Yup, aluminum was a bit too tough for my benchtop CNC. So I found a supplier of tooling board that I hope to try out soon, they promised to send me some samples. Tooling board is a high density polyurethane block that you can easily machine and hopefully use to make injection molds. 

But, it is quite expensive.

Thats why today, I tested out the CNC gcode with a piece of MDF wood board. It worked quite good in the beginning, but when I got to some depth the thinner details snapped right off. Hopefully tooling board wont have this issue, or I will need to replace all the smaller details with metal pins which would be very time consuming.

But in total, I'm quite happy with the experiment. Slab some sealer on the mold and I can use it to try the injection molding machine. If it works and the entire mold gets filled up with plastic, I will know if I can use some sort of polymer molds instead of aluminum. If not, I probably need to spend a few thousand dollars on contracting someone to make the molds.