I am soon to commission a new large(for me) match plate. It will produce a mold cavity 48” long, 10” high and 3” deep. I know from past experience with new match plates that the first run or two could have trouble with leakage of metal if everything is not fitted perfectly. That would not be so bad on smaller patterns. But I hate waste the labor from start to finish that is caused by such an occurrence. I am aware that there are commercial sealants made. But sourcing them especially in small quantities can be very challenging. So, does anyone have a recipe for such that can be made from commonly available materials? If I have to gin up my own without outside guidance, I am inclined to mix up some Bentonite and oil in a consistency like common caulking compound and place a 1/8” bead of it around the mold cavity. But, I’d prefer not having to reinvent a wheel that is likely well-proven. Denis
Ammen called them "dough rolls" and described them as a mixture of flour and molasses rolled out on a board like macaroni or clay snakes. I did not have very good luck with them and since almost all my castings are loose pattern I rely on the as rammed match at the parting line and just weighted my cope. As I remember they created excess flash.
Why not try some Dow Corning silicone. Admittedly I've only used it for bronze on resin sand and sodium silicate bound sand but a narrow 1/8" bead like you mention would flatten between the two halves and possibly embed in green sand. Afterwards the cured silicone would pull out of the sand like strings, allowing easy removal.
Yes, excessive thickness of the material both in amount applied and consistency is the main concern I had. That is why I was talking about making a rather thin mixture that was soft enough to emulate caulking as material of that consistency should smash flat pretty easily. I was figuring on using a frosting piping bag as an applicator. I am quite sensitive to whatever material is used being too firm and itself causing separation, flash, and possible runout. That is because I got the "bright idea" of making a hoop out of tig welding wire to surround the mold cavity. I put the hoop between the cope and drag and pressed down really hard and I could still, using a flashlight, SEE the shiny metal between the halves. I was surprised that the wire did not just press into the sand completely. But it did not. Certainly a "dough roll of any size and firmness would prevent good apposition of the cope and drag worse than my hoop. So whatever I use it will be soft like toothpaste. At least that seems right. I have seen sealer it applied in a videos of a commercial setting and it looked like the material they squeezed out of tubes was, in consistency, about like artists'oil paint you buy in tubes. I suspect that would work well. Two problems with the silicone are 1) expense as it would take a couple tubes to do one mold and 2)the stuff takes quite a bit of squeeze to extrude from the tube. I like the expected ease of separation from sand. That would be pretty slick. I THINK the bentonite and oil would diffuse into the sand with mulling. Denis
Core sealing from the navy handbook might work by adjusting the percentage. 3 % bentonite 6 % dextrine 91 % silica flour (200 mesh or finer) Water to develop the correct pasty consistency. After this, all joints are sealed with a filler mixed as follows: 3 % bentonite 3 % dextrine 94 % silica flour (200 mesh or finer) Water to develop the consistency of a thin putty.
I began to wonder how much distance you could get from a standard 300ml tube of silicone caulking: if you assume a bead 4mm 0r 3/16" diameter, the maths would be 3.141592 x 0.2² = 0.1257 square centimetres. That would give you 0.1257 mls per centimetre, so 300mls / 0.1257 = 2.38 metres of caulk or 94 inches. If you assume your mould is (48" + 10") x 2 = 116" so you'd need more than one tube of silicone: 1.25 tubes. With regard to the application, there are battery power caulking guns that would make application consistent and there may be some budget brand of clear silicone to cut costs but you'd be looking at the cost of 1.5 tubes of clear silicone per mould. The advantage would be having a commonly available product that requires no labor in advance to prepare. If you can get away with a 1/8" bead of silicone then one tube would easily do the job: 3.8 metres or 149 inches.
Meteor, Thanks for finding that. I had looked in the manual but failed to find it. What page? I know I will not be able to source the 200 mesh silica and probably not the dextrine at other than boutique priciong and quantities. But a key clue is contained in the "thin putty" consistency recommendation. That sort of confirms my suppositions. As I recall the material used in the video it was just thick enough to not quite run and just stand up in a slight mound like a typical baking batter might. Making sure the material is of "thin" consistency should prevent it from keeping the cope and drag from closing. I suppose I could just use water and bentonite as the expected contact with molten metal is small. Therefore, not a lot of steam should be generated. Just mixing up some bentonite and water would surely be easy and the ingredients are at hand. Denis
Good work on your calcs. Using single-part silicone may not be out of the question from an economic standpoint. I did try mixing up a simple paste from Western Bentonite and also from Southern Bentonite. It is possible without much difficulty to mix either one into a soft paste just a bit thinner than common tooth paste. It is interesting that Western mixes into a paste much more easily than Southern. The Southern definitely had more of a tendency to be lumpy and took roughly three times the mixing compared to Western. I like the idea that Bentonite should just immediately dry into a hard mass on exposure to molten metal. I guess I should add “I think.” I also checked to see how the paste behaved after being dried out and heated. I was hoping it would not become a rock-like mass that would be a nuisance in molding sand. And it does not. I tried adding back some water to the dried out hard Western Bentonite and it immediately softened and fell apart—-nice! I did not try the Southern as I think I’ll use Western because of its relative ease of mixing. As I was writing this I saw a post by Mark came up with a pretty easy way to make dextrin. So, if I need to mix up the foundry manual recipe, the dextrin issue should be solved. Silica flour is available on line I see. For instance PMC has 5 pounds 400 mesh for 35 bucks. So, maybe I’ll give that recipe a try if the Western Bentonite proves to be in any way unsatisfactory. Thanks, Mark. Denis
Here are a couple very short vids that may provide an idea of what the Southern Bentonite paste looks like as I mixed it and guessing this might be right Denis
With your pattern and flask work, I'm surprised you would need such things Denis.... But if so, instead of things that ultimately will alter the composition of your sand, why not just take a small router with a V-bit and rout a vee groove in the plate around the perimeter of the pattern so when molded, the correspnding green sand rib does the job? If they're loose patterns you wouldn't have to be married to the idea, and you could always fill the feature if the board/plate was of value to you. Best, Kelly
Dang, that’s not a bad idea,Kelly. The most compelling thing about it is no extra steps are added to molding process. Once the groove is in place, I’m good to go for as many molding sessions as that plate is used for. Nice! Comment on sand composition modification using Bentonite slurry: That is a reasonable concern, but I think the actual percentage of Bentonite added per molding session is not very great. We’re talking maybe 5 ounces in a 350 pound sand mold that already contains 18 pound of clay Still, over time it could add up. The counter argument is the added Bentonite could be considered a replenished for Bentonite that is depleted each pour. Exactly how much depletion occurs and if it actually occurs is unknown to me. I think it occurs as I do find myself periodically adding Bentonite when I THINK it is needed. Bottom line is I think I’ll try the groovin idea. The only potential negative may be inadequate compression of the sand ridge causing increased flash and increased metallostatic pressure on the mold. Adjustment of groove depth should minimize those problems. Denis.
Some small scale experimentation could potentially sort that out. A shallow pass with a small (1/8 - 1/4 D perhaps?) round nose bit might work too. Maybe more durable and release better? Dunno. Best, Kelly
Cyanoacrylate glue works fine and is instantaneous for gluing cores. I use a medium viscosity type and often accelerate it with activator. Thin cyano glue can be dribbled into a crack in a core for a good repair. Sodium silicate cores seem to speed cure of the SS cores on their own. The glue has never generated excess gas or in any other way caused a problem with surface finish. Denis
I'm sure I've seen someone say they have had success using plain old PVA glue to hold Core halves together (Elmer's glue here in the states) . I've never tried it and I could be misremembering (I am getting older the kids tell me).
Yes, I use PVA glue. The only issue is that it takes a while to dry, so I prepare my core the day before, Cheers Charlie
