I'm Steve Cox, a member of the Utimaker Community. I'm an experienced engineer having spent many years in the automotive industry but I'm now focussed on the world of 3D technologies, specifically 3D product design and 3DPprinting. I'm an Autodesk Certified Instructor for Fusion 360, so many of the images in this post are taken from that design software but this post is not specific to that software but is about designing for 3D Printing and Additive Manufacturing. This is a first of a series of blog posts in this area that will be focussing on how engineering is interacting with the latest 3D technologies.
Additive Manufacturing (AM) and 3D Printing (3DP) - whilst the way they produce an object from nowhere can often seem like modern-day magic, the truth is that in many ways they are no different to any other way of making things.
Every method that we use to manufacture things has it’s own rules that we need to consider when designing. These rules are known as DFM – Design For Manufacture.
This approach takes into account the pros and cons of the chosen manufacturing method to produce a design that can be made repeatably, reliably and to meet the intended function and life expectancy of the product.
This way of thinking when applied to AM (or 3DP) is now becoming known as DfAM, or Design for Additive Manufacture.
In reality there are two aspects of DfAM, the first we will deal with in this post where we will concentrate on the use of DfAM applied to detail features of the design to ensure manufacturability. The second aspect is using DfAM at the conceptual design to realise some of the unique capabilities that AM has to offer, and that will be covered in a later article.
The rules of DfAM tend to be slightly different for each type of AM/3DP technology. Here we will be assuming that we are using Fused Deposition Modelling (FDM) 3DP but, for instance, in metal AM residual stresses build up in the part during manufacturing due to the high local energies applied by the laser or electron beam. These have to be taken into consideration if warping and possible early-life failure are to be avoided. So, in metal AM, the use of DfAM can involve designing out thick sections where heat build-up can be greatest. This is seldom a significant issue in FDM 3D printing.
Two of the main DfAM considerations in FDM 3D printing are layer orientation and overhangs which we will take a closer look at here.
When a detail design is being prepared for manufacture one of the first things to consider is the loads that will be applied to it, and 3D printing is no different. There can be potential weaknesses in 3D prints in the “welded” joints that exist between every layer which provide multiple potential crack propagation opportunities.
So at the detail design stage the loading direction may need to be taken into account, which can in turn lead to a decision being made on the print direction to be used very early on and that will then set the tone for the rest of the design.
In this particular case the stress analysis in Fusion 360 on a loaded side wall of a design shows that the peak stress occurs on the inside face of that wall near to it’s base which, if we were to print it in this orientation, will coincide with the end of a layer and hence one of these potential crack propagation sites :
Which can lead to this :
The better way of 3D printing this design to withstand this loading condition would be to orientate the printing direction by 90 degrees to ensure that the load is being applied along the layer lines rather than across them.
The strength of a part with this layer orientation will be many times greater under the loading condition described previously, though the amount is difficult to objectively state since simulation software taking into account the layer construction of AM is still an emerging area of activity.
So this is a DfAM consideration to think about at the very start of your design - what are the main load bearing directions and is it possible to optimise the design to ensure that the way that you will make the part which does not result in loads being applied across a layer?
This is the single most effective step that you can take, but it may not always be possible to do that, in which case you need to employ mitigation factors into your design.
The usual best practice in any design is, where possible, to add a fillet (or radius) at the base of the wall to counteract these high stresses. This reduces the local stress moves the higher stress point further up the side wall and is an optimal way of adding strength with the addition of minimal material.
However, in AM/3DP it is often a better option to use an angled face rather than a curved face to achieve the same effect
The reason for this different approach is that the "staircase" of layers in more uniform in the case of the angled face, whilst with a fillet radius the smooth blend into the base results in a longer first layer step which reduces it's effectiveness.
So this is another aspect of DfAM where strategies used for other methods of manufacture may need to be subtly modified to make them most effective when using this particular method
Once the print direction has been selected then the design of overhangs, and preferably the elimination of as many of these as possible, can be addressed. Fewer overhangs means less requirement for support which leads to a more efficient print time, lower material usage and reduced post processing time for removal of supports
This is the most obvious way to eliminate an overhanging feature :
Things like this are simplistic and often easy to spot, but you may find that your design is more complex than this and there is a tendency to design from experience with traditional manufacturing methods and put in features that aren’t good for AM almost without thinking.
For instance in this example of a flanged coupling the features with blind tapped holes for the connection have been designed with a feature that would cause no problem for a moulding process but produce an overhanging area for 3D printing (highlighted in red when viewed in Cura)
With some re-examination it was possible to re-imagine these features like this which result in no overhang and hence no support.
Rather than fill this post with lots of examples of individual examples of this kind of comparison my recommendation when engaging with DfAM is to regularly check your design in the slicing software as your design develops, looking for those overhanging areas using an inspection tool that highlights those areas, or looking through the layer stack for areas that look difficult to print. The layer stack should be something that’s looked at before every print as a matter of course and is also a great way of spotting issues at the design stage that you may be easily able to address.
In Fusion 360 the ability to go from the design workspace to the slicer software (such as Cura) to check for printability can be done with a single click of a button, and without the need for any time-consuming exporting and subsequent importing of .stl files. This can make the iterative process of Design → Check → Modify → Recheck much quicker, and result in a faster convergence to an efficient design for additive manufacture
The approaches we have looked at here are when DfAM is applied at the detail design stage and looks to address, and deal with, the drawbacks of 3DP/AM.
In a future post we will look at applying DfAM at the conceptual design stage where the advantages that AM has to offer can really become very valuable.
This approach can be much more powerful and result in designs that really do provide unique and extremely effective solutions that would have been unthinkable just a few years ago.