Hi
Since i’ve noticed nobody replied to your post yet and i randomly ( after another sleepless night thinking over my feature )
browsed to your post, since i have a hub here i feel a bit responsable.
Let me clarify what my 6 hour long sleepless research proved to me.
3D printing and mold making seem to work hand in hand. You need a master object for a mold.
Simply put, a master is the original, the one you will clone endlessly with your mold.
First of all you need to make a square box larger then your original.
You put in the original and surround it with a liquid that solidifies.
There are multiple options that i’ve found online :
- composimold : seems like a good beginner start, you can make a mold, use this mold for concreet or w/e you want as material. Afterwards unlike other materials you can put the mold into a microwave and make it into gelly again.
You can use your original mold for over 50 molds without damaging the mold. And the website says you can reuse it to about 10 times in the microwave to create other molds. Looks fair. I might buy this product soon for testing.
- sillicone. The internet says this is quite expensive, and often the mold breaks or is unusable after a cast.
No further research done on this part. since the first option already looked more interesting.
3D comes perfect in this case because you can design very creative things with the process in mind.
You can for example easily leave a hole to pour in concrete.
My sleepless night today was all about how do i make concrete end products out of a 3d model that i make myself or find online. So my conclussion is this.
step 1: 3d print the model ( i’m looking more at vases to start with, they seem pretty easy, not all to complex shapes)
pay attention a good vase needs a hole ( or multiple ones ) at the bottom to let the water drain out. So leave a hole in the design. Online 3d print files often miss this hole, easy with a cut tool and a cylinder to subtract the cilinder from the original file. No design knowledge is needed what so ever for this.
step 2 : make a box larger then your master model.
Step 3: 3d print a cylinder for the hole, you need to put this cylinder in the hole so that compimold will not fill the cylinder up when commencing your mold. Pretty logical.
Step 4, put some compimold into your box ( make sure it is firmly shut at all sides so no compimold can leak )
also make sure that you can break the box apart easy. Wooden plates with screws works quite well i see online, since you can unscrew the box in 2 minutes and have your mold.
step 4: after some compimold is put in the box, lower the original 3d model.
I see online that they use some sort of support like 1 cm high screws or whatever to make sure the master is
perfectly lined out and leveled out inside the box.
Step 5: pour in the rest of the compimold untill the box is full and minimum 1 - 2 cm above your master. ( if this is a full object)
ofcourse in my case, for a vase, i need a mold for the outside and inside. So i pour untill the tip of the vase and not over.
Step 6 
in case of vase ) : make sure the cylinder is leveld well with the bottom of the vase and fill the inside of the vase.
Step 7 let it cure and dry , patience
Step 8: since we’re working with gelly time molds slowly wriggle the edges of the inside of the vase untill that part of the mold comes lose, pull it out and you have 1 side of the mold.
Step 9, demount the box, your left with a squar mold with ur vase still in it, pull the vase out, you have the other part of tyour mold.
Step 9. place the bottom mold and fill it a bit with concrete, now put in the second part of the mold that will make sure you have a hollow inside of a vase, and pour concrete all around.
Step 10: let it cure
Step 11 : remove the compimold mold, inside and outside by wriggling thus not damaging your mold, thus makes for reusability of your mold. ( dont use sharp edges, this will damage the mold )
Step 12 : enjoy the mold.
resources i found that explain this by pictures, cause it looks confusing now that i look this all over :
http://www.composimold.com/make-your-own-unique-molds-and-casting.html
https://www.google.be/search?q=concrete+molds+for+vases&espv=2&biw=1680&bih=949&tbm=isch&imgil=K2g1o9C3PdnypM%253A%253BIKkFypUDZsvXqM%253Bhttp%25253A%25252F%25252Fdesign-milk.com%25252Fconcrete-palmas-vases-look-like-geometric-pineapples%25252F&source=iu&pf=m&fir=K2g1o9C3PdnypM%253A%252CIKkFypUDZsvXqM%252C\_&usg=\_\_dwVio8gfmMNqRelXiHjbEtwO4b0%3D&ved=0CCQQyjdqFQoTCPDv88L8gcgCFcJAGgodqnQCQQ&ei=4b38VbD7KcKBaarpiYgE#imgrc=K2g1o9C3PdnypM%3A&usg=\_\_dwVio8gfmMNqRelXiHjbEtwO4b0%3D
- → that thing behind the vase is the inside mold to make the vase hollow that i talked about above.
- → very interesting and learning video, shows you the entire process, he uses silicone , not compimold. Silicones are apparantly more expensive ( no research done on prices )
concrete casting for a counter sink in a nautilus pattern, shows the entire process from start to finish, very interesting to learn off. Also explains the need to leave pouring holes etc if not included in the design.
Hope this all helps!
Find me at BeCreative Hub, i will probably order some things and test this out soon with some vases 
