Optimizing Part Design for Speed and Material Savings With High Speed Additive Manufacturing—Part 2

In Part 1 we presented several basic best practices to optimize part design for high speed extrusion to maximize 3d printer extrusion rate. We learned that by avoiding sharp corners and merging linkable bodies where possible, we can maximize print speed, and just as importantly, maintain a consistent material temperature and flow to print the best quality parts. But to truly capitalize on the possibilities additive manufacturing presents, engineers must update their design practices to match the latest advancements in high speed extrusion technology. Let’s explore a few more advanced design techniques you can employ to further reduce print times and realize greater material savings without compromising quality.

• Create functional “Z” overhangs. Overhangs are easily cut or milled out of solid blocks when using traditional manufacturing methods. Not so with 3D printing. When printing a part with a larger cross-section sitting over a smaller area, be it square, cylindrical, or odd-shaped, the goal is to allow for a very smooth and supported transition from the smaller to the larger cross-section layers. In additive manufacturing, overhangs are created using chamfers or fillets. Overhangs are usually necessary when creating parts with cutouts, a counter-bore for flush mount integration with a piece of hardware, lettering in the Z direction, and part interfaces such as a lap shear joint. They may be printed right side up or upside down, but it is the angle of the overhang that is key to creating a smooth transition.
  • When the vertical and horizontal lengths of the overhang are equal, use 45-degree chamfers for the smoothest possible transitions. Chamfers use more material than fillets, but eliminate the need for support and allow for printing at faster speeds. If a fillet is used, you will need support at the perimeters to prevent layers of material that jut out slightly over the previous layer from drooping due to under support.
  • When the vertical length is shorter than the horizontal length, there is limited height to transition from the small to the large cross-section of the overhang (like a dinner plate). Support will be needed to mitigate the steep angles whether using a chamfer or fillet. [Tip: Print a section of the part that includes support material. Test how it works and how easy it is removed before committing to the entire part.]
  • When the vertical length is longer than the horizontal length, designers have additional space and freedom to build the overhang with much smaller angles. Use custom transition profiles available in most CAD programs to design longer chamfers or larger radius fillets using more layers for stronger, more graceful transitions without support. Always aim for this design condition whenever possible.
• Make effective use of cutouts. Cutouts are usually integrated into part design for material savings and/or lightweighting. If the polymer is strong enough to handle the load without 100 percent infill, faster print times and lower cost-per-part can be achieved, even if the cutout removes only two or three grams from the part. But cutouts have several functional purposes as well, such as for gripping, to allow fluids, gasses, light, or wires to pass through the part, to be a seat for a switch or PCB, or to connect with another part. Cutouts may be in the X/Y plane or in the Z plane, and as such there are different considerations for each.
  • When making cutouts in the X/Y plane, (parallel to the build surface), primary considerations are perimeters and infill. Make sure your design uses the longest toolpaths possible to maximize printing velocity. Use rounded fillets and wide angles to minimize accelerations and decelerations at every turn on every layer. [Tip: When printing small parts and a cutout is not functionally necessary, it may actually be faster to print the part with a light amount of solid infill to reduce needless cornering and increase toolpaths. Large parts will still benefit from material savings using cutouts.]
  • When making cutouts in the Z plane, (perpendicular to the build surface), designers should be more concerned with travel movements and layer height, or Z resolution. Use cutout shapes that are conducive to reducing the effects Z resolution or bridging will have on the final part. Unless the cutout must be in a particular shape or dimension to accommodate a switch, for example, consider substituting larger rectangle cutouts for triangles, dome shapes, and hexagons to minimize bridging distances and the potential for drooping. Because high speed extrusion is so fast, it supports distances up to 15 to 20 mm and can make cutouts much larger than can be created by traditional FFF machines. If the Z plane cutout is purely for material savings, design the part with multiple smaller openings in an array to create a lattice-type wall to save material.
• Designing for part interfacing using high speed extrusion. Additive manufacturing, and especially high speed extrusion, allows for many creative ways to join two parts together. The potential use of dovetails, lap shear joints, fastener holes, slots, clips, hooks, and dowels gives designers more freedom to innovate. Here are a few tips and tricks to maximize part design for interfacing with other parts when using high speed extrusion:
  • Build robust attachment points. Small clips, thin dowels or narrow locating slots printed with a single pass are delicate. They will not be thick enough to connect parts with necessary strength and will snap off easily. Make sure your connecting features are at least two toolpaths thick.
  • Design tolerances of 0.05 mm to 0.1 mm into the part sketch. Tolerances ensure a snug fit between parts. Too small and the parts will not fit together after printing, potentially requiring post-process sanding; too large and the parts will have too much movement.
  • Understand how materials may expand or contract during or after printing, and how parts will be joined. Various materials like PACF and TPU absorb moisture over time. This should be accounted for in your design by undersizing the male connection feature by about 50 microns to offset swelling. When working with more stiffer materials like PEEK, or designing parts that will be glued together, tolerances for female parts can be increased to 100 to 150 microns to account for the adhesive.

In sum, printing a part with maximum speed and quality using high speed extrusion is really about ensuring your design employs the longest possible toolpaths to print at the highest velocity with a consistent filament temperature.

Implementing these techniques will help you achieve ideal 3d print extrusion rate and optimal layer bonding while saving time, materials, and money. We invite you to learn more about best practices optimized for high speed extrusion additive manufacturing by watching our webinars, Designing for High Speed Extrusion, Parts I and II.