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FDM Best Practices
This best practice concentrates on factors that impact the quality, value, and performance of FDM® production parts. There are a number of design
considerations, material choices and performance objectives that should be taken into account when designing for additive manufacturing. Tailoring these decisions to the FDM manufacturing process allows the benefits of FDM to be maximized.
Build orientation should be considered as a precursor to any detailed design. Failure to consider build orientation early on will result in the need to compromise part quality and requirements at later stages of design. The current state of additive manufacturing technology results in parts that are highly anisotropic, making build orientation a critical design factor. Build orientation dictates the directional behavior of design requirements, including:
There are occasions when the best-case build orientation for a given design requirement often conflicts with the optimal build orientation of other part requirements. As such, a part should have build orientation selected as a precursor to any detailed design, as well as incorporating other multiple design requirements. Failure to select a build orientation early in the design phase results in a compromise that meets only the minimum design requirement, or worse, may eliminate or reduce certain requirements to accommodate the build process.
The ideal time for choosing build orientation is after the concept design is completed but before detailed design requirements are implemented. Stratasys offers several material options with a range of mechanical properties. The primary consideration is to optimize for a weighted set of requirements including cost, buildability, accuracy, aesthetics, mechanical properties and surface finish. From there, decisions can be made during the detailed design to mitigate the effects of anisotropy on secondary or lower-priority requirements.
If strength is the highest priority and the material will be highly stressed, build orientation can be selected based on mechanical properties. In this case, negative impacts to the other design requirements should be mitigated in the detailed design. Ultimately, an experienced designer will be able to easily visualize an optimal build orientation for a preliminary design that satisfies a weighted set of design requirements, and make design decisions to meet the less-critical design requirements considering the locked orientation. Iteration from the pre-selected build orientation is often necessary as a design evolves.
Generally speaking, mechanical properties are best in the X-Y build plane and weaker across layers. Multiple loading conditions may require an orientation that provides the best compromise for each case.
Aesthetics and surface finish are best when features are aligned vertically. Minimizing the angle between the vertical axis and a wall ensures that layer stacking is as concentric as possible.
Build time is reduced by minimizing Z build height and minimizing the numbers of layers requiring both model and support material.
A feature will have the best accuracy if its OML (outside mold line) is traced out on the X-Y build plane.
FDM has the unique ability to produce certain geometric features without the use of support material.
Additive manufacturing typically uses a special support material to hold overhanging geometry in place as the model is created. While some additive manufacturing processes completely surround a part in support material, FDM has the unique ability to produce certain geometric features without the use of support material. This enables a fully enclosed lattice structure, fill patterns, or hollow parts. Typically, an overhang does not require support material if its walls or features exist at a 45-degree angle or less from vertical. This is known as a self-supporting angle, which varies slightly depending on the material and toolpath parameters used. An overhang that spans a gap of a half inch or less does not
require support material. If an overhang spanning a gap does not meet requirements for a self-supporting angle, support material will be generated and the user must manually delete.
There are a number of reasons a designer should eliminate support material within a design. Support material is one of the highest contributing factors in a part’s cost, the predominant issue being part production time:
Besides process time and cost, a designer should consider aesthetics and finish in the elimination of support material by:
Generally, self-supporting angles should be maintained whenever it does not conflict with other design requirements such as weight, aesthetics, or functionality.
The FDM process involves the deposition of a thermoplastic at elevated temperatures, so a part’s tolerance will depend on its behavior while shrinking. When preprocessing part geometry, shrink factors are automatically applied. Thermal mass and part geometry, among other factors, dictate how appropriate the default shrink factors are for matching actual part behavior. Since standard shrink factors are applied to an infinite possibility of geometries, the final tolerance of a part will vary from part to part. A high level of confidence exists for any given geometry that a part tolerance will meet or exceed. The specifications are listed below:
± 0.005 in. or ± 0.0015 in. (± 0.127 mm or ± 0.0015 mm), whichever is greater.
Fortus 900mc™/Stratasys F900™:
± 0.0035 in. or ± 0.0015 in. (± 0.089 mm or ± 0.0015 mm), whichever is greater.
Holes built using FDM, smaller than 1 in. (25 mm), are typically slightly undersized. When tighter tolerances are required, these holes can be drilled and reamed to ensure accuracy. The minimum hole size for an FDM part depends on the material used since different materials expand and shrink at different rates. All materials are capable of producing holes down to 0.0625 inch (1.6 mm).
To reduce material usage and decrease build time, support structures can be manually removed after support generation from horizontal holes that are less than 0.200 inches (5.0 mm) internal diameter. Re-designing holes as self-supporting diamonds or tear drops, will eliminate the need for support material regardless of the hole size, if the design requirements allow.
Minimum feature size is a function of slice thickness, toolpath width, and orientation. As a rule of thumb, the minimum feature size is 0.016 inch (0.4 mm). This is available on 0.005 inch (0.127 mm) slice thickness configurations. For features that approach or exceed the minimum suggested thickness, it is advisable to preprocess the part in the correct orientation to validate that the geometry can be filled sufficiently. Note that custom toolpath parameters are often necessary to accommodate small features. The custom toolpath parameters can either be applied globally to the part or locally to the small feature.
Minimum suggested text size on the top or bottom build plane of an FDM model is a 16 point, boldface. Minimum suggested text size on vertical walls is 10 point,
boldface. If vertical text is embossed inward, the supports for the text can usually be eliminated, because of the ability of FDM to bridge small gaps. In most cases,
outward-protruding text can also be produced without support material, if the protrusion is less than .020 inch (0.51 mm).
FDM is a layered manufacturing process with anisotropic resolution characteristics. Minimum wall thickness must be calculated relative to the build directions, rather than using a thickness value normal to the part surface. Minimum wall thickness in the Z direction is equal to the layer thickness. Note that when approaching the minimum thickness, the as-built thickness will be rounded down to the nearest multiple of slice thickness. For example, a 0.007 inch (0.178 mm) horizontal wall produced with a 0.005 inch (0.127 mm) slice thickness will likely be rounded down to 0.005 inch (0.127 mm), with the remaining 0.002 inch (0.051 mm) being filtered off. Thickness in the X-Y planes are much less fixed, as they depend on useradjustable toolpath widths. Thicknesses in the X-Y build plane are constrained to a minimum of 2x the desired toolpath width, without using advanced “Single Bead” FDM design techniques (see Lightweight Structures Design Guide). FDM is constrained in producing toolpaths of
In general, support generation is fully automated and the user does not need to provide special attention to its creation. However, some cases require manipulation of the support structures in order to ensure build reliability. Additionally, a user may wish to modify support material structures as a means to reduce build cost, build time, or part quality. Many of these tactics are detailed throughout this document, while the remainder are provided here.
In order to reduce build time, a designer should minimize the number of layers that require both model and support material. If a part contains self-supporting angles throughout the majority of its height, however, a non-self-supporting feature at the top of the part will require support material from the base of the part up to the feature.
Surround support offers greater part stability, however, at the expense of build
time and reduced part surface quality. In general, it is not recommended to use surround support unless a part is otherwise unreliable in its build.
FDM offers a designer a path to design freedom, ultimately producing parts with better performance levels and functionality compared to traditional manufacturing processes
Beyond the obvious benefits of using FDM to reduce costs and accelerate the design process and part delivery, the real opportunity with FDM rests in the mind of the designer. It removes the restraints of conventional manufacturing and has produced parts that have:
With additive manufacturing’s design freedom, designers can produce semi-dense
parts, enclosed lattice structures, or fill patterns.
Producing lightweight structures with FDM parts requires multiple design decisions not covered in this document. See the FDM Lightweight Structures Design Guide for direction on designing parts with a high stiffness-to-weight or strength-to-weight ratio.
Topology optimization is a mathematical tool that uses a physics-based algorithm to design distribution of material within a user-defined boundary, based on a set
of requirements. The algorithm can optimize for aerodynamic efficiency, electrical performance or heat transfer, among others. Structural topology optimization has the best defined workflow, with several simulation companies offering a streamlined software that automates the modeling process. Typically, the results of a topology optimization will yield a geometry that is highly complex and organic in shape.
Traditional manufacturing processes are constrained in these cases, requiring interpretation and adoption of results to suit the limits of the manufacturing process. The design freedom allowed by additive manufacturing provides a path to directly utilize the results of the optimization. In some cases, the results can be directly sent to a printer after some cleanup of the output mesh.
FDM provides a method to consolidate an assembly of multiple components, to create a single component. For example, because it does not impact the cost greatly, an aircraft environmental control system duct can be printed with integrated attachment brackets. However, it is advised to combine components of an assembly only when doing so does not violate any engineering constraints, such as material performance, part cost or build time. With any part consolidation, the build orientation must meet the requirements of the individual parts within the combined part.
Similar to the consolidation of parts, an FDM part should be designed to integrate mounting features and packaging of peripheral components. This integration allows for high levels of modularity, as well as the ability to reduce the footprint of assemblies.
Designing for FDM Production Parts – Key Takeaways:
Orienting for Strength, Speed or Surface Finish
Designing for Production Parts