In “Selecting the Right Sign Adhesive for the Job,” you learned how various substrates affect adhesive performance. Now that you’ve selected the right adhesive for your project, how can you apply it to get best results? In this four-part blog series, we’ll show you how.
Complete adhesive coverage in a bonding area is important, but how much is too much? With adhesives, more is not necessarily better.
A relatively thin bondline is usually recommended for several reasons. Just as Part Two of our series showed that adhesive beads should not be placed in joints where they will experience cleavage or peel forces, these forces can also be introduced by excessive bead thickness. Thick beads of adhesive may experience a shift in axial loading and consequent stress at corners of the joint. They may also have more air voids than thin beads, which can reduce structural strength. Similar problems may occur if the bondline is not uniform. Furthermore, thick beads may cure differently than thin ones, which in turn changes internal stresses.
The addition of glass beads into an adhesive formula allows precise control of the bondline thickness. The glass spheres are added at a very small percentage of the formulation weight to maintain the desired thickness for the application when the material is dispensed. When added at such a low percentage, glass beads can maintain the appropriate bond line thickness without negatively affecting the properties of the material.
The rule of thumb is to dispense a bead with a diameter about 1/6th to 1/4th the width of the bond joint. Using this rule of thumb, it’s possible to estimate how much adhesive you’ll need for a given job. Check out our online Structural Adhesives Volume Estimating Tools to find estimated linear foot coverage based on cartridge size and bead diameter. Applicators should also allow for variation in bondline thickness around the periphery of the part and increase the bead size dispensed in areas of poor fit to assure adequate coverage.
Thin-gauge aluminum—as thin as 2 mm—is often used as a sign substrate and is usually bonded to another type of material. Particularly common are aluminum composite material (ACM) panels, which are composed of a polypropylene or polyethylene core sandwiched between aluminum sheets. A familiar problem in these assemblies is bondline read-through. Read-through creates rippling, or panel surface distortion, that is all too easy to see against a sign’s flat surface. It is caused when structural adhesives heat up (exotherm) during curing and soften the ACM panel’s core. The softening, combined with adhesive shrinkage, pulls at the facing.
To prevent read-through, sign designers should avoid using stiff adhesives that exhibit excessive exotherm and shrinkage during cure.
Signlok 810 Low Read-Through (LRT) acrylic adhesive was formulated to be low shrinkage and low exotherm while still offering high-strength. It bonds to painted surfaces and can cross bond to all metals commonly used in sign brackets, posts or extrusions. In addition to ACM panels, the acrylic structural adhesive bonds thin, flexible plastic and composite substrates with no bondline read-through.
Adhesive thickness also plays a part. When bonding sensitive materials, it’s recommended to maintain a thin and consistent bondline of 0.010 inches (10 mils). Applicators should remove squeeze-out and weight the bondline appropriately so that full surface contact results between the substrates and gaps are avoided.
Applying the recommended amount of adhesive while bonding signage—and maintaining a consistent bondline—are strategies that will improve not only the appearance of the sign, but its structural performance as well.
Read more in our blog series on adhesive application tips for the sign industry: