The Hollow tool is used to create internal cavities inside closed solids, with a consistent wall thickness. Although you could use tools like Offset to create internal shells, these mesh-based operations can have self-intersections. The Hollow tool is built on top of the same infrastructure used in Make Solid, and as such does not have these problems. Hollow can also add escape holes so that if you 3D print your model, you can remove any internal powder or uncured resins.
The property panel for Hollow is shown to the right. Like Make Solid, the Hollow tool requires that you click the Update Hollow button to recompute the internal shells. However, unlike Make Solid, we do not create a new Mesh scene object when you click Accept. Instead the new internal shells are appended to the input mesh.
The Solid Accuracy and Mesh Density sliders have the same effect as in the Make Solid tool. We strongly recommend you read up on Make Solid to understand how Hollow works.
The rest of the parameter settings are explained below.
The way we compute the internal shell in Hollow is to simply run Make Solid in Accurate mode with a negative Offset. So, the Offset Distance just specifying this offset, which is effectively the wall thickness of the hollowed object. In the image below we Hollow with the default 2mm, then increase to 3mm, and finally perform a plane cut on the final object to see the shell (click to enlarge).
Note that at both distances the bunny's ears have been filled. This is because they are less than 4mm thick (2 x 2mm walls = 4mm). Also, for the 2mm offset (middle-left), you can see a few small floating shells at the base of the ear. Currently we do not automatically discard these artifacts, although if you add holes they can easily be removed.
One useful capability of Hollow is that you can add "escape holes" which connect the interior and outer shell, so that if you 3D print a hollowed object, internal support material such as powder, resin, or dissolvable supports can be removed. These holes can be added automatically or manually.
To manually add an escape hole, simply left-double-click in the location you want it to be. A semi-transparent green cylinder will appear, as well as a red sphere. Left-click-drag on the red sphere to re-position the hole on the surface. Use the Hole Radius and Hole Taper sliders to control the shape of the hole-creating elements. Note that these settings cannot be performed per-hole. To discard an unwanted hole element, double-click on the red sphere. When you Accept the Hollow, we will perform a Boolean subtraction of the hole geometry. The images below show an example where we begin with two simple holes, change their position and shape, and then Accept.
You can also automatically generate a fixed number of holes, with locations determined by Meshmixer. Set the Holes Per Hollow slider to specify the number of holes for each disconnected internal cavity, then click Generate Holes. Note that we may not be able to fit the requested number of holes on very small cavities.
Tips and Tricks
Hollow is dependent on the dimensions of your model. We assume that 2mm is a reasonable minimum wall thickness for 3D printing, and use that as the default Offset. If your object is very small, then 2mm walls may mean that no cavity would exist. About 90% of the users who contact us about issues with Hollow are using it on tiny, tiny objects. You can use the Units/Scale tool to check the size of your object.
In some cases the Boolean subtraction of the hole geometry may fail. This usually occurs when the holes are placed such that they do not fully intersect the interior shell, or when the outer shell is has poor triangle quality. See the Boolean tool for suggestions about how to resolve these failures.
We mentioned above that tiny internal cavities sometimes are left behind. If you are adding holes, these are easily removed. Simply select a bit of the outer shell and hit e to expand the selection to all connected triangles, then i to invert the selection. This will result in disconnected interior shells being selected, which can then be discarded with x.