Automotive Fasteners - Living Hinge Design

nylon living hinge design

The world we live in right now is shifting towards an all-electric future.  But there’s a big problem with this shift… electric vehicles are substantially heavier than traditional internal combustion vehicles. New EVs weigh hundreds and sometimes even thousands of pounds more than similar internal combustion vehicles.

So, this means tiered automotive manufacturers must figure out how to produce lighter and stronger products to help combat this issue. One shift the industry has made has been converting metal to plastic. This is nothing new. What has changed is more strict requirements. Off-the-shelf fasteners won't always do the trick. The fasteners must be expertly designed and molded properly, especially when you’re talking about underbody parts that fasten critical components like brake lines. In some cases, this means having an in-depth knowledge of what could potentially be a failure point.

One failure point in particular regarding automotive fasteners can be living hinges.  So, in this post, we will take you through not just the basics of a successful living hinge but also design tips, material breakdown, and what you need to get right with the tooling and injection molding process.  With that said, let's get into it...

Basics of Living Hinges

What are Living Hinges

Injection-molded living hinges are thin, flexible joints that join two bodies together that is made from the same material as the bodies.  The thin material allows the two bodies to bend along the line of the hinge, as you can see in the image here.

automotive clip with living hingeautomotive clip with living hinge

Applications for Living Hinges on Fasteners

When it comes to automotive applications, fasteners that include an integrated living hinge will typically be used for fluid routing tube clips or wire harness fasteners.  The main reason is that the fastener needs to wrap around these products and maintain its hold. Designs to meet these applications' needs will typically come in the form of a clamshell design.

By wrapping around these parts, the fasteners can be installed/removed at any point without needing to remove the entire system. Just imagine having to remove an entire brake line system to slide off/on a new clip. Not ideal.

Common Challenges of Living Hinges

The challenge for most living hinges is developing a design that is thin enough to effectively bend without snapping yet thick enough to withstand the stresses exerted on the living hinge during molding and the flexing movement. They also present additional challenges for the molding process, as the material will need to flow through these areas. When not done properly, you can experience material hesitation, resulting in it cooling rapidly and not flowing properly across the part.

Advantages of Living Hinges

Injection-molded living hinges have several advantages. Compared to metal alternatives, they don’t corrode, are less expensive in mass production volumes, and weigh less.  They also reduce part numbers/SKUs and remove the need for additional assembly tools (screwdriver, impact driver, etc.), which helps reduce manufacturing costs.

Disadvantages of Living Hinges

Two of the main disadvantages you'll find with including living hinges on automotive fasteners are:

  • They are more prone to snap, especially when stored in cold, dry environments.
  • They can be more complicated to effectively injection mold due to the thin profile the molten plastic will need to move through in order to completely mold without being short shot or having performance-related issues.

Design Engineering Living Hinges

Let’s get into some of the technical aspects of creating an automotive fastener’s living hinge.  In this portion of the post we will take you through the different types of living hinges and design elements for moldability that impacts several critical areas of the fastener’s performance and ability to pass testing.

Living Hinge Designs

When engineering a product that will incorporate a living hinge, there are several design options that have been commonly used in various industries, ranging from shampoo bottle caps to container boxes. With automotive fasteners, it’s typically narrowed down to just one of the most common options: the flat hinge.

Flat Hinge

A flat hinge will be one of the most common forms of living hinges you’ll find. This version is a straight hinge design that falls along a single axis with two sides connected via a thin hinge.  This design is a basic, straightforward option used in the majority of automotive fasteners that require living hinges.

flat living hingeflat living hinge

Living Hinge Designs Not Typically Used for Automotive Fasteners

The flat hinge design will most likely be your go-to option for the majority of automotive clips & fasteners due to its simplicity, and there is no real need to open/close the hinge multiple times.  The simplicity of a flat hinge is beneficial as well in relation to tooling, as other options can sometimes have more features, resulting in more complex/expensive tooling.

While these are still unlikely to be implemented in an automotive fastener design, we still wanted to take you through a few of the common living hinge options:

  • Double Hinge: The double hinge is two flat hinges separated by a thin opening strip. This design allows the hinge to be more flexible and reach up to 360 degrees of rotation.
  • Butterfly Hinge: Butterfly hinges allow for a certain range of motion and work by snapping open, close, or somewhere in between.
  • Bi-Stable Hinge: The bi-stable hinge is essentially a butterfly hinge with three hinged sections instead of one, making it the most flexible hinge type.

Design Guidelines and Moldability of Living Hinges

As mentioned before, design is critical to the clip's success, including the part’s moldability.  A factor that needs to be kept in mind with molding living hinges is understanding the hinge’s wall thickness and how it’s impacted by material flow and cooling.  The strength and durability of a hinge will come from the high polymer orientation gained when the material is forced to flow across the thin feature under high stress.

Hinge Thickness

When designing the hinge portion of your fastener, you're going to want the thickness of the hinge itself to be somewhere between 0.008” - 0.015” thick. Avoid going thinner than 0.008” as this can create excessive local shear, shear heat buildup, and short shots/under-packed fasteners. You can also modify the thickness of the hinge to alter its stiffness and feel.

Corner Radius

In the world of injection molding, a “corner radius” describes the rounding out of a sharp corner. When designing a living hinge, including a decent corner radius will help to improve the material flow through the hinge during the injection molding process. This in turn, helps reduce the risk of short shots and stress concentration. It also benefits the design during the cooling process as the hinge will shrink during that stage, and shrinkage around a sharp angle can add additional stress that could lead to cracking.

When determining the radius, a general rule of thumb is to make it around 0.5 times the adjacent wall thickness.

living hinge corner radiusliving hinge corner radius

What to Avoid

  • Sharp Folds: It’s important that the hinge’s design avoids being a sharp fold and provides enough relief to avoid a concentrated area of stress that will likely cause the fastener to snap and fail.
  • Oversized / Undersized Hinge Length: This isn’t a common issue for automotive fasteners, but it is still worth noting. Oversizing a living hinge is mainly an issue regarding wasted material, space, and weight. Undersizing the hinge will cause issues with the fastener's ability to close easily and effectively.

Injection Molding Clips with Living Hinges

Once a fastener is designed and approved, it’s time to get into what goes into the production process. This includes elements of the injection molding process, such as understanding complex tooling, injection molding presses, material flow, and temperature restrictions.

Injection Molding

In regards to automotive clips and fasteners that include living hinge designs, you’re most likely going to see these be injection molded due to speed and ability to produce in mass quantities at low pricing. Injection molding involves melting plastic resin pellets and using pressure to inject them into a mold. Several factors need to be considered when it comes to injection molding, such as tooling, material, and the number of parts needed.


Critical Tooling Elements for Molding Living Hinges

There are several fundamentals of injection molding tools that are extremely important, but for this post and our focus on living hinges, we will go over a couple of the most critical areas of tooling that will have the most impact on the success of the fastener.

Gate Placement

Gate placement’s role involves ensuring the material flows through the living hinge at a consistent rate and direction.  The proper material flow will reduce knit lines that can result in part failure. Out of everything in this section, gate placement is one of the most critical aspects to successfully injection molding a fastener with a living hinge feature.

You'll likely never want to place the injection gate on the hinge directly for automotive fasteners as you risk the resin freezing off and not packing correctly.

For most fasteners with living hinges, you'll want to gate at the thickest portion of the clip and flow from thick to thin toward the hinge.  Ideally, the gate would be placed in such a way as to maximize the amount of linear material flow, running parallel to the hinge length. If not, transverse stress introduces weak points to the hinge, which is not ideal for optimal hinge performance.

Living Hinge Gate Placement Example

Go ahead and check out the image in this section. In this example, the arrow points toward where the gate location was placed. So, take a second and think about why this works for this automotive fastener design.

Why this placement works well:

  • The gate location will help maximize the amount of linear material flow parallel to the hinge length.
  • It's centrally located, so the material will NOT have to flow through the hinge to pack the thickest portion (the base of the fastener) as it is located between the hinge and the base.
  • Gate is also located in one of the thicker sections of the clip, ensuring that the gate will not freeze off from the rest of the clip. This results in proper filling and packing of the entire clip.
gate placement for automotive fastenersgate placement for automotive fasteners

Material Flow

When it comes to injection molding, you are taking a hot molten material and packing it into a cooler cavity, as steel plates will take away heat resulting in material cooling. The material flow and melt quality are critical elements to factor in.  For example, nylon is a semi-crystalline material, which means it is more difficult to melt than amorphous resins.

The goal with molecular chain flow is to get the molecules to orient themselves parallel to the living hinge length, which will primarily be based on gate location.


Cooling

Injection molders need to ensure proper cooling flows through the tooling, including around the living hinge section. High mold temperature near the hinge area that lacks proper cooling will most likely result in a weak hinge. Cooling will also take up the majority of your cycle time.


Ejecting

The injection molding process includes the need to eject the fasteners out of the mold. Ejector pin location should not be located on the living hinge itself for two main reasons:

  1. The living hinge doesn't have enough surface area to put a pin on it
  2. Ejector pins could damage or even tear through the hinge

Most automotive fasteners will be used in underbody / underhood applications, so you won't need to be concerned about the visibility of ejector pin marks as long as they do not impact the part's performance.

ejector pin location for living hingesejector pin location for living hinges

Material Selection

Most living hinge products you’re probably more accustomed to seeing, such as the ones on bottle caps and storage containers, are typically made with PP and PE (HDPE/LDPE) due to their toughness and low price. But these materials will not work for an automotive fastening application for various reasons.

Nylon

Nylon is typically the default material selection for automotive fasteners, but it has limited life when it comes to living hinge applications. Luckily, most automotive fasteners/clips will rarely, if ever, need to be reopened.

Nylon is a high-performance thermoplastic, which makes it an excellent material for automotive clips and fasteners for various reasons. Nylon has high tensile strength, wear resistance, and heat resistance; and it possesses excellent chemical resistance for certain chemicals.

While glass-filled nylon may be an excellent material option for many other automotive applications such as barbed hose connectors, it is not recommended for fasteners, especially those with a living hinge design. The glass filler makes the living hinge more brittle and stiff.

The form of nylon most commonly used is an impact-modified version with high flow that is typically used for fasteners and clamps, such as Ultramid® A3L.

One note about using nylon in living hinge designs: unlike polypropylene, nylon hinges should NOT be flexed right out of the mold. Lack of moisture will actually cause the hinge to more than likely snap. But what other impact does moisture play on nylon?

Moisture’s Impact on Nylon

Do not use the dry-as-molded properties of nylon.  You need 50% relative humidity properties when you calculate the product design. We’ve covered this topic in a previous post, but it is important to note that nylon is hygroscopic. This means that it’s able to absorb water from its surrounding.

Concerning automotive clips, moisture needs to be factored in, especially when they have snap features and/or living hinges.  There are 3 areas in particular we covered, but to quickly summarize, they are:

  • Design: Do not use the dry-as-molded properties of nylon.  You need 50% relative humidity properties when you calculate the product design.
  • Drying: Nylon must be dried before processing. Once it is processed, it will be at its driest point. If not done properly, the end product will end up being weak. it is processed, it will be its driest point. If not done properly, the end product will end up being a weak part.
  • Packaging: In many cases, especially during the winter and in drier climates, you’ll want to have these parts packaged with moisture inside to condition them during shipment/storage.

Prototyping

When it comes to automotive applications, some of the primary reasons for wanting to prototype a living hinge fastener is to ensure proper moldability and testing, such as insertion force testing.  Because of this, you’ll rarely see 3D printing as the method for prototyping one of these components. The main reason for this is that 3d printed material options are limited and are typically fairly brittle, which won’t give you the ability to test to automotive requirements.

If we’re concerned about a design, then you would do a prototype tool made of steel or aluminum  Likely, you’ll want to go with steel due to costs. This typically takes roughly 4-6 weeks, dependent on the simplicity vs. complexity of the design.


Conclusion

Injection molded automotive fasteners have numerous factors that are factored into achieving the end product that meets OEM’s strict requirements. Converting metal to plastic has its challenges. Because of this, it is essential to have a solid understanding of engineering design, material knowledge, tooling, and manufacturing fundamentals.  

Automotive fasteners differ in many ways from products with living hinges you're more accustomed to seeing in your day-to-day life. Because of this, they shouldn't be treated the same. If you're looking to start your next automotive fastener project, whether you require molding expertise or a fastener designed from scratch, reach out to us today by filling out the contact form below.

Connect with our team of pros!

We'd love to help you solve your company's biggest production challenges.

Let us know what project you're working on and how our team can help you create the best solution!

Connect with our team of pros!

We'd love to help you solve your company's biggest production challenges.

Let us know what project you're working on and how our team can help you create the best solution!

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