I went to the library yesterday and got out ISBN 3-478-93273-4 "Mineralguss für den Maschinenbau".

It reinforces the gems of knowledge Matt has shared:

Aggregates from <0.1mm (Sand dust) up to 16mm (on the larger castings from 80mm section thickness). Whereby you want to aim for 1/5 to 1/8 your smallest feature.

Resin ratios by weight of 7% to 10%.

Shaking frequencies up to 70Hz and accelerations to 25 m/3 (2.5g) This is basically your industrial 2 pole motor with a unbalance. If you pick up a cheap motor and VFD off ebay, you can vary the speed until you hit the sweet spot for each casting.

The largest castings use a combination of a shaking table and shaking motors bolted to the outside of the form.

For optimal bond, any steel inserts you bond into the part should be sand blasted.

The practical method for prototypes and us homebuilders is wooden forms with steel sections moulded in which are subsequently milled or ground to tolerance. The big guys are going more and more to cast to tolerance, but also do grinding of the mineral to tolerance. If you have a surface plate and an eye for detail, grinding and scraping alignment rails could be a garage job.

Tolerances to +- 0,5mm should be posible with wooden molds.

Edge distance for inserts should be >3D.

If you intend to reuse the form, it should taper about 5°.

Round all internal edges.

A rule of thumb would be a polymer concrete machine base is made the same size as a cast iron part, but solid where the C.I part would be cored, should have nearly the same weight and approx 3.5x the stiffness.

As the shrinkage of epoxy occurs around the time it gels, much if the shrinkage is compensated by further filling of the still liquid mix. The final shrinkage is given as 0.02 - 0.03% linear.