Selecting a material that will withstand the elements for a building’s exterior graphic display is about performance, not aesthetics. Choosing incorrectly can lead to both frustration and replacement expense. After two years of having to constantly wonder what your sign or graphic could have looked like in it’s prime, you’ll be even more frustrated when you total up the cost of buckled panels, fastener fatigue, laminated faces delaminating, and the replacement cost that is an order of magnitude more than what you initially budgeted.
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Read the Site Conditions Before You Read the Specification Sheet
Every site has it’s own set of conditions, and those conditions should be used as the primary filter when narrowing down materials. Coastal strip retail is not the same as a land-locked downtown district, despite both being in the same country, and both presumably having the same average temperature.
For example, salt water is the most aggressive catalyst for the deterioration of materials. It eats away at the inside of metal fasteners from the outside, gets trapped in microscopic crevasses in plastics, and reacts with moisture and UV light to accelerate the breakdown of laminating adhesives. In this environment, you want to specify in marine grade aluminium alloys, or high-density polyethylene as your minimum standard if you want the installation to last. Regular aluminium composites with a polyester finish won’t last as long as a panel with a PVDF coating would in this environment.
In high solar exposure environments, however, you need to worry about photo-degradation. UV light breaks down plastics, and affects the pigments in prints. Under 500-1000 hours of continuous UV exposure (ASTM G154), unprotected plastics will have a reduction in impact resistance of up to 50%, and noticeable yellowing. In high altitude or arid regions, 1000 hours of cumulative exposure can be seen in a single year.
In high-density urban environments, wind-load is the main concern. It’s not the sheer force of the wind throwing your sign around that breaks displays down, but the repeated flexing of the panel face in response to wind shear across the face of the material. This places alternating stress on the face material, fasteners, and bond lines, leading to fatigue failure.
The Differences Between ACP, Acrylic and Polycarbonate
It’s easy to think of the three substrates as being in competition with one another, but they’re actually solving different problems. Aluminium composite panel, or ACP, (also known as Dibond) is the material you choose when you want large substrates in a lightweight material without the risk of flex or bow that plastic signs will exhibit. It’s a sandwich of two aluminium surfaces with a low-density core. ACP’s primary application is in larger architectural facades, where it’s undesirable to have the panel flex under it’s own weight (i.e. a building ID sign spanning multiple meters). It’s easy to machine and shape, and has excellent print performance via direct-to-substrate printing, or graphic vinyl application.
Acrylic (PMMA, or plexiglass) has optical qualities that make it desirable in applications where image sharpness and brightness are a priority. It’s the medium of choice for dimensional lettering, cabinet faces, and illuminated displays. It’s optical clarity is unmatched, but it’s surface hardness means it tends to be more resistant to yellowing than polycarbonate or PVC, particularly when produced in cast sheets as opposed to extruded acrylic. On the other hand, it’s more prone to impact failure in low temperatures, and tends to shatter rather than flex under impact. It’s this impact resistance, along with a high surface hardness (it’s harder than polycarbonate) that makes it desirable in applications where vandalism is an issue. It’s a poor choice for any sign where the display surface is mounted lower than eye-height due to it’s fragility under impact.
Polycarbonate (Lexan) is the impact resistant option of the three. It’s impact resistance is up to 250x that of glass, and significantly better than acrylic. In applications where the sign is likely to be subject to impact, it’s the preferred medium to use. This includes areas of known hail damage, high-foot traffic areas, bus shelters, and places with a known history of vandalism. The same resilience makes it a desirable choice for applications where the sign face must be mounted below eye level. The trade-off is a relative decrease in surface hardness, and increased likelihood to scratch under abrasion. It’s also more likely to yellow over time without a properly applied UV coating. It’s a poor choice for applications where optical clarity is a consideration.
Thermal Expansion: the Silent Killer of Signage
Expansion and contraction of materials in response to temperature changes is the root-cause of more sign failures than almost any other factor. It’s easy to forget, but most materials expand at different rates when exposed to temperature changes. Most mounting conventions also do little to account for it
PVC substrates such as expanded PVC (Sintra Forex) and acrylic tend to have expansion rates several times greater than aluminium. This is a problem when applying a large format PVC banner signage or poster to an aluminium subframe. If the temperature varies by +/- 15C over the course of a day, a rigidly mounted PVC banner is going to buckle in response to the attempt to contract or expand, even if it’s mounted on all four sides at corner points. Bubbling of the face material is inevitable, and may give the appearance of structural failure even if the mounting surface itself hasn’t failed.
There are a number of solutions, most of which are ignored in the name of cost.
Drilling oversized holes in the mounting frame allows the substrate to float slightly relative to the frame, reducing the stress placed on it. Using flexible silicone adhesives rather than rigid epoxy allows for some movement in the bond line. Floating frames where the sign is mounted in a channel rather than directly to a subframe are the most tolerant option available for large format banners or posters in environments with wide temperature variation.
It’s this consideration, among others, that makes specialist sign makers in Perth such as Signage Perth, take the mounting system into account when designing their systems around Western Australia’s extreme climate.
Substrate and Graphic Durability Are Two Different Considerations
It’s easy, when discussing the lifespan of a sign, to conflate the durability of the display medium and the graphic image. This is a mistake, because they’re two very different factors when determining a sign’s overall endurance. Take ACP as an example. An ACP panel mounted externally will typically last for 20+ years without structural failure. It’s the graphic layer applied to the ACP that will begin to yellow after only 3-5 years if the wrong ink and overlaminate combination is used.
Graphic durability is a consideration when specifying what kind of graphic media to apply to a substrate. When using vinyl graphics, the overlaminate is just as important as the ink choice if the graphic is to have any longevity at all. It protects both the graphic ink and the vinyl itself from the elements, abrasion, and UV-degradation, assuming it’s a high quality UV curable vinyl laminate that’s been designed for long-term durability. Without laminating the graphic overlay, the benefit of using an outdoor rated substrate is reduced.
When talking about direct-print UV inkjet graphics on ACP or HDPE panels, the same principles apply. The UV inkjet must be able to resist UV degradation, which means using either a high quality UV ink formulated for outdoor use, or a solvent-based outdoor ink. Inks formulated strictly for interior or short-term graphics will rapidly yellow, crack, and peel off the ACP or HDPE panel.
Hardware Considerations: Galvanic Corrosion and Fastener Fatigue
Galvanic corrosion happens when two dissimilar metals are in contact with one another in the presence of an electrolyte. The presence of moisture acts as a catalyst for the reaction, and often results in one of the metals corroding far more quickly and severely that either would on their own. One of the easiest ways to avoid galvanic corrosion is to avoid putting two dissimilar metals in contact with one another. This extends to fastening systems, as a metal fastener placed in contact with a metal subframe will corrode at an accelerated rate when exposed to moisture.
The other solution is to use metals that are know to be resistant to galvanic corrosion when placed in contact with other metals. For an aluminium composite panel, this usually means using either anodized aluminium, or stainless steel fasteners and washers, depending on the application. Washers, in general, help dissipate the load placed on a fastener, and reduce the opportunity for corrosion by isolating the subframe from the fastener. There are specialized washers designed for use in signage applications, but larger-than-normal washers can often serve the same purpose.
Some sign-making applications require the use of two metals in contact with one another. The classic example is steel flats. The best way to prevent galvanic corrosion is to spread the load across a larger surface area by using larger washers. When working with plastics, washers also serve the additional function of spreading the load out over a larger surface area to reduce stress on the panels due to wind-load or thermal expansion and contraction.
Sustainability and End-of-life Considerations
Sustainability is becoming a bigger concern for external display systems, particularly with signs that contribute to a building’s broader sustainability credentials and accreditation. End of life considerations vary wildly between materials, and can make a real difference to your organisation’s sustainability targets, depending on the material you choose.
Aluminium is exceptionally sustainable, being easily recyclable at the end of the panel’s life. Clean aluminium sheeting and ACP with removable aluminium layers can be placed back into the recycling stream with confidence. HDPE has similar benefits, as it can be recycled in many cases (depending on the application) and has a long service life that often outlasts the signage requirements, decreasing the frequency of disposal. Both materials have a clear advantage over materials with polymer cores, or treated plastics that require specialist recycling at the end of their life.
Foam PVC is a tricky material at end of life, primarily due to the lack of recycling facilities in Australia to process it. The recycling value is reduced further when the material has undergone the foaming process to make it lightweight and flexible. The material still has value to many end-of-life sign recycling companies, but for a project concerned with long-term sustainability, it’s drawbacks outweigh the benefits.
The difference between a premium substrate and a budget one can have a considerable impact on a sign’s overall lifespan, both in terms of the image and the material. Premium-grade ACP and UV-stabilized cast acrylic, for example, have significantly higher resistance to UV degradation than their lower-cost counterparts, and will last significantly longer. Maintenance tends to be a factor as well, as premium materials often have simpler and more cost-effective cleaning regimes than budget materials.
The substrates that are initially more expensive tend to have a significantly lower total cost of ownership over their service life, and the maintenance regimes for most are a contributing factor to this.
Exterior substrates that have been treated to resist UV degradation and other environmental factors can often be cleaned with mild, pH neutral detergents that won’t damage their protective finishes, or the graphics applied to them. This is an important consideration as dirt and grime can detract from the appearance of almost any sign or graphic display. On most premium exterior substrates a twice-yearly cleaning regime is sufficient to remove the build up of dirt, restoring the display to it’s former condition. It also helps extend the lifespan of the protective finish, and prevents premature yellowing or cracking of graphics.
It’s also during this kind of maintenance regime that fastener fatigue and bond line failure in laminated graphics can be discovered before the failure becomes severe. Finding these issues early means they can be corrected cheaply and without downtime for the sign. Similar issues with laminated graphics can also be found much easier on premium materials, as opposed to budget substrates. The ability to correct these issues at an early stage significantly reduces total cost of ownership.
Budget substrates tend to require more intensive maintenance as a class, while also offering diminishing returns on investment. Corflute or untreated sheet PVC tends to suffer from chalking, cracking at stress points from fasteners or expansion joints, and quicker delamination of graphics than premium substrates. This increase in required maintenance has a corresponding effect on total cost of ownership, and for the same 10 year service period tends to greatly exceed the intial investment difference between premium and budget substrates. The math rarely favours budget substrates when it comes to total cost of ownership.
Getting the Specification Right in the First Place
Exterior display medium failures are few, but they’re not random events. They’re the result of a premium material being placed in an environmentally inappropriate situation, an environmentally appropriate material being mounted with an incompatible mounting system, and the entire system (material, mounting system, graphics) being specified incorrectly. In every case, the failure in question has a solution, and the cost of getting it right during the specification stage is miniscule compared to the cost of correcting failures later on
First, make sure to choose a material suited to the environmental conditions at the installation site. Then, cross-check the graphic display solution with the material to ensure compatibility. Finally, make sure the fastening system doesn’t introduce any new problems to the mix. And, if the sign is going to carry the company brand for the next decade, remember that the difference between a premium and budget material often isn’t as great as it first appears when you consider long-term savings in maintenance and downtime.
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