Rough surface finish from green sand

Discussion in 'Sand Casting' started by Mister ED, May 24, 2020.

  1. Mister ED

    Mister ED Silver

    Well, I finally got my furnace fired up again and also finally quit playing with foam. My first time using green sand, all went really well ... except for surface finish.

    I made 3 batches of sand (25lb in my Mini-Mite muller). First was 4% western bentonite, and dumped in the 5% water that the Navy manual called for ... even I could see that was way too wet. Second batch I went with 5% bentonite and ended up around 2.5% water ... much better. Third batch I went with 6% bentonite (to get me at a 5% avg) and similar on the water. I let the first batch dry out a couple days and then mixed and mulled the entire heap. When I was done, I thought it was good, just very little sand sticking to my hand and pretty decent green strength. Oh, sand was bone dry with a 70 or 75 AFS grain fineness number.

    So, I rammed up a flask (using Old Foundryman's flask patterns). Used diatomaceous earth as parting agent (I have a 25 lb box ... need to use it somehow). Pattern pulled easily from the mold and pour went smoothly. I did not put any vents into the mold ... not sure if I should have, but meant to.
    20200522_135611[1].jpg
    Castings filled excellent even with the small sprue and shallow gates.
    20200524_091543[1].jpg
    But whats up with the surface finish? Still too much moisture? The roughness is on all 4 sides ... but slightly less on bottom. If I look past the roughness, I can see the print lines from 3D printer ... so the sand is giving me detail. Interesting, while I am typing I also realize the underside of the overflow from the sprue is very smooth is that maybe because the top surface dried out in air for a couple hours?
    20200524_091658[1].jpg
     
  2. Melterskelter

    Melterskelter Gold Banner Member

    Overall, it looks like you had very good results. I wish I knew enough to say for sure why you got the roughness that you did. It looks like the aluminum was seeping into the sand somewhat. That makes me wonder if you rammed it hard enough. I have heard all the recommendations about not ramming too hard, but I have yet to have trouble from too hard ramming. To soft is something I have had trouble with from time to time.

    One thing I like to see in my mold is that when I am cutting gates and runners or pouring basins that the sand actually carves and makes a sort of hissing sound as I cut it with my spatula or spoon. This is hard to describe. But the sand should be very firm to finger pressure and when piercing it with an ice pick for vents it should have distinct resistance all the way down into the sand.

    Good work on you pour. Must feel good to have the furnace going again.

    Denis
     
  3. Petee716

    Petee716 Gold Banner Member

    Any idea how hot your pour was?
     
  4. Mister ED

    Mister ED Silver

    Thanks Denis. I may play around some more today or tomorrow. And will head your advice. I did not ram real hard and will take a look at that.

    Petee - 1390*F Based on my probe (same config as the old post on AA, type K thermo couple in the graphite rod). It was still equalizing, but was only going up in 0.1* f increments at the time of that reading.
     
  5. Petee716

    Petee716 Gold Banner Member

    Well that would seem to rule out pouring too hot although you could get away with less.
     
  6. Tobho Mott

    Tobho Mott Gold Banner Member

    The sand I bought years ago and have been using for facing is AFS GFN 115-130 and has been giving me a very nice finish. My homemade sand was made with 75 mesh silica, but I don't have a GFN for it. It gives an OK but notably rougher finish when I don't use facing sand.

    Capture+_2020-05-24-12-40-33.png

    The photo above is a recent example, I don't know how this stacks up vs other people's 75 mesh greensand, but although the castings are perfectly usable, I don't find this finish particularly beautiful either. Giving myself the benefit of the doubt and assuming this is about an average result for the grain size I used, maybe it could be useful a reference photo to compare with your results after you try ramming a bit harder.

    Good luck!

    Jeff
     
  7. Patrick-C

    Patrick-C Silver

    Well with my small amount of experience, I'd say the no vents would cause rough surface area. Had it been me pouring something with that much surface area there would be about 15-20 vents top and bottom.
    Patrick
     
  8. I get good results with no vents. Green sand is very porous. No vents should give you porosity, not leakage into the sand.

    It sure looks like the results I get when I pour too hot.
     
  9. Mister ED

    Mister ED Silver

    On the venting ... OldFoundryman used one vent between the tabs on the flask (on the underside of my pics).

    On the temp, maybe my cheap Chinese meter is wonky. Thermocouple was custom made by guys that cast aerospace items.
     
  10. Tobho Mott

    Tobho Mott Gold Banner Member

    You have more reason to trust your pyromeyer than I do to trust the hot steel rod dip test I use to determine when I've reached pouring temperature...

    Did you get a chance to try ramming it up harder yet?

    Jeff
     
  11. HT1

    HT1 Gold Banner Member

    your sand is probably too wet,
    was the sand bone dry before you added water ???
    remember the Navy uses a Moisture tester, and if the sand started out with moisture that is a part of the overal moisture. additionally, sand that is not dry causes the binders to bunch up rather then evenly coat the sand grains, so the sand will not have any green strength even though you have enough binders, the way to overcome this is to overmull the sand BEFORE ADDING WATER. binders must be added to DRY sand of you will be force to Mull silly long times .

    as to your surface finish, you have not given enough info for me to speak with authority . but there are really only 2 Major factors:
    how smooth is the pattern,
    how fine is your sand.

    and one minor :
    Pouring temp

    Now wild ass Guess, did you use too much parting powder??? No really, that surface finish looks very much like too much parting powder.

    though that open mold looks like the pattern has no parting powder at all???


    Im very confused

    V/r HT1
     
  12. Mister ED

    Mister ED Silver

    HT1 - You must have missed this sentence above ... "Oh, sand was bone dry with a 70 or 75 AFS grain fineness number." It was purchased dried and bagged, and has been sitting in my garage for a year. Now that being said ... can sand pic up atmospheric moisture? I never though of that. And I realize that the 70-75 AFS will not give me the smoothness of a babies bottom ... but I wouldn't think it would be 50 grit sandpaper either.

    Western bentonite was added to the dry sand and mixed for 10ish minutes (Mini Mite muller). Then water was added, mixed another 10. I only added 2.2% water ... much more and it was coating my hand after making a ball. Green strength appeared good to me.

    Pattern is a fairly smooth 3D printed part. Smooth with lines running lengthwise. you can actually pick up some of the layer lines in the photo above ... it you look past the bumps.

    I used diatomaceous earth as parting compound. The pic of the open mold is actually a test I did the prior day, it doesn't even have sprue or riser ... sorry for that confusion. Too much parting compound ... I don't know that I did ... but maybe.

    I was going to cast two more today, and experiment with three new batches of sand ... even drier (just to try and rule out moisture). Had everything mulled up, flasks rammed (hard) ... then ran into an issue with my Corny Keg fuel tank. For some reason I am not getting fuel through the outlet fittings, I think I need to get nitrile O-rings.
     
  13. HT1

    HT1 Gold Banner Member

    Ok you guys know GFN is roughly synonimous to sand paper grit number (very layperson visualisation aid)

    your pictures finish is rougher then a casting done in 60GFN if you want a finer finish and you keep getting otherwise good castings you neeed a mold wash

    in the Navy Foundry in San Diego they used a PUNB sand system with 60 GFN sand , just with sand they got a slightly better finish then you, with a mold wash, they got a VERY good finish about what you would expect from 140GFN petrobond... because occasionally they would do plaques .

    I'm still leaning towards too much parting compouns, because your finish is not consistant, though you do not have the "blobs" what PP parting often causes, try a very fine camel hair artists brush and brush down the mold and blow it out, the soft brush will knock loose excess parting, air blows it away... you can also use a finer parting bag, but that's "advanced technique"

    V/r HT1
     
  14. Mister ED

    Mister ED Silver

    Ok, I will get a different brush and use it a little more diligently. Any issues using the diatomaceous earth for parting? I do not see it listed in the Navy manual, but I had googled it in the past and seen it was being used. I don't think most "talcum" powder is talc anymore. But maybe the DE is causing some of this issue.

    That GFN number is confusing as heck, I remember struggling with that last year when I bought the sand. In the 1958 manual it says that the GP sand (at that time) was 63. So I figured I would be good, or at least OK, at the 70 that I bought and the sand mine had it (below is a breakdown from their website).Here is what I have below. Maybe I will need something different, but from what I saw in the mold, I thought I would be OK. Heck, I thought I was doing good not getting "playground sand" from the box store and shifting through it.

    I am out of commission until I get some parts here for my fuel tank.
    Capture.JPG
     
    Last edited: May 26, 2020
  15. Tobho Mott

    Tobho Mott Gold Banner Member

    GFN confuses me too. Never could figure out how to find or synthesize GFN 115-130 sand in my efforts to replicate Smelko Foundry Products' greensand recipe. Not sure if the "140 mesh" silica I passed on from the pottery supply would have been a better bet than the "75 mesh" I settled on, 140 sounded like it might be more into petrobond territory.

    Last time I was parting dust shopping, they had both cheap talc baby powder and more expensive corn starch baby powder at the drug store. Still using the cheap option here. Not sure about diatomaceous earth as parting. Sounds vaguely familiar, too bad we can't just look it up on AA. :(

    Edit - I almost had a stroke trying to decipher this grain distribution chart from a local quarry's finest grade of sand at all, much less figure out its GFN:
    https://greelysand.com/wp-content/u..._Greely_Spratt-Pit_Hydro-Sand_Sieve_83734.pdf

    Jeff
     
    Last edited: May 26, 2020
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Has it been sitting around for quite a while? It's nearly all silica but it's very porous and also an absorbent so may have an affinity for collecting things that would be liberated under heat. In looking at your pictures, it looks like the roughness is eruption from the casting surface which means into the mold surface. Not sure what to make from that but to me it points more to the sand.....leaning toward too wet. Did you let it rest for a few days after the first mulling and hit it again?

    Best,
    Kelly
     
  17. Mister ED

    Mister ED Silver

    Rested for 3 days (I think, maybe 2). I actually had to let the first batch dry out. What I don't understand is that I was only at about 2.2% added water on the first 3 batches that I made (that's what was used in the flask). More water than that and my hand would be fully coated with a layer of sand (like in one of Jeff's videos with the first batch in his muller).

    You might be on to something with the diatomaceous earth absorbing moisture. It has been around for 3 years or so.

    I have a coupe camel hair brushes ordered, along with some parting compound.
     
  18. Melterskelter

    Melterskelter Gold Banner Member

    Just as a general data point, I re-mulled some sand that had really dried out after it’s last use. I measured the amount of water I added and it worked out to a shade more than 4%. I added 2 pounds 4 oz to 50 pounds of sand. The sand moisture content prior to re-milling must have been close to zero based on its feel and appearance.
    100 mesh Olivine
    5% Souther Bentonite

    Denis
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Not a sand guy but just as an observation, wouldn't you expect some differences in optimal amount of water and bentonite from very coarse to very fine mesh sand due to the large difference in surface area.......coarse sand needs less of both for a given mass of sand?

    Best,
    Kelly
     
    HT1 likes this.
  20. Tobho Mott

    Tobho Mott Gold Banner Member

    I was going to point out that adding the "right" weighed out amount of water to the dry ingredients turned my homemade greensand into mud too, but you already saw that.

    Now, after much trial and error, I just fill a McDonald's plastic Lego Movie cup up to about the top of Emmet's head and add that to half a 5 gallon bucket of dry greensand in the muller, that usually seems to come out about right for me.

    Jeff
     

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