Checking C4D Geometry for 3D Printing

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Instructor Rick Barrett

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  • Duration: 10:45
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In this tutorial, you’ll learn about the importance of solid, watertight meshes and how intersecting objects may be misinterpreted by a slicer.

Perfect geometry isn’t as important as you might think with 3D printing, but geometry issues leave the interpretation of your model up to the slicing app. In this tutorial, you’ll learn about the importance of solid, watertight meshes and how intersecting objects may be misinterpreted by a slicer. You’ll also learn about non-manifold edges, and how to identify them using Cinema 4D’s Mesh Checker.



One of the most intimidating things for me about 3D printing was the idea that my geometry had to be perfect. But in my experience, the workflow is a lot more forgiving than that. Commercial services and slicers have tools to automatically fix your bad geometry. Some slicers are much more forgiving than others. But in general, keeping an eye on some specific geometry concerns will help to ensure that your model comes out of the 3D print process looking like you intended rather than the way that the slicer interpreted your geometry. So with that in mind, let's take a look at some of the key geometry issues that can throw 3D printers for a loop, and the tools that will help you identify and fix those within Cinema 4D. The first thing that I'm sure you've heard is that your 3D meshes need to be watertight in order to be 3D printable. All that basically means is that your object has to be able to exist in the real world. If we look at, say, a simple cube, this is a watertight mesh. It's solid. But if I go in here and select this top polygon and delete it, what I have now is a cube with walls that are a single polygon thick. There's no thickness here, so there is no room for it to lay down plastic. And it's also not going to be able to tell what's the inside of the object versus the outside. You can identify issues with the water-tightness of your objects using Cinema 4D's Mesh Checker. An easy way to get to the mesh checker is by hitting Shift M, or you can access it from the mode menu of the attribute manager. Just choose Modeling, and then find the mesh checking tab. Go ahead and hit Enable Mesh Check. We'll go ahead, and for now, turn everything off except for boundary edges. And now you can see that this edge where I deleted the polygon is green. That tells me that this is a boundary edge that's causing this object to not be watertight. Now, there are different solutions for this depending on your model. In this case, for instance, I can enable the extrude tool and extrude all of those polygons out. Now, there are few things you want to watch out for. If you want to keep all of the edges together, you need to increase the maximum angle; in many cases, above 90 degrees, and make sure preserve groups is on. Also in this case, I want to make sure to create caps. And now, I have a solid cube with a whole in the middle. Another thing you need to consider is intersecting objects. A lot of times, when we're not deforming geometry or don't need to do anything special with it, we'll just cram it together and figure that it looks good enough. But the 3D printer actually needs to know that it should merge those objects together. The .stl file format fundamentally only contains one object. So when you export to .stl, all of your objects are going to be merged down into one polygon object anyway. The problem is if those polygon objects aren't connected, the slicer is not going to know how to handle them. Let's take a look at a scene in Cinema 4D. What I've done here is just created an ice cream cone and a ring by overlapping two primitive objects. And if we look in a slicer like Simplify3D, what we'll see is that where those objects intersected-, here we have the ring intersecting into the cube- it's actually creating a void here where the ring is going inside of the cube. So rather than fully connecting the ring into the cube, it's actually creating this void. We get the same problem here with the ice cream cone. Here, it's actually creating a void rather than continuing to fill with the standard infill material. And so that's going to cause issues of strength in your models. It's going to cause you to possibly use more material than you need to; it's just generally not a good idea. Now again, I said that some slicers are more forgiving than others. Let's take a look at MakerBot desktop, and I'm going to load that same file in. We'll just do the print preview to see how it's sliced, and you can see here that it's actually figured out that it needs to properly merge these objects together. You're getting the infill material here on the inside of the ring. And as we go up here with the ice cream cone, I think it will be a little bit more obvious, the infill material just continuous up into that parameter. So MakerBot here is a little bit more forgiving in actually merging your objects together. But again, this is where you've got to remember that by not cleaning this up in Cinema 4D, you're leaving it up to the slicer to decide. The nice thing is that you can export to your slicer and see if it work or not and always go back into Cinema 4D and fix the issues you need to. So let's take a look at a couple of the fixes for this. I've got them already hidden here in layers. One thing you might think of doing is using a connect object. In some cases that will work, because the connect has a built-in weld functionality. However, in this case, because the objects are fully embedded in one another, the weld actually, there are no points that it can weld to actually connect this and make them one single solid object. It's connecting them into one object, but they still have the interior void. You also have the Boolean object that you can use, which with a union Boolean, especially if you turn on Create Single Object, it's actually going to combine those two objects together. And then finally, there's a plugin by Nitro4D called Magic Merge, which is basically, I think, just doing a union Boolean and making it editable immediately. So that's an option as well. So these last two options of using Booleans or Magic Merge are going to be your best ticket. And just to confirm that, let's double check how that looks in our slicer. I'm going to go ahead and load the full file here and prepare to pint. And we'll go ahead and animate our layers. Here you can see, this is our original. This is our connected object, so you can see that the connect object did nothing. But when we get down here to the Boolean object and the Magic Merge object, you'll see that the infill is continuing all they way around the ring, which is what we want. So this is where it can be important to actually Boolean your objects together and combine them properly in Cinema 4D. Now again, like I said, some slicers are going to be forgiving enough that you don't need to worry about this. In fact, in some of our 3D projects, we're not going to worry about it. But it's good to know that this is a consideration that you should keep in mind and look for when you send your 3D models to print. The next thing you need to be aware of is reversed normals. The reason this is important is the slicer actually uses the normal to determine what's outside versus what's inside of a model. You can visualize the normals by simply selecting the polygons in Cinema 4D. Any normals that are reversed are going to appear in a blue-purple-type color, whereas the normals that are facing outwards are going to appear orange. Another good way to visualize reverse normals is to turn on the Backface Culling in the options tab of your viewport. In this case, if the normals are reversed, the polygon is going to simply disappear. To fix reverse normals, you can simply select all of your polygons and choose to align normals in many cases. Or if only specific ones are reversed, you can select just that one and choose Reverse Normals. The next issue you need to worry about is non-manifold edges. This is probably the most confusing one and also the trickiest one to identify and fix. A non-manifold edge is basically a case where more than two polygons meet in a single edge. So in this example here, you can see that three polygons are all meeting in this one edge that's marked red. So that edge becomes non-manifold because this geometry can't exist in real life. Again, you're basically creating a wall of infinite thinness that can't be printed. A lot of times, non-manifold edges can be created inside your geometry as the result of an extrusion, a Boolean, or a symmetry-type operation. Here for instance, I have a cube that looks okay from the outside, but I've turned on the non-manifold option in my Mesh Check. Again, shift M will go to the mesh checking. And with non-manifold on, you can see those are red and there are four non-manifold edges on this model, these four right here. The reason why is that there's actually a polygon inside of here. So if I go ahead and select these three on the outside and hide them, there's that polygon. If I delete it now, I've gotten rid of my non-manifold edge. So now, I can unhide all. Here in this cube, we have a similar type situation where we actually extruded an edge and then extruded back into itself and then ended up deleting the polygon, so we basically got this one polygon that is sort of folding in on itself. So what we could do in this case is, for instance, select that boundary edge. The easiest way to do that is using the Mesh Checker. We'll go ahead and select the boundary edge. If we scale that down, and then we could take this outside polygon and delete it. Now here, we have a boundary edge that needs to be taken care of, which we could do by closing the polygon hole, but we've taken care of our non-manifold edge there as well. So that's the basics of non-manifold edges. Again, they're really easy to identify-- Again, they're really easy to identify thanks to the mesh checking feature. And then it's just a matter of a little bit of modeling work to clean them up. So these are some of the key geometry issues that can often throw off a slicer or 3D printing process. Watch out for these when you send your models off for 3D printing, and you'll have a lot greater success.
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