Hot take: Everyone says to always preheat the sand for a full hour, but I lost a whole day on a simple core box because of it.
We had a rush job for a small bronze gear, maybe 20 pieces. The core box was a basic two-part aluminum job. I followed the old rule and preheated the sand mix in the muller for 60 minutes like always. The cores kept crumbling on the draw, every single time. I rechecked the binder ratio, the moisture, everything. After 8 hours of this, I was ready to scrap it. On a hunch, I mixed a fresh batch and only preheated for 20 minutes. The cores came out perfect, first try. I think that extra heat was baking out the binder before it even got to the box. Has anyone else run into a core mix that just can't take the long preheat?