Mechanical fastening has long been the standard method of keeping manufacturing assemblies together, but adhesive bonding has seen a rise in popularity as an alternative technique.
Mechanical fastening with screws and rivets, welding or soldering and brazing are effective, yet insufficiently distribute load stress, which compromises structural strength. Additionally, they present a risk of galvanic corrosion and can add weight to a product.
Proponents of structural bonding with adhesives tout advantages in weight, greater structural strength and minimization of galvanic corrosion as primary benefits of using bonding agents such as epoxy resins versus traditional mechanical fastening.
Additional benefits of adhesive bonding include:
- Greater fatigue, mechanical shock and thermal shock resistance;
- Contiguous contact between substrates, which promotes improved load-bearing and sealing properties;
- Ability to bond dissimilar substrate materials, as well as materials with different coefficients of thermal expansion;
- Gap-filling capability reduces required tolerances;
- Thermal and/or electrical insulation or conductivity;
- Wide service temperature range;
- Long term durability.
Despite these benefits, engineers need to carefully plan and execute the application of adhesive bonding – it’s not like a piece of tape you can just tear off and reapply.
It’s important to be mindful of joint designs, performance properties of the adhesive, surface preparation requirements, curing requirements and polymer chemistry options.
Whether you need an epoxy resin, hot melt, contact sprays or other chemistries will depend on your application. It’s important to work closely with your adhesive supplier to clearly define how you can meet your application requirements.
Surface Preparation, Joint Design and Application
Just as important as your choice of adhesive is how the adhesive is applied.
To achieve the strongest structural bonds, substrates must be properly prepared, cleaned and roughened to remove oils, greases, dirt, moisture and any forms of oxidation. To accomplish this, physical abrasive treatments and chemical cleaning are usually essential.
“What solvents you use could depend upon your substrate, but acetone flashes off quickly,” said Venkat Nandivada, manager of technical support at Master Bond Inc.
“It’s easier to remove any residuals using that, before it has to be dried before the adhesive is applied. Roughening is also important, especially with metals, as it helps to improve the bond. If mechanical roughening is not possible, there are chemical etching solutions and even plasma treatments.”
Specially formulated primer coats can also be used to maximize the adhesive’s effectiveness.
Adhesive manufacturers have invested significant R&D into pre-treatments for metallic and non-metallic substrates, so it is important to follow their recommendations to ensure the best results.
Adhesive bonds can be applied in several different ways, to suit the product’s design. For example, adhesives could be applied by hand or with automated dispensing systems.
“It depends on the geometry of the substrates and the amount of adhesive being applied, among other factors,” said Nandivada.
“For example, if you’re covering a flat area, you could use a spatula or a brush to spread the adhesive. An automated dispensing system can apply beads and then clamp the product with enough force to squeeze out whatever excess is there and leave a thin, strong bond line.”
Nandivada stresses to be mindful of just how much force is used to squeeze out excess adhesive, however, as squeezing out too much may compromise the strength of the bond. Excess adhesive can be removed later using solvents like acetone or isopropyl alcohol.
This type of application sounds simple, but different cases may require different styles of bonding, called joint styles.
Joint designs fit unique applications, to respond to stresses in different ways. The goal is to minimize stress concentration and maximize bond strength by distributing load over as wide an area as possible.
Some popular joint designs include butt, scarf, lap and offset lap joints.
Butt joints are used when stress forces are concentrated along the bond line and force, perpendicular to the bond, is minimal.
Scarf joints maximize bond area, but parts joined this way require a closer fit.
Lap and offset lap joints are best for thin sections and rigid parts. When using lap joints, however, bonded parts are slightly offset, causing peel and cleavage forces to develop when the joints are under load. Offset lap joints can minimize these forces.
Additionally, thickness of the adhesive plays a significant role.
“For structural applications, we would recommend a few thousandths of an inch in bond thickness,” said Rohit Ramnath, senior product development engineer, at Master Bond.
“You can go thinner or thicker, but the optimal strength is achieved with somewhere around that point to seven thousandths. When you are filling a gap, you cannot have too much thickness. However, for thicker adhesive layers, you may see a drop in strength.”
To ensure reliable results in the application of the adhesive, industrial automation solutions can be employed for maximum precision.
“If you have a two-component epoxy, for example, you can use a meter mix dispenser to measure, mix and dispense exactly the amount of adhesive that is required,” said Nandivada. “If you have a one-part system, it becomes even easier.”
Curing and Testing Adhesive Bonds
Single part systems can be cured with latent heat curing, initiating at 180° to 250°F (82° to 121°C) or a dual-curing mechanism utilizing UV blasts, followed by heat.
In many cases, clamping or fixturing may be necessary to keep the part in place until the adhesive sets.
Pot Life, the amount of time an adhesive can remain viable after being mixed and set time, how much time the adhesive needs to form a good bond, will differ between adhesives due to their individual chemistry.
When choosing an adhesive, it is important to determine what types of stresses the product will experience and select an adhesive resistant to those stresses; for example, thermal expansion.
When testing to failure, the failure mode could either be cohesive or adhesive, Nandivada explained. If you have extremely rigid adhesives, there is a chance for the adhesive itself to crack, but this depends on the stresses and overall conditions.
“There are toughening agents that can be added into a polymer matrix and you can make the adhesive really resistant, to handle thermal stress mismatches between metals across a wide temperature range,” said Ramnath.
The application and curing of epoxy adhesives offers a safety advantage over solvent-based adhesives, Ramnath added.
“You will of course require personal protective equipment while handling epoxy systems. For many adhesives though, you won’t need to be concerned about respirators or anything like that if it’s used in a well-ventilated room.”
Savings and Other Advantages Using Adhesive Bonding
When it comes to the price tag, it’s always application-determined whether the cost will beat out mechanical fastening. However, Nandivada explains there are real savings in larger volume orders with adhesives, as well as other long term benefits.
“In larger quantities, adhesive bonding would be more cost effective compared to a mechanical fastening technique and it would make a difference labor-wise. You need to look not just at the adhesive cost, but the assembly cost overall, as well as processing, handling and so on.”
Ultimately, adhesive bonding stands out for its unique strengths in reducing weight and eliminating significant stress-related weaknesses inherent to mechanical fastening.
Additionally, the technique provides opportunities for bonding small parts, where traditional fastening methods just can’t work.
Today, adhesive bonding is used across the automotive, appliance, aerospace and defense industries as a reliable and cost-effective alternative to mechanical fastening.
You can learn more about adhesive bonding and Master Bond, by visiting their website. If you have a specific application to discuss, contact Master Bond’s technical experts at http://www.masterbond.com/contact.
Master Bond has sponsored this article. All opinions are mine. --Kagan Pittman