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Bonding with adhesives

Photos courtesy ITW Plexus

Photos courtesy ITW Plexus

By Krys Wesselink
Adhesive-based bonding offers some distinctive sign-fabrication advantages that are not possible with mechanical fasteners. A well-planned adhesive application can enhance a sign’s outdoor durability, making it more watertight, airtight and impact-resistant, while also offering the opportunity for design enhancements, including the joining of dissimilar materials, unique shapes and smooth lines free of screws, rivets, clips or other fasteners. Indeed, most sign shops today use adhesive bonding for at least some of their projects.

Chemical bonds allow lighter materials to perform at higher levels, reducing both weight and cost. Shops can assemble signs less expensively, but charge a premium price for a higher-quality end product that also reduces the subsequent need for on-site repairs.

It may seem difficult at first to visualize how an adhesive bond could possibly provide superior resistance to impact and environmental forces when compared to a mechanically fixed part. Many products, however, depend on chemical bonds for this very reason, including those in the automotive industry.

Making the right choice
Adhesive selection for sign fabrication is often made at the last minute, unfortunately, with a prevailing sentiment along the lines of “we’ll just glue it,” only to find the devil is in the details and finding the appropriate product is not as easy as originally thought. To take full advantage of this technology, a bonding application should instead be determined as part of the planning process well ahead of time.

There are several issues to consider in terms of both the bonding process and the performance of the finished sign. While the number of possible combinations of substrates, production processes and client requirements is nearly infinite, the number of adhesive types is not.

Adhesives can join dissimilar materials, such as bonding polycarbonate resin thermoplastics to extruded aluminum.

Adhesives can join dissimilar materials, such as bonding polycarbonate resin thermoplastics to extruded aluminum.

So, broadly speaking, the following questions need to be answered before selecting an adhesive for a sign:

  • Which materials are being used and how will they fit together?
  • How will the sign be built (e.g. working vertically or laying flat) and how much time is there for the fabrication process?
  • Will the sign be installed indoors or outdoors?
  • What environmental factors will affect the sign?
  • How much stress will it need to withstand and for how long?
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Taken together, such variables as substrate materials, workflow and long-term performance may seem to conflict with each other. Achieving the right mix can be like a jigsaw puzzle, but with forethought and expert advice, what initially seems complicated will come into clearer focus.

As mentioned, adhesive bonding offers options that other methods do not. Welding, for example, cannot combine two different metals. A chemical bond, on the other hand, can be achieved not only between two different metals, but also in combinations of materials like plastics, glass and wood, in a far simpler and broader way, allowing for more creative designs.

Compared to welding, chemical bonding eliminates the worry of distorting sign substrates with heat. It also provides insulation against galvanic corrosion. Glass, in particular, would be very challenging to incorporate into sign designs without adhesives, as would any two materials with significantly different thermal expansion and contraction properties, flexibility or other physical properties.

It is important to choose the right adhesive for the materials being bonded.

It is important to choose the right adhesive for the materials being bonded.

Time and temperature
In bonding applications, two ‘countdowns’ begin as soon as the adhesives are applied or their components are mixed.

The first counts down the curing process. The second counts down ‘open’ or ‘working’ time, during which the operator can lay down the beads of adhesive and work with them, marrying the two surfaces together. After this working time ends, it will be too late to guarantee adequate adhesion between the first and second surface.

So, it is important to select an adhesive whose capabilities match the sign fabrication workflow, based on how much time the operator needs to assemble the parts and then can wait before moving them again.

Fortunately, there is usually no need to wait for full cure. Instead, it is likely sufficient to wait for ‘fixture time,’ when the bond has achieved a fair percentage of final cure strength.

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Besides time, temperature is another important consideration when fabricating a sign, especially if the work processes are being handled outdoors. Ambient temperature affects all chemical reactions—heat speeds them up, while cold slows them down. So, adhesives must be applied within their specific temperature guidelines.

Ambient temperature is not the only concern in this respect. Adhesives also need to be stored at stable temperatures to ensure they remain usable, as do the corresponding sign substrates. Cold substrates can act as heat sinks and tend to develop a thin film of condensation that can prevent good adhesion.