Threaded components, holes and bolted connections are often an important part of prototypes and production parts that are utilized every day — so likely at some point, you’ll require a 3D printed or CNC part with threads. Adding threaded surfaces to plastic 3D printed parts — even high strength specialty materials, including carbon fiber-reinforced nylon — requires more design considerations than simply modeling the threads in 3D or tapping an undersized hole with a threading tool. To start with, refer to manufacturers of threaded brass inserts for hole size specifications needed for the insert. 3D printed plastics generally have lower material yield strength than common production prototyping metals, and the small feature dimensions of popular thread sizes (both metric and English) mean that it’s fairly easy to strip out built in threads when excessive force is applied.
PrintForm often manufactures customer parts using metal threaded inserts when the need for strong threads arises in the product development process. Threaded inserts are cylindrical metal components, commonly made of brass, steel and aluminum with a pre-formed thread of the specified pitch, depth and size along the ID (inner walls) of the cylinder. The insert is embedded into a larger part and resists loosening forces because of knurled, knobbed or threaded external surfaces designed to mate with and grip the “nominal” material of the part. Threaded inserts are also more functional and wear-resistant than threads made from the base plastic of a prototype, and if the part’s application includes unscrewing and re-inserting a bolt or other threaded connection frequently, a mating insert is the best choice.
Threaded inserts are relatively easy to find through a variety of hardware manufacturers, including McMaster-Carr, in the press-in, expanding and heat-sinkvarieties. Heat set types are generally the most robust when installed correctly, since melting and realigning the surrounding plastic of a part ensures the best adhesion between the part and the threaded insert itself. This in turn results in stronger strength ratings of the final thread, particularly when compared to press-in inserts.
FDM & SLS 3D Printed parts are considered the most compatible technologies for thermally placed inserts. Heat-setting is also the method most commonly used to add threads for true production injection molded parts. In these cases, plastic parts are first injection molded, with holes of the appropriate size left in the part at the location where threads are desired. As a post manufacturing process, the threaded inserts can be melted into the part via thermal or ultrasonic fashion.
When installing inserts, important printer settings are manipulated and need to be thoroughly inspected prior to ‘pressing print’. This applies to both the larger industrial machines on which PrintForm builds customer parts, as well as your engineering department’s in house desktop printer. An important factor to keep in mind is that increasing the number of shells in your print leaves more plastic around insert locations, thus offering a more robust connection.
Many times a soldering iron is used to apply direct heat to specific areas with high precision.
At PrintForm, we allow the soldering iron to heat for 4-5 minutes before installing inserts. This ensures that our technicians have to use minimal force to install inserts. Before installing your inserts, it’s also important that the part is properly secured with the use of a clamp, grips, and many times, custom jigs and fixtures (also 3D printed in some cases). The point of this set up is to eliminate the potential movement through the process of heating, melting and manipulating the part material. Movement equals errors. Remember: Mitigate, Minimize, and Eliminate Movement! (which pretty much applies to all assembly processes in product development land). With a properly heated soldering iron, thermal energy does most of the work. As brass transfers temperature relatively quickly, inserts are usually cool within a minute or so.
Things we avoid: Installing the hardware too quickly, Pushing down on inserts with too much force, Holding inserts in place with hands when installing (we always use pliers, and if possible, the mating bolt partially screwed in to dissipate heat).
Once the inserts are in place and ready for use, be sure not to over tighten the mating bolt or component. This will force the insert free from the surrounding plastic.
About the Author: Bill Artley is the Vice President of Operations at PrintForm and has written many articles & white papers on product development, sourcing, additive manufacturing, and materials. He is a contributing author and peer reviewer of The 3D Handbook (http://a.co/h9fL1b5), a top 10 Amazon Best Seller in its category.