Modelling Guidelines

This article covers the elements of a 3D printed model that must be considered along with the design approaches.

Like all manufacturing methods, FDM has limitations and constraints on what can be printed. This article introduces these parameters and discusses how to adapt your digital model for representational or functional modelling, and how they affect your final physical model.

If you have read our Design Approaches article, you should start to have a clearer idea on what your model needs to achieve.

pageDesign Approaches

This article expands on those parameters by introducing modelling guidelines to help shape your 3D models, whether you are adapting one or modelling from scratch. The following matrix summarises for the main types of models, which modelling guidelines are important.

Overview: FDM 3D Printing

As with all tools, it is important to understand how the functions of a 3D printer can impact the possibilities of your printed geometry. The diagram below shows the key elements of a 3D print. For the most optimal result, the following capabilities and limitations should be thoroughly considered when designing a model.

A well-constructed 3D CAD model does not necessarily mean it is suitable for 3D printing. There are many factors and we recommend that you read through 'Design Approaches' before continuing.

Volumes vs Surfaces

3D printing generally requires solid geometry with thickness to print. To a computer, a surface is actually an infinitely thin object and will not be understood as 3D printing geometry.

In Rhino, there are some commands to get you from surfaces to closed polysurfaces, including the commands[ExtrudeSrf], [OffsetSrf] and [OffsetMesh] for Meshes.

Scaling and Detail

When scaling models, it is important to bear in mind the minimum thicknesses and detail resolution for 3D printing. For that reason, it is recommended that you design your model within the scale that you have decided instead of simply scaling down a full scale sized model. If you have an existing model, you can begin by scaling it down but you will need to adjust the model to these constraints.

Abstraction & Level of Detail

Just like how different scales of an architectural plan warrants more or less information, the level of detail in a 3D print should also only be as high as required for the print scale - is is both a conceptual exercise as well as addressing technical limitations of minimal thicknesses and printer resolution as mentioned above.

You may have to completely remodel certain sections in more/less detail to get the most of the print.

If you are going down in detail with a mesh, you can also try the [Reduce Mesh] command in Rhino.

Substitution

Substitution can also be used to completely replace 3D printed parts where it is not viable. There are many common architectural model-making supplies that can supplement your 3D printing, and actually reduce the hassle of dealing with an unwieldy 3D print.

This model of the Villa Savoye substitutes out the 3D printed columns with the filament itself - 3D printing columns at this scale would have been extremely fragile at this scale and hard to handle.

Using filament as column substitutes

Thickness

Ensure your geometry is of a certain thickness so that details are properly captured and that it will not break. Each layer of a 3D print is printed to the thickness of the nozzle, which is 0.4mm - this can be considered an adequate baseline for minimum resolution in most cases.

If geometry is too thin, it may not be sliced properly! When slicing, use the preview to verify it will print as expected - by default, each 'line' of filament is 0.4mm.

Note how the 0.2mm column does not even get sliced. Even though the smaller elements may get picked up by the printer, they are very fragile!

To ensure some level of strength, rigidity and good finish for a successful print, NExT Lab recommends a minimum thickness of 2mm for all geometry, especially if it has support material. Otherwise, you may go down to 1.2mm for free-standing or flat components.

Shells

When slicing, the geometry is interpreted as bottom, top and shell layers - these can be adjusted via the appropriate setting.

Making a multiple of the nozzle size usually guarantees adequate strength. The standard nozzle size is 0.4mm.

Where the part is thicker/larger than the layers specified - it will be infill.

Layer Height Resolution

The layer height is essentially as the name suggests: the height of each layer of plastic deposited during the process. A higher layer height has less resolution but prints faster while lower layer height captures more details but is slower.

Our Makerbot printers can print down to 0.1mm layer heights. However, bear in mind that in the XY plane, the resolution is limited by the nozzle diameter which is 0.4mm.

Lower layer height captures slopes and curves more accurately

NExT Lab only uses 0.4mm nozzle diameters. We recommend a minimum detail dimension of 1.2mm for the best results.

Overhangs

Overhangs are shapes that extend outwards beyond the previous layer (cantilevers). We know that FDM is a process where each layer is deposited over the previous layer below. Therefore, at certain overhanging angles, there may be insufficient material from the previous layer for the next layer to print on. This will affect any layers beyond this as it will droop.

Overhang angles at more than 45° will required supports

This can affect print finish of that surface so you may want to adjust for orientation of the print to eliminate/minimise overhangs.

Figure - No Supports under overhangs (3D Hubs) left vs. Supported model on right

Support Material

Supports are sacrificial layers, generated to counter the effects of gravity on filament during printing. Essential part of 3D printing is understanding support angles, overhangs and bridges.

Angle	Result
< 45°	No intervention required
45° - 60°	Usually no intervention required
60° >	Support Material is required

In situations where an overhang is necessary and the inclusion of supports will hinder a successful outcome, it is recommended to either incorporate artificial bridges (to be eliminated during post processing), or design the model undersides with a 45° angle.

Supports require time and effort to remove, please manage your time accordingly.

Bridges

Bridging in 3D printing is the horizontal extrusions supported by structures on either sides. Unlike overhangs, you can potentially print bridges without supports; however, this is heavily influence by two main factors: bridging distance and amount of cooling. Bridges are not perfect; there will inevitably be some sagging underneath the bridge as the filament is being extruded over thin air. Our Makerbots can create bridging gaps that span 20mm without supports, with gaps under 2mm having no perceivable sag.

Figure - Bridging (maybe highlight the sagging bits)

Orientation

Support Material

Ideal orientation minimises support material and achieves the best surface finish. Less material used means a quicker return time and reduces overall cost price.

Model Strength

If model strength is key to your application, orienting your model in the X-Y direction will improve stability. The weakest point of a 3D print is where the layers are deposited and adhere to each other.

Orientation may be manipulated as a final step in the Makerbot Print software, where the Print Preview selection could assist in comparing results.

Warpage

As the material expands and shrinks whilst it is being printed, small deformations can happen - if a model requires precision, then these aspects should be considered:

Large Surface Area Prints

As your prints increase in size, a key thing to look out for is large surface area prints that may cause warping. Makerbot Print gives us the option to add helper discs at the corners of the print to mitigate this. There is also a detailed write up on warping that you can read.

pageCombating Warping

Corners

Due to the nature of plastic flow, it is very difficult to achieve true sharp corners in 3D printing. Instead, it is recommended to incorporate fillets and chamfers in your design, particularly with parts that need to fit together. Fillets and chamfers can also help combat the 'elephant feet' effect where the first layer or two that touches your build plate is slightly larger than the rest.

Helpful Commands: ChamferEdge, FilletEdge

pageFillets & Chamfers

Hole Orientation

In general, you want to orient holes to be parallel with the build platform (XY planes). Your holes are most accurate this way. Please note that the fitting of your hole is heavily dependant on tolerances (see Tolerances) and can also be potentially affected by elephant foot effect (see Elephant Foot effect).

Sometimes, you will have to print some holes horizontally (XZ and YZ planes). As such, it is important to take note that the finish quality may not be as nice and that tolerances may potentially be looser/tigher (see Tolerances). This is due to contouring nature of FDM 3d printing.

Large Models

If your model is too large to fit in the build platform or requires moving joints, you should consider designing alignment and joint elements that allow your split models to come together.

Connecting/Moving Joints

Check out our technique page on how to achieve this in Rhino.

pageSplit Models & Joints

Alignment Pins

When you split your 3D print into multiple parts, consider using alignment pins/dowels that help connect your parts together. The male part (the pin) should be sized smaller than the female part; a tolerance of 0.5mm is recommended.

Alignment pins.

Check out our technique page for more info.

pageSplit Models & Joints

Case Studies

While this article covers each design parameter individually, you will likely have to resolve them simultaneously. We have some example projects below that might provide some insight into how you may implement these modelling parameters together.

page|3DP|AR| Prosthetic Habitats