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Sagging adhesive that drips down the substrate or squeezes out of a joint line is an all-too-common occurrence. The adhesive moved before it set, and the bond you designed is not the bond you got. While it may seem like a small issue, it can become a big problem for manufacturers. Fortunately, these adhesive slumps, drips, and sags are avoidable. Our team shares some tips on how to prevent adhesive sagging with a little more control.
What Is Adhesive Sagging?
Adhesive sagging, also sometimes called adhesive slumping or adhesive dripping, is the flow of uncured adhesive after dispensing. It is typically caused by the pull of gravity on the adhesives's own weight. Because the adhesive has not yet cured, its resistance to flow is lower than the gravitational pull. In technical terms, the adhesive begins to sag when gravitational stress exceeds its yield stress, or the force required to make it flow. An adhesive with sufficient yield stress holds its shape on a vertical surface. One with little or no yield stress flows downward easily and falls victim to adhesive sagging.
Adhesive sagging can lead to the following three results:
- Adhesive on the substrate. Drips of adhesive can appear on the substrate as the bead deforms and travels down a vertical surface before it cures.
- Adhesive oozing out of joints. Under clamp pressure, low-viscosity materials can escape the bond line and run. The outcome is a thin, starved joint and a cosmetic mess.
- Drip off the nozzle or bead. If the material is fluid enough to separate and fall, it can leave strings and spatter on the part below.
The end result may be an aesthetic or functional issue, depending on the application. Either way, adhesive slumping can be costly for manufacturers.
The Top Ways to Fix Adhesive Slumping
While adhesive sagging and slumping can be detrimental, they are largely preventable. Our team is sharing the top tips to prevent adhesive sagging in your application.
1. Switch to a Thixotropic or Non-Sag Formulation
Many people will switch to an adhesive with a higher viscosity, but this is usually not the solution. Instead, a thixotropic or non-sag adhesive is the way to go. Especially for vertical applications, a thixotropic adhesive may solve the problem completely.
A thixotropic adhesive, also sometimes called a non-sag adhesive, is a paste-like adhesive whose viscosity can be manipulated to meet the needs of the application. Thixotropic adhesives will generally thin out and flow freely when under pressure or stress, such as when they are being pumped through a static mixer or a gun. Once the pressure is released, their viscosity will change, and they will thicken while at rest. This property is what makes thixotropic adhesives such a good solution to adhesive sagging.
Thixotropic and non-sag adhesives are usually two-part epoxies, MMAs, or structural polyurethanes. Look for a product that is described as thixotropic, non-sag, non-slump, low-slump, gap-filling, or suitable for vertical applications in the technical datasheet. Many structural adhesive datasheets will also list a maximum bead thickness that will not sag at a stated temperature. This number is tied to ASTM D2202 or a manufacturer's internal vertical-flow test.
2. Change Your Dispensing
An adhesive bead that behaves fine on a horizontal bench can slump when the same part is stood up vertically. Try changing your dispensing in the following ways:
- Reduce the bead height and increase the bead count. Two smaller parallel beads carry the same volume as one tall bead with far less material weight that is being pulled down by gravity. A tall, round bead has a high center of mass and a small contact footprint. It is also prone to rolling and running.
- Use a triangular or V-profile bead. For vertical surfaces where the nozzle allows it, a wide base with a tapered top resists tipping better than a fat round bead.
- Do not over-apply. Excess volume is the most common reason adhesive oozes out of joints. If you have a gap to fill, select a gap-filling, non-sag adhesive rather than using more of a general-purpose product.
3. Control the Temperature
Viscosity is largely dependent on temperature, and adhesive slumping is more common when the adhesive or work environment is outside of the acceptable temperature range. Excess temperature drops viscosity, extends open time, and gives the material a longer window in which to run. Ultimately, this can lead to dripping, stringing, or sagging of the adhesive. On the other hand, some two-part adhesives will gel more slowly when they are cold, which leads to a longer sag window.
Avoid leaving adhesive containers in the sun, out in the cold, or exposed to temperatures outside of their recommended storage range. Warm substrates can also lead to adhesive slumping, so consider the temperature of the substrate before application.
4. Shorten the Window Between Application and Set Time
Adhesive slumping is a war between the pull of gravity and the time it takes the adhesive to set. To minimize the impact of gravity and reduce the amount of adhesive sagging, choose an adhesive with a faster set time. An adhesive that reaches handling time in a few minutes gives gravity less time to work against it. That being said, you should still be mindful that the open time is compatible with your application.
You should also look to assemble parts promptly after dispensing the adhesive. Apply the adhesive in a sequence that allows you to align and join the parts before it has time to sag. On larger vertical panels, consider tacking or adding smaller amounts of fast-setting hot melts or cyanoacrylates to hold the parts in place while the structural adhesive cures.
5. Hold the Part in Place
It is imperative to keep the bonded assembly from moving while the adhesive sets. Movement in the bond line after the adhesive has begun to set can weaken adhesion or damage the joint. Even a non-sag adhesive can fail if the parts creep, slide, or get bumped during curing.
To hold the parts in place, use a clamp or fixture, but be mindful not to overclamp. Glass beads, shims, or fixture stops can help maintain the right bond-line thickness while preventing overclamping. Temporary screws, cleats, magnets, or braces can also be used to help support the assembly’s weight. For repeat assemblies, a jig can ensure consistent positioning and reduce variability.
Always refer to the fixture time listed in the technical datasheet to avoid moving or handling the parts too early. Overhead assemblies especially require continuous support throughout the fixture period to prevent adhesive sagging or movement.
6. Clean Up Squeeze-Out Properly
If any material does escape the joint, cleaning can help, but timing your cleanup is important. Most structural adhesives are easiest to remove when partially gelled. Wiping fully liquid squeeze-out can smear it and contaminate an adjacent bonding surface.
For easy cleanup, mask the joint line before you dispense and pull the tape up while the adhesive is still uncured. You should also never use a solvent near an uncured bond line unless the manufacturer says it is safe, as you risk pulling the solvent into the joint.
Getting It Right
A lot of adhesive slumping issues tie back to human error. Some manufacturers choose the wrong adhesive while others make mistakes during the dispensing process. Especially for vertical or overhead assemblies, avoiding adhesive sagging should be a key consideration from the start.
If you need help troubleshooting adhesive sag or preventing it, our team is here to assist you. With more than 50 years of experience in the adhesives industry, we can guide you through product selection and dispensing. Connect with our team to get started.
