Getting a good result from a 3D print starts with good design. In the same way that injection moulded, or machined parts have to be designed a certain way, when using FDM it helps to consider the manufacturing process early one. There are several tips and tricks when it comes to getting the most out of FDM printing and in this article, we will highlight a few of them to help you design good quality parts.

Here is a break-down of what will be covered:

  • Part orientation
  • Overhangs
  • Bridges
  • Supported structure
  • Fits and Tolerances
  • Dimensions

Part Orientation

This shows three versions of the same part, all of which have been printed in different orientations

FDM printing builds up the part in layers. As a result of this, most 3D printed parts have anisotropic properties, meaning they are much stronger in the XY direction than they are in the Z direction. In practical terms, this means that it’s much more likely to split between the layers than through them.

In the image on the right are three versions of the same part (A, B and C), all printed with different orientations. Despite being the same part, the different orientations really affect the strength of the part. Part C will be the weakest, due to weaker bonds between layers, whilst part A will be much stronger (see video below). These breaks show just how much stronger the horizontal planes are than the bonding that holds them together.

If you can’t avoid having tall thin parts that extend in the Z direction, then one option is to add some supportive ribs to that part of the design. Adding ribs can hugely increase the stability and strength of parts like this and are very easy to add to existing models. We’ve combined all this information into an explanatory video, which shows just how much stronger orientation and supportive ribs can make a part.

Video showing the difference in strength that can be given by changing orientations and adding ribs

Overhangs

The overhang angle is given by alpha – the angle from the vertical

Overhangs in 3D printing occur when layers have to be printed so they hang over the previous one, creating a wall that slopes outward. As you can imagine, extruding filament onto an overhang can have its problems, the biggest of which is that extruding ­into thin air is generally a bad idea. With nothing to stick to, the layers tend to sag down and give a rough surface finish to the part.

A good rule of thumb with overhangs is to keep them all below 35°, as this means that at least most the extrusion is still supported by the previous layer. If you do have to go over this angle, then there are several methods that can help you still print that part, such as: adding support structure, reorienting the part or printing the model as separate parts.

Print quality is significantly worse at higher overhang angles. Here is an example of the difference between a 20 and a 70 degree overhang.

Bridges

Bridges are like overhangs, but specifically they refer to an extrusion of horizontal material that connects two raised points. The connection between two points means that bridges can be much longer than overhangs, although the longer the distance between the two the more likely the bridge is to fail.

In general, it’s not recommended to use bridges for distances over 25mm, as the bottom layer of filament sags, meaning that the layers above it have nothing to bond to. This can lead to the bundle of filaments you can see making up the 130mm bridge in the image below.

Shows how our printers cope with various bridge lengths

Supported Structure

Support structure is one of the ways that FDM printing can allow you to overcome the problems caused by overhangs and bridges. Supports essentially consist of low infill structures that are built up next to the print without being connected to it. They support overhanging sections of the model higher up the print, and hugely increase the variety of geometries that can be printed.

The number of supports is set by the support overhang angle, this decided which overhangs get supported– we tend to use a value of 50° – 60° depending on geometry, any surface over that angle will be supported. In general, its best to avoid support structure unless absolutely necessary for two reasons:

  • Supports reduce surface finish quality
  • Supports use more material

Areas that are directly in contact with support structure have a different, rougher finish compared to other surfaces on the part. Whilst this isn’t always obvious, and rarely causes physical problems, if you’re designing the part to have a smooth exterior finish then avoiding support structure will be beneficial.

Supports also uses up excess material. As the cost of FDM printing is entirely related to the amount of time it takes, and hence how much material is used – avoiding designs that need supports will also produce cheaper parts.

The difference in finish between a supported surface and a non supported surface can be quite noticeable. Best to design without supports

Fits and Tolerances

The subject of fits and tolerances for 3D printing is incredibly extensive and there are several good articles that go more in depth on the subject. Here we will quickly cover why getting accurate tolerances on 3D prints is difficult and give a few different values for different types of fit.

When molten plastic is extruded out of the hot end of a printer, it adheres to the layer beneath it and then rapidly starts to cool. As it cools the plastic shrinks, minutely changing the dimensions of the part that has just been printed. Most of these variations are too small to notice, however, when designing parts that fit together its important to take this shrinkage into account.

3D printed parts very slightly change shape during printing, so tight fits can come out differently

The most basic types of fit that are commonly used between 3D printed parts are interference fits and clearance fits. Interference fits means that the two parts are in complete contact with one another, resulting in a very tight fitting, difficult to remove joint. Clearance fits, on the other hand, are much looser. They are often used between sliding connections where the two parts still need to move relative to one another. We tested several different hole sizes with a 3D printed peg and found that increasing the diameter of the hole by 0.3 – 0.5mm gave an acceptable clearance fit, whereas an increase in diameter of 0.1mm gave a better interference fit.

With fits and tolerances, the best way to get it right is to prototype and see what works best for your design. This is especially true if you’re using a combination of 3D printed and traditional parts, as the tolerances might be slightly different than the ones we prescribed.

Dimensions

When designing parts for 3D printing, you need to to consider the maximum part size that you can produce. At Prodpoint, our printers have build plates that can print parts up to 250 x 210 x 200 mm. Normally this is big enough for almost everything that our customers require, but in the rare cases that customers need to print larger parts, out Design and Development Service can help you segment the part and tweak it so it can be assembled after printing. Segmenting a part can also often give a better finish than printing as a single piece, as you often avoid using support structure and messy overhangs.

Conclusion

Properly designing parts for FDM printing takes a new approach to design. However, compared to other manufacturing processes, FDM printing can be much more forgiving. Here is a recap of the points covered in this article:

  • Add ribs to strengthen thin vertical parts of your design.

  • Keep overhangs below 55 degrees where possible.

  • Use bridges instead of overhangs, up to 25mm is generally good.

  • Minimising support material will save cost and improve quality.

  • Fits and tolerances: 0.1-0.2mm for a push-fit,  0.5mm for a clearance fit.

Thinking about the points mentioned in this article should really help you get the most out of your 3D prints. However, if you’re still having trouble with getting your CAD model FDM ready, our Design and Development team will be able to help. You can get in touch with us directly (01394 822025) or through our contact page here.