Since the beginning of time we have maintained a restriction that prevents you from overlapping multiple geometries in 3DTin. Whenever you move a geometry and try to collide it with another geometry, you get the orange warning box – “Avoiding Geometry Overlap”. This limitation can be frustrating sometimes. Imagine you want to stick a conical nose to the spherical head of a Joker. If you cannot push the nose into the head, then it will look floating – barely touching the head at a single point of contact, totally artificial.
We have solved this problem for you now. You can now merge all the objects into other objects every which way you want. As a result you can now make some interesting shapes like this one.
This change can drastically improve the quality of the models. They can now look more realistic, instead of looking like mere collection of primitive blocks. I can’t wait to see how you put it to use.
[Technically speaking, this operation is called Union of two solids. It’s one of the 3 standard CSG /boolean operations (others being subtraction and intersection)]
Update:
There are some limitations that I left out in hurry yesterday. You can merge any number of geometries in the browser if you want, but if your ultimate goal is 3D printing you should use geometry merging with restraint. The boolean operations on solids are time consuming. So while exporting the STLs we do not perform merging if more than 10 geometries are overlapping in a group. We just export them as is. Such exported files may look the same, but they may contain invalid polygons that may confuse a 3D printer.
3DTin 
Congratulations with the new and important feature !i.materialise doesn’t have a problem with the merged geometries.You can export the model to us and when you see a price appearing our 3D printlab, it means the merge is ok and we can print it.In case you don’t, you are offered the chance to ask us for a price for it.WiM