Designing 3D Printed Parts with Inserts, Threads, and Snap Fits

As 3D printed parts become more commonplace in industrial applications, functional integration is becoming increasingly important. A secure mechanical attachment to the larger system is one of the key success factors. With a secure attachment, the part—and the entire system—gains strength and reliability

Over the past five years, there have been significant improvements in the reliability of additive parts. HP has been at the forefront of these advances with its Multi Jet Fusion (MJF) technology, a system designed for high-volume manufacturing of series parts. Both the quality of the material and strength of the bonds between the layers have resulted in mechanical performance that is on par with conventional manufactured parts. Similar performance improvements have been achieved in other plastic additive manufacturing technologies, including DLP, FDM, DLS, and SLA. That reliability has contributed to a broader adoption rate for 3D printing. That led to the next step of integration into full-part assemblies.

This article will outline the three most effective methods for connecting 3D-printed parts to other components, including metal inserts, threaded connections, and snap-fits. Each method has advantages and disadvantages, depending on factors like the required strength, the frequency of reuse, and the placement among surrounding parts. When key factors are accounted for during the Design for Additive Manufacturing (DfAM) phase, 3D-printed components can serve as robust, cost-effective components in a wide range of products and machines.

Metal Inserts: The Industry Standard for 3D Printed Connections

Combining 3D-printed parts with metal inserts is the safest way to strengthen an existing design or assembly. The standardized, metal components allow for repeated fastening and unfastening of mating parts with screws, ensuring a secure and reliable attachment. The insert systems are available in various materials (stainless steel, brass, aluminum) to be selected depending on the functional requirements of the final application. Additionally, the inserts are available in various geometries and can be integrated into the part in different ways. One of the disadvantages of metal inserts is that they always bring additional costs for purchasing and installing them in the part. However, when the objective is to repeatedly attach and release parts from each other with screws, using metal inserts is the optimal solution to ensure that 3D-printed parts meet the long-term performance requirements.

Performing a few checks up front to ensure that the metal inserts will function as expected. 3D-printed parts require sufficient additional material around the connection point to ensure a secure hold. While the exact distance varies depending on the insert size and the type of installation, as a general guideline, we recommend a minimum of twice the inner diameter of the insert as the surrounding material. Therefore, a 1/8″ screw would require a minimum of 1/4″ of material surrounding the hole, with a tolerance range of +/- 1/64″ for the hole’s alignment. Having enough surrounding material is important to ensure the desired level of strength.

It is also important to ensure that all inserts are accessible and correctly placed. Since they are added to the original part by heating and melting them into place, pressing them into the structure, or screwing them in with a counter torque, these areas must be accessible to the finishing team. The resulting connections are robust and able to withstand the wear and tear of daily use.

Brass Inserts

In general, brass inserts are the most suitable choice for industrial, automotive, and agricultural applications. Brass metal inserts have excellent heat dissipation properties so post-heat treatment, the inserts cool rapidly and set precisely. Steel, which has a longer cooling time, is susceptible to tilting because the slower process is less consistent across the mass. Comparatively, brass inserts have a lower cost point because they are easier to mill. This is not as critical for standard inserts but is particularly noticeable when ordering custom sizes where additional machining expertise or time is required.

Brass is an ideal material for a range of applications involving hot and cold liquid flow systems. Its heat dissipation capabilities help avoid placing undue stress on the part. Additionally, the material boasts excellent corrosion resistance to salt water, mild alkaloids, and non-oxidizing acids. It is a highly robust material, suitable for deployment in many standard applications.

Steel Inserts

For applications requiring enhanced durability and performance, stainless steel inserts are an excellent alternative. The most significant advantage of stainless steel is its strength, which makes it a suitable material for applications that require high mechanical performance. Additionally, it exhibits superior resistance to aggressive, turbulent saltwater, oil, and acidic environments. It is also well-suited for applications involving food contact, due to its ability to withstand harsh cleaning solutions and advanced surface treatments like chemical smoothing. The cost difference is significant, so steel inserts are always limited to specific performance requirements. However, both stainless steel and brass inserts are compatible with nylon PA 12 MJF parts.

Insert Type	Considerations for Use
Brass Inserts	• Standard applications • Exposure to salt water, alkaloids, or non-oxidizing acids • High accuracy requirements • Cycling of hot/cold
Steel Inserts	• High-strength applications • Exposure to moving salt water, oil, aggressive acids, food contact, aggressive cleaning chemicals • High-heat exposure

Selecting the Proper Metal Insert Design

Once the metal type has been selected, the insert shape can be designed based on the mechanical demands of the application. The objective is to achieve an optimal balance between the speed of installation, the required accuracy of the insert location, and the torque/pressure that will be applied over the long duration of use. As with the selection of the insert material, the more mechanically demanding the application is, the higher the cost of the insert will be, primarily due to the personnel costs during installation. The chart below provides a differentiation between the four primary types: press-in inserts (round and hexagonal), self-threading inserts, and heat-staking inserts.

Insert Type		Considerations for Use
Press-In Inserts (Screw to Expand)		• Quick installation process • Strength through addition expansion with screw • Highly dependent on hole size • Repeated use or removal of screw can lead to loosening of attachment • Recommended for non-critical applications
Press-In Inserts (Hexagon Shape)		• Quick installation process • Higher torque strength through hexagonal form • Good pull-out resistance • Highly dependent on hole size • Recommended for non-critical applications with slightly more mechanical stress
Self-Threading Inserts		• Fairly simple installation process • High degree of placement accuracy • Excellent pull-out resistance through outer ridges and cutting directly into part • Moderately dependent on accurate placement of hole size
Heat-Staking Inserts		• Highest overall mechanical performance • Stable over repeated usage • Most complex installation process • Chance of inaccuracy due to tilt during melting • Forgiving of small placement holes but looses stability if placement hole is too large

Once the mechanical specifications of the component have been defined, selecting the optimal insert design for the intended application is a relatively straightforward process. In developing MJF 3D printing, HP has prioritized repeatability since the inception of the project. Once the appropriate hole size for a specific insert type has been identified, it can be added to the design with a high degree of reliability. The final consideration is that different orientations can require slightly different scaling. If you are working with an external partner, they will have the required scaling parameters. However, for first-time production with an internal team, it is recommended to produce a first set of tests to finalize the scaling parameters. This is something that Endeavor 3D offers in their program development process.

There are a few additional design parameters to consider when creating a part that will function optimally with metal inserts. The first step is to ensure that the overall tolerances of the production technology, +/-0.2 mm (ISO 286, IT Grade 13), are considered in the design of the part. This will guarantee a good fit and the best possible mechanical performance. Secondly, the strength of the part is directly affected by having enough material around the insert location.

The final design aspect is to ensure that the pressure at the connection point is transferred to the metal insert rather than the plastic component. This results in the maximum strength during usage. Furthermore, it avoids placing pressure directly around the insert, which can result in it loosening over time. This can be solved with the design of the mating part, as shown in the diagram. Alternatively, for a slightly higher cost, inserts can be purchased that have a metal lip to further guarantee that the pressure falls onto the insert rather than the plastic.

When all of these factors are considered in the product design, metal inserts play a crucial role in ensuring that 3D-printed parts perform at a level that matches conventionally manufactured parts.

Adding Functional Threading to 3D Printed Parts

For applications with lower mechanical demands on the part, such as covers and housings that are only bolted together once or twice, creating threads directly in the part can be sufficient. Materials used in 3D printing processes, such as Nylon PA 12 or Nylon PA 11, are industrial grade. The performance of the manufacturing process in terms of bonding between layers has also improved significantly over the last 5-10 years, moving 3D printing from prototyping to production. Those developments make 3D printing functional threads possible.

The difference is the underlying strength in today’s 3D printed parts so that the material can be used directly as a mounting point at those threaded locations. The ability to have secure attachment points, without the additional labor and coordination of metal insert sourcing and installation, reduces costs and long lead times. Whether mechanically cut or directly printed, additively manufactured parts with functional threads can be integrated into larger systems with ease, adding value to products and strengthening production streams.

Cutting Threads into 3D Printed Parts

Self-tapping screws are an excellent alternative to working with metal inserts. The pilot hole diameter is always specified by the screw manufacturer. Because the thread is cut directly into the plastic, it has a similar pull-out strength as the self-tapping metal insert. As long as the fastening is performed only once or twice for the primary connection, strength is assured. The potential for failure comes from repeated use of the connection, as the screw will tend to cut away small amounts of plastic with each new use, weakening the strength of the hole.

A second method is to mechanically cut the threads into the plastic part. This is done using a pilot hole to guide the location in the part. Then, a drill is used to

cut the hole with threads for the final screw. The advantage is that this is a relatively low labor process and allows a hole to be used more often than with a self-tapping screw. The disadvantage is that it can be difficult to mount the part for effective tapping. Slight tilts in the part can make the final performance problematic. For many parts, we recommend using a jig made with a low-cost 3D printing process such as FDM that includes a negative of the part to effectively “hit the mark” with all of the parts. Alternatively, a tool like Trinckle’s Paramate can be used to make jigs without engineering expertise, to ensure the final production process is simple and repeatable.

External threads can also be made using a hand cutter on the plastic part. These are simple tools that rotate around a properly sized plastic section protruding from the part. The advantage is that it is a quick process that can cut much finer thread sizes. In some cases, it is also advantageous to print the basic thread geometry and then cut it again to get sharper angles on the threads and a more precise fit into the mating part. Because these sections are externally accessible, they are “easy wins” for producing accurate threads. Since the threads are plastic rather than metal, they will tend to loosen if reused over a long period, so the ideal application is when they are joined once or changed fairly infrequently.

Designing Threads for 3D Printing

The final option for additively manufactured parts is to directly 3D print the thread geometry.

The advantage of 3D printing threads is that no additional machining is required on the parts. The disadvantage is that the 3D-printed threads produced by MJF must consider the design of round vs. sharp edges. However, if these constraints are considered during the design validation phase, the parts can perform to the standard of other cast parts. Often, 3D-printed parts with pre-designed threads also receive a secondary treatment of infiltration or chemical smoothing so that they are completely air-tight or leak-resistant.

The most common thread applications with 3D printing are for items like caps for industrial cases or pressurized systems with wide, self-locking threads that require a custom fit. This is highly relevant for aftermarket replacement items, spares, or repairs.

Rapid Assembly using Snap-Fit Union Joints

Snap-fit assembly is the fastest way to mate parts, given the right design approach. Because the parts can be “snapped” into place, little time is required to join an assembly. The same principles can be used to attach third-party components such as sensors, electronics, or controllers, which instantly add much more value to the part when integrated into a single design. This is where 3D printing has an advantage over injection molding or polyurethane casting because there is a significant cost associated with creating a tool for these types of specialty elements. The additional design work to add snap-fits from a design library in 3D printing is negligible to the final cost of the part. As a result, these types of components typically represent a strong business case for MJF products.

Snap-fit 3D printed parts have become much more common, due in no small part to Multi Jet Fusion’s popularization of PA11 material. PA11 is much more ductile than traditional PA12, leading to increased use in items such as buckles, cases, covers, electronics, and medical devices. The improvements made to the interlayer strength of MJF also increased the use of snap-fit components, as they can now better withstand counterforces in the Z-direction.

Designing 3D Printed Snap-Fits for Function

The main reason that 3D-printed parts with snap-fit connections are not already more common is that several design guidelines must be followed for the part to work successfully. The first principle is to work with a minimum thickness of 1 mm (3/64″) throughout the snap-fit elements (thickness of both the base and the locking element). Depending on the application requirement, this rivals injection molding.

The second principle is to round corners and eliminate sharp edges in the parts. These design approaches help ensure long-term function and accuracy by accounting for limitations in the manufacturing process. Rounding corners improves the overall structural strength of 3D printed parts. It is much less likely that localized stress will cause a crack in the plastic if there are no ” focal points”. This is similar to the guidelines for injection molding, but since we are not dealing with milling, where rounding internal shapes happens by default, the rounding should be added to every corner at best.

The third principle is that sharp corners at the top and front of a typical snap fit should also be rounded. This is to ensure strength and guarantee the 1 mm thickness in all areas. By removing sharp edges at the design stage, the engineer ensures proper interaction with the other aspects of the system. Eliminating the front corner where the part is inserted also plays a big role in reducing breakage during assembly, as the snap-fit is more forgiving in “guiding” the element into place. If it is intended to be disassembled later, a similar angle should be placed on the back of the snap to help guide it smoothly back out of the fit.

The fourth principle ensures proper tolerances between the snap-fit and the surrounding elements. Just like a good injection molded design, 3D-printed parts must be designed with a proper fit. A good rule of thumb is to start with the process tolerance of +- 0.2mm. This is also important because a good fit limits the amount of stress placed on the snap-fit during assembly.

The fifth principle applies the same approach of working with the final production engineer to determine the part orientation for the snap fits that will optimize function. As described in the Assembly Consolidation article, unlike other manufacturing processes, 3D printing depends on much more collaboration between manufacturing and engineering to achieve the best results. The basic rule of thumb is to determine the critical tolerance for the functional fit (width, length, or height). Since the X-Z plane provides the highest level of accuracy, the critical dimensions should be placed flat in this orientation. If multiple dimensions are critical, consider an additional round of scaling. The more snap-fit elements that appear on a single part, the more attention must be paid to fit during the initial validation phase. An open dialogue between the production team and the CAD engineer is the best way to adjust design and tolerance expectations on the way to success.

The final principle involves correctly calculating the forces that will be applied to the part to ensure long-term performance. The exact strength depends on the design type (discussed in the section below) as well as the choice of 3D printed material and the function within the final system. However, HP provides a very good overview (Design > Union Joints Design > Snap Fits) that can be referenced for early projects until a good baseline is established.

Selecting the Optimal Snap-Fit Design for 3D Printing

There are different types of snap fits that can be incorporated into the design depending on the function required, the space available within the design, and the frequency of disassembly and reassembly of the fit. We will review the five most common snap-fit types that can be implemented into the system.

Snap Fit Type		Considerations for Use
Cantilever Snap-Fit		Selection Criteria: Most common snap-fit type. Cantilever snap-fits are simple to design and provide a robust fit to the matching part. The direct relationship between thickness and connection strength makes it easiest to calculate and adjust. Disadvantage: Pressure is concentrated at base of beam which can be problematic in repeated use. Particularly if there is a short base which limits the elasticity, this can cause breakage.
L-Shaped Cantilever Snap-Fit		Selection Criteria: When the part geometries don’t allow for enough elasticity/bend then an L-shaped cantilever provides a solution for adding length. The slots to the side are optional but also help increase the flexibility. Disadvantage: Strength is reduced (due to increased length) and is harder to calculate during design phase.
U-Shaped Cantilever Snap-Fit		Selection Criteria: When space is limited or for optical reasons the top surface should not be disturbed, a U-shaped cantilever at the side is a good option. It is extremely flexible and therefore a good solution for items that need to be regularly opened and closed but don’t require much force (like caps and lids). Disadvantage: Strength is again reduced leading to limited applications.
Annular Snap-Fit		Selection Criteria: Usually suited for cylindrical or ring-shaped parts (without flat edges) and where the part can be turned into place. Typically strong fit with good alignment of the mating part. Disadvantage: Requires push and/or rotational force to join and remove the connection, not ideal for regular removal. Can be loosened or crack under repeated use.
Torsional Snap-Fit		Selection Criteria: For use as a latch in parts where the release needs to be performed regularly and with a minimum of effort. Can be a strong connection depending on the placement. Rotation access can be 3D printed in place eliminating assembly. Disadvantage: Most intensive to design and ensure proper function. Space and design intensive to ensure the rotation access is well mounted.

Strong Connections Bring 3D Printed Parts to Industry

With strong connections, the functional performance of 3D-printed parts can match that of traditionally manufactured parts. Using the three connection methods of metal inserts, plastic threads, and snap fits, 3D-printed parts can be smoothly integrated into active industrial systems. There is almost always a way to meet performance requirements with the right design. The guidelines above help ensure a high probability of “quick win” success. Parts that are well designed are critical to developing or maintaining a market advantage, as 3D parts connected to larger systems represent significant cost and time advantages. Now is the time to take advantage of MJF technology and use 3D-printed parts with functional integration through strong connections.

Designing 3D Printed Parts with Inserts, Threads, and Snap Fits