Hello, I have a little challenge that perhaps readers could offer views upon. The challenge is to form an elbow shaped void in a mould. The bowl is approx 14 mm dia at the widest point and reduces in dia to 8mm as it passes through a the bend. My question is what is the best method to anchor the core such that it remains true in X, Y and Z axis? I have tried using a simple box arrangement that is added to the core such that the box locates the core into the sand mould. This did not work as the core managed to twist and run out of true. Any alternate suggestions appreciated. The form of the sand core may be seen om 2 views on the upper l/h section of the pic, the lower r/h shows the square locater added to core with mating impression in the mould. Thanks in advance.
It's not clear from your drawing. Are both ends of your core are supported in the mold? Or is it just the one end with the square core print? Anyways, I have kind of an odd ball idea but, how about putting a small piece of eps foam under the core? It would support the core and help to prevent it from dropping into the mold cavity. I've never seen or heard of anyone using foam like this so, I hope some of the more experienced casters here will give their opinions on the pros and cons of doing this.
If both ends of the core extend into the mould enough to give good support, that should work. There's no need to have square ends on the core that just adds complexity. My only experience with cast elbows are thin wall 3" and 4" diameter aluminium elbows for turbocharging plumbing made with resin sand moulds and cores.
Maybe make a small hole through the core print, then pin it down with a flat head nail pushed through the hole? Jeff
Apologies, only one end is supported, that where the square is shown. I had intended for the core to locate at the other end by way of the core extending to the side of the mold , however as this section is just 8mm dia, the cast sand core kept snapping at the point where the bowl transitions to 8 mm dia (the extension being approx 35 mm length. The shorter core would at least form the elbow and then the extension would be formed by 8mm dia twist drill. I had thought of using a resin core, however I lack practical experience.
What is your current core material made from?, there have been members using a few percent of epoxy resin for their cores for strength and another method would be to embed some wire inside the core, with the wire bent at 90 degrees to follow the centre. Anything from 1mm to 2mm thick would boost core strength.
Core is from sand, appreciate if you could elaborate on the making with a few percent epoxy? Wire maybe difficult to pass through the core when in the corebox although maybe worth a try.
The best way would be to have the wire already in the pattern before packing in the core sand. There's a few methods for making cores including resin bound and baked linseed oil. I haven't directly used epoxy but forum member ironsides has used it successfully, you'd have to experiment with say 2-10% resin mixes and evaluate the strength as you want the least amount of resin that gives enough strength yet breaks down from the heat of the molten metal otherwise a core that is too strong could crack the cooling metal instead of letting go just as the metal solidifies.
Wire can work, but it doesn't have to be wire, it can be string. Called strum in foundry lingo. Your shape requires a core print at both ends, as mentioned already, and prints can be cylindrical, or even flat shaped (with internal strum or wire reinforcement). Make your corebox in two halves, fill each half, lay in the string or wire, and assemble. Then bake (if traditional) or gas, or wait for cure if not. You can also harden them first, then connect the halves with core cement. I'd like to suggest getting yourself the two small paperback casting books by Terry Aspin for really clear explanations and superb drawings and photos of all kinds of core making and mold cavity support, geared toward the home molder, as well as lots of useful info on foundry practice. Drawings were by Aspin who was a great illustrator for Model Engineer. ps. make your core prints longer than what you did last time -- it looks short for good support -- besides doing it at both ends.
Many thanks for your steer, I would indeed wish to have core prints at both ends, however I found that around the point where the bowl transitions from a tapering bowl into a straight cylindrical length of 8mm dia , the core would break. Hence I thought of forming only part of the internal cavity with a tapered bowl shaped core and then forming the longer 8mm di section by drilling through. The strum is not something I have heard of before so will further look into this.
Try adding a bit of wire to the center of the core. (with prints at both ends) this should help keep the core supported without making it harder to extract. (like Hacksaw says...or the string!)Or up your sodium silicate to sand ratio (or epoxy if going that way) try a bump of 5% (so if you start with 10% go to 15%. One of the Larger brass hammers I made I think I had 20% Sodium silicate in. It was like chipping rock when it was done so I don't recommend that for anything more complicate than as short straight hole.
