3D Printing Considerations
As you design your part, consider how it can be optimized for the layer-by-layer 3D printing process. Below are six steps to keep in mind when designing your parts:
1. Determine loading conditions: Composite 3D printed parts are stronger on planes parallel to the print bed, especially if you are reinforcing with continuous fiber. Analyze how your part will be loaded and design the part such that the largest forces traverse the XY plane. Some parts may need to be split into multiple printed pieces to optimize for strength.
2. Identify critical dimensions: 3D printers have higher precision in planes parallel to the build plate. What are your critical dimensions or features? Critical features print optimally when in plane with the print bed.
3. Maximize bed contact: Greater surface area on the print bed minimizes the need for supports and improves bed adhesion. Which face of your part contacts the bed? Try to orient the part so that the largest face lies on the print bed, unless strength or geometry needs dictate otherwise.
4. Reduce supports and improve overhangs: Fewer supports reduce printing and processing time. How can you design to minimize supports? Are the supports in your part accessible? Use angled overhangs to reduce supports and improve support removal.
5. Fillet or chamfer edges: Adding fillets ensures smooth edge transitions and reduces stress concentrations at corners. Filleting edges normal to the print bed reduces the potential for warping, while chamfering edges flush with the build plate makes part removal easier and prevents edges from splaying out on the first layer. Chamfers on interface edges like holes will further help line up fits more easily.
6. Consider printer bandwidth. Consider when you use your printer and how to make efficient use of its bandwidth. Print longer jobs overnight and shorter jobs during the day. You can also create builds by printing multiple parts together that start and end during a workday. Below are guidelines and four example days of prints to help you:
Identify What Needs to be Printed
Think critically about which aspects of your design need to be 3D printed. Some features could be implemented more efficiently with other manufacturing methods. When appropriate, integrate other parts into your design to save on print time and cost or to improve important features. Below are a few examples of where simple hardware integrations or design changes can improve part success.
Threads and inserts: Instead of printing or tapping threads into plastic, add a metal heat-set insert where you need threads. These inserts get pressed in with a soldering iron to reflow the plastic around them for local isotropic strength. Inserts are stronger and last longer than printed or tapped plastic threads. Read our blog post on heat set inserts to learn how to design for and install them!
Wear surfaces: Dowel pins provide a hardened steel wear surface for areas of parts that interact with abrasive surfaces. In this example, robotic end effectors grip a threaded pipe coupling. The dowel pins prevent the threads from cutting into the printed plastic, increasing the lifespan of the grippers.
Alignment: Use pressed-in dowel pins or shoulder bolts to precisely align multiple components. Press-fit dowel pins are used to line up this fixture with its baseplate, while screws secure it. Use dowel pins for alignment before gluing or bolting the components together to attach multiple printed parts precisely.
Concentricity: Bushings or sleeve bearings like the one inserted into this printed brake lever provide high cylindrical precision and smooth concentric clearance fits. Off-axis loads distribute to the printed part with the bushing’s larger surface area. The bushing cavity can be reinforced with continuous strand composite fibers for higher torsional resistance.
Splitting up parts: Sometimes it is more effective to split up a part than to print it as one piece. This part is split in two, with each piece printed from its highlighted face to prioritize the strength of each segment. Here are some reasons to consider splitting a part up:
- Parts with many iterations or customizations can be designed with a core base geometry and interchangeable modules
- Elements of parts that undergo increased wear or strain can be isolated into components that can be changed out regularly
- Designs requiring specific strength profiles across multiple axes can be printed in sub-components in different orientations and joined post-print
- Complex prints with critical features on multiple planes can be split into sections to reduce supports, decrease print time, and ensure print success
Design 3D Printed Unit Tests
In software development, a unit test is used to confirm that a small section of code works before its integration into a larger program. 3D printed unit tests work much the same way. A 3D printed unit test is a small test print that confirms feature success before committing to a long, costly print. Unit tests can be designed to experiment with different clearances and select the fit most appropriate to your application. Isolate the critical segments of large parts and print multiple versions with slightly altered dimensions or configurations to test how they interface. Update your final CAD model with the specification you find works best, and print it with confidence that it will succeed.
1. Identify critical features in your CAD model that either require tolerance verification or need to be tested to confirm they print as expected.
2. Isolate features in question as a part file or body separate from the main CAD model. Try to make it a small section that can be printed quickly — aim for less than 6 hours in print time, so you you can print and test over the course of a few hours.
3. Design segment variations if you want to test different tolerances on the feature in question. Each unit variation can be its own print or you can combine them into a single part to keep them organized.
4. Print and test segment variations to determine which variation fits the way you like it and best suits your part needs.
5. Update the original model with the desired dimensions tested with your variations and print out the full part.
3D Printing Tolerances and Clearances
Whenever you’re interfacing or joining multiple parts, it’s important to know how you want your parts to fit together. Below are some recommended starting fits between 3D printed parts. Specifics may change based on the materials and parts you are using and their geometries. Listed dimensions are diametral, indicating the overall change in dimension between the two interfacing parts.
Press Fit Tolerance: 0.00 mm - 0.05 mm (0.00" - 0.002"): Parts require some applied force with a cold press to assemble.
Close Fit Tolerance: 0.05 mm - 0.10 mm (0.002" - 0.004"): Parts can be assembled or disassembled by hand with negligible clearance or force.
Free Fit Tolerance: 0.10 mm - 0.20 mm (0.004" - 0.008"): Parts can slide and/or rotate easily when assembled.
If you'd like to read the full composites design guide, you can access it on the Markforged support page.