I’ll see if I can post some pictures tomorrow. This is comission work for someone else, so it’s not entirely my design so i’ll have to clear posting any pictures with them first. The process is pretty straightforward, just time and labor intensive. I have a large pile of rectangular lego bricks and baseplates. I build a wall out of the bricks about 2-3 rows taller than the part I am building the mold for. I then put a 1/2" thick layer of modeling clay on the bottom of the form and press the part in so it is slightly embedded into the surface, well enough to make sure no silicone gets under it. I then add two pieces of 3mm fillament from the part out to the edge of the mold to act as vents, and 1 piece of unmelted hot glue stick to occupy the space where the pour spout will be, cut to fit between the part and the outer wall. I then use 4 or more marbles to make allignment points for the mold, pressing them halfway into the clay. I make sure the vents and the pour spout are embeded half way into the modeling clay as well, then I pour in some Smooth-on Mold max RTV silicone to fill the mold form up to about another 1/2" to 3/4" above the highest part of the mold core. Once that sets, I pull off the form and flip it over, removing the clay layer entirely. I put it back into the form, but now upside down, and coat the entire surface with petrolium jelly. The vent and pour spout bits stay in, but the marbles are all removed at this point so the other side piece will flow into the holes. Anywhere that isn’t coated with the petrolium jelly, the new silicone will stick to, so I can use that to repair any mold flaws.
Once all that is done, I pull the cores out and inspect the mold. Typically i’ll need to trim some flashing away from the pour spout and the air vents. I blow into the pour spout. if I feel air coming out the air vents, the mold is ready. If this is a 2 part mold, I cut a pair of thin particle board sheets for either side of the mold to act as a pour jacket. This prevents the mold walls from bowing outward. I hold them in place with light clamping action and or tape. Then its just a matter of pouring in the resin. For this project I’m using a polyester resin. They bought me 3 minit cure time resin, which is horrible. never try to cast a large project with resin that flashes solid in 3 minutes. It’s not fun.
To pour, I mix the resin A and B part, then if it’s not rediculously fast curing, I stick it in the vaccume pot to draw out any air bubbles up to about 29 bar. Then I pour about half the resin into the mold and cover the vent holes. i then rotate the mold to coat the inner corners and then fill it the rest of the way witih resin. If you do it right, you get a laser blaster part.