Time to read: 11 min
Fictiv’s guide to fasteners for 3D printed parts includes an overview of threaded inserts, self-threading screws, captured hex nuts, as well as how to design threads and cut threads with a tap.
In this article, we’ll examine plastic fastening assemblies for CNC machined and 3D printed parts, including:
- Screws
- Rivets
- Snap fits and tabs
- Plastic welding
- Adhesives and tapes
- Specialty options
This guide will help you choose the best, most cost-effective plastic mechanical fastening methods for your plastic product design with all the technological advances 2023 has brought to the industry.
The approach for choosing the right fastening option should be based on product requirements including cost, quality, manufacturability, and plastic assembly techniques, as well as technical factors that impact performance, such as corrosion resistance, material compatibility, and conductivity. Read on to learn more about the different types of fasteners you can utilize to assemble your plastic 3D printed or CNC machined parts.
Types of Fasteners
Screws
Screw fasteners are among the most common non-permanent fastening methods for assemblies, ranging from plastic automobile components to housings for consumer electronics.
The basic design of a screw fastener is well known—an inclined plane wrapped around a metal dowel. But the design intricacies for screwing into different materials are broad, with different geometries best suited for different materials.
Let’s review some examples of screw fastener designs specifically engineered for plastics.
Standard Duty Plastic Screws
Almost all screws designed for plastics have a narrower thread profile, a coarser thread pitch, and a larger deviation between the major and minor diameters compared to similarly sized screws for metals.
This difference in geometry (decreased thread angle) reduces radial stresses in bosses and increases the pull-out strength of the fastener. This reduction can even allow thinner bosses to secure the screw and eliminate the need for supplemental locking devices. When using a metal screw in a plastic boss, the failure point will almost always be the plastic, so these thread profile changes are important.
In addition to thread profile changes, some manufacturers have optimized the screw design even further to improve various factors such as radial stress. The screw shown below is known as a “Hi-Lo” fastener. We can see that half of the threads have a larger major diameter than the other half, hence the “Hi-Lo” name.
The “Hi” threads help improve the pullout resistance and stripping torque, much like a standard screw for plastic. On the other hand, the “Lo” threads help reduce driving torque. These screws may be chosen when a designer is constrained by wall thickness, driving torque or needs improved pullout resistance.
Vibration Resistant Screws
Standard-duty screws will not always work in dynamic situations. A specialty fastener will be a good option if a product is required to operate in a high-vibration environment. This will result in increased costs, but it will likely improve the quality and durability of the product enough to outweigh any cost increase.The first type of screw we’ll look at in this category is known as a plastite screw. This screw improves on the standard plastic screw with a tri-lobular shape, as shown in the drawing below.
As we can see, the screw threads will appear as if they’re triangular instead of circular. This geometry works with the cold-flowing (creep) properties of plastic to increase the removal torque, which in turn, improves the security of the connection.
Another option for high-vibration environments is the BosScrew. This screw combines aspects of a “Hi-Lo” screw with a unique thread profile that includes small “steps” around the profile, as shown in the image below.
The small indentations around the “Hi” thread are intended to take advantage of the cold-flowing properties of plastic, like the plastite screw. However, this screw also has a very high drive-to-strip torque ratio, which reduces the chances of manufacturing defects while maintaining high product performance.
It’s important to note that many screw manufacturers have recommended boss design practices to assist in the development process.
Threadlocker (Loctite)
Another common method for preventing unintended screw loosening from vibration and material relaxation is by utilizing threadlockers. You may know them by brand names such as Loctite or Vibra-Tite.
Threadlockers are adhesives applied to threads before assembly, which provide a non-mechanical attachment method to bond the threads in place after they have been cured in the assembled position. They work well in many applications but must be chosen carefully, especially with plastics.
Some adhesives are not compatible with plastic polymers and will result in the degradation of the plastic and eventual mechanical failure. Many Loctite formulations (242 and 271) tend to cause stress cracking in plastics, especially thermoplastics. Henkel Adhesives recommends Loctite 425 for plastic applications.
Rivets
Rivets are a low-cost and quick assembly mechanical attachment method option for attaching plastic components that are typically used when aesthetics and precision are not the driving requirements.
There are a few rivet designs that are specific to plastic assembly techniques, much like we saw with screws. The image below shows an example of rivets being used to secure a backing nameplate behind a beverage dispenser.
The rivets were utilized here because the plate is made of HDPE, a material that does not glue well with epoxies, and the lid was too thin to screw into.
Next, we’llwalk through some more examples of different rivets, but the main features to remember when using rivets in these plastic assembly techniques are large contact areas and reduced assembly forces.
Peel and Bulb Style
Peel and bulb-style rivets distribute the load across a larger area than a standard snap rivet fastener. They provide a great alternative that takes less force to assemble than rivets for harder metals, reducing the stress induced in the plastic parts assembly.
Below show examples of fasteners with peel and bulb-style rivets:
The peel-style rivet is shown on the left; it splits apart, peeling after insertion. The bulb is shown on the right; it maintains most of the shape through the material, with tabs on either side. Both rivet styles work to distribute the load over a larger area than a standard pop rivet.
Nylon Rivets
Nylon rivets are shaped much like standard pop rivet fasteners but are much softer than those typically used in metals. They also have the benefit of being non-conductive.
These rivets are less likely to damage holes in plastic during assembly due to the lower strength of nylon bodies when compared with the aluminum and steel bodies of most pop rivets.
Some other important factors to consider when looking into nylon rivets are the dimensional stability through moisture absorption and the maximum operating temperature. Nearly all mechanical properties will also be lower than their metal counterparts if strength is of concern.
Snap Fits, Tabs, and Interference Fits
Snap fits and tabs are snap-and-go fasteners with one of the lowest-cost and most commonly used plastic fastening techniques for high-volume plastic parts assembly.
This method uses features molded into the plastic part assembly to fasten pieces together. Interferences are loosely grouped into this category because they rely on elastic deformation to retain components.
Snap fits and Tabs
Snap fits are molded in features that fasten when two or more components are pressed together and are very common in injection molded parts, such as cell phone cases and battery covers.
The battery cover on my Samsung Galaxy uses snap fits to hold it in place and provide the environmental seal. The image below represents a CAD model of a snap-fit cover:
The feature circled in red will act as a snap when inserted into the blue piece. The tab circled in blue will constrain the other degrees of freedom so the snap doesn’t bear the entire load.
The cross-section below shows more detail on how the snap will retain the cover:
Snap fits look simple, but there’s a bit of science behind them. Here’s our snap fit design guide that will help with this process if you’re starting from scratch and don’t want to take the “guess and check” approach.
Interference Fits
Interference fits involve the installation of a fastener into a part by forcing one of the parts to deform elastically.
An example would be press fitting a dowel pin in a hole that is slightly smaller than the dowel pin. These applications do not typically load the interference fit feature in the insertion axis.
Plastic Welding
Plastic welding is exactly what it sounds like—the welding of two thermoplastics together. There are multiple processes for welding two plastic components together, much as there are with metals. A few of the most common processes are detailed below.
Ultrasonic Welding
Using high-frequency ultrasonic vibrations to join plastic mechanical parts together is known as ultrasonic welding. It generates enough energy at the joint between two plastic components to melt the pieces. This process of joining plastic parts is low-cost (aside from the cost of capital equipment) and can be a quick mechanical attachment method.
One example of an ultrasonic welded plastic product is a Tervis tumbler. The image below shows that the joint between the clear outer plastic housing and the blue inner plastic housing is an ultrasonic welded joint.
While this joint is simple and requires no additional bonding materials, it does require special considerations, including proper joint design, material selection, and access to the equipment. The materials that are bonded together not only have to be compatible with the process, they also have to be the same or very similar to work together. So it’s important to consider this in your material selection when designing for manufacturability. When it comes to joint design, there are several options and methods, but they typically require what is known as an energy director between the two parts to improve the energy transfer and, subsequently, the bonding process. This energy director is typically a molded-in triangle around the face of the joint, as shown in the image below.
After the welding process is complete, the gray and blue parts will sit flush against each other due to the energy director melting. A great resource for joint design has been published by Branson and can be found here. Furthermore, the machine will likely require special tooling.. If production quantities are low, this may not be the optimum process.
Thermal Welding
The thermal welding process uses the direct application of heat from a tool like a heat gun. In this process, a filler rod of the same or very similar material is heated and joined together. It’s pretty similar to the process used by an extruder head on a 3D printer to build parts.
The thermal welding process produces a weld bead in the joint that is being joined together, so it may not be aesthetically pleasing. There’s a lot of information about the thermal welding process on the Plastic Welding Tools Website.
Adhesives and Tapes
Adhesives are widely used and can be strong enough for applications like car exterior body panels. The type of adhesive you use should be carefully chosen based on the materials to be bonded, environmental conditions, and performance requirements. Not all adhesives work with all plastics and environments, so pay careful attention when selecting a type.
Glues and Epoxies
Glues and epoxies are the first adhesives that pop into your mind when discussing adhering components together.
There are many different formulations, so it’s important to select the proper glue/epoxy and test various samples. Your selection will depend on both the application and materials. While single-component adhesives are the simplest to use, two-part epoxies typically perform very well and have a broader range of applications.
Epoxies can also coat 3D-printed parts to smooth the lines between build layers. Smooth-on produces a product called XTC-3D specifically for this purpose.
Note that some challenges are also associated with using glues and epoxies. They are sensitive to the type of material, and some materials will be more difficult to bond than others. For example, it will be difficult to adhere to PTFE, polyethylenes, acetals, and some other low-friction plastics. The bonding process is also sensitive to surface cleanliness, preparation, humidity, and other contaminants.
Tapes
It’s very common to use tapes for the assembly of plastic components. These parts range from electronics to ductwork.
My favorite tape is the 3M VHB double-sided series often used in automotive applications, but its uses extend much outside that of automobiles, and the adhesion is excellent on smooth surfaces. However, the appropriate tape must be used for the intended application.
Some important industrial tapes are:
- Kapton (polyimide) Tape: Used in high-temperature applications and electronics assemblies. It’s common to use this on the build surface of a 3D printer because ABS adheres well to it.
- Self-Fusing Silicone Tape: This tape doesn’t have an adhesive on either surface but it fuses with itself when wrapped around an object under tension. The tape can create a flexible and weather-resistant, low-pressure seal.
When using tapes, it’s important to make sure they are suitable with the material, much like you do with glue and epoxy. While this is another low-cost method of mechanical fastening of plastic objects, it’s also one of the least aesthetically pleasing and precise methods.
Warning!
It’s also important to understand some adhesives and tapes’ impact on plastics. For example, it is common for cyanoacrylates (superglue) to attack some plastic surfaces.
This may cause minor problems like cloudiness on the surface of the plastic or even premature failure of a bond due to stress cracking. Before using any tape or adhesive, it’s important to research the impact it may have on your parts carefully.
Specialty Options
The items below don’t fit exactly into the categories above, but they are still valuable to know about for product design. The use of these options may be the performance differentiator that your project needs!
Metal Inserts
Metal Inserts are features embedded into the plastic through many processes, including ultrasonic welding, heat staking, and press fits. In some designs, they are even molded into the part.
Metal inserts are typically threaded features that allow reusable machine screws to be used with the assembly instead of special plastic screws. This makes the threaded connection more durable and reliable when the fastener must be repeatedly removed and reinstalled.
The image below is of a threaded insert in a plastic knob.
This knob is used with a threaded rod and clamp to hold down material on the work bed of my CNC while I machine parts. This assembly (shown on the right-hand side of the image) could hold a substantial amount of force that would not be possible if the plastic knob mounting did not have a threaded insert.
Pro-Tip: Fictiv, you now have the ability to upload parts, generate or upload 2D drawings, and annotate exactly where you want the threaded inserts to be installed, for 3D printed parts.
Elastomer Bands and O-Rings
It’s relatively common to use rubber O-rings, rubber bands, and other elastomers to retain and fasten components. This is typically one of the lowest-cost options but not very structurally sound.
Elastomers are commonly used to hold lids down and retain covers, but are rarely used for high load bearing applications. As an example, the straps on a Yeti cooler are a custom molded elastomer:
Using an elastomer in that application allows for the product to have looser manufacturing tolerances and a constant load on the lid to compress the gaskets.
Main Takeaways
As we have seen, there is a broad range of mechanical fastening methods, so I encourage you to think through mechanical fastening options at the beginning of the process. For more info on materials and processes you can use for your project, check out the Fictiv Capabilities Guide.
As mentioned at the beginning of the article, your sourcing approach should be based on your product requirements, so it’s important to define your product requirements first and then select the best mechanical plastic fastening approach from there.
If there is a level of uncertainty, the best option is to test the most promising methods in their actual application. Having the optimum fastener can make the difference between a beautiful product and an “okay looking” product, or it may determine if you are within budget or not when your design is scaled to production quantities.
Regardless of your product type, I encourage you to discuss these plastic fastener options with your fellow designers and teams to ensure the design is going down the best path for success.
