FDM Design Considerations
Bosses and Ribs
FDM parts can often be solid rather than a hollowed out design supported by bosses and ribs. This can reduce build time and use less support material. It is not necessary to reduce wall thickness of a boss, rib or gusset in FDM parts. This will increase the amount of stress the feature can withstand.
Draft Angle
Since FDM is an additive process, draft is not required.
Fastening Hardware
Recommend utilizing a cap screw or a flanged cap screw. The flat surface eliminates multi-directional stresses from cracking the part. Washers can also be used to spread the load over the largest possible surface area. Lock nuts, embedded nuts, or metal inserts are all stronger fastening options than adding threads to the FDM plastic.
Fillets
Although fillets are not necessary in FDM parts, they can be used to reduce stress concentrations and increase the overall strength of the part. Design fillets with an outer radius equal to the inner radius plus the wall thickness to maintain consistent thickness.
Holes
Holes (those in bosses as well) on an FDM part are generally undersized. When tight tolerances are required, holes should be drilled or reamed to ensure the diameter is accurate.
Orientation vs. Strength
Designers should note that extruded plastic has its strongest strength in the tensile mode along the x-y plane. Since the layers are held together by “hot flow’ across the strands (one stand is cooling while the other is laid upont it), the lowest strength is in the Z-axis for both tensile and shear modes.
Shrinkage
The FDM process adds shrink rates to the part when processed, so shrink factors do not have to be designed into the part.
Text
Minimum suggested text size on the top or bottom build plane is 16 point boldface. Minimum suggested size on vertical walls is 10 point boldface.
Threads
When designing built-in threads, avoid sharp edges and include a radius on the root. Sharp edges can have the effect of concentrating stress in plastic parts. Creating an ACME thread design with rounded roots and crests has been found to work well when using FDM. Also, use a “dog point” head of at least 1/32 in. (0.08 mm). This dog point design makes starting the thread much easier. Small threads produced by FDM are not recommended and not possible for hole or posts smaller than a 1/16 in. (1.6 mm) diameter. An easy alternative is to use a tap or die to thread holes or posts.
Undercuts
Since FDM is an additive process, undercut for design features such as O-ring grooves are not an issue.
Wall Thickness
Minimum wall thickness for FDM parts varies depending upon the slice thickness that will be used to build the part.
Single Contour Width:
0.010″ (0.25 mm) slice thickness = 0.020″ (0.50 mm) minimum wall
0.013″ (0.33 mm) slice thickness = 0.026″ (0.66 mm) minimum wall
Note: building multiple layers while using the minimum contour width will cause the feature to be brittle. Warping may occur if there are large extents of minimum-thickness, vertical walls without support features like ribs or a support material tower.
Stratasys encourages the use of the recommended minmum wall thickness (below), which will eliminate brittleness.
General Recommendation:
0.010″ (0.25 mm) slice thickness = 0.040″ (1.02 mm) minimum wall
0.013″ (0.33 mm) slice thickness = 0.052″ (1.32 mm) minimum wall
Warp
Since Stratasys FDM systems add small amounts of molten material in a heated environment, warp is not a common problem. However, to avoid potential warping (deformation of vertical walls) when building thin-walled sections of a model, designers might chose to add ribs to the walls (similar to what would be done with standard injection molding processes).
Secondary Operations
Since the FDM processes uses engineering-grade thermoplastics, the parts produced are capable of withstanding a number of post-manufacturing processes, including machining operations such as drilling and tapping, sawing, turning, and milling. Other post processing operations may include smoothing, burnishing, sealing, joining, bonding, and plating.