Printing in parts.

So this is a newbie question no doubt. I've modeled several pieces for a client who prints in sandstone, but this is my first attempt at printing in anything else and on Shapeways...

I can't seem to find out any info on how to print in multiple parts. I hear people use sprues to connect their parts together into I'm assuming is one stl. But I've also read that when printing in certain materials you don't need to do that. My objects in Zbrush are separated into 5 parts, head, hat, hammer, glasses, torso and legs. I assumed that would be enough to be detected as a part, but obviously I'm wrong. Shapeways sees it as a solid object. Which makes sense, so I'm not sure on what the best way to proceed is.

1. Parts will be fused when they are (a) touching or even overlapping in the model file (the site software will fuse them) or (b) closer than the minimum distance stated under "Clearance" on the relevant materials page (the printing process will fuse them)
2. Depending on the selected material, multiple physically separate parts in a single file may be not allowed or limited to "natural pairs", i.e. two matching objects of a type typically used in pairs like earrings or cufflinks. This is mainly a cost issue, as the fixed "handling fee" is calculated per model. Also, very small objects are harder to pick up and clean (lots of manual work still involved), so not having them sprued together to form manageable units can lead to a rejection.
See topic Materials under the MAKE heading for material-specific rules.
3. If you want your model to consist of parts made in different materials, these parts have to be in separate files.

Thanks for the helpful response. Answers my question perfectly.

I do have another one regarding wall thickness.

I understand the minimum guidelines set in the material pages. But what I'm curious about is how how sturdy something would be at a given thickness and size. The piece I'm referring to is going to be either 6-7 inches tall in WSF. I set the wall thickness to be about 1.3 mm. While this meets the requirements I am not sure how the end product would feel or how durable it would be.

We cannot say much without seeing the model, but I can tell you that WSF is pretty darn strong. Take a look at this video to get a little bit of an idea of how strong it is. WSF is super weak compared to the same Nylon plastic that is injection molded, however the injection molded version is so strong that even a little bit of that strength is impressive when testing the 3D printed version.