From Snap Fits to Adhesives: A Comprehensive Guide to Mechanical Fastener Options
Written by
Sean Thomas

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On
May 27, 2016
In
Engineering

Recently, Fictiv pulled together a guide to fasteners for 3D printed parts, which includes an overview of threaded inserts, self threading screws, designing threads into your model, captured hex nuts, and cutting threads with a tap.

Today, we’re going to dive in a bit deeper with a more comprehensive guide to all the options available for fastening plastic assemblies, 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 fastening methods for your product design.

The approach for choosing the right fastening option should be based on product requirements. These requirements can be driven by factors such as cost, quality, manufacturability, and technical factors that impact performance, such as corrosion resistance, material compatibility, conductivity and more.  

Types of Fasteners

Screws

Screws are among the most common fastening methods for assemblies, ranging from automobiles to plastic housings for consumer electronics.

The basic design of a screw is well known—an inclined plane wrapped around a metal dowel. But the design intricacies for screwing into different materials are broad.

Let’s go through some screw designs that are specifically engineered for plastics.

Standard Duty Plastic Screws

Almost all screws that are designed for plastics have a narrower thread profile, a coarse thread pitch, and a larger difference between the major and minor diameters when compared to similarly sized screws for metals.

This difference in geometry is to reduce radial stresses in bosses and increase the pull-out strength of the fastener. When it comes to using a metal screw in a plastic boss, the failure point will almost always be the plastic, so these thread profile changes are important.

Stanley Engineered Fastening has more information regarding metal screws in plastic parts here.

Difference in thread profile for screws designs for plastics

In addition to thread profile changes, some manufacturers have taken the screw design even further to improve on various factors. 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 to 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. If a product is required to operate in a high vibration environment, a specialty fastener will be a good option. This will never be the lowest cost choice, but it may 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 knows 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, this means the screw threads will almost 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 reliability 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 from shakeproof below.

The small indentations around the “Hi” thread are intended to take advantage of the cold flowing properties of plastic, similar to the plastite screw. However, this screw also has a very high drive-to-strip torque ratio, which reduces the chances for manufacturing defects while maintaining high product performance.  

It’s important to note that many screw manufacturers have recommended boss design practices that will assist in the development process. An example of boss recommendations can be found in the Stanley Engineered Fastening guide.

Threadlocker (loctite)

Another common method for preventing unintended screw loosening from vibration and material relaxation is through the use of threadlockers. You may know them by brand names such as Loctite or Vibra-Tite.

Threadlockers are adhesives that are applied to threads before assembly, which bond the threads in place after they have been cured in the assembled position. They work well in many applications but need to be chosen carefully, especially with plastics.

Many loctite formulations (such as 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 option for attaching components together that are typically used when aesthetics and precision are not the driving requirements.

There are a few rivet designs that are specific to plastics much like we saw with screws. The image below shows an example of rivets being used to secure a backing name plate behind the tap on my own portable kegerator.

I chose to use rivets here because the plate was made of HDPE, a material that does not glue well would with epoxies, and the lid was too thin to screw into.

The following paragraphs will walk through some more examples of different rivets, but the main features to keep in mind when using rivets in plastic 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 pop-rivet. They also take less force to assemble than rivets for harder metals, which reduces the stresses induced in plastic parts during assembly.

Below, we can see what peel and bulb style rivets look like:

I’m sure the images give it away, but the peel style rivet is shown on the left, while the bulb is shown on the right. Both of these 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 rivets, but are much softer than the rivets 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 the absorption of moisture 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 one of the lowest-cost and most commonly used fastening techniques for high-volume plastic parts.

This method uses features molded into the actual plastic part to fasten pieces together. Interference 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:

We can see how 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. You can also download my CAD file for this snap design and adjust it for your particular product needs.

Download my snap fit design CAD model

Interference Fits

Interference fits involve the installation of a fastener into a part by forcing one of the parts to elastically deform.

An example would be press fitting a dowel pin in a hole that is slightly smaller than the dowel pin is. These applications do not typically load the interference fit feature in the axis of insertion.

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

The process of using high frequency ultrasonic vibrations to join plastic parts together is known as ultrasonic welding. It generates enough energy at the joint between two plastic components to melt the pieces together. This process is low-cost (aside from the cost of capital equipment) and can be very quick.

One example of an ultrasonic welded plastic product is a tervis tumbler. In the image below, we can see 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. The main ones being proper joint design, material selection and access to the equipment. The materials that are to be 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 knows 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 all the way around the face of the joint as shown in the image below.

After the welding process is complete, the grey part and the blue part will sit flush against each other as a result of 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 to hold the If production quantities are low, so 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, the parts to be joined, as well as a filler rod of the same or very similar material, are heated and joined together. It’s actually 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 at the Plastic Welding Tools Website.

Adhesives and Tapes

Adhesives are widely used and can even 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 probably the first adhesives that pop into mind when we talk about adhering components together.

There are many different formulations, so it’s important to select the proper glue/epoxy and even test a variety of 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 be used to 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 there are some challenges associated with using glues and epoxies as well. 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. Additionally, the bonding process is sensitive to surface cleanliness, preparation, humidity and other contaminates.

Tapes

It’s very common to use tapes for the assembly of plastic components. These parts can range from electronics to ductwork.

My absolute favorite tape is the 3M VHB double sided series often used in automotive applications, but 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 temp applications and electronics assemblies. It’s actually common to use this on the build surface of a 3D printer because ABS adheres well to it.
  • Self-Fusing Silicone Tape: This is a type of tape that doesn’t actually have an adhesive on either surface, but that 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 (usually) low cost method of fastening objects, it’s also one of the least aesthetically pleasing and precise methods.

Warning!

It’s also important to understand the impact that some adhesives and tapes have 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 carefully research the impact it may have on your parts.

Specialty Options

The list of 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 that are embedded into the plastic through a multitude of 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) can hold a substantial amount of force that would not be possible if the plastic knob did not have a threaded insert.

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 typically 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:

The use of an elastomer in that application allows for the product to have looser manufacturing tolerances as well as a constant load on the lid to compress the gaskets.

Main Takeaways

As we have seen, there is a broad range of mechanical fasteners, so I encourage you to think through 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 start 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 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 “ok 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 fastener options with your fellow designers and teams to make sure the design is going down the best path for success.

Written by
Sean Thomas
I'm a mechanical engineer with over 10 years experience in the defense & energy industries. My passion is in highly analytical and sketch to scale development.
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