Optimizing Design for Additive Manufacturing
Avoiding the pitfalls of 3D printing requires knowing the process limitations — and how to work around them. An expert at a leading AM specialist shares insights on getting it right.
As additive manufacturing (AM) technology and its applications expand, engineers are recognizing that different industrial 3D printing processes have different constraints that can affect designed parts in production. Some constraints are universal across the different processes, and some are more specific to the type of process used. It is thus essential to understand the technology you are working with to maximize its potential as a production method. With this understanding it is possible to design around the general limitations of AM as well as the specific process constraints that could impact a product or part.
While design for manufacture (DfM) is not a new concept, the rules for designing for additive manufacture (DfAM) require design engineers to take a different approach. This article is dedicated to sharing some of the most common pitfalls encountered when designing parts for the selective laser sintering (SLS) and multi-jet fusion (MJF) 3D printing processes and how to avoid them.
Wall thickness is a critical consideration for parts being designed for AM both in terms of the part itself and any post-processing that may be required. We recommend a maximum wall thickness to prevent shrinkage deformation during cooling, while our minimum wall thickness is recommended to ensure parts withstand our automated post-processing techniques without damage.
- 1 mm (0.039 in.) is our guaranteed minimum wall thickness for unfinished parts, but it is important to note exceptions where thickness should be increased. Exceptions include vulnerable unsupported structures, skeletal structures, weight-bearing walls and specific functional/ performance requirements.
- 1.5 mm (0.059 in.) is our guaranteed minimum thickness for parts that will be post-processed using our polishing or shot peening options. Again, there are exceptions for vulnerable unsupported features, weight-bearing walls and performance requirements.
- Our recommended maximum wall thickness is 5 mm (0.196 in.). Anything beyond 10 mm (0.393 in.) may require the wall of the part to be hollowed out to prevent shrinkage deformation, build failures and excessive print times.
In most cases, 3 mm (0.118 in.) wall thickness provides a rigid part with a little or no flex, while 1.5-mm walls result in some flex depending on length and/or structural support.
The nature of the powder bed fusion (PBF) processes that 3DPrintUK uses means that parts come off the printer with a granular surface, and sometimes layer lines can be visible. This is why post-processing options such as polishing or shot peening are often required, to achieve a smoother injection moulding-like finish. With this considered, if specific surface details are designed into a product or part, certain design rules are pertinent here.
Surface details can vary depending on the surface to which they have been applied. The top side may include a raised burr of the laser around the outer edge, whilst the underside can appear more muted. Thus, for most parts the best outcome is achieved by applying the text on the side skins for the best and most consistent visibility. Other recommendations are as follows:
- Minimum 0.5 mm (0.019 in.) wide and deep. The same depth and width measurement results in superior clarity.
- Embossed text is safer than extruded text, as small details and edges can be vulnerable to break.
- Avoid embossing or extruding the surface details too far. Try to keep them to around 0.5 mm; further out can result in damage in post-processing and too deep can result in trapped powder.
Solid vs. hollow parts
Hollowing can prevent the part from deforming and achieve higher levels of accuracy and reliability. However, based on 3DPrintUK’s experience working with PBF processes, the combination of powdered material and hollow parts can result in trapped powder within the sintered shell. When designing a hollowed part, there are a few design options that avoid trapped powder. These include designing in powder escape holes, removing unwanted surfaces altogether (i.e., bases), or including a locating lid.
For customers, we will automatically hollow larger parts before printing when above 15-20 mm (0.590-0.787 in.) on a case-by-case basis. For customers who do want it printed solid we advise accordingly.
Interlocking / mechanical parts
When designing interlocking or mechanical parts, including a clearance between parts is essential. This is because a gap between the sintered surfaces prevents them from fusing together and becoming a merged part. The tighter the tolerance, the more likely it is to fuse together. Some general design rules for this are:
- In nearly all cases, a clearance between moving parts must be at least 0.5 mm to guarantee a result.
- Contacting surfaces must be kept to 5 mm or below to guarantee them not fusing. Longer shafts are likely to be too difficult or impossible to free up.
- Think about how the trapped powder between the surfaces can be removed. Sometimes a little force is enough to remove the powder. Designing powder removal holes may also be needed.
- Dense volumes can refract more heat and harden the powder between the surfaces. This means that the clearance may have to be increased if this is flagged as a concern or if the part does not function as intended.
Holes and channels
One of the key advantages of 3D printing is the ability to design and produce complex geometries without the need for expensive or “impossible” tooling. However, designers still must consider the complexities they design into parts, especially when it comes to holes and channels running through them. The nature of the PBF process comes into play here, specifically the amount of heat that parts are exposed to during the build process. Thus, holes and channels with small diameters can result in fused powder within them.
To prevent this, the recommendation is to design them greater than 3 mm. For long internal channels over 50 mm (1.97 in.) the same problem applies. It can be difficult to remove all of the powder, therefore diameters greater than 5 mm are recommended for internal channel features. The same rule applies to curved holes.
Maximum build size
This might seem like an obvious, but all 3D printers — whether desktop, mid-range or full production systems — have a maximum build size. Remarkably this is often overlooked. Across 3DPrintUK’s fleet of industrial-scale 3D printers our maximum build sizes are: 300 x 300 x 600 mm (for SLS PA12); 350 x 255 x 350 mm (for MJF PA12), and 180 mm x 120 mm x 120 mm (for SLS flexible TPU).
3DPrintUK will always position parts in a build to get the best possible outcome — no matter the original orientation of the file. The only exception to this is if a client locks the orientation while placing the order.
Designing for 3D printing is a vital facet of successful outcomes with the AM technology. Hopefully this overview will provide a useful primer for anyone coming to the technology for the first time or some fresh insight to anyone working with the technologies on new products or parts.
Nick Allen is managing director of 3DPrintUK , a U.K.-based specialist in low-volume production using powder bed fusion (PBF) 3D printing systems with polymer materials. The company bridges the void that exists between prototyping and injection molding.
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