Help in pattern separation from the sand

Discussion in 'Sand Casting' started by my_account_todoist, Apr 15, 2021.

  1. oldironfarmer

    oldironfarmer Silver Banner Member

    Hmmm, natural sand has too much clay? I guess that depends on the source. You might find bentonite clay is worth the extra effort to add to green sand for long term sand durability.

    So you are saying you were liberally adding parting compound to the parting line all across the sand?
     
  2. Oldironfarmer, yes, yes I was. The photo is admittedly a little poor demo of it, but I've tried many different quantities from "just a light dusting" to "as much as sticks to it" to "leave even a bunch of loose compound lying on a surface already brushed with the stuff".
    Every single time the result is the same, and it seems to make little difference whether I use talc or chalk powder - it almost feels like it is acting more as a glue than as a separator, when it comes to rammed sand. That said, it does do a fine job at keeping loose sand from sticking to it.
    Never yet parted open a mold and been able to see the white of the powder, it's always covered by a millimetre or two of sand.
     
  3. Melterskelter

    Melterskelter Gold Banner Member

    You could put a little "science" into this by taking a sample of your mulled sand weighing it prior to and after baking all the water out. Using 4% bentonite and 100 mesh olivine sand as a datapoint, you will typically have about 4% water content of mulled sand. It might be interesting to see what your content is.

    I am guessing that where you live bentonite is not available and standard foundry sands can not be had. Starting with unknown ingredients makes the already tricky job of mixing good foundry sand much harder.

    When my sand is working right (most of the time) this is what the parted sand looks like. You can see the brush marks in the talc.
    Wishbone mold.JPG

    Denis
     
  4. Denis; one word there jumped at me: "mulled". Now, I've not been mulling my sand - I've figured between all ramming it and breaking it up, it should be reasonably mixed. Mind you, I'm working with a total volume of 10 litres right now - I've another couple hundred litres dry sitting in boxes. But this means the little sand I am using does get "well used".
    However.
    The way I am applying the first layer of sand has been hugely deleterious. Using a sieve to do it is fine in itself, but I'd never given thought to there being a difference between pushing it through the sift and just sifting it. I've done largely the latter, on count of its ease and the perceived superior evenness in the coat.
    However, as I was doing this, I just noticed that what ends up reaming in the sieve is the higher clay content stuff - the distinction became much clearer to me after all that experimenting yesterday. From this one might deduce that what falls into the pattern is the lower-clay content material.
    That these two are thusly separable at all does slightly call into question whether some form of mechanical mixing, some kind of an improvised muller, would be helpful after all.
    But trying out taking some rammed up sand and "grinding it" with the sieve already yielded a marked improvement in the strength of the bottom of the mold. No longer was it so oddly crumbly compared to the top.... it's honestly a bit surprising I've never noticed this actively in the past, given how marked the difference is.
    Now I'm back to struggling a bit with pattern removal (oh, the endless cycle) but the features that I am able to remove no longer crumble at the slightest disturbance like they used to. I'd say they are still a bit lower on clay than the average of the sand, but they are no longer so completely free of it.

    I could acquire bentonite clay from a pottery supplies store, and even with shipping it would only cost a few dozen dollars. Definitely something I will consider if whatever clay I have here or can easily find in the garden won't behave. (If I go to the same spot and dig a few feet down, I'll find a thick grey moist mass of clay that's readily sculpted by hand. Hence my slight desire to learn what the properties of the materials so readily accessible for me are before relying on commercial suppliers - after all, I'm not under a particular time pressure to get things working, so as long as I'm having fun I'm happy).
     
  5. Just tried putting a batch in an old bread kneading implement - well, really it's an old Kenwood multipurpose kitchen machine affair, but this one's a bit limited in functionality since it was gotten off the flea market to use as spare parts.
    Initial results are promising. One quick and dirty trial actually allowed for the pattern to be removed in tact! Too bad it was so quick and dirty that it wasn't really possible to use it as a real mold. So I went for a second try, and all was very promising - I even got a bit of the powder to remain visible after separation! Too bad that the thread on the back of the pattern was by then stripped enough that I couldn't extract it in a clean fashion, so in the end the mold had to be scrapped. Now filling in the hole at the back so I can put a screw into it once more. 20210430_152432.jpg
     
  6. Melterskelter

    Melterskelter Gold Banner Member

    That pattern piece above looks like rusty steel.

    Two thoughts: sanding it fairly smooth but not necessarily mirror-smooth or spraying on some paint will probably help get cleaner pattern drawing.

    And if it is iron/steel, a magnet makes a good “draw spike.”

    Denis
     
  7. Ahaha, that's definitely a fun way to interpret the photo. No, it's a dark grey plastic. Some sand has remained stuck to it at the back, but less than usual (alas, the usual is worse). Might consider a slightly heavier dusting of talc at the back of it going forward. Though it might be that the issue will get better as the sand dries up a little - I added the clay as a suspended liquid, which pushed the moisture a bit high. Once I have a moisture level that works, I'll probably start storing my sand sealed - but as it stands, I'm working in a bone dry basement so the sand moisture content is always in quite a bit of flux. Does help in getting a feel for the moisture level, I guess. But long term not good for repeatability. At the moment repeatability isn't really my concern though, as so far I'm only making mistakes ;)

    That said, the magnet thing definitely gives me some ideas..... I might integrate magnets into some future patterns, because that does sound like a breeze to work with.
     
  8. oldironfarmer

    oldironfarmer Silver Banner Member

    I think it's obvious your sand is still too wet. Notice Denis' picture. You should get that.

    You should not be pushing sand through the sieve. Although sometimes I do that, it's best to have it mulled where is will shake through.

    I think your sand is not well coated with clay. Even though it was dug from the ground, and mixed wet, you should not be getting clumps of high clay content. You need to mash, mash, and mash the sand to get every grain coated. Spread it out to get it to dry quicker.

    The crudest test for sand is grab a handful and squeeze tightly. It should not stick to your hand when you open your hand. You should be able to hold it 1/4 of the way from one end and tap on the other end without failure. If it sticks to your hand it's too wet. If you can't hold it from one end it's too dry, is not mixed well, or does not have enough clay.
     
    Tobho Mott, dennis and HT1 like this.
  9. 3Dcasting

    3Dcasting Copper

    Yes, I do think so the sand is very wet and does not have enough clay. With enough clay less water will be necessary to hold.
     
  10. oldironfarmer

    oldironfarmer Silver Banner Member

    You may find it takes less clay than you think.
     
  11. Billy Elmore

    Billy Elmore Silver

    Those clumps you took out probably should have been left in and mashed/mulled up. They probably held clay that you needed for green strength...the strength that holds you molds together and keeps the sand from pulling out with the pattern. Definitely need a good way to mix/mull the sand to get it as evenly coated with the clay and water as you can. Mold doesnt look like it was rammed very hard to me either, which will cause tear offs around the pattern. I look at molds being squeezed at a couple hundred tons of pressure everyday though so I might be too critical.
     

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