3D printing threads - A guide to methods and best practices 

As 3D printers become increasingly available and popular, new opportunities to quickly produce prototypes and finished products are opening up. A recurring challenge for many who design and manufacture 3D printed parts is creating durable and suitable threads. In this article, we'll look at different methods of producing threaded joints in 3D printing, what to consider when designing threads in CAD, and how to optimize your material selection and print setup for best results. 

Why threads in 3D printing? 

The ability to assemble different parts using screws and nuts is central to mechanical designs. By printing the threads directly or otherwise integrating them into your 3D printed parts, you can: 

• Save time: Avoid separate operations for attaching nuts or other solutions. 

• Optimize the design: Customize screw joints exactly to the needs of the product. 

• Create advanced geometries3D printing allows for injection molding-like geometries even in smaller series. 

Despite these advantages, precision and strength can be a challenge, especially in the FDM/FFF method where the layer-by-layer build-up can easily produce unevenness in the thread profile. 

Different methods for thread manufacturing in 3D printed parts 

Direct-printed threads 

Drawing in the threads directly in CAD and then printing them in the same process is for many the most logical first choice. But there are some things to consider: 

• Selection of thread dimension: Coarse threads (e.g. M6 and up) tend to work better, as finer threads (M3, M4) will be more difficult to print with sufficient precision and strength. 

• Tolerances: 3D printers, especially FDM printers, may have some over-extrusion or uneven layer loading. This may mean that you need to slightly increase the hole diameter of the thread (about 0.4-0.6 mm) or adjust the outer diameter to allow a regular screw to fit in the thread without cutting the plastic. 

• Print orientation: If possible, position the threads so that they are printed along the Z-axis (vertically). This will reduce the risk of bearing loosening in the ”flanks” of the thread profile, while providing a more even geometry across the bearings. 

• Bearing height: To bring out the fine details of a thread profile, you may need to use a slightly lower layer height than normal, for example 0.1 or 0.15 mm instead of 0.2 mm. This results in smoother surfaces and less ”step effect”. 

The advantages of printing threads directly are that you can get a fully integrated solution in a single print. The disadvantages are the risk of uneven fit and lower strength in finer threads. 

Thread tapping 

For those who want more precise and robust threads, finishing with a tap can be a good option. 

• Prepare holes in CAD: Construct a hole with the core diameter that applies to your desired thread (example: for M6, the recommended drill diameter is often 5.0 mm). 

• After printing: Use a tap (hand or machine tap) to cut the thread manually. 

• Material selection: The method works best in stiffer plastics such as PLA, PETG or ABS. In softer materials such as TPU, the result is often less reliable because the thread can deform during use. 

• Strength: You usually get better strength than with direct-printed threads, because the tap cuts a more precise profile with straighter flanks and less risk of porosity. 

This method requires an extra step, but can be worth the time to get a good fit and reliable threads. 

Thread inserts (thread bushings or heat-set inserts) 

In industry, it is common to reinforce plastic parts with metal inserts to improve strength and extend the life of bolted joints. The same technique can be applied in 3D printing. 

Heat-set inserts (heating inserts): 

Construct a hole diameter and chamfered entry according to the manufacturer's recommendations. 

Using a soldering iron, the threaded insert is heated and slowly pressed into the plastic. When the plastic cools, the insert is securely in place. 

The result is a high-strength metal thread that can withstand both repeated assembly and high tightening force. 

Press activities: 

Some threaded inserts do not need to be heated, but can be pressed or tapped into a hole with the right tolerance. 

Glue or other fixings can sometimes be used for extra security. 

Benefits and advantages: 

Very strong and professional solution. 

Relatively easy to replace if the insert is damaged. 

Withstands multiple assembly/disassembly cycles without wearing out the thread. 

The disadvantage can be the extra cost of the inserts and the need for suitable tools (soldering iron, crimping tool or similar). 

Built-in metal components (”captive nuts”) 

An alternative hybrid solution is to design your 3D printed part with a built-in pocket (”captive nut”) where a hexagonal nut is placed during printing or fitted afterwards. When you then screw in the bolt, the nut will be held in place in the pocket, providing a strong metal thread connection. 

• Placement of the nut: You can ”pause” the printing when you reach the height of the nut, place the nut in the pocket and then resume printing. Or design the part in two halves that snap together around the nut. 

• Advantage: Requires no special effort, and you can easily replace the nut if necessary. 

• Disadvantage: Requires extra planning in CAD, and sometimes also manual handling during the printing process. 

CAD design - right from the start 

When designing threaded joints in CAD, it is useful to use built-in threading tools and library functions. Many modern CAD systems (such as Fusion 360, SolidWorks, Onshape, etc.) have parametric functions for threads where you can specify dimension, pitch, fit and tolerances. 

• Standard threadsISO (M-threads) and UNC/UNF are the most common. Make sure to choose a standard that corresponds to the screws and nuts you intend to use. 

• Test parts: Start by printing a small test part or two to verify fit and strength before making a larger series or complex part. 

• Adjusting tolerances: Is the thread too tight or loose? Experiment with hole diameter, outer diameter and/or extra play. Each 3D printer is unique and often requires some calibration. 

Material selection for threads 

• PLA: Very easy to print, but can be brittle. PLA threads may crack under high tensile or clamping force. 

• PETGTough and more durable than PLA, relatively easy to print. Good ”all-round” choice for functional parts. 

• ABS/ASA: Slightly more demanding to print, but more heat resistant and durable in mechanical applications. 

• nylon: High toughness and strength, but can be more difficult to print. The threads often become very durable. 

• Carbon fiber or glass fiber reinforced plastics: Provide high strength, but are often very abrasive to the nozzle. Requires hardened nozzles and customized settings. 

The basic principle is that the harder and tougher the plastic, the better it will hold in a threaded joint situation. If the part is to withstand high mechanical stress or many assemblies/disassemblies, it may be wise to use metal inserts. 

Summary and recommendations 

1. Direct-printed threads works best for larger dimensions (M6 and up) and at lower loads. Adjust tolerances and orientation for better results. 

2. Thread tapping provides a more accurate and robust thread, especially in stiffer plastics. However, it requires manual finishing. 

3. Thread inserts, Heat-set inserts, in particular, are often the most professional and durable option if you need a really strong dressing and want to withstand many assembly cycles. 

4. Built-in nuts (”captive nuts”) is a simple hybrid solution that provides metal threads without special inserts. 

5. Testing on a small scale to find the right tolerances and settings in both CAD and slicer before committing to larger projects. 

6. Choose your material according to requirements for strength, temperature resistance and ease of printing. 

Threading 3D printed parts can be both time-efficient and economical for prototypes and sometimes even for final products, provided you take into account tolerances, orientation and material selection. With the right approach and careful planning, you can create functional, durable and professional threaded joints in your 3D projects.