In this Blender tutorial, I’m going to show you how to add parting lines to a mold so you can 3D print this mold and then pour plaster into it and use it for slip casting.

The first thing to do is to remove all undercuts from your model. You can do this by following the instructions in the previous video. Once you have your undercuts removed, you need to decide where you’re going to start making your parting lines. This should be thought of a little bit while you’re removing undercuts, but now is better than ever for this particular object. I want to have a parting line going around the face. This will keep most of the detail.

Select your model and make sure you have two materials for this particular model. I have an undercuts material and a default material. If I tab into edit mode and I select Material Preview, I want to select the default material. Press A and assign. Now, if I tab back into object mode, the default material is yellow. I’m going to apply the undercut material to places that are undercut, so I know where to put my mold. I’m going to use the 3D print toolbox to show where undercuts are. I’ll rotate my model so it’s face down because the 3D print tool evaluates undercuts from looking down.

So x -90 tab in the edit mode, press in the 3D print toolbox; it’s over on the right overhang. Make sure it’s set to zero. Otherwise, you won’t get all of the overhangs. I’m just going to select undercuts and then assign. Now, if I tab back into object mode, I can see where the undercuts would be. Anything in yellow now is not part of the mold. You need to make sure that your head is the right size for your 3D printer using the 3D print toolkit. You can select your object, and then under bounds, you can set a length limit instead of 280 cm or 0.28 meters. I’m going to make mine have bounds of, let’s say, 8 inches. And if I say okay, it’s going to make the entire head be 8 inches. If I want to make it a little bit smaller, I could say 7 inches and then say okay. And if I click on the item, you’ll notice that the scale factor has changed to 0.165. This scale needs to be applied so it’s actually the right size.

Press control A and select scale. Now you have a head that is the size that you need. You can always make a backup of your original size, but that way, you’re not making a huge head because you want to make a small slip cast mold. I can make my parting line somewhere along this purple line, but how am I going to make a parting line? I need to have something that I can use to split the mold. I’m going to get a circle and use the shrinkwrap modifier to have the circle stay on the edges of the model.

Press shift A to add a mesh circle. You’ll probably want more than 32 vertices, so maybe add another 12 to get 44. And it’s a little too big, so select it. Press S to scale. Look from the side of your model. GZ to move it up. This probably could be rotated a little bit, just to get started. Look from the top and press S x to get it a little bit closer in size. Then S y. Press control A, rotation, and scale. And this is going to be used for our parting line. But we want it to actually follow the purple a little bit easier. In order to do that, with the circle selected, use the wrench. Add a modifier from Shrinkwrap. We want this to be on the surface, and the target will be the head.

As you can see now, it shrinkwraps right to the head. We want to be able to control this. So if I tab into edit mode, notice now the shrinkwrap isn’t applied. You need to click this little triangle over here in the modifier. And then you can see it in edit mode. If I press one, I can select vertices and move them around with the G key. And then they will stick to the surface. As you’re working, you may notice that sometimes the vertices jump around or they’re hard to control. Just remember that you’re not actually moving them here. You’re moving them in space. So if I turn off the modifier, notice where I’ve moved those vertices, and they’re snapping in.

So see, I have these two really close together, even though it’s going to this point right here. Sometimes, it’s helpful to go ahead and apply the Shrinkwrap modifier. So now, if I tab into edit mode, they’re just there now instead of a circle. Then add the shrinkwrap modifier right back. So under deform Shrinkwrap, target the head tab in edit mode. Once again, we need to select the little triangle so we can see it happening. But now we know where these objects are, and we’re not moving them as far. Sometimes, Blender just gets a little upset trying to calculate where everything goes, so you can apply and then reapply the shrinkwrap modifier as many times as you need.

Then just select the vertices, press G, and then move them into position to create a parting line along the undercuts. We’ve already done all the work to figure out where this should be. Sometimes you may need more points. Just press Ctrl R, and then you can have more points. Press G. Move them into position. You want a smooth parting line, so you don’t have to follow exactly where the undercuts are because you can always modify them as well. Here, I need some more, so press control R with the mouse. We’ll have some more vertices, G to move. Probably need a few more here. Control R, G to move. And then continue doing this until you have all the vertices in place where you need them.

So sometimes, as you can see, this isn’t going where I want it to be. I want it to be on top of the ear, so you can always toggle off. In this particular case, I think I need to select that one. Press G, Z, and move it straight up. And then it’s going to try to be on the top there. And then probably, it’s not a bad idea to go ahead and apply the modifier. I need to be in object mode to apply the modifier. So click Apply. Add modifier, deform, shrinkwrap. Pick the object as the head tab in edit mode to shrinkwrap modifier preview. Now, it should be easier to move this around because it’s much closer to that location.

Blender gets confused, so just go ahead and get everything close and then apply the modifier again. Make sure you pivot around to the other side. Get everything into position. Now, this is looking pretty good. It’s going around the parting line on the spots that I marked with the undercut. And now I can apply the shrinkwrap modifier just so everything is in place. Make sure you’re in object mode. Apply the modifier. Now we need to extrude this so it becomes a parting line. In order to do that, tab into edit mode. Press A to select all of the points, E then S. This will scale this out. We want to scale pretty far, and now we want to continue to use the Shrinkwrap modifier because we want to move this around. We don’t want our points to move.

In order to do that, we need to use a vertex group. So if you click one of the points down here, hold alt or option, then click again. It’ll select all of the points in the middle. If you look in the menu on the right, this little green triangle is the data panel. You can create a vertex group, click plus, and we could label this group. It’s not necessary, but we can say “inner ring” and then click assign. Now, all of those are assigned to that group. If we apply a modifier, add modifier, deform, Shrinkwrap. And then we select the head. Notice everything gets smashed to the head. We don’t want that. We only want the interior vertex group. So click vertex group, inner ring. And this time, it’s only going to the inner ring.

Now, we want to extrude in one more time. So to do that, we’ll hide the head. I have the shrinkwrap modifier on, and I press E, S. Notice that it all gets messed up because it’s trying to scale to there. That’s because we need to make these points not part of the shrinkwrap modifier. So make sure you don’t have the modifier turned on. Type E, S to scale. Then in the vertex group in the data panel, click this green icon and the vertex points that we have selected. Now, we want to remove them from the inner ring. Notice now they’re no longer part of the shrinkwrap. That means the shrinkwrap modifier is only being applied to this inner ring. So now we can go back and use all the other tools that we want to without disturbing our shrinkwrap.

Press E one more time, then S to scale. Then we can go E, S and then type M at the center. So now we have everything joined. If I bring the head back, you can see that we have a parting line going all the way through our object. There’s a couple of places we may want to tweak that parting line so it isn’t going through the ear like this. So we can easily do that by selecting the other points. I can hide the head here, select this point, bring it back, and then G, Z, and I can customize that parting line as long as I don’t go through it on the other side. So that makes it really nice because this point is always going to be stuck right on the object. So I can move that one.

Hide the head, grab this point, bring the head back, G, Z. Bring that down so it’s not in the ear. I want to get this one now. Hide the head, bring it back. G, Z. We can make very custom parting lines and forms inside Blender. Hide the head again. Select this point. Bring the head back. G, Z to move it down. I think there’s only one more in the way, just this one here. So then hide the head again. Select this point. Bring the head back. G, Z. And now I can still select this point because it has the shrinkwrap, and I can move it around.

And what’s nice is you start to notice that maybe you didn’t make the best decisions on where to put things. So I can bring this one up, and notice that that still stays there, and I can make a nice smooth parting line. So go around and smooth things out. There’s no specific way to do this, but anytime you see something big like this, you could be potentially creating undercuts, so just move it around, and then that’ll help bring these down. You can look in the cardinal directions. It’s a good idea just to kind of level it up as you go around. Grab these, press the G key. The goal is to have this be a gradual flow.

So now I have evened that out a lot better, and it looks nicer. Another thing you can do is look up from the top. Notice that this isn’t a square. So if I tab into edit mode, I select all the ones over here, kind of estimate where you think that will be, kind of look at the corner and see where that is. So this one’s not the corner. Then type S, X, 0. That’ll scale everything in the x direction to 0, G, X. And I can move those over. Select over on this side. Same idea as highlight all the ones on the bottom here. And I can move those to where I need them to be. And then I could do the same thing up on the top. Probably have to make some other adjustments, but this at least gets you in place.

Be careful. If you have two corner pieces, they’ll become right on top of each other, so make sure you only have one on the corner as Y, 0. There we go. Now we have a nice parting form that we can use for our mold. I’m going to show you how to do the entire process with just this single parting line. But note I would add additional parting lines for the ears and nose as needed, but it’s just repeating this process to make more pieces. This is a little rough, so we want to add a subdivision surface modifier to this. So click Add modifier, generate subdivision surface, and crank it up to maybe three.

And now you can see that we have a nice smooth parting line. Kind of give it a look around to make sure you don’t see anything that is going to be an undercut. And remember, since Blender is a mesh modeling program, anything you see in flat preview is what will 3D print. So make sure you crank up the subdivisions as far as you need. There’s no need to do it until the final export. Right now, this is nice and smooth, so that would be better. So up to at least level five, I’m going to leave it at three right now just to make it easier to process.

We need to have a solidify modifier so our parting line has some thickness. Add a modifier, generate, solidify. Now we need to make the thickness correct. For this, we want to make it very thin. This is just 0.2 millimeters, but that’s what we want because we don’t want the parting to have much size. We want the plaster to be right next to each other, so we want just a little bit of tolerance. The next thing we need is a mold form. So press shift A, create a new cube. It’s going to be large, so zoom out. Make sure you’re in object mode. And then if you are in Material preview still, click Viewport Shading. Then you can use the See-through object, scale the cube down.

It’s important that the cube is intersected by your mold object as well as your parting line. So here I have the neck poking through, and the parting line is all the way through as well. Make sure you look to the other side. G, Z. Move that up to where it needs to be. S, Y, S, Z. And we need some space, so make sure that you have enough space for the plaster to be there. Maybe a few centimeters there. S, Y. So I think that’s in a pretty good location. S, X. Make it out just a little bit bigger like that. Now press control A to apply your scale, and you can turn off See-through.

So now we have everything we need to be able to make a two-part mold. This is funky enough that it doesn’t need keys. In fact, I probably want to make sure that some of these aren’t so tight that they’re creating undercuts, but it seems pretty smooth. This one is almost problematic, so I should probably click this and then make that not so crazy. Move this up like this. Up like this. Now I have a much smoother curve. So if you notice anything in your model that is creating something that might be a problem, make sure you fix it now. It’s a lot easier now than later.

Now, that’s much smoother. You can go around and check things. For example, this probably needs to be moved over a little bit more. And so the nice thing about having this as a digital model is everything is fluid and editable. So I think that looks okay. This is probably pretty sharp. Bring that down a little bit. So these can act as keys to register the mold. If you have a flat spot, you may need to add extra registration keys, but this should work for now. The next thing to do is to subtract the model first from the cube because we can only have one boolean at a time, and we want to make sure the model is a nice clean boolean.

So select the box. Add a modifier. We want to generate a boolean. It’s going to be different, and the object this time is going to be our head. Then we’re going to add one more modifier, generate boolean. So also going to be a difference. And this time, it’s going to be our parting line. So you can make a backup of this cube by clicking in the outliner. Command C, command V. And you can put that in your archive for backup later. That’s not a necessary step, but it can be helpful because once you apply these modifiers, they are applied. So on this first cube right here, I’m going to apply the head boolean and then apply the parting line boolean. Then I can hide the head and hide the parting line.

Now you can see that this mold has two pieces. This is what we actually want. We want this to be in plaster because we’ll pour our plaster into here, and then we’ll get a slip cast of our head. But how do we get this to accept plaster first? Tab into edit mode, then right-click, separate by loose parts. You should get two main pieces. There may be some stray geometry. You can see right here I have this little stray piece, so just hide everything. And then the piece that you don’t need, for example, this right here, I can just delete that.

Now I have two cubes, and I can label them as needed. If I select one of them and tab into edit mode, press three for face mode. G, Z to move it up. So now we can see how that mold looks. Select faces, and I need to think about which way I’m going to pour the plaster. And in this particular case, I want to pour the plaster in here, so I’m going to press X to delete that face. Come down here. Select this face, X, delete that face. So now you can see how this is going to be the mold that accepts the plaster. And I want to make a solidify modifier on this, but I want to solidify to the outside.

So if I tab, select modifiers, add modifier, generate, solidify. Right now, it solidifies inside. That’s because the offset is negative one. If we make the offset one, it’ll go to the outside, but we don’t want it to be 10 millimeters thick. We just want 0.002 or 0.016. Should be plenty. That’s 1.6 mm. We can leave that there. But make sure you apply the modifier when you export. So now this will accept plaster and it’ll be the negative shape of the cube. We just saw. We do the same thing on the other object, tab in the edit mode, press three X, delete that face. This side has three faces. Select all three X, delete faces. Tab back into object mode. Add a modifier, generate solidify. Make sure the offset is. And now I have two pieces of my mold and is ready to 3D print. And then ready for plaster, and then ready for slip casting. To make sure that there’s no undercuts on the ears, you can add additional pieces to your mold and then just make sure you repeat the same process. Hopefully this allows you to make negative 3D print slip casts, plaster molds. Happy 3D modeling!