As a point of reference, these are the pattern and core box for the pistons shown above that John O made. You can see how much draft there was to get the piston out of the sand. The core boxes have a piece attached at the bottom only because the molder at the foundry preferred to make the cores in two pieces and past them together. They could have been made in one piece but I didn't think it expedient to disagree with the guy who was doing the job.
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I am by no means a professional pattern maker, but if I am reading this right, I would say that the added plywood at the base is so that each side of the box could be filled and swiped to make the half-line flat, where they can be glued together.
With that much detail in the core, filling from the top to make a one piece core would possibly miss a lot of the details.
Here are some pictures of core boxes to explain my thinking.
The small one could be rammed from the top and completely fill the box, while the larger one has details that could only be filled by making two halves.
I hesitate to make it look like I know what I am doing as both of those are at the foundry now and I can only hope they work!
Foundry work has become easier with resin-bonded sand reducing the talent needed for pounding green sand, but it is still challenging.
I have been ramming petro-bonded sand for my small model and experimental furnace for aluminum and bronze, but I leave the iron for the Big Boys.
My brother is making a core out of 3d printed plastic that we will try to melt out of the mold with the molten metal later this year.
The Big guys are 3d printing sand cores with no need for draft to remove from a box and even have undercuts!
Fascinating stuff!
Mike