Okay so this core was intended to create the initial rough bore for two cylinders and also create the crankshaft housing space in the block. That bottom block casting shape is trapezoidal (a trapezium to you guys, we don't know what that is here, and to the north
![:wack:](https://madmodder.net/Smileys/default/Just_Cuz_15.gif)
). The dowels are 3/4" dia. (19mm) to give you scale. And I made this pattern about a decade ago.
To make a core box for it, I tried casting it in plaster of Paris as a split mold. I did the usual of forming a box first, larger than the pattern, fastening the pattern in place, the box half way with wet plaster, waiting for cure, and then forming the upper half the same way.
That worked, but molding a core afterward from it didn't. At the time I was using baked sand with molasses water and wheat paste as binder for cores. I had not yet seen Ironman's Portland cement core, or plaster casting technique for a core box, made by sinking into a greensand false cope. Both neat innovations.
The biggest problems in moulding cores of this size and complexity are:
1.) that they want to break when trying to release them
2.) adding wires and reinforcement is difficult
3.) ramming is difficult
4.) connecting the two halves is difficult if trying to make a 1 piece core, or
5.) cementing the two halves together as a 2 piece is even harder to reinforce, and the core halves even more fragile
So here are some ideas, observations to improve:
1.) If making a plaster core box, I like the greensand "false cope" idea. It avoids the likelihood of a fragile parting edge in the resulting plaster mold because of the meniscus formed in the first half.
2.) In a split plaster mold ALL flat vertical surfaces of the pattern should be drafted to the parting line, which is the split line of the mold. This means a double taper, either way on vertical surfaces that cross the parting line. This is much more important on a core box than on actual metal casting, where you can get away with rapping an un-drafted vertical plane. You can't rap a core, or a pattern cast into a plaster mold.
![:poke:](https://madmodder.net/Smileys/default/P0king_Emoticon.gif)
My pattern did not have appropriate draft.
3.) In making the outer wooden box for pouring the plaster, you should shape it irregularly (non-rectangularly?) so that there is a flat face adjacent to the end of any cavity. Let me explain......what you want is to be able to ram the sand from any necessary direction while the mold is closed up.
If you look at my pattern, and if the two dowels were right at the edge of the assembled plaster pouring box, once the plaster was cast, it would leave two open holes where you could ram sand down while the mold was closed up.
All well and good. But also you need to ram sand into the trapezoidal cavity - which means ramming from right angles to the dowels. Well if you build your box with one of those trapezoidal ends tight to the box face, it will also leave a cavity that you can ram into.
Well you could make a complicated box to do all of that. But, simpler would be to make a rectangular box, but pad out the sides with blocks of wood to meet the pattern faces where needed, which would --- well, act like prints to leave spaces where you want them. Loose prints for the core pattern, I guess they could be called.
to be continued.....