Looking for some advice on minimizing what I believe to be inclusions of loose sand in the surface of my castings. Some photos displaying what I'm up against: https://imgur.com/gallery/YfKNuHS This is from a pattern that is symmetrical across the part line, and you can see the issue is much more pronounced on the bottom (drag side) of the casting, leading me to assume it's the result of loose bits of sand being picked up by the leading edge of the metal and depositing on any bit of the mold surface they can grab onto. This particular example is in aluminum bronze, but I have been seeing this across all metals. The sand used is a 200 mesh ground silica (Sold as "Sil-Co-Sil") that I picked up from a local pottery supplier and mulled 10:1 with powdered bentonite and water. This stuff makes an excellent mold, it can just about pick up fingerprint-level detail, but I haven't found any examples of it being used as a casting sand so I'm unsure if my problem is more a product of my technique or the sand itself. I started out using the Teton Black greensand from Amazon, and that was generally what I was hoping to replicate, though I can't exactly recall if I was having this same issue back then since there were at least a dozen other flaws in my process at the time. I also have a batch of 60 mesh greensand sifted out of home center sand but I exclusively use it as a backing for the 200 mesh in my larger flasks because the surface texture is simply too poor to even know if I'm having the same inclusion problem. I typically make my molds using a tapered sprue former and a matching runner/surge trap former. The only parts that I'm cutting are the gate from the runner to the pattern and the pour basin at the top of the cope. I suspect that those 2 areas must be where the majority of the free sand is getting picked up by the metal, but I take a lot of care to blow everything out and make sure there are no sharp corners or uneven surfaces in the mold. For the most part I'm very happy with my castings using this system, and this seems to be the last annoyance for me to conquer, at least in terms of decorative castings. I am certainly open to trying a sand that is intended for the foundry, but I'm struggling to locate a supplier here in central FL so if anyone has a lead on that I'm all ears.
Those are nice clear macro photos. (Thanks Tops) I wish the answer could be as sharp and clear as the images. I think the smaller speckled pinprick defects may be and probably are sand. However, the larger smooth round defects look more like bubbles to me. In my foundry I have not been able to completely eliminate all loose sand, so I typically make a couple of what I call "sand traps" at the ends of lower runs of the mold cavity. That is because, despite being as careful as I can be, I still get the occasional sand booger that breaks off from some random spot. With the carved depression at the end of a run there is a place for it to land and still cause a defect. But it is a defect in an area of sacrificial metal that I trim off as part of fettling. I find this greatly reduces the incidence of "fatal" casting defects. But there still are tiny specks of suspected sand here and there much like you show. I think it may be hard to completely eliminate them. I'll be following the thread to learn what others have done in this regard and if they have a "foolproof" solution. Denis
Looks like loose sand. If it is not present after pulling the pattern then it may be falling in when placing the cope onto the drag. Try adding radius to all sides of casting and gating to give sand more body to hold itself in place and eliminate sharp edges that may cause sand to be brittle. Adding sand traps (like Dennis suggested) to gating will help if it is being picked up by metal stream inside gating system. Pics of pattern may help aid in finding root cause of sand.
Billy, I actually put the traps at the ends of the path of the metal which does a pretty good job of collecting the sand bits. My observations seem to suggest in most cases small chunks of sand seem to be pushed along at the head of the meniscus of the metal as it flows into the cavity. Does that square with your experience? Denis
Yes...some of the loose sand around the gating system will be picked up by the metal front which is why I add traps at the bottom of sprues and ends of runners. Sometimes it will lay in the bottom and iron will run over it if the velocity is low enough. This kind of looks like it is loose sand laying in the casting cavity though.
Denis, By bubbles do you mean air getting churned in via turbulence? I'm sure my sprue and gates have room for improvement, but I would think that if this were the issue there would be porosity throughout the final casting, and I'm seeing no evidence of this when cutting the feeders. I have read about something called blowholes, whereby steam generated in the sand forces its way into the metal instead of out through the surrounding grains. My sand seems to have fairly low permeability so perhaps I'm seeing a minor effect of this type, but I preheated the mold generously and the defects are concentrated in a way that seems consistent with sand being dragged by the metal. What you are looking at in the photos is in fact the end of the run for the metal, so it makes sense that issue is most pronounced here. The geometry of the part makes for a perfect 'sand trap', but unfortunately I have limited machining ability so it is this same geometry that I need to preserve from the casting. I think I will try adding some 'traps' ahead of this area along the flat section of the part where they will be easier to remove to hopefully capture more of the sand that makes its way down the mold. I don't mind the 'specks' in the surface, in fact I find them rather appealing in the sense that the final product unmistakably displays the method of creation. Its the crater-like defects surrounded by an otherwise smooth surface that I hope to eliminate.
Billy, Adding a trap directly below the sprue sounds like a good addition. I have one at the end of the runner but if sand is being pulled from the basin/sprue it can only help to increase the capturing areas. As for sand falling out of the cope upon closing, I do at least 2 trial closes before the pour, once to ensure the cope is stable, and again after preheating. Of course once the mold is closed for good there is no way of knowing for sure, but I never find enough fallen sand to account for the amount of inclusion I'm getting. I think if these defects are entirely the result of loose sand, it has to be a majority of grains/clumps that are getting pulled from the mold surface by the flowing metal. I wonder if dusting the mold with a bit of graphite would help reduce the friction between the metal and sand?
do a google search for " foundry in Florida " call your nearest foundry and quickly explain your a hobbiest looking for a foundry sand supplier Can you help me please. If you get any results please come back and share V/r HT1
With respect to ‘bubbles” I was thinking of steam more than turbulence. I suppose the larger ovoid smooth surface defects could be sand clumps rounded by tumbling like rocks in a stream. But the sure seem to be perfectly smooth. Here is a photo of a couple of traps carved into the cavity and subsequently filled with iron—-no sand this time. Without the traps those otherwise straight walls would not be bulging at the ends. The traps were about 3/4” long and maybe 3/8” deep. Denis
I'm with you on that. I see one that looks like a grain of sand, the rest look like steam pockets. Sand a little wet?
When I removed this from the mold there were bits of sand lodged in most of the pockets, hence my assumption. I guess it makes sense that a little puff of steam would also lift a bit of sand into the metal though. I did pre-heat the mold with a torch prior to pouring but perhaps not well enough. I will try mixing the sand a bit drier next time to see what happens.
UPDATE: I've cast this piece a few more times since starting this thread and am seeing much less of this pitting defect in the finished products. I have made several tweaks to my process but I believe there were 2 major changes that helped the most. 1: Sand moisture. I have been reducing the amount of water I add back into the sand after use to great effect. I had been seeing bits of sand in the voids pictured above, but I now believe these were being pushed up into the metal by steam pressure. My overall surface finish is much more uniform and those pesky little pockets have become few and far between. The sand has lost some strength leading to more breakout and subsequently flashing along the part line, but for me I'd much rather have some extraneous metal to grind off than voids in the finished product. 2: Gating. I have widened the sprue and gates to give the metal a greater flow rate into the pattern, and done away with a pouring basin in favor of a direct pour into the tapered sprue. These changes were more geared toward solving a couple short pours I experienced, but I suspect that the increase in pressure of the liquid metal is helping to push any loose sand that may be present out to the very edges of the mold instead of depositing it along the surface. This has had its trade-offs as well, as the wider sprue and direct pour allows some air and oxides to enter the mold, but I have added a 'catchment area' just prior to the gate into the pattern which must fill first, by which point I have a stable supply of clean metal coming in.
If you feel good about the moisture level in your sand but do not have the strength you want, it is possible you need more clay in your sand. Would be better to be on lower side of moisture as long as you are making good molds.
