Steve I hope your health is going the positive way.
You made a great pattern on those little engines.
A friend of me has made for an new Zenoah engine an exhaust core mold by CNC in delrin.
This one is in three parts and has to be glued together.
The problem with the petrobond forming sand looks the same I have sometimes.
I use two types off pertrobond sand (there are five type’s available).
One is quite hard when the mold is ready, the other one seems all the time a little flexible when the mold is ready.
With the first type I had never trouble but with the second the errors you have I get a lot off times.
Maybe a little more mulling will work in your case.
I will order the firs type off sand the next time.
