A note from Erik: Some of my friends from the Original Prusa Owners’ group might be wondering where I’ve been for the last few weeks. I’ve moved to another group! You can now find me and most of the brilliant and helpful people who taught me everything I know about 3d printing at the Prusa Community Forum on Facebook – tinyurl.com/originalprusa

I’m leaving in 10 seconds unless you show me what you’re talking about and how it is useful.

Ok then. Say you have a part like this drone flotation accessory.



You want this part to be as light as possible so you choose a nice 10% gyroid infill and you slice it.

So far so good. But as you step through the layers you realize that there is a problem.

There is so little material above that hole due to the low infill percentage that this hole will not be strong enough to perform its task. And its task is to keep your fancy drone from sinking to the bottom of the lake, so you’re a bit concerned about that. But you don’t want to print the whole model at a higher infill setting, because that will make this part heavy, and gravity is the natural enemy of drones.

Captain Whatshisname to the rescue

Disclaimer #1:

I have no idea what the feature this post is about is officially called. It might be “modifier meshes”. It might be “mesh modifiers”. I will never be able to remember which it is, so I will call them MMs for the sake of ambiguity. The fact that the name of the feature is this hard to remember (and never referenced by Slic3r itself I might add) should be your first clue that there be dragons ahead.

Disclaimer #2:

MMs are the swiss army knife of custom slicing, but for some reason they are stored at the bottom of a well filled with hot garbage juice. As of version 1.4.0 the feature is buried deep in the UI, and the UI is an abomination that contains far too much unnecessary “fun”. Nevertheless, this feature is really powerful, unique to Slic3r, and worth learning about.

Step 1: How to find the place where the beginning is

With the 3D tab open, either double click on your model or right click it and choose “Settings…”



Step 2: Mary had a little Lambda



This is the settings window. Take a moment to marvel at how clean and unhelpful it is.

We want to change the infill above our hole to be something denser than 10%. But how do we select only that part of the model? We do this with MMs. You can think of a MM as a 3D selection of part of your object. This selection can be a very simple shape like a cube or a cylinder, or it can be a complex shape, like a custom designed STL file. In this example we will be using a cube to select the area above the hole.

If we wanted to, we could model up a cube (or any other shape) in our favourite 3d modeling package, save it as an STL, and import it using the “Load modifier” button, but we are quite lazy so we’ll generate one to use in Slic3r instead.

To generate a cube shaped selection, for some hilarious reason you will need to click the button labeled “Load generic…”

You will be greeted by the Lambda Object window (and the stench of hot garbage juice). From the drop down box you can select some simple shapes that can be generated. Here we are creating a 25mm * 25mm * 20mm box. It is an intelligence test to figure out if you can remap the cartesian coordinate system to a new set of axes labelled L, W, and H. If you’re like me this will involve some trial and error. Click OK.

We now have a box shape that can be used to select the section above the hole. It just isn’t in the right spot at the moment. Please note that you have to select the MM cube before you can move it, either by clicking on it or on the “lambda-box” object in the top left menu. This step is mandatory since the box you just created won’t be selected automatically like you expected it would be. After selecting the box, you get to play the XYZ slider minigame, located in the bottom left corner of the screen. There is no way to directly move the box with your mouse, you need to play the minigame. You can also position the box more finely by entering postitive or negative numbers in the fields next to the sliders. Decimals are allowed.

Minigame complete: The MM is now roughly where we want it. Now what?

Step 3: X marks the spot

After you have completed the XYZ slider minigame and have your MM positioned where you want it, you get to go on a treasure hunt. There is a menu that lets you make changes to the slicer settings that will be selectively applied to the box you’ve put around your model. But where on God’s green earth is it?

Here. In the little green icon in the section with absolutely no hints about what it is for to spoil the thrill of the hunt. Remember that this is an expert method, tilt your head at an angle that says “Seriously?” and remember that X marks the spot.

Before you click on it, be sure to select the lambda-box / MM / generic box / cube object that was created, otherwise you will apply your changes to the wrong part of your model.

After clicking the green plus sign / x marks the spot icon, you will see some choices about print settings. In this case we want to increase the density of the infill, so are selecting Infill > Fill density.



An “infill” modifier section has now appeared. We can either change the percentage by changing the number in the box, or we can click the red “I can’t even” circle to delete this setting from our MM object if we made a mistake.

In addition to changing the infill density, I also want to change the infill pattern, since 100% gyroid infill will cause my printer to dance around a lot especially when printed at high speed. I want to print with rectilinear infill instead for this section. How do I do that again?

Once again, X marks the spot. I click the green circle with the plus sign in the middle a second time (it is hidden by the menu in the picture below, sorry about that).

I select Infill > Fill pattern to get another field that I can edit

Now I can choose a different infill pattern that will be applied to the parts of my model covered by the MM.

Once I’ve done that, I try to locate the OK button. It *way* over in the bottom right corner of the screen. Now to shower off the garbage juice, slice the model and see if it worked properly.

Step 4: Was it worth it?

Here is the section below the hole. As you can see, it has the original 10% gyroid infill. Looking good so far. Now for the moment of truth:



Here’s the section above the hole. It is now filled with 100% rectilinear infill, which will make it much less likely that this hole will tear through and my drone will sink to the bottom of the lake.

Other uses

There are lots of reasons you might want to change print settings on certain parts of your model.

You might need more perimeters for strength in one section.

You might want to turn off infill altogether in places to save time and material.

You might want to do something decorative by exposing infill or changing its pattern or density on the surface of your model.

You can even use this method to selectively add or remove supports from sections of a model. In principle it works exactly the same as the example above – create a selection shape, choose print settings to be modified, then change their values. I have done this in the past to remove supports and would have loved to provide you with an example of this here in this post. Sadly, the option to alter this particular setting with a MM appears to be broken in Slic3r PE 1.4.0, which I really love otherwise.

There are rumors circulating that Prusa is working on a much better way to selectively apply supports than this anyhow, and with major changes to the UI coming as Prusa Control’s very lovely interface gradually replaces Slic3r’s clunky one, I’m sure we will soon get the powerful features I have described in this post without having to hold our noses. Maybe these features will even have a proper name by then.

Got questions? Come find me and people way smarter than me at tinyurl.com/originalprusa