Design for 3D Printing

Basics

For a successful print, it's important that submitted prints follow the outlined requirements:

Minimum of 1mm thickness of all elements
1:4 spanning ratio
Submission in the form of a closed geometry

Before submitting a print, read through all the necessary information for the selected printing type.

Scale and Details

Resizing geometry/models down to an appropriate scale is essential when preparing a 3D print model. Display models are rarely produced at a 1:1 scale, and different printers will have different boundary sizes to adhere to.

Not all digital models can be 3D printed

Due to digital constraints of geometry, or physical limitations of 3D printers, taking a digital model and simply scaling it down for 3D printing will not always work.

NOTE

Understand what thicknesses are being digitally worked with by using a ruler when adapting a design for 3D printing.

E.g. Standard paper is 0.1mm, and is impossible to print successfully.

Adapt (or rebuild) a digital model

It will almost always be easier to rebuild a model for 3D printing. A properly adapted model can mean a higher quality return, and even save a lot of money in the long run.

Adapting a model could mean:

Simplification and/or reduction of design
Converting thin frames and/or columns to surface details
Elimination of elements

Note in the Above Image

Delicate balustrades eliminated, stairs and columns thickened, roof and wall details simplified.

Thickness to Span Ratio

In order to ensure weak points will be less susceptible to damage when getting handled during the post print processing phases, it's important to consider the thickness to span ratio.

Spanning beams and columns

For beams and similar long, thin structures maintain the 1:4 thickness to span ratio by inserting a supporting element at intersections.

E.g. 2mm thick beam will require a column at every 8mm length

Planar surfaces

For thin planar surfaces, consider including a ribbing designed into the underside.

Creating Closed Geometry

Geometry types

Models for 3D printing should only be composed of solids. In Rhino it's important to understand what type of geometry you are working with:

Type

Description

Point [1]

Represents a coordinate, a single location in space

Line [2]

Whether it is straight or curved, is best thought of as a collection of points. It traces a path through 3D space. A line cut in section creates a point.

Surface [3]

Surfaces have a front and a back, but no inside (thickness) so they cannot be 3D printed. Surfaces can be joined together to create a polysurface. A surface cut in section creates a line.

Solid [4]

A solid is geometry which has a volume. Multiple surfaces can be joined together to enclose a volume, which is then called a closed polysurface. If there is a hole in the surface or the surfaces do not completely enclose the volume, they do not make a solid. Closing these holes to form a solid is known as making the model watertight. A solid cut in section creates a surface.

Volume

In Rhino, geometry with a volume is known as a solid.

Surfaces cannot be 3D printed as they do not have a volume or thickness.

Objects which enclose a volume will have an inside and an outside. If you slice through a surface it will create a single line (Image 5). If you slice through a solid, it will create a closed profile (Image 6).

The easiest way to create solid models in Rhino is using the commands under the solid tools tab or the solid menu option.

Closed polysurfaces

In Rhino, joining surfaces together creates a polysurface, and if they completely enclose a volume, it creates a closed polysurface.

For the model to be 3D printable, the surfaces must be joined together into a single, closed polysurface (Image 8). To join surfaces, they must share an edge. If you are having trouble joining surfaces into a solid, it could be that some edges do not line up.

Did you Know? When you select an object in Rhino, the properties panel will tell you whether it is open or closed geometry.

Watertight models

“Watertight” is a common term used to describe closed or solid geometry. A watertight object has no holes, gaps or unjoined edges. Edges which are around an opening or not properly joined to another surface are called naked edges (Image 9). A watertight model has no naked edges. You can highlight naked edges in Rhino using the showedges > zoom command.

Connected solids

Ensure components within a model are joined before 3D printing. Components not joined together may come out as 2 separate pieces (Image 12).

Solid objects are combined using the BooleanUnion command. To boolean objects together, they must either overlap or have co-planar faces (Image 12). If the command does not work it is an indication that they may be slightly separated or not connected. In this case, move the objects closer to together and try again.

Did you Know? Use the intersect command to check whether objects are overlapping or sharing co-planar faces.

Optimization

In the following titles, the guidelines for an optimised 3D print model will be outlined.

Different 3D printing methods will require different things to be considered. This may include:

Cost efficiency
Quick turnaround of time
High quality finish and/or surface detailing

By considering and adapting a design to the following topics, chances of obtaining a good quality print for a reasonable cost, are maximised.

Hollowing a Model

When should I hollow my model?

Hollowing a model is essential to keep outrageous costs and material waste down.

Prints at the Maker Spaces are typically charged by material consumption, either calculated as the gram or volume (cm3). A hollowed model can at times reduce this number by up to 70%.

Models should be hollowed wherever possible, regardless of the printer type.

Some printers may require hollowed geometry to ensure a successful print.

E.g. 3D Resin Printing

2mm rule of thumb

As a rule of thumb, hollow models enough to leave a 2mm shell thickness to the final geometry. Always hollow out models when printing large, blocky components including bases and columns.

If needed, consider substantial structural components that may be required to be stronger and weightier to print successfully.

Here are some methods to hollowing out geometry.

Rhino: 'Shell' command

Hint

Effective for simple geometries, with minimal surface facets.

Rhino: 'Offset' command

Alternative Software: MeshMixer

Hint

Effective for complex geometries

MeshMixer is a free program available for download, ideal for use when working to manipulate or clean up mesh geometry.

STEP 1

Download MeshMixer and run the program on your computer.

STEP 2

In Rhino, BooleanUnion all intersecting geometry and export as a .stl file format.

STEP 3

Import the geometry into the MeshMixer work space.

STEP 4

Enter the EDIT tab and select the HOLLOW command. MeshMixer will run a default hollow setting before providing parameters for control. Ensure the OffsetDistance parameter is set to a minimum 2 mm.

STEP 5

To ensure openings are then created for excess powder to exit the hollow from, toggle the Holes per Hollow parameter, then click generate holes > update hollow to create openings. The locations of these openings will be controlled by MeshMixer.

STEP 6

Alternatively, to customise the openings, double-click left on desired points on the model surface before clicking Update Hollow.

STEP 7

Accept.

Reducing the Mesh Size

It is essential to keep file sizes to a minimum for NExT Lab to receive and process a 3D print file.

The most common reason for an inflated mesh file size will be due to the resolution of a created mesh upon conversion from a polysurface. Keep in mind that a higher resolution does not always mean a smoother surface finish, or better quality print.

The following are some ways to reduce a mesh.