w =(b*n) + (k*(n-1)) + 2k
where:
w = width of block mold cavity
b = final blank width
n = number of blanks in the block
k = kerf width of blade used to cut blanks
Plug and chug to see what width might minimize the waste.
I use a 1/16" kerf circular saw blade in the table saw to cut blanks. I like that better than a thinner bandsaw blade, because it leaves a cleaner cut.
The "2k" is for cleaning the edges of the two outsides of the block. You could figure 1/16" for each outside edge regardless of the kerf for cutting the blanks.
My math says that, for 3/4" square blanks, a 5" internal cavity would yield 6 blanks. For 13/16" blanks, a 5-5/16" internal cavity would yield 6 blanks. For 7/8" blanks, a 4-7/8" internal cavity would yield 5, 7/8" blanks and 1, 5/8" blank.
As I mentioned, PR "shrinks" as it cures. Also consider that a slight meniscus forms when the casting is under pressure. I would make the depth 1" and not 7/8" because then the pour isn't right to the brim and the risk of spillage declines.
I think most people can "eyeball" a 1/8" margin at the top. After a few tries, filling the block mold to the right depth is a matter of correct measuring.
I'd suggest considering, if you have the tooling to do it, routing a finger groove in the ends of the block mold to make picking it up and moving it easier. That's what I'll do in the next set of block molds I make.
Length. . . I think 5-1/4" is long enough, and I'd discourage 5-1/2" unless the plan is to square them after demolding. Blanks of 5-1/4" fit nicely in a SFRB for shipping.
All intended to be helpful. Good luck.
