Helps make clean ring casts when sand casting and allows casting some negative space features like prong settings.    Simply fit your ring mold or model onto the core and build up your flask as normal.   Then replace the core and model with a sand core made from the matching mold.  

I've made a few models but you will need to do some partial prints to figure out the tolerances for your printer and materials.  Usually the core doesn't need a change but the inner diameter on the cube needs an adjustment.     You can use the included fusion file to make other sizes or customize it as you see fit.   I've made parameters so you can adjust it quickly.