If the adhesive sets before the parts are properly placed and aligned, it leads to a poorly positioned bond or high internal stress.

Consequences

• Misalignment: Parts cannot be adjusted after initial contact, leading to scrap or rework.
• Incomplete Wet-Out: The adhesive gels before it fully spreads across the entire bond area, resulting in a starved joint with low strength and gaps.
• High Cure Stress: Rapid curing generates heat and shrinkage stress quickly, increasing the risk of cracking or bond failure upon removal from the fixture.

Solutions

• Increase Working Time: Switch to a UV adhesive formulated with a slower photoinitiator package, increasing its “open time” or working time before it fully sets under light.
  
• Reduce UV Exposure: If using a high-intensity lamp, reduce the UV irradiance during the initial “tack cure” phase. This slows the polymerization slightly, allowing extra time for minor adjustments.
  
• Separate Application/Cure: If the parts require complex alignment, use a UV adhesive that relies on a specific high-intensity UV wavelength for cure. Apply the adhesive under a different (non-curing) wavelength of light (e.g., standard room lighting shielded with UV filters) to maximize working time, then move to the high-intensity light for immediate cure.

2. Problem: Cure is Too Slow (Movement Before Set)

If the adhesive remains liquid for too long after placement, the parts are susceptible to shifting, or the assembly process is significantly bottlenecked.

Consequences

• Part Shifting/Slump: Gravity, vibration, or handling during transfer can cause parts to shift relative to each other before the adhesive develops sufficient strength (“green strength”) to hold them in place.
  
• Slow Production Rate: Long cure times tie up fixtures and floor space, limiting the throughput of the assembly line.
  
• Flow-Out: As noted previously, on vertical or inverted joints, slow curing allows the liquid adhesive to flow out of the joint due to gravity.

Solutions

• Increase Cure Intensity/Time:
  • Increase UV Irradiance: Use a higher-power UV light source (e.g., switch from an LED to a high-intensity arc lamp) to speed up the reaction kinetics.
  • Increase Exposure Time: Slow the conveyor or increase the dwell time under the lamp to ensure the part receives the full required dose quickly.
    
• Implement Tack Cure: Use a small, focused UV spot lamp for a flash cure (1-3 seconds) on a small, non-critical area of the joint. This rapidly achieves enough green strength to fix the parts in position, preventing movement, before the final, full cure is performed.
  
• Use Dual-Component Systems: If the assembly is complex and needs high green strength before UV exposure, consider a dual-cure system where a small amount of a fast-setting catalyst is mixed in, which initiates a quick, initial cure while the UV remains the primary final cure mechanism.