There's a few methods
Clam shells - Cut the larger parts in half hollow them out turn hem into 2 'clam shells'. This is what armorcast did and imo probably the best results. There's a catch though, you have to cut the master in half (that's very risky) or take a mould of the master and then cut that in half mucho expensive (I dont have the economies of scale to make it work)
Rotary casting - this where you put in a small amount of resin in the mould and spin the mould on all axis evenly coating the mould and leaving a hollow centre. Lots of commercial processes use this. But there are draw backs its not pressure cast, the apparatus is expensive, complex and bulky, its hard to get right 100% of the time (for an amateur like me). So too much hassle for the likes of me.
That leave fillers specifically Fillite Powder. I'll copy paste what it is;
Fillite is a glass hard, inert, hollow silicate sphere. Fillite is primarily used to reduce the weight of plastics, rubbers, resins, cement, etc., but also imparts further benefits in many situationsSo its teeny tiny glass bubbles dust. This was my chosen option because;
- Its cheap
- Its the same process as resin
- It makes the resin as much as 50% lighter while adding strength
Left pure resin - Right fillite/resin
(click to enlarge)
The fillite gives the resin speckled colour. The torso came out at 300g compared to the 360g of pure resin version. That was a conservative mix so i could probably get the weight down even more.
Rough maths means a 1kg mech could be taken down to 600g which results in a 25% saving on postage costs according to the postage calculator.
It also has another benefit on models that are top heavy (mainly chicken walkers). I can fillite cast the top half while pure cast the legs making the model much more stable!