So much discouragement? We're modelers, gentlemen! We do what we like! (And often whether it's practical or not. I mean, what's really practical about modeling in the first place?) We learn from our personal mistakes, and grow in knowledge from getting our hands dirty and spending money we really don't have! We enjoy the journey! We enjoy the challenge! This is half the fun of what we do!
Hans is right in the point that it is not inexpensive to cast your own parts. Even MicroMark's smallest volume casting kit will set you back around $80 or so dollars. But Hans, if you have been casting for 20 years and still are getting an overall 60% failure rate, I'd suggest in the spirit of being helpful that there might be something wrong with your mold setup or technique (not a jab, don't take it as such). Even when I was casting without a pressure pot, my failure rate was never more than around 10-15%.
When I started using a pressure pot, my failure rate dropped to less than maybe 5%, quite acceptible. A pressure pot will indeed add to the cost, however. The pot will cost around $60 on sale from Harbor Freight, a decent compressor will be a minimum of around $150-$200.
If pressure pots and compressors are not a practical expenditure, you can minimize trapped air by "injecting" the resin (rather than pouring) into the mold, either with a plastic syringe or with a hairdresser's plastic hair color applicator squeeze bottle (available at most hair supply shops).
After basically teaching myself about molding and casting, I wrote a book about it for those starting out, to get them up and running faster than I did. So far I have sold over 100 copies, so there must be an audience for this subject. I started to design and make aftermarket sets for my own projects, and fund them by selling copies to other modelers.
Above: Home made cast resin cowl for Polikarpov I-16 (1:48)
Currently at my work, we're building a studio scale (around 6 ft long) Battlestar Galactica model ship, which is encrusted with resin copies of model kit parts, a few thousand of them. Their failure rate is maybe 1-2%. (But in all fairness it's important to remember they de-gas their silicone (under vacuum) before pouring a mold.) Molding and casting goes on all day long. I have since learned a great deal about mold layout. Both Hans and Gerald are quite right that it would be impractical to attempt to mold an entire tree of parts. Instead, you'll want to clip the parts, then arrange them closely on a slab of modeling clay. These are just rough details, we can go into greater detail if needed when you're actually ready to construct your molds.
Smeagol, I encourage you to follow your passion and curiosity on casting. Who knows? Your desire to learn today might cause you to end up being the newest big wheel in aftermarket resin tomorrow. For me, my dabbling in casting helped to get me a job later when my professional career (real estate appraisal) collapsed from the housing/lending crisis. I say go for it, dude.
Regards-