Kathrine Lonkin, application engineer at SPIROL, explains
Like injected moulded plastic assemblies, threaded inserts are commonly used in 3D printed assemblies to provide reusable threads and enhance joint strength
This white paper provides recommendations, design guidelines, and performance expectations for various threaded inserts used in Fused Deposition Modelling (FDM), Selective Laser Sintering (SLS), and Stereolithography (SLA) 3D printing processes, which are the most relevant additive manufacturing technologies for assemblies that benefit from the performance enhancements afforded by threaded inserts.
THREADED INSERT RECOMMENDATIONS
Threaded inserts come in various types, often categorised by their installation method, such as post-mould heat/ultrasonic-install and cold press-in. The optimal insert type for a 3D printed application is determined by the host plastic material.
For thermoplastics (FDM and SLS)
Thermoplastics can be heated and cooled multiple times with minimal change to their properties. Therefore, threaded inserts can be installed into thermoplastic 3D printed assemblies using heat (thermal) or ultrasonics, or they can be simply pressed into or threaded into the assembly:
- Heat/Ultrasonic Inserts: Recommended for assemblies requiring the highest tensile and torque performance
- Press-In Inserts: Deliver moderate tensile and torque performance and are the simplest to install as they do not require specialised equipment
For thermosetting plastics (SLA)
Thermosetting plastics remain permanently solid after curing and decompose rather than melt when subjected to high temperatures. Therefore, it is advisable to install threaded inserts in a “cold” condition:
- Press-In Inserts: The most effective choice for thermosets
- Self-Tapping Inserts: A viable alternative that may require less force to install, but limits back-out torque performance
- Expansion Style Inserts: Most simple to install, however, performance is the least of all Insert styles
MATERIAL PROPERTIES
Insert performance is influenced by the properties of the host material.
- Tensile Strength: Measures the material’s property to resist plastic deformation
- Flexural Modulus: Influences material stiffness and ability to distribute bending forces, critical in preventing deformation and ensuring proper load distribution
Additionally, thermal properties will affect the installation process, as higher conductivity materials promote more uniform heat distribution, improving plastic flow and ensuring complete fill around the retaining features for a secure bond. Likewise, the heat deflection temperature for a given plastic will influence the temperature for heat or ultrasonic installation of the threaded Insert.
DESIGN GUIDELINES
The performance of a threaded insert in 3D printed materials is influenced by several key factors. This section outlines essential design guidelines, focusing on host hole size, material properties, wall thickness, infill, and layer thickness to ensure optimal performance of the threaded insert in 3D printing applications.
Hole size is critical for the performance of threaded inserts. SPIROL’s threaded insert catalogue provides guidelines for hole size, but 3D printing tolerances often exceed the recommended +0.08mm. Typical tolerances are as follows:
- Industrial FDM printing: ±0.2mm
- Desktop printers: ±0.3-0.5mm
- SLA printers: ±0.1mm
- SLS printing: ±0.3mm
To mitigate variation, one option is to drill holes in the plastic host, which reduces variation but may compromise reinforcement in FDM-printed outer walls. Alternatively, iterative development can be done until the optimal hole size is achieved.
Threaded inserts require sufficient solid wall thickness in the surrounding area to meet performance expectations. For optimal insert performance, design the number of outer wall loops to create a solid feature (boss) of at least 1.5 times the insert’s knurl diameter. For an M6 threaded insert, this occurs at 6 wall loops (at 0.4mm nozzle diameter). Maximum performance potential is reached with a boss of at least two times the insert knurl diameter.
Regarding FDM, insert performance increases linearly with infill density. For optimal insert performance, design with an infill density of greater than 50%. In general, thicker layers result in weaker inter-layer bonding as there are larger voids. Best insert performance typically occurs around 0.16-0.2mm layer thickness for FDM and SLS, where tensile strength is optimised.
INSERT PERFORMANCE COMPARISON
Let’s compare the performance of three different series of M6 threaded inserts; Symmetrical (SPIROL INS 29 Long) and asymmetrical (SPIROL INS 19 Long) heat-installed, and a press-in threaded insert (SPIROL INS 50) in common 3D printing materials. Detailed comparative test results are provided for FDM, SLS, and SLA printed components. These insights will help designers choose the optimal threaded insert based on specific performance requirements.
KEY TAKEAWAYS
Heat/ultrasonic inserts provide the highest strength in 3D printed thermoplastics, while press-in inserts serve as a viable alternative, particularly for thermoset materials, where heat installation is not feasible. Thermoset plastics inherently limit insert options to press-in methods. Key findings indicate that threaded insert performance improves with increased infill density and wall loops in FDM printing, with optimal results at a layer thickness of 0.16-0.2mm, 50% or more infill, and number of wall loops equal to or higher than the nominal metric insert size.
Results show that at high infill and wall thickness, FDM 3D printed components carry approximately 70-80% of the performance in an equivalent moulded material. While SLS components exhibit high theoretical strength, their microporosity can carry approximately 50-70% of the performance compared to moulded plastics.
For more information visit: www.spirol.com
