Bondline read-through can present a challenge when laminating signs. (Laminating is the process of bonding two or more layers of material together with an adhesive.) Read-through creates panel surface distortions that are all too easy to see against a sign’s flat, glossy surface. The problem is exacerbated by the very thin substrates that are increasingly used in modern signage—think, for example, of aluminum composite materials (ACMs), which are composed of a polypropylene or polyethylene core sandwiched between thin-gauge aluminum.
ACMs are made of lightweight, untempered aluminum with a soft, low-melting-point plastic core. During the curing process, traditional structural adhesives heat up (exotherm), softening the ACM panel’s core. This softening, combined with adhesive shrinkage, pulls down the facing or painted surface. The result is read-through that is visible on the surface. Therefore, sign designers should avoid using a stiff adhesive that exhibits excessive exotherm and shrinkage during cure. Newer acrylic structural adhesives are a better option, being low shrinkage and low exotherm while still offering high-strength.
The Benefits of No-Mix Adhesives
Large assemblies with a tight tolerance, like mounting a sign face, may benefit from the LORD No-Mix Series. No-mix adhesives, also known as “honeymoon” adhesives, allow an indefinite open time. Examples include LORD 201 and 204 adhesives used with LORD Accelerator 4. These products perform well on acrylic, bare metals, painted metals, aluminum composites, polycarbonate, vinyl and high-density urethane foams.
LORD 201 adhesive has a thinner consistency and is best for brushing applications. LORD 204 adhesive is non-sag and best to spread with a notched trowel. The adhesives are applied on the opposite mating surfaces of the substrates to be bonded. Curing does not start until the parts are brought together. Recommended bondline thickness of LORD 201 and 204 adhesives is 10 mils and can be no more than 20 mils.
Following a few best practices during sign assembly will help ensure a smooth, mar-free surface: It’s critical to limit the amount of adhesive that squeezes-out during application. Excess adhesive that remains around the attachment point of the substrate will intensify the read-through problem. It’s also important to use the right dispensing system. Options are available for high- and low-volume applications. Hand-held applicators are ideal for small- to mid-sized sign shops and automated dispensing systems can be implemented for high-production operations.
Laminating Sign Foam with LORD 7650 Adhesive
Sign designers can use readily available grades and standard foam thicknesses to manufacture a variety of designs by layering the foam and laminating the layers. Laminating reduces the cost and the need for storage space and inventory of various grades and thicknesses of sign foam.
LORD 7650 is a good adhesive choice for foam. It can be applied with a brush or roll at a recommended dry film thickness of 2-4 mils (.002-.004 inches) or a wet film thickness of 3-6 mils (0.003-0.006 inches). LORD 7650 adhesive can also be applied by spray if done in a spray booth with proper ventilation.
Once the adhesive has been applied, applicators should wait approximately 20-30 minutes for a good tack to develop as the solvent evaporates. Next steps are to mate the substrates, slide to reposition and apply uniform pressure with a board and/ or weights. The assembly should remain under pressure for 10-24 hours at 75°F to reach handling strength. LORD 7650 adhesive will fully cure in one to five days, depending on humidity. Once fully cured, the bonded area can be cut on a router to the desired shape and finished with a primer or sanding.
Careful craftsmanship—along with use of the correct adhesive—can result in high quality signs as well as efficient operations and minimization of inventory. For more information on how to assemble signs using adhesives, read the rest of our blog series: