What is bondline thickness, and why is it important?

When designing a joint, there are several factors you need to consider. Bondline thickness is one of those factors.

Here at Forgeway, we help manufacturers in the commercial vehicle, aerospace, and rail industries with their joint design. When it comes to bondline thickness, we know that it can be difficult to understand what it actually means and why it’s so important.

This article will discuss the definition of bondline thickness, provide some examples, and then analyse why it’s so important. By the end of the article, you will have a good understanding of bondline thickness. 

The next time you need to design joints, you will no longer struggle to understand what bondline thickness is, and what it should be for your specific application.

What is bondline thickness?

Bondline thickness is the thickness of the adhesive between the substrates. The thickness will vary depending on the structure you are bonding. However, bondline thickness is usually between 0.5mm and 4mm.

Bondline thickness
Bondline thickness is anywhere between 0.5mm and 4mm

In this article, we are talking about the bond line thickness for structural applications when using structural adhesives. Bondline thickness for smaller DIY projects, contact adhesives and packaging adhesives would be very different.

When designing a bonded structure, you need to consider the different types of stresses each bond will experience. The bond could face exposure to either static or dynamic loading. Therefore, the bondline thickness must be suitable to cope with the stress it is going to experience.

A static loading is where there is constant and unchanging stress on the adhesive. An example would be if you were to bond a shower panel to the wall. The stress on the bond will not fluctuate as the wall isn’t likely to move and neither is the shower panel. The bondline thickness needs to accommodate for the fact that the shower panel shouldn’t experience any movement. An effective bondline thickness for joints under a static load is between 0.5mm and 2mm.

A dynamic loading is where there are constantly changing forces on a bond over time. A typical example of a dynamic load would be bonding a glass window to a vehicle. The bond between the vehicle structure and glass window will be exposed to constantly changing forces over time and the amount of stress on the bond will fluctuate. An effective bondline thickness for joints under a dynamic load is above 2mm and rarely more than 4mm.

Applying sealant to a vehicle

Why is bondline thickness important?

There are three main reasons why bondline thickness is such an important part of the joint design:

  1. Ensuring the right level of movement in the joint. When two substrates are rigid and the structure will be exposed to a lot of movement, you need a flexible adhesive to transfer (or dissipate) the stress put through the substrates. A typical example of this is when bonding glass to metal in a vehicle. If the adhesive is too rigid or the bondline thickness is too thin, the adhesive will not be able to adequately dissipate the constantly changing forces put on the structure and will therefore lead to the glass breaking. You need to ensure the bondline is thick enough to transfer this stress.
  1. Ensuring the optimum strength from your adhesive. Most adhesives will have an ‘optimum’ bondline thickness to achieve maximum strength. This will obviously vary depending on the adhesive. Typically, ticker bondlines will break at lower strengths than thinner bondlines. This isn’t to say thinner is always better. You must also consider the exposure to movement the joint will experience.
  1. Ensuring you get the right aesthetics from the joint. If the bondline is too thick, the adhesive itself could be exposed. This will ultimately alter the aesthetics of the structure and could lead to increased amounts of yellowing, or polymer degredation over time.
Polymer degradation can occur

When do you need to consider bondline thickness?

There are two stages where you need to consider the bondline thickness. 

  1. The design stage
  1. The application stage

Ensuring the bond design is correct

At the design stage, you need to ensure that the bondline is the correct thickness to perform its job correctly. Testing is the most efficient method of guaranteeing the bondline is set at the correct thickness for your bonded structure.

Seeking the help of an adhesive expert who will help you speed up the process is another way to determine how thick the bondline should be. 

If you are working under the guidance of DIN 6701 or DIN 2304, the designer of the bonded structure needs to ensure (and be able to demonstrate) that the design will be able to cope with the loads and stresses it will be exposed to. Or in other words, mitigate the risk it poses if an adhesive failure should occur.

So ensuring that the bondline thickness is sufficient to mitigate this risk is an important part of the design process. 

Whilst DIN 6701 and DIN 2304 are German standards, the same guide and method of determining risk, and mitigating that risk should be applied in any bonding situation; especially where there is a threat to life.

DIN 6701 helps ensure the bondline thickness is sufficient to reduce risk

How to make sure the application of the bondline thickness is correct

At the point of adhesive application, you need to make sure that the correct bondline thickness is actually put into practice on your production line. 

The operative applying the adhesive should be aware of the correct bondline thickness according to drawings or standard operating procedures for example. Nonetheless, there are mitigating steps you can take to ensure that the correct bondline thickness is achieved every time.

  1. Design your substrates to have an ‘in-built’ bondline. This will help ensure you always achieve the same bondline thickness during the assembly process. For example, an edge capping rail for a commercial vehicle may have a recess designed into it to accommodate a 2mm thick bondline. When it is assembled the adhesive will squash down to 2mm every time. 
  1. Put spacers/packers between the substrates. This is an easy and effective method of ensuring the correct bondline thickness. The spacers will make sure the bondline between substrates will be the thickness you desire.
  1. Put spacers in the adhesive itself. These spacers come in the form of glass or ceramic beads. They don’t affect the performance of the adhesive and work in much the same way as conventional spacers. However, the adhesive manufacturer has to include these in the formulation during the manufacturing process. This isn’t something you can do yourself.

Bondline thickness; making sure you get it right

The joint design process is never easy. Nonetheless, it is a critical part of making sure the bonds will not fail. When it comes to bondline thickness, you must know the exact thickness your application requires.

Here at Forgeway, we have helped hundreds of companies measure and determine the correct bondline thickness for their application. We know that it is never the same and there are multiple factors that can affect it.

This article will have given you an overview of bondline thickness and why it’s so important. It’s now down to you to put that into practice. But it’s not quite that simple, we know that.

If you want the help of a team member from us here at Forgeway, a member of our team would be more than happy to help.

Alternatively, if you want further help with joint designs, you can read chapter six of our Ebook which goes into more detail about joint design. You can download chapter 6 by following the link below.

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