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implemented immediately after assembly, or indeed sometimes in-line as part of the process. Yields are high, since the process is readily controlled and robust.

SPECIFYING THE ADHESIVE So what does a design engineer need to think about in order to specify a light curing adhesive? • Substrates

DESIGNING-IN light curing adhesives

T been well documented1

he advantages of using adhesives over mechanical fasteners have . They include

distribution of load and stress over wider surface areas, elimination of joint fatigue, improved impact resistance, reduction in finishing and improved aesthetics. But, specifying a light curing adhesive

can offer even more benefits to the design engineer. In addition to the performance advantages listed above, these are single part, which means that no mixing or mixing equipment is required, and there is less waste. They also cure very quickly, typically in seconds, and ‘on demand’, in that they only start curing when exposed to light of the correct wavelength. This reduces the need for jigs and tools, and minimises WIP and factory space needs.

BONDING MATERIALS Light curing adhesives produce true structural bonds to glass, metal, ceramic and many plastics, including ABS, PC, PMMA, PA and PVC (as well as many less well-known polymers). They are used in assemblies with multiple substrate types (such as glass/steel/plastic) and, with the correctly chosen formulation, tensile, shear and peel strengths are high (2,500 to 4,000psi at break). In general, they are resilient, tough,

provide good impact strength, and are available in a hardness range of Shore A80 to Shore D80 (and in the Shore 00 range for light curable gaskets). There are viscosity variations to suit wicking and thin bondlines, or to fill gaps; to be self-levelling, or to be applied to a vertical surface. The operating temperature window is typically -55˚C to 150˚C.

Historically, these materials were cured

with UV light, reacting to light in the near-visible, long-wave UV-A range (315-400nm). Since many plastics have UV blocking ingredients in them to help prevent the embrittlement and yellowing caused by ambient UV light, plastic bonding adhesives use a synergistic combination of UV and visible light to generate fast and effective cures. UV curing equipment based on metal-halide lamps produce broad spectrum light suitable for curing most materials. The latest developments in curing technology include curing equipment based on LEDs, which produce a narrow spectrum light, centred around a specific wavelength (i.e. 385nm or 395nm). While not a drop-in replacement for broad spectrum lamps, UV LED curing lamps are becoming more popular, and adhesive formulations are emerging which are optimised to cure with their output. Due to the on-demand cure, single component formulation and viscosity choice, light curing adhesives are very easy to employ in automated processes, and a wide variety of dispensing technologies can be used with repeatable precision. As the cure is so fast, quality assessment procedures can be


Light curing adhesives can bond many types of plastic

Why should a design engineer consider using light curing adhesives? Peter Swanson, managing director of Intertronics, looks into their benefits and examines the considerations

curing adhesives produce true

structural bonds to glass, metal, ceramic and many plastics, including ABS, PC, PA and PVC”

Curing lamps based on LEDs are becoming more popular

As previously discussed, the adhesives can be used with many substrates, including a wide range of plastics. But, in common with all adhesives, low surface energy polymers like PTFE or polypropylene are not readily bondable with light curing adhesives. Polyolefins may be surface treated in order to improve wettability and therefore adhesion, and so attain adequate bond strength. Also, the technology has poor adhesion to rubber or silicone so, if possible, choose other substrates in your design. • Bondline design Whilst the adhesives are gap filling or can wick into tight bondlines, optimal bondline thickness is on the order of 0.125mm. With the correct viscosity choice, the adhesive can be very forgiving to tolerance variations in the fit of the parts. This can save money by being able to specify less accurate mould tools than if the assembly were designed to be solvent

welded or press-fit. • Light transmission The acrylated-urethane light cure chemistry needs to be exposed to light of the correct

wavelength in order to cure. Light must reach the adhesive in the entire bondline; the light cure does not propagate into shadowed areas by itself. This means that most suitable applications are when at least one of the substrates is clear. However, the latest developments in plastic bonding adhesives include formulations with a UV/visible light cure and a secondary ambient moisture cure system for shadow areas. There are many proven applications in

industries like electronics, automotive, medical devices, electronic displays, optics, glass and appliances – and all manner of plastic-based assemblies. In fact the technology brings with it inherent productivity gains which can save money and increase competitiveness.2

Image courtesy of References • 1

Intertronics T: 01865 842842 “Where Adhesives Beat Mechanical Fasteners” - Nicole Langer,

Machine Design, August 2005 • 2

“Light-Curable Adhesives” – Virginia P Read, Appliance Design, February 2010 DESIGN SOLUTIONS | SEPTEMBER 2014 13 Enter 206

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