Lots of good info in this thread! I think that it is important not to over-complicate the process. Sure, as a manufacturer we have tried to perfect our formulation. Many manufacturers optimize in areas such as strict aggregate sizing, specific aggregate mineral formulations, very high granite to epoxy ratios, complicated shake and vacuum arrangements, high precision molds, and many others. Many of these optimizations are only practical when the costs and efforts associated with such optimizations can be amortized over the the product's lifespan.

As a hobbyist or someone without a need to create many machines, you are free to use materials and processes that may not be practical to a manufacturer. For example, you can simply add additional internal reinforcement to counter effects of voids created by ungraded aggregate and less complicated compaction and deaerating techniques. Also, because you are adding reinforcement you can use a higher percentage of epoxy and smaller aggregate to ease mixing and pouring and still have equivalent rigidity. Since you are not running a production line, you don't have to be concerned about the small additional cost of extra epoxy.

Here are some important notes on granite, epoxy and molds to help ensure a good casting(some of these have already been mentioned):

Granite
  • Size: 3/8 to sand
  • Rounded edges ease mixing and compaction
  • Clean and dry aggregate well


"Decomposed granite" from landscaping places usually has a good mix of sizes and relatively smooth edges. If you want to get kind of fancy, you could probably separate the granite by size using appropriately sized wire screens, and then combine in an optimized ratio. This really isn't necessary, but I know some of us are overachievers...

Epoxy

  • Low viscosity
  • Slow cure
  • Minimal amount of additives


lgalla did a great job with his specifications:
Raw epoxy is solvent free and ships non hazmat.
Cheap reactive dilutents are used to thin the epoxy and thin the price.They also thin the properties.Ask your supplier if their epoxy contains Nonyl Phenol.
If your supplyer cannot ship non hazmat the epoxy has dangerous additaves.
Raw, neet epoxy has low vapour pressure{smell is not bad]
Reactive dilutents have very strong odour,might i add unbearable.Again if it ships non hazmat it is probably OK epoxy.To confirm request MSDS from the supplier.
We need thin epoxy to maximize filler loading.This would be called Laminating epoxy.Thinnest ? Probably 600cps which would be the consistancy of #one Canadian maple syrup.Suddenly I feel like bacon& pancakes.
Epoxy glue is 10,000cps or better.No use to us granite guys.Similar to honey.
We want a long pot life to maximize mixing time and reduce heat build up.Many times we have mixed or de gassed for too long and the epoxy is really hot.Now we are sweating granite bricks trying to get it in the mold fast before gell.
Molds
  • Masonite or similar with a smooth surface and adequate thickness for noncritical mold sections
  • Aluminum or steal on areas to be precisely replicated
  • Take the time to produce your precision mold surfaces to a higher degree of accuracy than you want your final product
  • Don't place inserts closer than 3/4" from an edge
  • Use a wax based release that is brushed on. Apply a coat, let dry, buff with a soft rag, then repeat two to four more times
  • A gel coat may also be used


I hope this helps. I will post some tips later about the molding process.

Matt