Thread design specifications for 3D printed parts: Selection of self-tapping threads and insert nuts

Application requirements for 3D printed threaded connections

Threaded connections are one of the most commonly used connection methods in mechanical assembly. 3D printed parts also require threaded connections to be assembled with other parts. The challenge of printing threads lies in the mechanical properties and manufacturing accuracy of the material. The strength and hardness of most 3D printing materials are lower than metals, and the thread profile is prone to wear and damage. Printing accuracy limits the minimum size and accuracy level of threads. According to the load size, disassembly frequency and reliability requirements of the application scenario, an appropriate thread design needs to be selected.

Design points for direct printing of threads

Direct printing of threads is to design the thread profile during modeling and print it integrally with the part. This method is suitable for applications with low load and low disassembly frequency, such as decorative parts and light-load connectors. When designing, you need to consider: thread size. It is recommended that the pitch is not less than 1mm and the thread depth is not less than 0.5mm. If the thread is too small, it is difficult to guarantee the printing accuracy. For the thread direction, try to make the thread axis perpendicular to the printing direction to obtain better tooth profile accuracy. The starting point of the thread and the starting point of the tooth profile need to be chamfered to avoid deformation of the first layer affecting screwing. For fit clearance, the tolerance zone of printed threads should be wider than that of metal threads. It is recommended that the fit clearance be increased by 0.1-0.2mm.

Design specifications for self-tapping threaded holes

Self-tapping threads are to screw self-tapping screws directly into the printed optical holes so that the screw profile forms threads on the hole wall. This method is suitable for applications with medium load and medium disassembly frequency, and has high assembly efficiency. Design key points include: hole diameter design. The hole diameter should be 0.3-0.5mm smaller than the nominal diameter of the screw to ensure that the screw can cut into the material to form a connection. Hole depth, the hole depth should be 2-3mm greater than the screw screwing length to prevent the screw from bottoming out. The thickness of the hole wall and the thickness of the material around the hole should be no less than the diameter of the screw to ensure sufficient load-bearing area. Material selection: softer materials (such as PLA, ABS) are suitable for self-tapping, while harder or brittle materials (such as resin, carbon fiber materials) are easy to crack.

Application advantages of insert nuts

Insert nuts embed metal nuts into printed parts to form a high-strength threaded connection. This method is suitable for applications with high load and high disassembly frequency, and has the highest reliability. Insert methods include: hot melt inserts, using hot melt tools to press the inserts into the reserved holes, suitable for FDM printing of PLA, ABS, nylon and other materials. Ultrasonic inserts use ultrasonic vibration to press the inserts in and are suitable for engineering plastics. Adhesive inserts, where the insert is fixed in the hole with structural glue, are suitable for all materials. The insert hole needs to be reserved during design. The hole diameter is 0.1-0.2mm larger than the outer diameter of the insert. The depth of the insert is determined according to the load, usually 1-1.5 times the height of the insert.

Reliability testing of threaded connections

Reliability testing is required after the design of threaded connections is completed. The first is the screwing test to check whether the threads can be screwed together smoothly and whether there is any jamming or slippage. The second step is the pull-out force test, which measures the axial load-bearing capacity of the threaded connection. The pull-out force of the self-tapping thread should not be less than 50% of the material's shear strength. The third is the disassembly test, repeatedly screwing in and out the screws (usually 10-20 times) to check whether the threads are damaged or loose. The fourth is the vibration test, which simulates the vibration of the working environment and checks whether the connection is loose. Choose the most appropriate thread design based on the test results.