Hi,new here and already a question........I want to cast a few things that are hollow more or less. I have watched bunches of videos on casting objects like manifolds and tubes. My question is how is the core supported in the mold to get the walls to be the right thickness all around? The videos usually just show them being laid in the mold..Magic???? lol....Thanks in advance..
The pattern (a cylinder for example) is made with round bosses protruding from either end. When the pattern is removed from the sand, the bosses leave openings in the sand on either end where the pattern was. The core is made using a "corebox", which is a box that is bored to the exact diameter of what you want the inside diameter of your cylinder to be. The corebox has matching bosses on it, and so when the core is placed in the mold, its bosses fit perfectly in the openings in the sand left by the bosses on the pattern. The core is held in place exactly by the bosses on either end. If the dimensions for all these parts are not exact, you will get core shift, and your cylinder wall thickness will not be uniform. .
Edit - Pat beat me to it while I was looking for a good pic. Core prints on the pattern create a place in the mold to support the core (also called a core print). Lemme see if I can find a pic... My pipe tomahawk split pattern has coreprints for the eye and the bowl which you can see here. Make sense? Good luck! Jeff
These are good descriptions, Dave. For clarity, I'll repeat what has already been said. If you want a 1" diameter hole 2" long through a part then you make a core box to form core sand in which is maybe 4" long. Then you add a round stub 1" long on each end of your pattern. The stubs will make a core print when sand is molded around the pattern. Then the core is placed in the molded cavity bridging the gap between core prints. Obviously the stubs on the pattern have to be in alignment so the core fits in well. If you have a complex core, like a water jacket, it is important to use a binder in the core sand which will weaken when it gets hot. The best cores are weakened by the heat and fracture when the metal shrinks around them so they pour out. It's a fine line to get a core which stands up to handling and pouring but will also come out. For sodium silicate bound cores, the strongest sodium silicate is not always the best.
Dave, welcome. Tell us a little more about what foundry equipment you have, the metal you wish to cast, and the type and quantities of parts you'd like to make. Best, Kelly
Thanks for the explanations...This helps out a lot...Just working with one of the homemade foundries I found on line...Will be mostly casting aluminum..My son and I make replica vintage motorbikes from the teens and 20's...We decided to try to do a non running display as close to an actual bike as possible..So we wanted to cast the parts for a motor (not a working motor) cylinder,head and case...solid parts don't look to hard to do but I have never cast a hollow part like a cylinder. we'll let you know how it works out.....fingers crossed!!!
If you are going for "as close as possible" then I would suggest making the patterns in a way that they could produce an actual working part. My reasoning is that it takes a lot of time to develop a pattern, and that even if you don't produce a running engine, a pattern that could produce a replacement part would have great value to someone in need of one....
I would love to make actual working parts but I don't have the ability to do the necessary machining in my shop. As for patterns ,I don't have a problem with making them. The bikes we make use off the shelf 2 stroke motors. I just wanted to have a little fun try something different..The pic is the type of bike we make. Not an exact reproduction but a replica....They are fun to make and ride also and I make most of the parts other then the frames. I'm retired so I have a lot of time on my hands...
I too struggled with how the core was supported in the mould but finally managed to get my head around it after reading "Backyard Foundry for Home Machinists" by Terry B. Aspin (https://www.amazon.co.uk/Backyard-Foundry-Machinists-Terry-Aspin/dp/1565238656). Great book for explaining the basics of sand casting. That bike looks amazing BTW!
