Here I have a glazed (i.e. sand not sticking) cup for butter and want to cast a replica of this and use this one as a pattern. The inner walls are straight, so it *should* be able to remove from the mold. I tried it, but cannot prevent that the sand (silica sand + bentonite + water) sticks to the inside, so it breaks off. I tried to put some wood chips into it to force it attached to the drag (it is placed upside down in the mold), but still it breaks off. Does anyone of you have an idea to prevent the sand breaking off ?
Add draft, or use a core for the inside. Another way would be to double roll so the interior is facing down to allow rapping to free it. Added re enforcement might be needed if you invert to pour.
Using some silicone or wax on the pattern might help a little. But, if there is no draft, even with non-stick, there are undoubtedly slight irregularities that may lock the sand and prevent clean withdrawal. Even if the walls are mechanically perfectly straight, a very unlikely condition, drawing that out of green sand is very unlikely. I agree with the core idea or adding draft by adding material to the walls of the ceramic. I think you will be time ahead to just turn a very similar pattern from wood or aluminum but with 1.5 degrees of draft inside and out. Denis
this a delicate act and doable, but not repeatable, brush the interior of the cup down with powdered graphite , the sand will have to be perfect , a little towards the dry side , you will need to reimforce the green sand core with a gagger, A lagging pin with the button, or a pair wired together with a fine wire works really well I cannot really describe the use of Gaggers , if they are too big the core will crumble, if they are too small they wont hold the core in place , a snarl of wire also works pretty well ive used dowels with nails run trough it, but I dont think that will be small enough in this case as mentioned above the mold has to be handled so the core is in the drag when poured, and when you pull the pattern the core has to be in the drag, this normally requires the double roll, Now rapping the pattern was mentioned, this is a delicate operation, because you have to rap hard enough to pop the sand loose breaking the vaccum effect as you lift without breaking the core, be extremely careful rapping side to side as that will break the core loose, I would rap around the bottom edge of the cup towards the center (sort of down and in at a 45) using a small rawhide mallet , about 10 inches of 1 inch softwood dowel would be your DIY replacement for tyhe mallet. remeber in this case lift and rap simultaniously , if you have a helper directing some compressed air under the rim of the cup can help pop the pattern loose, not too much presure or you may bore a hole in the sand, Hope this helps, you have one of those Master sand rat problems on your hands , V/r HT1 P.S. if the core breaks off and you have it orientated in the drag, you can place it on the mold with some core glue, and then use a mold wash , if the core does not float you may get the part . Ive done this on parts so big you really do not want to start over like 60 inch flasks Again good luck
@HT1: "This a delicate act and doable, but not repeatable" What do you mean with 'not repeatable' ? Probably my sand was slightly too wet. But the wire or nailed dowel trick I'll try to keep the core in place. Thanks for the tips.
you are going to an insane amount of effort for one piece, with a proper pattern, you could do 3 in the time it will take you to do one, if you are lucky, it will never really get better, because you are on the bleeding edge of what can be done. if your goal is to get one, it can be done, if you wanna start making these as regular work, get a real pattern V/r HT1
If you use a core that needs to be anchored to prevent float, I would recommend a simple method I used that seemed quite positive. I had a silicate core about 1X1X3 inches that had to be held down in iron. So, I drilled it with three 1/16th holes each one splaying out in different directions about 20 degrees from vertical. I then pushed tig wire through the holes and into the green sand for 1.5 to 2 inches and "puttied over" the holes with moist green sand. That provided a firm hold and the holes really did not show in the cast part. Denis
I recently had a similar situation with my bell mold. I used the double roll method. Meaning: I put my cope on top of the empty drag in the same orientation as a finished mold except I placed a molding board between the two. This was just to elevate the alignment pins. I placed the pattern on the board with the cavity down. I rammed it as normal along with my sprue pin. As I was ramming, I placed a dowel on the center of my circular pattern so it protruded above the cope. I finished ramming and then struck it off. I removed the sprue pin and the protruding dowel (shifting the dowel a bit to enlarge the hole) and flipped the cope. Placed the drag on, covered the sprue hole with a piece of paper, dusted and rammed the drag. I placed a large screw in the cavity to reinforce it. I think it helped. I rolled the whole mold over, inserted the dowel down to the pattern and rapped it a couple times. This was to ensure that the pattern released from the cope and stayed on the drag. Very lightly rapped around the pattern with a light dowel, then grasped it while tapping until it released. Pete
