Aluminium precoated coil has been widely used in fabricated letters because it's lightweight, easy to bend, and available in long strips. The problem is not always the glue itself. In outdoor use, the limiting factor is often the coating system beneath the glue.
At Aussie 3D Fabrications, we tested coated coil samples under controlled pull conditions to better understand where these failures start. The goal was not just to find a stronger adhesive, but to separate adhesive failure from coating failure.
1. How We Tested Adhesion
The test samples used 20 mm wide, 80 mm high coated coil tabs bonded to an acrylic surface. Each sample was pulled until failure and the peak force was recorded in Newtons (N). Higher force readings indicate a stronger joint, but the number only matters when it is read alongside the failure mode.
Testing was performed in two conditions:
- Ambient temperature, representing cooler workshop or indoor conditions.
- 70 °C elevated temperature, representing the kind of surface heat an exterior sign can experience on a hot wall or fascia.
Where duplicate samples were tested, we compared the results for consistency. Large differences between repeated pulls are important because they point to variability in the coating, surface preparation, adhesive application, or curing behaviour.
2. Why Failure Mode Matters
In a simple pull test, it is tempting to rank samples by the highest Newton reading. That can be misleading. A joint can record a high peak load while the paint film is already cracking or separating from the aluminium at a much lower force.
For signage fabrication, the key question is: what actually failed?
- Adhesive failure: the glue releases from one of the bonded surfaces.
- Cohesive failure: the glue itself tears internally.
- Coating failure: the paint or coating separates from the metal substrate.
Coating failure is the main concern with precoated aluminium coil. If the glue remains attached to the paint but the paint lifts from the aluminium, using a stronger glue does not solve the problem. The weak link is below the adhesive layer.
3. What the Heat Testing Showed
The test results showed that adhesive performance changes significantly with temperature. Some combinations that performed well at ambient temperature dropped sharply at 70 °C. Other combinations improved under heat, which confirms that adhesive selection cannot be judged from workshop conditions alone.
This is important because outdoor signage materials rarely stay at air temperature. Darker surfaces, enclosed letter returns, and sun-facing elevations can all raise material temperatures enough to change adhesive behaviour.
4. What the Coating Observations Showed
Several coated aluminium samples showed paint cracking or delamination at relatively low force, even when the final pull value was much higher. In practical terms, that means the adhesive may have been stronger than the coating-to-metal bond.
That distinction matters in fabricated letters. The return is not just being pulled once in a laboratory. It is exposed to heat cycles, vibration, transport handling, installation loads, and long-term weathering. If the coating starts to release at the interface, the finished letter can fail even though the glue is still bonded to the paint.
5. Why Aluminium Coil Is Higher Risk Outdoors
Precoated aluminium coil can still be suitable for indoor letters, displays, and controlled environments. Indoors, temperatures are more stable, UV exposure is limited, and the bonded joint is not repeatedly pushed through hot-cold cycles.
Outdoors, the risk profile changes:
- Elevated surface temperature can reduce adhesive strength or soften the coating.
- Aluminium expands and contracts more than the coating system above it.
- Thermal cycling repeatedly stresses the coating-to-metal interface.
- If the coating releases, the glue joint fails regardless of adhesive strength.
The failure points are therefore not just workmanship issues. They are material-system issues involving the adhesive, coating, substrate, temperature, and load path.
6. How We Use This Data
Our testing changed how we assess materials. We look at peak pull force, but we also record whether the glue failed, the coating cracked, or the paint separated from the metal. A lower but consistent result with clean failure behaviour can be more useful than a high number with early coating delamination.
That is why we limit aluminium precoated coil to indoor applications and use our EcoSteel outdoor letter system for outdoor fabricated letters. The decision is based on the full test method: temperature exposure, tensile pull strength, repeatability, and failure-mode inspection.
For trade customers, the outcome is simple. The finished letter needs a bond that survives real outdoor conditions, not just a glue that records a high number in one room-temperature test.