Digital Alloys’ Guide to Metal Additive Manufacturing - Part 4
Design Rules for Metal Additive Manufacturing
December 26th, 2018
“Complexity is free” is one of the biggest myths in 3D Printing. Metal Additive Manufacturing (AM) can produce complex geometries not possible with conventional manufacturing, but the design of a 3D printed part must account for the constraints of the whole manufacturing workflow
If design rules are ignored, the print can fail, the printer can be damaged, and lots of material, money and time will be wasted. Most importantly, design for manufacturability (DFM) in metal AM is important because of its implications on: final part performance
This post will describe some critical metal AM design rules and the reasons they exist. It will also cover software tools being leveraged to improve the metal AM design process.
The physics of a 3D printing process drive the design rules. Each of the many metal AM processes in the market today (see chart in prior post
- Mechanics - How the feedstock, part, and energy are moved and controlled
- Thermodynamics - Heat flow and thermal history
- Metallurgy - The relationship between the printing process and the chemistry and crystal structure of the metal it produces.
A notorious example of how process physics translates to design constraints is the residual thermal stresses and anisotropic grain structures that can be created by temperature gradients in a cyclical, layer-by-layer melting process. Thermal stresses, if not accounted for, can result in warping, cracking or delamination of a part. Many of the design rules attempt to limit the effects of thermal stresses by constraining aspect ratios (the ratio between the length, width and height of a part or feature), wall thicknesses, and radii on sharp corners, and by optimizing part orientation within the build.
The printing process cannot be the only element of the workflow that is examined. Post-processing can involve many different steps (see Process Steps in Metal AM), each with its own set of requirements and constraints that must be considered in the design. For example, if CNC machining is a post processing step, the design must include locating and fixturing features on the part or build substrate.
General Metal AM Design Guidelines
While every metal AM process is different, there are some general design considerations that serve as a great starting point in a new design:
Metal AM parts are produced in a wide range of sizes from tiny intricate gears to airplane wing spars. If production is constrained to a certain printer then the build envelope and orientation of the part in the build are the ultimate limits on maximum part size. However, size constraints due to the physics of the printing process can be more restrictive than the printer’s build envelope. In the case of binder processes, the low green strength in the “as-printed” state and then the extreme shrinkage in the sintering step typically means large parts are off the table. Also, part size should be examined in context of the economics of each process as this can become a bounding constraint too.
Minimum / Maximum Feature Size
Minimum feature size is largely dictated by the resolution of the printing process. Process resolution is dependent on the process mechanics (e.g. laser diameter, binder deposition control, melt pool diameter) and the feedstock material (e.g. powder particle size or wire diameter). In the case of very thin structures like lattices, the overriding constraint can be the strength of the feature and its ability to survive post-processing.
Maximum feature size can be limited by residual stresses that build up in the part affecting that part’s accuracy and mechanical properties. In binder processes, there are highly restrictive constraints on max feature size due to the de-binding and sintering post-processing steps.
Tolerance is driven by the resolution of the printing process as well as other factors such as the material, print parameters, and geometry of the part. Part orientation can also play a large role. Warping, created by residual stresses, can be a major contributor of inaccuracy too. In processes that have a final sintering step, part shrinkage can be hard to control and predict. All of these variables must be assessed against the part’s accuracy requirements. If the required tolerances are not attainable from the printing process alone, secondary operations can often be utilized to bring the part back within tolerances.
Overhangs & Supports
For most metal 3D printing processes, there is a limit to how much a printed section can lean past vertical (the “overhang angle”). For unsupported overhangs greater than design rules allow, supports must be designed and printed. For most processes, supports are fully fused structures that have to be physically removed after printing. In this case, the part and supports must be designed and oriented so that supports can be accessed for removal.
For powder bed processes, design is constrained by the need to remove powder from internal voids, aka “trapped material”. As an example, a hollow sphere would require holes for draining powder after printing. In addition, the shape of an internal feature must allow for powder to be extracted. This can be an important consideration for applications like conformal cooling where there are long, circuitous channels within the part.
Orientation & Nesting
Design rules are influenced by the orientation in which a part is printed. Orientation defines the actual geometry of each layer in the build and can affect the process physics. Different print orientations are often attempted before choosing the best one as it is hard to always predict print results.
Nesting many parts in a build can greatly reduce individual part costs so it is usually an early design consideration in production metal AM. Nesting and optimal orientation can be a trade-off in some cases.
Design Optimization with Metal AM
Creating highly optimized part designs has been an impetus for the use of metal AM. Engineers use advanced, algorithmic design tools such as Topology Optimization (TO) and Generative Design (GD) to develop these geometries. These tools can optimize material placement within a given volume to meet load and boundary conditions. TO and GD are also employed to reduce material use, particularly important in powder-based printers as the relative cost of metal powder is high. Advanced lattice design software is also becoming popular. Industries like Aerospace and Automotive that demand high strength, light weight structures, find great value in these tools.
It is worth mentioning that design optimization tools are not yet advanced enough to understand and utilize all the design rules we discussed in this post. The skill and experience of the 3D printed part designers remains very important and is still somewhat limited in the industry.
Software tools exist to both optimize design (process agnostic) and perform analysis of a design to indicate whether it is a good fit for a process. Features of these tools can include orientation optimization to minimize supports, overhang angle detection, and simulation of process dynamics to predict warping and other build failure modes. Most of these tools are quite new and limited in functionality and process compatibility, but leading CAD/CAE software providers are investing heavily in this area.
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