it the aluminum gets stuck in the can and is just annoying.
For the can method:
- on a totally separate table away from the mold, pack sand into a can thats got the top and bottom removed.
- push a thin-walled pipe thru the middle of the sand to 'channel out'
- use fingers to round edges of the sand.
- Place risers gently over each hole, don’t knock and sand in there now!
- "putty" around them with sand. Casting sand is, in more than a few ways, high temperature molding putty.
- Get some ingot molds ready, here I'm using small muffin tins. After the first two uses of the tins, you need to shake talc on them to release the aluminum. With luck, your going to melt down a bit too much aluminum, and you do not want to leave it in the pot so that you can check it for problems before the next time you use it.
Step 10: Prep Aluminum and Pour
Aluminum absorbs hydrogen, and apparently carbon, so, if you just pour what you just melted down, the inside of the casting will be a mess that may not live long. Temperature of aluminum being cast is important, but I don’t have anything to measure it with, so I kinda skip that step. Its hard for me to emphasize the safety aspect enough here, I can't say everything, keep in mind that the molten aluminum is REALLY HOT. its so hot that even after 20 minutes of air cooling, swiping your finger across it to just flip it over, will leave you with a major burn. (which has alllllmost finished healing...)
Its important to also know that, as the sand is exposed to the 660c+ heat, it solidifies, up to about 2.5cm from the molten aluminum, at that point, its quite durable. (still reusable later)
- degass/flux aluminum. I'm using borax and "sodium free salt" poured onto a piece of aluminum sheet and crumpled into a ball. One of them is a flux, and one of them is a degaser. It does improve the quality of the cast metal, I'm still learning how much of which and how it helps. I take a ladle and push the ball to the bottom of the pot, it boils and bubbles, then stops.
- scoop off junk. There will be oxides and stuff on top of the aluminum, I made a perforated scoop to get it all off, I'm still not sure if it should be classed as garbage or scrap metal.
- pluck pot, pour aluminum into mold. Now, I have to apologize, I don't have any photos. Both the process of plucking the pot from my smelter and pouring the aluminum are dangerous and require both hands and full concentration. If you drop the pot, the reaction forces will send molten aluminum straight up. Make sure your equipment always has a complete lock on the pot while your moving and pouring it. Make sure your clothes are shingled!, that there is nowhere a blob of aluminum can land on you and end up in your shoe / etc! Aluminum isn't hot enough to require heat reflective clothing, other than that, have lots of safety gear. My pouring jig is a 2 person setup. You want the aluminum to come up "reasonably high" in the risers, a good sine things are going well is that the metal comes up in the riser as your pouring it. Pouring speed is a consideration, you do not want to erode the sand with the molten metal flow, but you do not want to pour so slowly that the aluminum is almost frozen by the time it gets to the shape details. If the flask springs a leak calmly stop pouring and set the crucible aside FIRST. do NOT dump water all over it! Use water sprayer to keep any fire under control (in my case the flask and the plywood under it, and by control, out is good, but 660c+ can reignite it at will) while the metal cools. Dumping water on molten aluminum (atleast 660c) could cause all sorts of nasty things to happen.
- Pour off any extra aluminum into your ingot trays
- wait. cooling needs to happen.
Step 11: Crack 'er Open
There is something about the moment you finish pouring the metal into the flask, that burning desire to know if it all worked or not. You HAVE to let the casting cool, I try to give a casting atleast 30 mins, at which point it will definitely be frozen, but still be burning hot.
- remove risers
- shake out flask (I have come to prefer doing this over a metal wheelbarrow, I can use a stick to move the casting around and scape off the extra sand.)
- wait for it to cool. This, depending can take another 2 hours to be handlable
- Cut off sprew/riser/runners
- machine / clean up surfaces. Hey, you got it!
Step 12: Tell!
I'm really surprised how many people seem to do this kinda stuff, but never really say anything about it. When I'm learning (always) I'm looking for all kinda of clues and hits people have that they may not even know as such, If you don't post about it anywhere me and countless others can't learn.
I'd rather see photos your a messed up casting on social media than your cat being so startled that its hanging by its claws from the ceiling cause you placed a cucumber behind it when it wasn't looking (tho that was a funny reaction).
If this inspires you to cast something, post it in the "I made this" this instructabel isn't about making *THIS* disc, its about greensand casting!
And Please, if there is something you think I should know, TELL ME
now, its almost 7am and I haven’t been to bed yet, gnight!