Pre preamble: Perhaps I am the only one who cares about this, but here goes. Preamble: One of the frustrating aspects of our craft is the difficulty we experience in comparing the results of processes we each use in our relatively far-flung and essentially isolated foundries. Green strength of molding sand. We can show each other videos of our gripping a blob of sand, squeezing it and then fracturing it. But, how does the strength of that sand compare to a "good" degree of green strength and how does that compare to optimal strength? Could some of the difficulties we might be encountering be due to sub-par green strength? Those are very difficult to answer questions. Yes, most of us know that organizations such a the US Navy have designed and built testing equipment. But most of us don't have the equipment or feel enough need to actually reproduce the excellent but somewhat complex and expensive designs that are known. I set out to come up with a perhaps crude but very simple and inexpensive tester that could be made by the great majority of sand casters, if desired, using cheap nearly universally available off-the-shelf items requiring minimal machining. I think for the proposed design a table saw would be nearly essential. But beyond that a hacksaw (hand or powered) is needed, some common strapping tape, a tape measure and an optional scale. It so happens I did spray a bit of silicone spray into the tube, but this is not necessary and probably should be dropped as it may present an extra item that needs to be sourced for many and probably does little to enhance the process. I think I started out using after I had some trouble early-on extruding the sand since I had to learn to spring open the tube. The proposal would allow me in Washington State USA to, for example, compare results with one of our Aussie friends with the investment of only a very few dollars and less than an hour of fabrication time. What was done: I bought a 3' section of 1" ID PVC plumbing pipe and split it lengthwise with a table saw making a single full-length cut. I selected an 10" segment of 3/4" steel bar (a loose fit in the pipe prevents jamming) I Placed the slit pipe in a vise to more or less close the kerf. (Complete closure is not easy or necessary. You just want the pipe to spring open a bit after packing sand.) Then common clear box-strapping tape was wrapped for a complete turn or more in four places more or less evenly spaced over the lower 12" of pipe thus holding more or less sprung partially closed. Mulled sand was loosely dropped, not packed, into the pipe with the open mouth of the pipe against the floor and the pipe vertically oriented until 1 foot of the pipe held sand. The slit makes it easy to see the sand level. In my case that amounted to 6 ounces of sand. Then, with pipe still vertical, I simply dropped the 10" segment of 3/4 bar down onto the sand column. Inverting the pipe returns the bar to the top of the pipe and the process was repeated. To be clear, the lower end of the bar was more or less even with the pipe opening at the beginning of each drop for a more or less 2-foot drop. I repeated this 12 times. Then the 8" bar was removed and the pipe placed horizontally on a convenient flat surface. The tape was slit and the pipe springs open a sixteenth or so releasing the pipe's grip on the sand column. Then a pusher of some sort (I used a piece of 1/2" all-thread) was used to prevent the sand column from moving toward me as I slowly drew the pipe toward me. This causes the sand to be extruded from the pipe and the column of sand fractures at some point as it extends unsupported (there is a less than 1/8" gap between the pipe ID and the table due to wall thickness of the pipe) from the pipe. The length of pieces is proportional to the green strength of the sand. It was true the several times I repeated this that pretty consistent results were obtained and the first piece extruded was a bit shorter than the second since the firmness of the pack would be expected to increase the closer to the top you sample the column. The sand I used to do the experiment was not quite ideally mulled as it was a sample I had in my home shop, not foundry. But it suffices to demonstrate the principle. If no one picks up on this and does some of their own testing, I will go no further. If there is interest, I will mull for what seems optimal in my foundry as I would be interested to know how that stacks up. I really have no way to otherwise know. This kind of information may be particularly helpful to the guy just starting out as in that situation most of us are trying to sort 20 different variables related to furnaces, burners, crucibles, sand mulling, patterns etc. The more that can be done to provide way points along that journey the better, I think. Denis
I'm keenly interested in the testing and the apparatus. It seems to work as you envisioned it. Something that keeps going through my mind is what aspects of strength are the most important. Ones I've seen most frequently mentioned are compression and shear (which I believe is related to tensile strength). In my research on the topic I've only really seen them mentioned and then they proceed to show how to test them, but there's never been much of an explanation as to why the respective properties are important to the molder/caster. The closest I got to an explanation was that the supervisor should keep careful and constant records of the test results and then continually ask the molder "how's it going." The type of test you've devised appears to be on the order of a shear strength test. It's value seems apparent, especially in regards to the sands ability to hold its shape when pulling the pattern and resisting dropouts and sand wash, etc., but the value of a compression strength test is not so clear. Yet it seems to be the more common of the two tests. Thoughts? Pete
Dennis, I think it is a neat experiment, But, I think there needs to be more control. The 1 inch ID is a fair start. But what was the size of the slit? A more controlled method...... -1 inch ID( 25.4) 12 inches long (long 305 mm) - 1/8 slit (3.2 mm) - Add 1 ounce loose sand (28.35 g) then pack - Pack with a 3/4” bar (19 mm) that is only 1’ long ( 305 mm) - only use a 22 pound weight with gravity as the controlled pressure(or 10 kg). No ramming ! - repeat x times. (Maybe 10 times) Horizontal sheer (removal) with gravity would be acceptable, But the mass and length would need to be measured. One last thing that could be measured is moisture content. Or conductivity. This could be done over the length or diameter or both, of the test piece.
Pete, Yes, I do not know exactly what this test measures whether it is tensile strength, compression strength, shear or more likely a combination of all. But it does seem to measure in some way an important characteristic of green sand that relates to its ability to maintain form in a mold. And very importantly, I think it uses simple methodology which can be copied by a worker whether living in NYC or some place where resources are very scant. It is ONE thing that we can measure and compare so that we get at least a notion of how one guys sand compares to someone else's. So, Pete, and anyone else interested, get a piece of PVC and a hunk of steel bar (or aluminum 3/4 bar that is 3 times longer than the 10" steel bar) and pound a few samples of sand. We may learn something. Furthermore, doing such a test could be an internal control so that I could repeat the test in a month or two to measure if my sand is starting to change at least in this parameter. Denis
Matt, the slit I made was about 1/8". My intuition suggests that its size could vary a fair bit and not alter results much. Perhaps you doubt that and might want to put that question to test. I don't think moisture content per se is as important as functional strength as one guy might use a formulation of sand quite different from someone else and have quite a different moisture content. That would not matter much if the functional strength is the same. Anyway moisture content is the easiest thing to measure using a scale and an oven. I want to ram the sand for strength testing as I ram the sand to make a mold. I can not see how conductivity of sand would be a useful factor for measurement though I suppose it could be done without too much trouble. If you can devise a simple and inexpensive test for pure shear measurement using very commonly available resources, please fabricate it, test it out, and write it up. Denis
Good initiative Dennis! When the core of sand emerges from the tube it is forming a cantilever. As the cantilever drops under the influence of gravity, it pulls on the sand at the top of the tube, which is a tensile force. As the break propogates the sand continues to separate under tension until probably some where about the middle of the core and at the same time that all of this is happening, the bottom half of the core is in compression. A slow motion video would show this quite clearly. Some shear probably happens when the break is near the bottom. So this leads to the question; what are you trying to establish by your experimenting? This experiment could be useful for someone trying a new sand, or adding bentonite to the sand they already have, etc. It seems to me that good tensile strength is highly desirable, which would mean that the longer the pieces before they break, the better the sand for moulding, Cheers Charlie
Your thinking mirrors mine exactly on the combination of compression, tension and shear all interacting. While this form of testing can isolate none of them, it does provide a simple objective way for us to compare the green strength of our various sand recipes. It is very common to see people (myself included) show that my sand I’d “desirable” by compressing a handful into a crude sausage and the break it, hopefully cleanly. But there is really no way that such a test conveys how strong the sand is. The test I propose is far from ideal, but better than the current alternatives—-none. I am aware of testing equipment that has been made to individually test compression, tensile strength, and shear. But when have you seen such equipment made/purchased and used? It is simply impractical for us small foundry types. Yet, I have always wished I could compare my sand to yours, say by squeezing a sample of mine and then yours. Not likely. So, as poor a test as it is, it is a functional measure of sand performance that is easy to perform. For me, I will use it from time to time to make sure my sand is not degrading. This can happen so slowly as to escape detection until it is really bad. And I also hope a few folks will test there sand so I/we can get a sense as to how our sands compare. If some sand is a lot stronger, then it would be good to try to understand why. Same would be true for unusually weak sand. I guess my preamble was too lengthy... Denis
My first batch of homemade greensand had waaayyyyy too much clay in it. Had great strength and I pulled off some deep moulds with very little draft. The issue became venting, rammed up too hard and it just wasn't permeable enough to breath. There are so many factors but it's nice to see some educated benchmarks.
This test can be done with different clay levels to see where you get the best results for compactability and green strength compression. It is all about how hard you can make the mold before it starts to break down and how well it will hold its shape during transporting of the mold and drawing the pattern out of the mold. It also has some value in surface finish. The tighter the mold can get the better the surface finish will be. It is a very important test for high density molding machines.
Depending on what type of surface finish you want and how deep the pockets are it may be just what you need. Maybe just adding vents would give you the best of both worlds, but that is kind of a generic thought without knowing much about your operation or casting design.
I don't have a muller and rely on mixing my sand on the moulding table and adding soluble-oil, using a bit of guess-work. Most of the time I get it right, but sometimes the sand is too oily, or too dry. Oily sand leads to a poor finish on the castings and too dry, leads to mould failure. Dennis's idea is a good one and will save me wasted time, if I test each batch. Better still would be to make a muller as well, Cheers Charlie
I agree. Sometimes these techniques work better than the high end equipment which can have failures that are unknown until you start seeing bad molds or scrap castings. We can make a mountain of scrap pretty quick.lol
