Designing for additive manufacturing (DFAM) –
ensuring expanding design opportunities for components
Just because a component has always been constructed a certain way, doesn’t mean that that is the only way it can be done. As scientists continue to push the boundaries of space, so design engineers are using 3D technologies, especially metal additive manufacturing (AM), to help expand the frontiers of component design. By taking time to understand the design possibilities of the AM process – and subsequently using the technology to its full ability – the potential of this process has only just begun to be realised.
What is design for additive manufacturing?
Metal additive manufacturing isn’t a direct replacement for casting and machining, but it is an important additional technology that can enable the creation of components that can’t be produced using the other processes. Designing for AM comes into its own when a part is created or redesigned to embrace improved functionality and it helps to reduce the component’s cost at the same time.
As an example of the value that DFAM can bring to the production process, let’s look at a redesigned manifold. A traditional manifold is made from multiple parts, with cross holes created through a solid main body, plugged by brazed or welded dowels. Thanks to metal AM, a significantly lighter manifold can be designed that uses less material, contains internal channels within the part, has no cross holes and optimises channel paths, improving flow.
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Design for additive manufacturing guidelines
As can be seen with the manifold example, designing for AM can significantly decrease the weight of a component, whilst maintaining its strength. With AM, design engineers are freed from the constraints of having to think in terms of straight lines and radii and can instead, optimise flow in components using sweeping curves.
There are however some key guidelines to follow to ensure that the design can be manufactured using the AM process:
Holes can be created during the AM production process, however in general, circular holes that have no support can only be created up to ø6mm.
Large diameter holes
Large holes are highly likely to result in a very rough surface at the top, or they will potentially collapse and cause a build failure.
Supporting a hole
To create larger holes a support structure (shown in blue) is needed to prevent collapse during the build process. This will then need to be removed post-build.
Using AM, holes don’t have to be circular. A variety of shapes can be produced and providing that the top section is below ø6mm, the need for support structures can be avoided.
Tear drop hole
If the top section of the hole can be angled or arched, there is also no need for a support structure.
When holes or channels are required to run in multiple directions, such as in an intricate manifold design, a diamond shaped hole helps promote uniformity along its length.
Due to the layering technique used in laser powder bed fusion (L-PBF), certain downward facing surfaces will need support during the build. It is similar in method to creating a part using layered pieces of paper – once 45 ̊from the vertical is exceeded, the pages bend and begin to collapse.
Keeping downward surfaces to a minimum and thinking about build orientation will help to reduce this issue however and produce more cost-effective parts.
What is the angle?
Ideally, any angled surfaces need to be self- supporting, as the metal powder used in AM provides very little support to the melted/sintered surfaces during the build.
The downward face of angled surfaces becomes rough as the angle becomes more acute and this can potentially require considerable post-finishing.
When the angle is too acute the surface will need a support structure (shown in blue). This is built as part of the component and is removed post-build, increasing manufacturing cost and time.
The minimum angle of a self-supporting part depends on the geometry of the component and material used, but in general the minimum from horizontal is 45 ̊.
Any downward facing surfaces will need supporting during the build and need to be removed at a later date, adding costs and time to the project.
If the top surface of the area needs to be flat, the simplest way to support these areas (shown in blue) is to fill the void down to the build plate.
If the top surface of the area doesn’t need to be flat, then a series of arch supports can be created (shown in blue). These are quicker to produce and cheaper than a single block support.
If the area is simply for weight saving or cooling, it can be turned into a series of arches or semi-circular slots, removing the need for supports all together.
The size of the laser focus point means that AM does not suit tapered edges. If this type of feature is required, it is recommended that this be machined in post-build.
When designing for AM, ‘stress raisers’ such as sharp corners are not recommended. Instead, radius corners can provide smooth transitions between the surfaces, reducing stress within this area of the part.
Sudden cross section changes should be avoided – and as a minimum have radii smoothing the transition – as they can cause potential stress points and trigger post-processing cracks.
AM gives designers more freedom – the ‘stepping’ that can cause potential stress points can be replaced with a freeform curve or angled surface to smooth the transition.
L-PBF is ideal for the creation of thin walls (>1mm in thickness) and features (as small as 0.4mm). However thinner features can be produced, depending on orientation and geometry.
Similar to castings, large expanses of thin walls are susceptible to distortion. With AM, a simple rib structure can be added to dramatically improve the stability of this part during the build.
Thick sections can cause issues, due to heat build-up during printing. It is advised that walls are kept <10mm for most materials and <5mm for Titanium – depending on geometry, thicker walls can be printed.
Depending on the component’s use, material can be removed from the internal areas. The part can be shelled to a nominal wall thickness – watching out for downward facing surfaces – or material can be pocketed out.
AM offers the potential to print multiple assembled parts, dramatically cutting time and costs. However, the heat involved when building the parts needs to be considered when designing pre-assembled parts.
Clearance gaps need to be included, to ensure the free movement of parts. These can vary between materials and geometries – 0.2mm is a good starting point. There also needs to be a way to remove any trapped powder.
Many into one
Remove part assembly by redesigning a traditionally designed assembly into a single AM produced part. Thanks to AM, internal features can be produced, opening up the potential to cut the number of parts and improving functionality.
AM enables the creation of highly complex internal geometries in a single piece – although internal structures do need to follow the design rules for over-hanging features.
Un-melted powder can be trapped inside the void of internal features, causing problems. To avoid this, it is important that an air channel (ideally two or more) is incorporated within the part or support structure, allowing an air-line to be used.
Threaded features can be created but there are limitations to their size, pitch and orientation. They can be cleaned up afterwards using a tap or die. As most CAD programs do not produce a physical thread detail within the model, these usually have to be manually created.
Built in threads can have large tolerances. For a precision fitment, machining threads post-build is recommended, allowing for more freedom when it comes to designing a part with threaded features.
AM is well suited to creating lattice structures that have many intersecting pieces, creating strength in multiple directions. As well as reducing the weight of components, they can be used as an alternative to arch structures to fill internal voids and increase their strength.
The density of a lattice structure can also be altered to create strength in specific locations within the component – however the top surface still needs to be fully supported to promote overall structural integrity. Quite a few main stream CAD packages now include options to create lattice structures.
Although the aim is to remove as many support structures as possible at the design stage or through the build set up, they are used on 99% of AM builds. However, following the ‘design guidelines’ detailed here will help to minimise the supports needed and reduce the energy required to build and then remove them.
Regardless of how tricky supports can be to remove, they are vital in:
- Attaching parts to the build platform
- Reducing deformation
- Dissipating heat
- Providing temporary support
Available metal powders
Powder is at the heart of L-PBF. As the AM sector evolves new powders are being added, from harden and temper tool steels through to high strength aluminium. When a powder is chosen, parameter sets need to be developed and refined to ensure the highest repeatable quality of the printed components.
Frazer-Nash Manufacturing provides an expert end-to-end additive manufacture service for 3D printing custom metal parts. Find out more on our Additive Manufacturing process below.
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