Material Extrusion: Design Considerations

Designing for Material Extrusion

• Material extrusion, also known as fused deposition modeling (FDM), is an additive manufacturing (AM) process where material is selectively dispensed through a nozzle, similar to a hot-glue gun.
• Thermoplastic polymers are the typical choice of materials.
• Support structures are needed for overhanging features.
• Support material can either be the same material as the print, a different material that is easier to remove mechanically, or a soluble material.

Material Extrusion Accuracy and Tolerances

• Accuracy and tolerance varies significantly between material extrusion systems, depending on geometric features and print orientation.
• Accuracy is the closeness of the part to the CAD model data.
• Tolerance refers to the acceptable degree of variation.
• For industrial quality material extrusion systems:
  • Layer thickness ranges from 0.1-0.3 mm (0.005–0.013 in).
  • Accuracy is ±0.1 or ±0.03 mm per 25 mm (±0.005 in. or ±0.0015 in. per inch), whichever is greater.
  • Tolerance is typically 0.25 mm (0.01 in).
  • Smallest feature size is around 1 mm (0.04 in).

Layer Thickness

• The choice of layer thickness is the first decision in printing material extrusion parts.
• Thinner layers result in better surface quality, specifically on rounded parts, and reduce the stair-step effect.
• A 0.1 mm layer takes three times longer to print than a 0.3 mm layer
• A thicker layer thickness is preferable for parts with flat geometric features in the vertical direction because they print faster without significantly worsening surface finishes.
• Thin layer thickness is preferable parts consisting of many curved surfaces in order to achieve curved surfaces that are as smooth as possible.

Support Material

• Choosing the type of support material is a key decision when printing material extrusion parts.
• Almost every material extrusion printer offers different support material options.
• Some systems also offer smarter choices of support structures.

Fill Style

• Material extrusion allows users to choose between printing a part as a solid or a sparse part.
• Sparse parts involve filling the interior void with a scaffold structure.
• Outer shell wall thickness can be specified in some systems
• Users can select the infill percentage to determine the scaffold structure's density.
  • A 0% infill results in a void, effectively letting the 3D printer software automatically shell the part
  • 100% infill means completely solid material
• In terms of mechanical strength, infill percentages above 50% can have diminishing returns.

Other Considerations

• Polymer material extrusion often leads to anisotropic material properties, with the part being weaker in the Z direction compared to the X and Y directions.
• Designers should account for this by designing based on lower material properties or ensuring highly stressed features are built horizontally rather than vertically.
• The "stair-stepping" effect on part surfaces can be reduced using post-processing techniques like acetone vapor chambers for parts printed in ABS, impacting part accuracy and material properties.
• Material extrusion AM may leave small air gaps between laid filament at certain wall thicknesses, depending on the software's decision to deposit an extra material strand.
• Holes in material extrusion parts are generally undersized, thus drilling/reaming the holes to the exact diameter or compensating in CAD by 0.2 mm (0.008 in.) diameter may be required.
• Self-tapping screws can strip away the contour material inside screw bosses due to the weak bond between contour lines and infill lines; this can be resolved by applying a drop of super glue between the contour and fill material.

Feature Type: Vertical Wall Thickness

• Warping may occur with unsupported walls. It is best to avoid using the minimum wall thickness in those cases.
• Wall thicknesses may leave small gaps between inner and outer walls. Always test your printer to figure out which particular wall thicknesses do so.
• Sharp transitions are not recommended, fillets are preferable.
• Even wall thickness is recommended on all walls.

Feature Type: Horizontal Walls

• Horizontal walls can theoretically be as thin as a single layer of material.
• To produce a horizontal wall with strength and consistency at least 4 layers of material are recommended.

Feature Type: Support Material Overhang Angles

• Overhang angles less than 45° require support material, which is normally added automatically by the system software.
• Excessive supports that are manually removed increase post-processing time.
• Soluble support structures require less manual labor, but still waste material.
• Horizontal holes can be modified into teardrop or ovals shapes to minimize the need for hard to remove internal supports.

Feature Type: Clearances Between Moving Parts with Soluble Supports

• Large areas of close proximity will slow down the removal of support material.
• Clearance between parts built separately and assembled later must be at least equal to the general build tolerance of the system.

Feature Type: Clearances Between Moving Parts with Break-Away Support Material

• The main challenge with printing moving parts on a printer without soluble support is in the difficulty of removing the support material from between the moving parts.

Feature Type: Vertical Circular Holes

• Holes are generally undersized, typically by around 0.2 mm across the diameter.
• Use a drop of super-glue between the contour and fill material to mitigate against stripping the column of contour material that surrounds the screw if using self-tapping screws.

Feature Type: Circular Pins

• Pins which are small in diameter and vertical, are prone to breaking-off if only supported at one end.
• Always fillet the pin where it joins the wall, even 0.5 mm is enough to strengthen the pin substantially.

Feature Type: Built-in Screw Threads

• Use rounds on root and crest of thread.
• Tapping is recommended for small threads.
• Fillet the screw boss at the point where it meets the wall to avoid stress concentrations.

Designing for Polymer Powder Bed Fusion

• Polymer powder bed fusion uses thermal energy to selectively fuse regions of a powder bed and does not normally require supports because the unfused powder provides sufficient support.
• Polyamide (nylon) is the most common material and parts created usually have some degree of anisotropy in their material properties, particularly for small features that are less than about 25 mm² in surface area in the vertical direction.
• There is pronounced granular roughness on the surface of parts that can be reduced by various post-processing techniques.

Powder Bed Fusion Accuracy and Tolerances

• Accuracy and tolerance vary between different manufacturers systems and also depend on geometric features and print orientation
• Layer thickness: 0.1 mm (0.005 in.)
• Accuracy: ±0.3% lower limit of ±0.3 mm (0.010 in.)
• Tolerance: ±0.25 mm (0.010 in.) or ±0.0015 mm/mm (0.0015 in./in.)—whichever is greater
• Smallest feature size: Around 0.5 mm (0.04 in.)

Layer Thickness

• The typical layer thickness for powder bed fusion is 0.1 mm
• The stair-stepping effect is less visible compared to other AM technologies.

Avoiding Large Masses of Material

• Uneven thicknesses of plastic and large masses of material can cause distortion and add substantially to the time spent making the part.

Powder Age and Refresh Rate

• Polymer powder bed fusion systems typically use a mix of virgin powder and used powder at a ratio of 20/80 to 35/65
• As powder ages printed parts can develop an 'orange peel' effect.

Feature Type: Wall Thickness

• It is occasionally possible to print walls thinner than 0.6 mm, but success is highly dependent on the rest of the part geometry and print orientation.
• Thin walls with a large surface area are likely to warp during the cooling process. In those cases consider adding ribs to stiffen the walls.
• Thicker walls and any large volumes of material will result in excess heat retention in the part and hence shrinkage, resulting in geometric deformation. A maximum wall thickness of 1.5–3 mm is also recommended.
• An even wall thickness is recommended on all walls.

Feature Type: Clearance Between Moving Parts

• The required gap between moving parts is highly dependent on the surface area of the faces that are in close proximity.
• If faces in close proximity have surface areas of only a few mm², then gaps as small as 0.2 mm between the faces are possible although 0.5mm is more common.
• Large areas of close proximity will slow down the removal of excess powder.
• Clearance between parts built separately and assembled later should be at least equal to the general build tolerance of the system.

Feature Type: Circular Profile Through Holes

• Small holes, both round and square, typically below 1.5 mm are related closely to wall thickness.
• As wall thickness increases, powder becomes increasingly difficult to clear from small holes and smaller through holes become less feasible.

Feature Type: Square Profile Through Holes

• Small holes, both round and square, typically below 1.5 mm are related closely to wall thickness.
• As wall thickness increases, powder becomes increasingly difficult to clear from small holes and smaller through holes become less feasible.

Feature Type: Circular Pins

• Pins with small diameters are prone to breaking off if only supported at one end.
• Always fillet the edge where a pin joins a face.

Feature Type: Hole Proximity to Wall Edge

• Larger holes require slightly greater distances to the edges of walls.

Designing for Vat Photopolymerisation

• Vat photopolymerisation uses a UV laser to cure a specific layer of a component from a tank of photosensitive resin.
• Some SLA systems fabricate the part top-down. Others work bottom-up where the laser source is below the vat, solidifies the part through a window at the bottom of the vat, and pulls the part out of the resin vat.

Resolution

• SLA resolution in the XY-direction is dependent on the laser spot size and can range anywhere from 50 to 200 µm.
• Resolution in the Z-direction varies from 25 to 200 µm.

Print Orientation

• The biggest concern is vertical cross-sectional area.
• The forces involved with a print sticking to the bottom of the tank are directly proportional to the 2D cross-sectional area of the print.
• Minimizing the cross-sectional area along the Z-axis is the best way to orient parts for SLA prints.
• Reducing the number of horizontal areas relative to the print orientation, hollowing out components and reducing the cross-sectional area are all steps that can be taken to optimize a design for SLA.

Support Material

• SLA does require support material for overhanging features because the uncured resin is not viscous enough to support features on its own.
• On most SLA systems the process of adding support material to the part is largely automated.

Overhangs

• Overhangs generally pose very little issue with SLA, unless the model is being printed without adequate support structures.
• If printing without supports is necessary, any unsupported overhangs must be kept less than 1.0 mm in length and at least 20° from horizontal.

Isotropy

• SLA is one of the few processes where the parts are relatively isotropic because the layers chemically bond to one another as they print, resulting in near identical physical properties in the X, Y, and Z-direction.

Hollowing Parts and Resin Removal

• Shelling the model to be hollow can significantly reduce the amount of material needed as well as reduce the print time.
• It is recommended that the walls of the hollowed part be at least 2 mm thick to reduce the risk of failure during printing.
• If printing a hollow part, add drainage holes to allow the uncured resin to be removed from the part.
• Drain holes should be at least 3.5 mm in diameter, and at least one hole must be included per hollow section.

Details

• Embossed details need to be at least 0.1 mm in height above the surface of the print to ensure the details will be visible.
• Engraved details must be at least 0.4 mm wide and at least 0.4 mm deep.

Horizontal Bridges

• Bridges between two points on a model can be successfully printed, but one must keep in mind that wider bridges must be kept shorter than thin bridges.

Connections

• If parts are being made that need to connect together, it is always best have a certain tolerance between the parts that fit together:
  • 0.2 mm clearance for assembly connections.
  • 0.1 mm clearance will give a good push or snug fit.
• If interlocked moving parts are being printed, then the tolerance should be 0.5 mm between the moving parts.

Feature Type: Wall Thickness

• Supported walls should be designed at a minimum of 0.4 mm thick.
• Unsupported walls must be at least 0.6 mm thick.
• Always fillet the corners where one wall meets another wall to reduce stress concentrations along the joint.
• In general, an even wall thickness is recommended on all walls.

Feature Type: Circular Holes

• Holes with a diameter less than 0.5 mm may close off during printing.