There are several things to be aware of when you're trying to design a model for 3d printing. They are:
Manifold edges are probably what people struggle with the most when they start out. It's a big, scary word that actually doesn't mean very much.
It's basically saying that all the edges of a model must be connected. Imagine you have a box. The box must be connected on all sides in order to hold liquid, right? Something like this would be NON-MANIFOLD:
The edges aren't connected, so the the computer (and therefore the printer) cannot build the shape. Now, this is an obvious example. Lets look at one that is less obvious:
Now, you may be asking what's wrong with this. After all it's an open box, right? Well, not exactly. See, a line and surface has 0 thickness. That is, it doesn't actually exist. The computer is simply telling you that you're working with a surface this size - but the surface has no thickness yet.
In the real world, everything has a thickness. Even paper and atoms can be measured, so we have to make the same true of our models. Lets give our box a thickness:
Alright, now our box has thickness. It's ready to print, right? No. Now we address the second problem. 3d printers need to know what surface is on the outside and which is on the inside. You'll notice the box has a blue-grey color and a white color. The blue-gray marks the interior of an object, and white the outside of it.
The rule of thumb is to make certain everything on the outside is white:
Alright, now this box is 100% 3d printable!
But, if you were to put it up on the market, it would probably cost around $10,000. Why? Material thickness. Shapeways charges by the amount of material required to print the model. For this reason, you'll need to make certain that your model is A) The size you want it to be and B)hollowed out (material optimization).
I'm not going to get into hollowing models because that is a full post on its own.
But let's discuss material thickness. Each material has a minimum thickness. If the material is too thin, it will break when it is cleaned or during shipping. Don't worry, Shapeways will detect the problem and reject your model so you don't waste money. But it won't tell you where the problem with thickness is.
There are a lot of different materials, and I won't go into them all right now. You can find this list (along with a lot of other helpful tips) here.
A plugin that is extremely helpful is the solid inspector. This plugin will scan your models for breaks in the surface and a few other problems and highlight them. This can save you a lot of time, especially if Shapeways is consistently rejecting your model.
Stay tuned for my next entry, where I'll explain how to make rounded surfaces!